JP2008103511A - Circuit and method for driving light-emitting element, and video display device equipped with the light-emitting element driving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light-emitting element driving circuit, where gamma correction can precisely be performed on a light-emitting element, such as a laser diode using a comparatively small circuit scale. <P>SOLUTION: Light intensity detection voltage DET of the laser diode 43 is generated. Reference voltages VREF<SB>0</SB>, VREF<SB>1</SB>, ..., VREF<SB>2m-1</SB>, corresponding to changes from a minimum value to a maximum value of input video signals DIN<SB>0</SB>, DIN<SB>1</SB>, ..., DIN<SB>n-1</SB>, are set; differential voltages CM<SB>0</SB>, CM<SB>1</SB>, ..., CM<SB>2m-1</SB>between respective reference voltages and light intensity detection voltage DET are generated. Since the drive current I<SB>LD</SB>is controlled, based on a differential voltage corresponding to an input signal level at that time for respective differential voltages, the output light intensity of the laser diode 43 becomes always constant, with respect to a prescribed input level, even if the temperature changes. Since the output light intensity of the laser diode 43 has a gamma characteristic proportional to the power of 2.2 of an input video signal level, gamma correction is performed accurately. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a light emitting element driving circuit, a light emitting element driving method, and a video display device including the light emitting element driving circuit, and in particular, drives a light emitting element such as a laser diode (semiconductor laser) based on an input video signal. The present invention relates to a light-emitting element driving circuit, a light-emitting element driving method, and a video display device including the light-emitting element driving circuit which are suitable for use in modulating output light intensity.

  In projector-type image display devices, a halogen lamp has been used as a light source in the past, but in recent years, a white light source using a laser diode has been developed, and a device using this laser diode as a light source has been put into practical use. . In such an image display device, light (laser beam light) emitted from a light source is scanned and projected on a screen by a scanning device such as a scanning mirror to display an image. In order to express gradation by such an image display device, it is necessary to modulate the light from the light source by some means. As one of the means, there is a method of modulating the output light intensity by directly controlling the current for driving the laser diode. In laser printers and copiers, a laser diode is used as a light source for writing an image with a laser beam. Generally, a drive current of the laser diode is directly modulated and controlled.

  Such a laser diode emits laser light when a current (threshold current) of a certain value or more flows, but has a characteristic that the drive current / light output characteristic, particularly the threshold current fluctuates due to a temperature change or a change with time. Therefore, even if the laser diode is driven at a constant current, there is a problem that the output light intensity varies with the variation of the threshold current. In order to improve this problem, automatic power control (APC) is used. In the automatic light amount adjustment, the output light intensity of the laser diode is detected, and the drive current is controlled based on the detection result. In general, in a video display device or an image writing device using automatic light amount adjustment, a light amount adjustment voltage is generated during a non-display period (blanking period) included in each frame period of a given input video signal, and a hold circuit In the video display period, a constant drive current corresponding to the light amount adjustment voltage is supplied from the laser diode drive circuit to the laser diode.

Conventionally, this type of light emitting element driving circuit has a current output circuit 1, a bias current source 2, a photodiode (PD) 3, and a current / voltage conversion circuit (IV) as shown in FIG. 4, a differential amplifier 5, and a sample and hold circuit (S / H) 6, and a laser diode 7 is connected to the current output circuit 1 and the bias current source 2. The current output circuit 1 responds to the input digital signal Din (“D ₇ D ₆ ... D ₁ D ₀ ”) with each current source (1/2, 1/4, 1/8, 1/16, 1/32). , 1/64, 1/128, 1/256) is combined to output a current Ia that increases or decreases in binary. The bias current source 2 outputs a bias current I _BIAS that is controlled based on the control voltage cv from the sample hold circuit 6. The photodiode 3 receives the output light of the laser diode 7 and outputs a current corresponding to the output light intensity. A current / voltage conversion circuit (IV) 4 converts the output current of the photodiode 3 into a voltage va. The differential amplifier 5 takes the difference between the voltage va of the current / voltage conversion circuit 4 and the reference voltage VREF and outputs a differential voltage vb. The sample hold circuit 6 samples the differential voltage vb of the differential amplifier 5 based on the sampling pulse SP and then holds it to output the control voltage cv.

In this light emitting element driving circuit, the input digital signal Din is input to the current output circuit 1, and the laser diode 7 is driven by the current Ia output from the current output circuit 1. At this time, the photodiode 3 outputs a current corresponding to the light intensity of the laser diode 7, and the current / voltage conversion circuit 4 converts the current into a voltage va. A difference between the voltage va and the reference voltage VREF is obtained by the differential amplifier 5, and the differential voltage vb is output from the differential amplifier 5. The differential voltage vb is sampled and held by the sample and hold circuit 6, and the control voltage cv is output from the sample and hold circuit 6. Based on this control voltage cv, the bias current I _BIAS of the bias current source 2 is controlled. Finally, the voltage va of the current / voltage conversion circuit 4 becomes equal to the reference voltage VREF, and the laser diode 7 receives the input digital signal Din. Emits output light with an intensity corresponding to. Thereby, the fluctuation | variation of output light intensity is suppressed with respect to the fluctuation | variation of the threshold current of the laser diode 7. FIG.

However, in this light emitting element driving circuit, since the slope of the input signal / output current characteristic of the current output circuit 1 is not controlled, it corresponds to the fluctuation of the slope of the driving current / light output characteristic of the laser diode 7 (external differential quantum efficiency). There is a problem that it is not possible.
As a light emitting element driving circuit for improving this problem, for example, there is one described in Patent Document 1.

The light emitting element driving circuit described in Patent Document 1 (in the same document, “light emitting element driving device”) is a laser diode driving device 10 as shown in FIG. 26, and the light detection voltage Vmon of the laser diode (LD). , And a plurality of comparators 12 ₁ ,... For comparing the detection voltage Vmon output from the output light intensity detector 11 with a plurality of reference voltages Vref1,. and 12 _n, the comparator 12 _1, ..., a plurality of sample and hold circuits 13 ₁ for holding an output differential voltage of 12 _n, ..., and a 13 _n. These sample hold circuits 13 _1, ..., the voltage held by 13 _n, voltage / current conversion circuit 14 _1, ..., current I ₁ at 14 _n, ..., after being converted to I _n, the arithmetic circuit 15 by arithmetic processing based on a predetermined step is performed, the threshold current Ith of the laser diode is required, the bias current I _bias on the basis of the threshold current Ith is outputted, the bias current I _bias is a current adder 17 It is given as one addition input.

The switch circuit 16 performs an on / off (switching) operation in accordance with data (DATA) input via the data input terminal 20, and generates a switching current Isw corresponding to the current I ₁ supplied from the current conversion circuit 14 _1. And output as the other addition input of the current adder 17. The current adder 17 adds the bias current I _bias and the switching current Isw and supplies the sum to the drive circuit 18. The drive circuit 18 is driven by supplying a drive current corresponding to the current supplied from the current adder 17 from the anode side of the laser diode. As described above, in this laser diode driving apparatus 10, since the APC control is performed on the bias current I _bias and the switching current Isw, the threshold current fluctuation and the slope fluctuation of the driving current / light output characteristics of the laser diode are affected. It becomes possible to respond to this.

On the other hand, when an image based on an input video signal is displayed on a display other than a CRT (Cathode Ray Tube), so-called gamma correction is required for the output light intensity of the laser diode in order to correspond to the light emission luminance characteristics of the CRT. There is. In gamma correction, the output light intensity of the laser diode is adjusted so as to be proportional to the level of the input video signal to the power of 2.2. Therefore, even in a projector using a laser diode, it is necessary to pass a drive current through the laser diode so that the output light intensity of the laser diode is proportional to the image signal level raised to the 2.2th power.
JP 2003-298178 (abstract, FIG. 1)

However, the conventional light emitting element driving circuit has the following problems.
That is, in the light emitting element driving circuit described in Patent Document 1, in order to perform gamma correction, it is necessary to control the driving current of the laser diode so as to be proportional to the power of the input video signal level to the power of 2.2. Requires a lookup table (LUT) describing the correspondence between the input video signal level and the laser diode drive current. Since the LUT is composed of a large-capacity memory, there is a problem that the circuit scale of the light emitting element driving circuit is increased and the chip area is increased when the light emitting element driving circuit is composed of an integrated circuit.

  The present invention has been made in view of the above circumstances. Even when the output light intensity characteristic of a laser diode has a function of performing gamma correction, the present invention can be realized with a relatively small circuit scale and can be accurately performed while performing APC control. It is an object of the present invention to provide a light emitting element driving circuit capable of performing gamma correction, a light emitting element driving method, and an image display device including the light emitting element driving circuit.

  In order to solve the above problems, the invention according to claim 1 relates to a light emitting element driving circuit for modulating and driving a light emitting element based on the level of an input signal, detecting the light intensity of the light emitting element, and obtaining the detection result. A light intensity detection means for generating a light intensity detection signal based on the reference signal, and a plurality of reference signals having a predetermined non-linear relationship with respect to the level of the input signal, from the minimum value to the maximum value of the input signal level Difference signal generating means for generating a difference signal between each of the reference signals and the light intensity detection signal in response to a change in the difference, and difference signal holding means for holding the difference signals generated by the difference signal generating means; And a light intensity control means for driving the light emitting element based on a difference signal corresponding to the current input signal level among the difference signals held in the difference signal holding means. It is characterized in that.

  The invention according to claim 2 relates to the light emitting element drive circuit according to claim 1, wherein the light intensity detecting means receives the output light of the light emitting element and outputs a detection current corresponding to the light intensity of the output light. And at least one current / voltage conversion circuit that converts the detection current output from the light detection element into a light intensity detection voltage, and the differential signal generation means includes the plurality of differential signal generation means. A plurality of reference voltages are set as reference signals, a differential amplifier that takes a difference between the light intensity detection voltage and each reference voltage and outputs a plurality of difference voltages as the difference signal, the difference signal holding means A sample-and-hold circuit that samples and holds each differential voltage and outputs a plurality of hold voltages, and the light intensity control means receives the input signal from the plurality of hold voltages. Select the held voltage corresponding to the bell, and a drive current based on the selected hold voltage characterized by comprising a current output circuit for outputting to the light emitting element.

  The invention according to claim 3 relates to the light emitting element drive circuit according to claim 2, wherein the light emitting element spontaneously emits light when the drive current is equal to or less than a predetermined threshold current, and laser when the drive current is equal to or greater than the threshold current. A laser diode that emits light, and the current output circuit outputs the drive current that causes a minimum output light intensity of the laser diode to be equal to or less than a light intensity when the laser diode starts the laser emission; It is characterized by being.

  The invention according to claim 4 relates to the light emitting element driving circuit according to claim 2 or 3, wherein the current output circuit has n bits (n; n) corresponding to a change from a minimum value to a maximum value of the level of the input signal. Natural number) digital signal is input, and the range of the input signal is divided into 2 ^ (n−k) areas based on the upper (nk) bits (k: natural number) of the digital signal, In the region, the output current of the current output circuit increases and decreases linearly and linearly based on the lower k bits of the digital signal.

  The invention according to claim 5 relates to the light emitting element drive circuit according to claim 4, wherein the current output circuit selects one of the plurality of hold voltages based on upper (nk) bits of the digital signal. The first selection circuit that outputs the first control voltage, the main current source that outputs the first current corresponding to the first control voltage, and the upper (n−k) bits of the digital signal A second selection circuit that selects a hold voltage output next to a hold voltage corresponding to the first control voltage from the plurality of hold voltages and outputs the selected second hold voltage as the second control voltage; and the digital signal A sub-current source that outputs a second linearly increasing / decreasing value by the lower k bits and adjusting the second current based on the second control voltage; and a first current of the main current source Current and the secondary current source second It is characterized in that it is composed of a current adder for adding the flow Prefecture.

  The invention according to claim 6 relates to the light emitting element driving circuit according to claim 5, wherein the current output circuit outputs a third current having a current value corresponding to the least significant bit of the lower k bits of the digital signal. The third current is varied according to the second control voltage selected by the second selection circuit, and the third current is on / off controlled based on a given control signal. An adjustment current source is provided.

  A seventh aspect of the present invention relates to the light emitting element driving circuit according to the second aspect, wherein the differential signal generating means selects one reference voltage from the plurality of reference voltages in a time division manner to the differential amplifier. A reference voltage selection circuit for sending is provided.

  The invention according to claim 8 relates to the light emitting element drive circuit according to claim 4, 5 or 6, wherein the digital signal is input during the non-display period of the input video signal having a display period and a non-display period in one frame period. Is supplied to the current output circuit, and an input signal switching circuit is provided for transmitting the input video signal as it is to the current output circuit as the input signal during the display period.

  A ninth aspect of the invention relates to the light emitting element driving circuit according to the eighth aspect of the invention, wherein the plurality of reference voltages are set in a relationship proportional to the power of 2.2 of the level of the input video signal. It is said.

  A tenth aspect of the present invention relates to a light emitting element driving method used in a light emitting element driving circuit that modulates and drives a light emitting element based on a level of an input signal. The light intensity of the light emitting element is detected, and based on the detection result. A light intensity detection process for generating a light intensity detection signal, and a plurality of reference signals having a predetermined non-linear relationship with respect to the level of the input signal, and setting a minimum value to a maximum value of the input signal level. A difference signal generation process for generating a difference signal between each reference signal and the light intensity detection signal in response to a change, a difference signal holding process for holding each generated difference signal, and each of the held signals A light intensity control process for driving the light emitting element is performed based on a difference signal corresponding to a current level of the input signal among the difference signals.

  An eleventh aspect of the present invention relates to an image display apparatus, and includes the light emitting element driving circuit according to any one of the first to ninth aspects.

  According to the configuration of the present invention, the light intensity detection means generates a light intensity detection signal of the light emitting element, and the difference signal generation means corresponds to each reference corresponding to the change from the minimum value to the maximum value of the level of the input signal. A difference signal between the signal and the light intensity detection signal is generated, and each difference signal is held by the difference signal holding means. Then, the light intensity control means is driven to modulate the light emitting element based on the difference signal corresponding to the level of the current input signal among the difference signals held in the difference signal holding means. On the other hand, a light emitting element driving circuit having a predetermined output light intensity characteristic and having little influence on the output light intensity characteristic can be realized even if the characteristics of the light emitting element fluctuate due to temperature change or aging. Further, each reference voltage as each reference signal is set to a relationship proportional to the power of the input video signal level to the power of 2.2 (that is, a gamma characteristic). A drive current that directly matches the gamma characteristic can be output, and a look-up table (LUT) for gamma-converting the input video signal becomes unnecessary. Therefore, it is possible to provide a light emitting element driving circuit in which gamma correction is accurately performed with a relatively small circuit configuration.

  A plurality of reference voltages (reference voltages) used for APC control are set. Further, in correspondence with an input video signal, a value of these reference voltages is a gamma characteristic, and APC control is performed for a plurality of video signal levels. Provided are a driving circuit, a light emitting element driving method, and an image display device including the light emitting element driving circuit.

FIG. 1 is a block diagram showing an electrical configuration of a main part of a light emitting element driving circuit for explaining the basic principle of the present invention.
As shown in the figure, the light emitting element driving circuit includes a driving unit 31, a light emitting element 32, a light intensity detecting unit 33, reference voltage sources 34 ₁ , 34 ₂ ,..., 34 _t, and a differential amplifier 35. ₁ , 35 ₂ ,..., 35 _t and a sample hold circuit (S / H) 36. The drive unit 31 supplies a drive current I _LD corresponding to the input video signal Vin to the light emitting element 32. The light emitting element 32 is configured by a laser diode and outputs light having an intensity corresponding to the driving current I _LD . Light intensity detecting unit 33 supplies a detection voltage DET corresponding to the intensity of the output light p of the light emitting element 32 differential amplifier 35 _1, 35 _2, ..., to 35 _t. Reference voltages 34 ₁ , 34 ₂ ,..., 34 _t are predetermined reference voltages VREF ₀ , VREF ₁ ,..., VREF _t− corresponding to changes from the minimum value to the maximum value of the level of the input video signal Vin. Generate ₁

Differential amplifier _{35 1, 35 2, ...,} 35 t , the reference voltage VREF ₀ and the detection voltage DET, VREF _1, ..., the differential voltage cv ₀ and _{VREF t-1, cv 1,} ..., a cv _t-1 Output. Sample-and-hold circuit 36, differential voltage _{cv 0, cv 1, ...,} cv t-1 sampled and held and output voltage hv _0, hv _1, ..., and outputs the hv _t-1. The drive unit 31 receives the output voltages hv ₀ , hv ₁ ,..., Hv _t−1 as control voltages, and controls the drive current I _LD based on these control voltages.

2 is a characteristic diagram showing the relationship between the input video signal level and the reference voltage, FIG. 3 is a characteristic diagram showing the relationship between the drive current I _LD of the light emitting element 32 and the output light intensity, and FIG. 4 is a light intensity detection. characteristic diagram showing the relationship between the light-receiving intensity and the detection voltage DET parts 33, FIG. 5 is a characteristic diagram showing the relationship between the input video signal level at the time the temperature TP1 of the drive unit 31 and the drive current I _LD, 6, FIG. 7 is a characteristic diagram showing the relationship between the input video signal level and the drive current I _LD when the driving unit 31 is at the temperature TP2, and FIG. 7 is a characteristic diagram showing the relationship between the input video signal level and the output light intensity of the light emitting element 32. is there.
The operation of the light emitting element driving circuit of FIG. 1 will be described with reference to these drawings.

First, as shown in FIG. 2, the reference voltage is set in a predetermined non-linear relationship with respect to the input video signal level. That is, the reference voltage VREF ₀ is the video signal level VIN ₀ , the reference voltage VREF ₁ is the video signal level VIN ₁ , the reference voltage VREF ₂ is the video signal level VIN ₂ , and similarly, the reference voltage VREF _t−2 is the video signal level. VIN _t−2 and reference voltage VREF _t−1 correspond to the video signal level VIN _t−1 , respectively. Therefore, the characteristic curve of FIG. 2 is divided into (t−1) pieces corresponding to the regions R ₁ , R ₂ ,..., R _t−1 , and the reference voltages are VREF ₀ , VREF ₁ ,. _It consists of _{t-1 t-1} .

Then, the drive unit 31 outputs a drive current I _LD corresponding to the input video signal Vin, and based on output voltages (control voltages) hv ₀ , hv ₁ ,..., Hv _t−1 from the sample hold circuit 36, The drive current I _LD is controlled. Since the light emitting element 32 is composed of a laser diode, as shown in FIG. 3, the light emitting element 32 emits little light when the drive current I _LD is less than or equal to the threshold current (I _0.1 , I _0.2 ), and the drive current is equal to or greater than the threshold current. In addition to emitting light with a light intensity proportional to I _LD , the threshold current and slope (ΔP / ΔI, external differential quantum efficiency) vary according to changes in ambient temperature (TP1 <TP2) and over time. Light intensity detecting unit 33, as shown in FIG. 4, the light receiving intensity P _0, P _1, ..., the detection voltage DET _0, DET ₁ that increases linearly with P _t-1, ..., a DET _t-1 Output. The sample and hold circuit 36 samples the differential voltages cv ₀ , cv ₁ ,..., Cv _t−1 output from the differential amplifiers 35 ₁ ,..., 35 _t at a predetermined timing. cv ₀ , cv ₁ ,..., cv _t-1 ) and output as output voltages (control voltages) hv ₀ , hv ₁ ,..., hv _t−1. And output as the output voltages hv ₀ , hv ₁ ,..., Hv _t−1 .

Next, operations (1) and (2) at ambient temperatures TP1 and TP2 will be described.
(1) Operation at Temperature TP1 At a certain time T1, as shown in FIG. 5, when the input video signal level is VIN _0, a drive current I _0.1 is output from the drive unit 31 and supplied to the light emitting element 32. The light emitting element 32 emits light with a light intensity P ₀ corresponding to the drive current I _0.1 . In the light intensity detecting unit 33, the detection voltage DET ₀ corresponding to the light intensity P ₀ is output, the detection voltage DET ₀ differential amplifier 35 _1, 35 _2, ..., is supplied to the 35 _t. Among these, in the differential amplifier 35 ₁ , the difference between the detection voltage DET ₀ and the reference voltage VREF ₀ is taken, and the differential voltage cv ₀ is output. In the sample and hold circuit 36, the differential voltage cv ₀ is sampled and output to the drive unit 31 as the control voltage hv ₀ . In the drive unit 31, the driving current I _0.1 is controlled based on a control voltage hv _0. Therefore, at this time T1, as the APC loop, “driving unit 31 → light emitting element 32 → light intensity detecting unit 33 → detection voltage DET ₀ → differential amplifier 35 ₁ → differential voltage cv ₀ → sample hold circuit 36 → control voltage hv ₀ → Driver 31 ″ is formed, and the value of the control voltage hv ₀ at this time is determined so that the light emitting element 32 emits light with the light intensity P ₀ determined by the reference voltage VREF ₀ .

Next, at a certain time T 2, when the input video signal level is VIN ₁ , the drive current I _1.1 is output from the drive unit 31 and supplied to the light emitting element 32. The light emitting element 32 emits light with a light intensity P ₁ corresponding to the drive current I _1.1 . In the light intensity detecting unit 33, the detection voltage DET ₁ corresponding to the light intensity P ₁ is output, the detection voltage DET ₁ differential amplifiers 35 _1, 35 _2, ..., is supplied to the 35 _t. Among these, in the differential amplifier 35 ₂ , the difference between the detection voltage DET ₁ and the reference voltage VREF ₁ is taken, and the difference voltage cv ₁ is output. In the sample and hold circuit 36, the differential voltage cv ₁ is sampled and output to the drive unit 31 as the control voltage hv ₁ . In the drive unit 31, the driving current I _1.1 is controlled based on a control voltage hv _1. Accordingly, at this time T2, as the APC loop, “driving unit 31 → light emitting element 32 → light intensity detecting unit 33 → detection voltage DET ₁ → differential amplifier 35 ₂ → differential voltage cv ₁ → sample hold circuit 36 → control voltage hv ₁ → Driver 31 ″ is formed, and the value of the control voltage hv ₁ at this time is determined so that the light emitting element 32 emits light with the light intensity P ₁ determined by the reference voltage VREF ₁ .

Similarly, the APC control is performed by switching the input video signal level and the reference voltage associated with the input video signal level, whereby the light emitting element 32 is determined by the reference voltages VREF ₀ ,..., VREF _t−1 . light intensity P _0, ..., to emit light P _t-1, the control voltage output from the sample hold circuit _{36 hv 0, hv 1, ...} , is determined hv _t-1. As a result, the drive current I _LD output from the drive unit 31 has the characteristics shown in FIG. 5 with respect to the input video signal level. Further, since the reference voltages VREF ₀ ,..., VREF _t−1 have a nonlinear relationship with the input signal level, the output light intensity of the light emitting element 32 also has a nonlinear relationship with the input signal level as shown in FIG. It is in.

(2) Operation at Temperature TP2 At a certain time T1, as shown in FIG. 6, when the input video signal level is VIN _0, a drive current I _0.2 is output from the drive unit 31 and supplied to the light emitting element 32. The light emitting element 32 emits light with a light intensity P ₀ corresponding to the drive current I _0.2 . In the light intensity detecting unit 33, the detection voltage DET ₀ corresponding to the light intensity P ₀ is output, the detection voltage DET ₀ differential amplifier 35 _1, 35 _2, ..., is supplied to the 35 _t. The differential amplifier 35 ₁ outputs a differential voltage cv ₀ between the detection voltage DET ₀ and the reference voltage VREF ₀ . In the sample and hold circuit 36, the differential voltage cv ₀ is sampled and output to the drive unit 31 as the control voltage hv ₀ . In the drive unit 31, the driving current I _0.2 is controlled based on a control voltage hv _0. Therefore, at this time T1, as the APC loop, “driving unit 31 → light emitting element 32 → light intensity detecting unit 33 → detection voltage DET ₀ → differential amplifier 35 ₁ → differential voltage cv ₀ → sample hold circuit 36 → control voltage hv ₀ → Driver 31 ″ is formed, and the value of the control voltage hv ₀ at this time is determined so that the light emitting element 32 emits light having an intensity P ₀ determined by the reference voltage VREF ₀ .

Accordingly, even if changes in the temperature TP1 TP2, a reference voltage _{VREF 0, ..., VREF t-} 1 light intensity P ₀ determined by, ..., so that the light emission element 32 emits light with P _t-1, sample-and-hold circuit The control voltages hv ₀ , hv ₁ ,..., Hv _t−1 output from 36 are determined, and the output current of the drive unit 31 has the characteristics shown in FIG. 6 with respect to the input video signal level. As a result, the light emitting element driving circuit operates so that the output light intensity of the light emitting element 32 is always constant with respect to a certain input video signal level even when the temperature changes, and the output light intensity of the light emitting element 32 is also increased. Operates so as to have a predetermined nonlinear characteristic with respect to the input video signal level.

FIG. 8 is a block diagram showing the electrical configuration of the main part of the light emitting element driving circuit according to the first embodiment of the present invention.
As shown in the figure, the light emitting element driving circuit of this example includes an input signal switching circuit 41, a current output circuit 42, a laser diode (LD) 43, and photodiodes (PD) 44 ₁ , 44 ₂ ,. 44 _2m , current / voltage conversion circuits 45 ₁ , 45 ₂ ,..., 45 _2m , differential amplifiers 46 ₁ , 46 ₂ ,..., 46 _2m , timing signal generation circuit 47, and sample hold circuit (S / H 48). The input signal switching circuit 41 inputs the input video signals DIN ₀ , DIN ₁ ,..., DIN _n−1 and, based on the control signal CNT1, in the video display period, the input video signals are directly input to the input digital signals D ₀ , .., D _n-1 is passed and sent to the current output circuit 42, while the lower bits (D ₀ ,..., D _k-1 ) of the input digital signal are all “1” per clock during the blanking period. 0 ”(“ 00... 00 ”) and all“ 1 ”(“ 11... 11 ”) are alternately switched and output, and the upper bits (D _k ,..., D _n−1 ) of the same input digital signal are 2 Increment "1" for each clock and output.

The current output circuit 42 includes input digital signals D ₀ , D ₁ ,..., D _n−1 , control voltages VM _m−1 ,..., VM ₀ and control voltages VS _m−1 ,. Based on ₀ , the drive current I _LD is supplied to the laser diode 43. The laser diode 43 emits light spontaneously when the drive current is lower than the threshold current, and emits laser light when the drive current is higher than the threshold current. Usually, when the laser diode 43 is caused to emit light, a driving current equal to or higher than a threshold current is given and used in the laser emission region. Further, the laser diode 43 has a characteristic that the threshold current and the external differential quantum efficiency (ΔP / ΔI) fluctuate with respect to temperature change and aging. The photodiodes 44 ₁ , 44 ₂ ,..., 44 _{2 m} receive the output light of the laser diode 43 and output a current corresponding to the received light illuminance. Current / voltage conversion circuit 45 _1, 45 _2, ..., 45 _2m, the photodiode 44 _1, 44 _2, ..., and outputs a detection voltage DET linearly proportional to the output current of 44 _2m. Differential amplifier _{46 1, 46 2, ...,} 46 2m , the reference voltage VREF ₀ and the voltage value of the detection voltage DET, VREF _1, ..., the differential voltage CM _0, CM ₁ and _{VREF 2m-1, ..., CM} 2m Outputs _-1 .

Based on the control signal CNT2, the sample hold circuit 48 samples the differential voltage CM ₀ by sampling the control voltage VM ₀ and the differential voltage CM ₁ and samples the control voltage VS ₀ and the differential voltage CM ₂ at every clock. Then, the control voltage VM ₁ , and similarly, the differential voltage CM _2m-2 is sampled and the control voltage VM _m-1 and the differential voltage CM _2m-1 are sampled and output as the control voltage VS _m-1 . Except at the time of sampling, these control voltages are held (held) at the voltage values at the time of sampling. The timing signal generation circuit 47 generates a control signal CNT1 for switching the outputs (input digital signals D ₀ , D ₁ ,..., D _n-1 ) of the input signal switching circuit 41 in synchronization with the clock CLK during the blanking period. In addition to the output, the control signal CNT2 for switching the differential voltages CM ₀ , CM ₁ ,..., CM _2m−1 to be sampled by the sample hold circuit 48 is output in synchronization with the clock CLK.

FIG. 9 is a block diagram showing an electrical configuration of a main part of the current output circuit 42 in FIG.
As shown in FIG. 9, the current output circuit 42 includes a selection circuit 51, a main current source 52, a selection circuit 53, a sub current source 54, and a current adder 55. In the current output circuit 42, when the upper bits (D _k ,..., D _n-1 ) of the input digital signal are all “0”, the selection circuit 51 selects the control voltage VM ₀ from the sample hold circuit 48. The control voltage VM _out is output, and the selection circuit 53 selects the control voltage VS ₀ and outputs it as the control voltage VS _out . Hereinafter, the upper bits _{(D k, ..., D n} -1) each time the increment is "1", the selection circuit 51 the control voltage VM _1, ..., select the VM _m-1 sequentially selecting circuit 53 is controlled The voltages VS ₁ ,..., VS _m−1 are sequentially selected. The main current source 52 outputs a current I _M that is directly proportional to the control voltage VM _out from the selection circuit 51, and the sub current source 54 has all the lower bits (D ₀ ,..., D _k−1 ) of the input digital signal all “0”. "", No current is output, and when the lower bits (D ₀ ,..., D _k-1 ) are all “1”, the maximum current I _SALL1 that is directly proportional to the control voltage VS _out from the selection circuit 53 is Output as _S. The current adder 55 adds the current I _M and the current I _S and outputs the output current I _OUT (that is, the drive current I _LD ).

10 is a time chart for explaining the operation of the light emitting element driving circuit of FIG. 8, FIG. 11 is a characteristic diagram showing the relationship between the input video signal level and the reference voltage, and FIG. 12 is a driving current I _{LD of the} laser diode 43. FIG. 13 is a characteristic diagram showing the relationship between the received light illuminance of the photodiode and the output current, and FIG. 14 is a graph showing the input video signal level at the temperature TP1 of the current output circuit 42. FIG. 15 is a characteristic diagram showing the relationship between the drive current I _LD and FIG. 15 is a characteristic diagram showing the relationship between the input video signal level and the drive current I _LD when the current output circuit 42 is at the temperature TP2. FIG. And FIG. 17 is a diagram showing the relationship between the input video signal level and the digital data.
With reference to these drawings, processing contents of the light emitting element driving method used in the light emitting element driving circuit of this example will be described.
In this light emitting element driving circuit, a light intensity detection voltage (detection voltage DET) of the laser diode 43 is generated (light intensity detection processing), and an input signal (input video signals DIN ₀ , DIN ₁ ,..., DIN _n-1 ). Corresponding to the change of the level from the minimum value to the maximum value, the differential voltages CM ₀ , CM ₁ , and the difference between the respective reference voltages (reference voltages VREF ₀ , VREF ₁ ,..., VREF _2m−1 ) and the light intensity detection voltage DET. .., CM _2m-1 are sequentially generated (differential voltage generation process), and each differential voltage is sampled and held (differential voltage holding process). Then, the drive current I _LD of the laser diode 43 is controlled based on the differential voltage corresponding to the level of the current input signal among the held differential voltages (light intensity control process), and the laser diode 43 is modulated. Driven.

FIG. 10 shows the operation during the blanking period (non-display period), and the period during which the synchronization signal SYNC is at a low level (hereinafter referred to as “L”) is the blanking period. In FIG. 11, a gamma characteristic curve is shown, and the reference voltage is proportional to the 2.2th power of the input video signal level. That is, in FIG. 11, the reference voltage VREF ₀ is equal to the input video signal level VIN ₀ (that is, the upper (nk) bits of the n-bit digital signal shown in FIG. 17 are “00... 00” and the lower k bits are "00 ... 00"). The reference voltage VREF ₁ corresponds to the input video signal level VIN ₁ (that is, the upper (nk) bits are “00... 00” and the lower k bits are “11... 11”). The reference voltage VREF ₂ corresponds to the input video signal level VIN ₂ (that is, the upper (nk) bits are “00... 01” and the lower k bits are “00... 00”). The reference voltage VREF ₃ corresponds to the input video signal level VIN ₃ (that is, the upper (nk) bits are “00... 01” and the lower k bits are “11... 11”). Similarly, the reference voltage VREF _2m-1 is set to the input video signal level VIN _2m-1 (that is, the upper (nk) bits are “11... 11” and the lower k bits are “11... 11”). Correspond. Therefore, this gamma curve is divided into 2 ^ (n−k) (= m) ranges, and the reference voltage consists of _{2m of} VREF ₀ , VREF ₁ ,..., VREF _2m−1 . The upper (n−k) bits n and k are set to n = 8 and k = 6 to n = 12 and k = 8, for example.

In the timing signal generation circuit 47, the output (D ₀ ,..., D _n-1 ) of the input signal switching circuit 41 is synchronized with the clock CLK during the blanking period (that is, the period when the synchronization signal SYNC is “L”). A control signal CNT1 for switching is output, and control for switching signals (differential voltages CM ₀ , CM ₁ ,..., CM _2m−1 ) to be sampled by the sample hold circuit 48 in synchronization with the clock CLK. A signal CNT2 is output. The input signal switching circuit 41, the image display period, the input video signal _{(DIN 0, ..., DIN k} -1) transmitted is directly input digital signal D _0, ..., the current output circuit 42 through a D _n-1 On the other hand, during the blanking period, as shown in FIG. 10, the lower bits (D ₀ ,..., D _k−1 ) of the same input digital signal are all “0” (“00... 00” for each clock). ) And all “1” (“11... 11”) are alternately switched and output, and the upper bits (D _k ,..., D _n−1 ) of the same input digital signal start from “00. Thus, every two clocks are incremented by “1” and output until “11... 11”.

As shown in FIG. 12, the laser diode 43 emits light spontaneously when the drive current I _LD is lower than the threshold current (I _0.1 , I _0.2 ), and emits laser light when the drive current I _LD is equal to or higher than the threshold current. In the laser diode 43, the threshold current and the external differential quantum efficiency (ΔP / ΔI) fluctuate with respect to the temperature change. At the temperature TP1, the threshold current is I _0.1 and the external differential quantum efficiency is ΔP ₁ / ΔI _1. At the temperature TP2, the threshold current is I _0.2 and the external differential quantum efficiency is ΔP ₂ / ΔI ₂ . The photodiodes 44 ₁ ,..., 44 _2m output a current I that increases linearly with respect to the received light illuminance P, as shown in FIG.

In this light-emitting element driving circuit, at temperatures TP1, as shown in FIG. 10, after starting the blanking period, at time T _1, the input signal switching circuit 41, the upper bits of the input digital signal (D _k, ..., D _{n -1} ) and lower bits (D ₀ ,..., D _k-1 ) are all “0”. Therefore, the selection circuit 51 of the current output circuit 42 selects the control voltage VM ₀ , the main current source 52 outputs the current I _M (= I _0.1 ) corresponding to the control voltage VM ₀ , and the sub current source 54 outputs the current. Do not output. Therefore, the current output circuit 42 outputs the drive current I _LD (= I _0.1 ) and supplies it to the laser diode 43. The laser diode 43 emits light with a light intensity P ₀ corresponding to the drive current I _LD (= I _0.1 ) at this time. Photodiode 44 _1, 44 _2, ..., 44 _2m, the detected current I _PD0 current / voltage conversion circuit 45 _1, 45 ₂ corresponding to the light intensity P _0, ..., and outputs the 45 _2m, the current / voltage The detection voltage DET is supplied from the conversion circuits 45 ₁ , 45 ₂ ,..., 45 _2m to the inverting inputs (−) of the differential amplifiers 46 ₁ , 46 _{2 ,.}

The differential amplifier 46 ₁ outputs a differential voltage CM ₀ between the voltage value of the detection voltage DET and the reference voltage VREF ₀ . In the sample-hold circuit 48, differential voltage CM ₀ is output as the control voltage VM ₀ is sampled. Therefore, at time T ₁ , as the APC loop, “control voltage VM ₀ → main current source 52 → laser diode 43 → photodiode 44 ₁ → current / voltage conversion circuit 45 ₁ → detection voltage DET → differential amplifier 46 ₁ → difference The voltage CM ₀ → the sample hold circuit 48 → the control voltage VM ₀ ″ is formed, and the voltage value of the control voltage VM ₀ is determined so that the laser diode 43 emits light with the light intensity P ₀ corresponding to the reference voltage VREF ₀ .

At time T ₂ , the input signal switching circuit 41 sets all “0” and lower bits (D ₀ ,..., D _k−1 ) to the upper bits (D _k ,..., D _n−1 ) of the input digital signal. All “1” is output. Therefore, the current I _M (= I _0.1 ) corresponding to the control voltage VM ₀ is output from the main current source 52 of the current output circuit 42, and the current I _S (= I corresponding to the control voltage VS ₀ is output from the sub current source 54. _SALL1 = I _S1.1 ) is output, and the current adder 55 adds the current I _M and the current I _S. Therefore, the drive current I _LD (= I _1.1 ) is output from the current output circuit 42 and supplied to the laser diode 43. The laser diode 43 emits light with a light intensity P ₁ corresponding to the driving current I _LD (= I _1.1 ) at this time. The photodiode 44 ₂ outputs the detection current I _PD1 corresponding to the light intensity P ₁ to the current / voltage conversion circuits 45 ₁ , 45 ₂ ,..., 45 _2m , and the differential amplifiers 46 ₁ , 46 ₂ as the detection voltage DET. ,..., _{462 to} the inverting input (-) of _2m .

The differential amplifier 46 ₂ outputs a differential voltage CM ₁ between the voltage value of the detection voltage DET and the reference voltage VREF ₁ . In the sample-hold circuit 48, differential voltage CM ₁ is outputted as a control voltage VS ₀ is sampled. Therefore, at time T ₂ , as the APC loop, “control voltage VS ₀ → sub-current source 54 → laser diode 43 → photodiode 44 ₂ → current / voltage conversion circuit 45 ₂ → detection voltage DET ₂ → differential amplifier 46 ₂ → The differential voltage CM ₁ → the sample hold circuit 48 → the control voltage VS ₀ ″ is formed, and the voltage value of the control voltage VS ₀ is determined so that the laser diode 43 emits light with the light intensity P ₁ corresponding to the reference voltage VREF ₁ .

At time T ₃ , the input signal switching circuit 41 sets “00... 01” and lower bits (D ₀ ,..., D _k−1 ) to the upper bits (D _k ,..., D _n−1 ) of the input digital signal. All “0” is output to. Therefore, the current I _M (= I _2.1 ) corresponding to the control voltage VM ₁ is output from the main current source 52 of the current output circuit 42, and the sub current source 54 does not output current. Therefore, the drive current I _LD (= I _2.1 ) is output from the current output circuit 42 and supplied to the laser diode 43. The laser diode 43 emits light with a light intensity P ₂ corresponding to the driving current I _LD (= I _2.1 ) at this time. The photodiode 44 ₃ outputs the detection current I _PD2 corresponding to the light intensity P ₂ to the current / voltage conversion circuits 45 ₁ , 45 ₂ ,..., 45 _2m , and the differential amplifiers 46 ₁ , 46 ₂ as the detection voltage DET. ,..., _{462 to} the inverting input (-) of _2m . The differential amplifier 46 ₃ outputs a differential voltage CM ₂ between the voltage value of the detection voltage DET and the reference voltage VREF ₂ . In the sample-hold circuit 48, differential voltage CM ₂ is outputted as a control voltage VM ₁ is sampled. Therefore, at time T ₃ , as the APC loop, “control voltage VM ₁ → main current source 52 → laser diode 43 → photodiode 44 ₃ → current / voltage conversion circuit 45 ₃ → detection voltage DET → differential amplifier 46 ₃ → difference The voltage CM ₂ → the sample hold circuit 48 → the control voltage VM ₁ ″ is formed, and the voltage value of the control voltage VM ₁ is determined so that the laser diode 43 emits light with the light intensity P ₂ corresponding to the reference voltage VREF ₂ .

At time T ₄ , the input signal switching circuit 41 sets “00... 01” and lower bits (D ₀ ,..., D _k−1 ) to the upper bits (D _k ,..., D _n−1 ) of the input digital signal. All “1” is output. Therefore, a current I _M (= I _2.1 ) corresponding to the control voltage VM ₁ is output from the main current source 52 of the current output circuit 42, and a current I _S (= I _SALL1) corresponding to the control voltage VS ₁ is output from the sub current source 54. = I _S3.1 ) is output, and the current adder 55 adds the current I _M and the current I _S. Therefore, the drive current I _LD (= I _3.1 ) is output from the current output circuit 42 and supplied to the laser diode 43. The laser diode 43 emits light with a light intensity P ₃ corresponding to the driving current I _LD (= I _3.1 ) at this time. The photodiode 44 ₄ outputs a detection current I _PD3 corresponding to the light intensity P ₃ to the current / voltage conversion circuit 45 _4, and an inverted input of the differential amplifiers 46 ₁ , 46 ₂ ,..., 46 _2m as the detection voltage DET. Supply to (-). The differential amplifier 46 ₄ outputs a differential voltage CM ₃ between the voltage value of the detection voltage DET and the reference voltage VREF ₃ . In the sample-hold circuit 48, the difference voltage CM ₃ are output as the control voltage VS ₁ is sampled. Therefore, at time T ₄ , as the APC loop, “control voltage VS ₁ → sub-current source 54 → laser diode 43 → photodiode 44 ₄ → current / voltage conversion circuit 45 ₄ → detection voltage DET → differential amplifier 46 ₄ → difference The voltage CM ₃ → the sample hold circuit 48 → the control voltage VS ₁ ″ is formed, and the voltage value of the control voltage VS ₁ is determined so that the laser diode 43 emits light with the light intensity P ₃ corresponding to the reference voltage VREF ₃ .

Hereinafter, the upper bits (D _k ,..., D _n-1 ) and lower bits (D ₀ ,..., D _k-1 ) of the input digital signal output from the input signal switching circuit 41 and the sampling of the sample hold circuit 48 differential voltage CM ₀ to be, ..., by switching the CM _2m-1, the reference voltage _{VREF 0, ..., VREF 2m-} 1 light intensity P ₀ corresponding to, ..., the laser diode 43 with P _2m-1 control voltage VM ₀ to emit light, ..., VM _m-1 and the control voltage VS _0, ..., is determined VS _m-1. As a result, the output current I _LD of the current output circuit 42 with respect to the input video signal level has the characteristics shown in FIG. 14, for example. Further, since the reference voltages VREF ₀ , VREF ₁ ,..., VREF _2m−1 are proportional to the 2.2th power of the input video signal level, the output light intensity of the laser diode 43 is as shown in FIG. It has a gamma characteristic proportional to the video signal level raised to the power of 2.2.

Further, the light-emitting element driving circuit, be varied from the temperature TP1 TP2, as shown in FIG. 15, the reference voltage _{VREF 0, VREF 1, ...,} VREF 2m-1 light intensity P ₀ corresponding to, ... the control voltage VM ₀ so that the laser diode 43 with P _2m-1 emits light, ..., VM _m-1 and the control voltage _{VS 0, ..., VS m-} 1 operates as determined.

In the operation of this embodiment, controlled by the same blanking interval voltage _{VM 0, ..., VM m-} 1 and the control voltage VS _0, ..., but APC control is performed for all the VS _m-1, a plurality of different blanking individually control voltage VM during the period _0, ..., VM _m-1 and the control voltage VS _0, ..., the VS _m-1 may be performed APC control. Further, although the timing for switching the output of the input signal switching circuit 41 and the timing for switching the sampling target signal of the sample hold circuit 48 are performed every clock, they may be performed every plural clocks, and the clock CLK It is not necessary to synchronize.

As described above, in the first embodiment, the light intensity detection voltage (detection voltage DET) of the laser diode 43 is generated, and the input signals (input video signals DIN ₀ , DIN ₁ ,..., DIN _n-1 ), A plurality of reference voltages (reference voltages VREF ₀ , VREF ₁ ,..., VREF _2m−1 ) corresponding to changes from the minimum value to the maximum value are set, and each reference voltage and the light intensity detection voltage DET are set. differential voltage CM _0, CM ₁ of, ..., CM _2m-1 are generated, the driving current I _LD is controlled based on the difference voltage corresponding to the level of the input signal of the current of each of these differential voltage Even if the temperature changes, the output light intensity of the laser diode 43 is always constant with respect to a certain input level, and the output light intensity of the laser diode 43 is a gamma proportional to the 2.2 power of the input video signal level. Characteristics Therefore, gamma correction is accurately performed with a relatively small circuit configuration.

FIG. 18 is a block diagram showing the electrical configuration of the main part of the light emitting element driving circuit according to the second embodiment of the present invention. Elements common to the elements in FIG. 8 showing the first embodiment are shown in FIG. The common code | symbol is attached | subjected.
In the light emitting element driving circuit of this example, as shown in FIG. 18, photodiodes 44 ₁ , 44 ₂ ,..., 44 _2m , current / voltage conversion circuits 45 ₁ , 45 _{2 ,.} differential amplifier 46 _1, 46 _2, ..., 46 _2m, instead of the timing signal generating circuit 47 and the sample hold circuit 48, one photodiode 44,1 one current / voltage conversion circuit 45 and the quarter single differential amplifier 46, A timing signal generation circuit 47A and a sample hold circuit 48A are provided, and a selection circuit 49 is newly provided.

The photodiode 44 receives the output light of the laser diode 43 and outputs a current corresponding to the received light illuminance. The current / voltage conversion circuit 45 outputs a detection voltage DET that is linearly proportional to the output current of the photodiode 44. The selection circuit 49 selects one of the reference voltages VREF ₀ ,..., VREF _2m−1 based on the control signal CNT3 output from the timing signal generation circuit 47A and outputs it as the reference voltage VREF. The differential amplifier 46 outputs a differential voltage CM between the voltage value of the detection voltage DET and the reference voltage VREF. The sample hold circuit 48A samples the differential voltage CM to be sampled every clock based on the control signal CNT2 to control the control voltages VM ₀ ,..., VM _m−1 and the control voltages VS ₀ ,. _m-1 is output, and these control voltages are held at the voltage values at the time of sampling except during sampling. In addition to the function of the timing signal generation circuit 47, the timing signal generation circuit 47A has a control signal CNT3 for selecting the reference voltages VREF ₀ ,..., VREF _2m-1 input to the selection circuit 49 in synchronization with the control signal CNT2. Is output. The other configuration is the same as that shown in FIG.

FIG. 19 is a time chart for explaining the operation of the light emitting element driving circuit of FIG.
With reference to this figure, the processing content of the light emitting element drive method used for the light emitting element drive circuit of this example is demonstrated.
In this light emitting element driving circuit, as shown in FIG. 19, at the time T ₁ after the blanking period starts, the input signal switching circuit 41 causes the upper bits (D _k ,..., D _n−1 ), And all lower bits (D ₀ ,..., D _k−1 ) are all “0”. Therefore, the control voltage VM ₀ is selected in the selection circuit 51 of the current output circuit 42, the current I _M (= I _0.1 ) corresponding to the control voltage VM ₀ is output from the main current source 52, and the current from the sub current source 54 Is not output. Therefore, the drive current I _LD (= I _0.1 ) is output from the current output circuit 42 and supplied to the laser diode 43. The laser diode 43 emits light with a light intensity P ₀ corresponding to the driving current I _LD (= I _0.1 ) at this time. In the photodiode 44, the detection current I _PD0 corresponding to the light intensity P ₀ is output to the current / voltage conversion circuit 45, and the detection voltage DET from the current / voltage conversion circuit 45 is input to the inverting input (−) of the differential amplifier 46. To be supplied. In the selection circuit 49, the reference voltage VREF ₀ is selected based on the control signal CNT 3 and supplied to the non-inverting input (+) of the differential amplifier 46. In the differential amplifier 46, the differential voltage CM of the reference voltage VREF ₀ and the voltage value of the detection voltage DET is outputted.

In the sample-hold circuit 48A, the differential voltage CM is outputted as a control voltage VM ₀ is sampled. Therefore, at time T ₁ , as the APC loop, “control voltage VM ₀ → main current source 52 → laser diode 43 → photodiode 44 → current / voltage conversion circuit 45 → detection voltage DET → differential amplifier 46 → differential voltage CM → The sample hold circuit 48A → control voltage VM ₀ ″ is formed, and the voltage value of the control voltage VM ₀ is determined so that the laser diode 43 emits light with the light intensity P ₀ corresponding to the reference voltage VREF ₀ . Hereinafter, the upper bits (D _k ,..., D _n-1 ) and lower bits (D ₀ ,..., D _k-1 ) of the input digital signal output by the input signal switching circuit 41 and the input signal of the selection circuit 49A (see By switching the voltage VREF ₀ ,..., VREF _2m-1 ) and the output of the sample hold circuit 48A (control voltage VM ₀ ,..., VM _m-1 , control voltage VS ₀ ,..., VS _m-1 ) voltage _{VREF 0, ..., VREF 2m-} 1 light intensity P ₀ corresponding to, ..., the control voltage VM ₀ such that the laser diode 43 emits light with _{P 2m-1, ..., VM} m-1 and the control voltage VS _0, ..., VS _m-1 is determined.

As a result, as in the first embodiment, the output current I _LD of the current output circuit 42 has the characteristics shown in FIG. 14 with respect to the input video signal level. Further, since the reference voltages VREF ₀ ,..., VREF _2m-1 are proportional to the 2.2th power of the input signal level, the output light intensity of the laser diode 43 is in proportion to the input signal level as shown in FIG. The gamma characteristic is proportional to the power of 2.2. As in the first embodiment, even in the light-emitting element driving circuit, when the temperature changes from TP1 TP2, a reference voltage VREF _0, ..., intensity P ₀ corresponding to _{VREF 2m-1, ..., P} 2m ₋₁ , the control voltages VM ₀ ,..., VM _m−1 and VS ₀ ,..., VS _m−1 are determined so that the laser diode 43 emits light of ₋₁ , and the same advantages as the first embodiment There is.

FIG. 20 is a block diagram showing an electrical configuration of the main part of the light emitting element driving circuit according to the third embodiment of the present invention. Elements common to the elements in FIG. 18 showing the second embodiment are shown in FIG. The common code | symbol is attached | subjected.
In the light emitting element driving circuit of this example, as shown in FIG. 20, instead of the current output circuit 42, the timing signal generation circuit 47A and the selection circuit 49 in FIG. 18, a current output circuit 42A having a different function and a timing signal are provided. A generation circuit 47B and a selection circuit 49A are provided. In addition to the function of the current output circuit 42, the current output circuit 42A adjusts the output current I _OUT based on the control signal CNT4 from the timing signal generation circuit 47A.

The timing signal generation circuit 47B outputs a control signal CNT1 for switching the input digital signals (D ₀ ,..., D _n-1 ) output from the input signal switching circuit 41 in synchronization with the clock CLK during the blanking period. At the same time, the control signal CNT2 for switching the sampling output (control voltage VM ₀ ,..., VM _m−1 , control voltage VS ₀ ,..., VS _m−1 ) of the sample hold circuit 48A in synchronization with the clock CLK, 2 clocks A control signal CNT3 for selecting one of the reference voltages VREF ₀ ,..., VREF _m inputted to the selection circuit 49A every two clocks, and an output current I _OUT of the current output circuit 42A every two clocks. A control signal CNT4 for adjusting only once is output. The selection circuit 49A selects one of the reference voltages VREF ₀ ,..., VREF _m based on the control signal CNT3 output from the timing signal generation circuit 47B and outputs it as the reference voltage VREF. The other configuration is the same as that shown in FIG.

FIG. 21 is a block diagram showing the electrical configuration of the main part of the current output circuit 42A in FIG. 20. Elements common to those in FIG. 9 showing the first embodiment are given common reference numerals. ing.
In this current output circuit 42A, as shown in FIG. 21, instead of the current adder 55 in FIG. 9, a current adder 55A having a different configuration is provided, and an adjustment current source 56 is newly provided. ing. In the current output circuit 42A, when the upper bits (D _k ,..., D _n-1 ) of the input digital signal are all “0”, the selection circuit 51 selects the control voltage VM ₀ from the sample hold circuit 48A. The control voltage VM _out is output, and the selection circuit 53 selects the control voltage VS ₀ and outputs it as the control voltage VS _out . Hereinafter, the upper bits _{(D k, ..., D n} -1) each time the increment is "1", the selection circuit 51 the control voltage VM _1, ..., sequentially selects the VM _m-1, selection circuit 53 controls The voltages VS ₁ ,..., VS _m−1 are sequentially selected. The main current source 52 outputs a current I _M corresponding to the control voltage VM _out , and the sub current source 54 is when all the lower bits (D ₀ ,..., D _k−1 ) of the input digital signal are “0”. When the lower bits (D ₀ ,..., D _k-1 ) are all “1” without outputting the current, the maximum current I _SALL1 corresponding to the control voltage VS _out is output as the current I _S. The adjustment current source 56 outputs a current I _ADJ corresponding to the control voltage VS _out when the control signal CNT4 is in an ON state. This current I _ADJ is the same value as the current value I _SLSB output from the sub-current source 54 when the lower bits (D ₀ ,..., D _k−1 ) are “00. The current adder 55A adds the current I _M , the current I _S, and the current I _ADJ and outputs the output current I _OUT (that is, the drive current I _LD ).

FIG. 22 is a time chart for explaining the operation of the light emitting element driving circuit of FIG. 20, and FIG. 23 is a characteristic diagram showing the relationship between the input video signal level and the reference voltage.
The processing contents of the light emitting element driving method used in the light emitting element driving circuit of this example will be described with reference to these drawings and FIGS. 14 and 16 of the first embodiment.

FIG. 22 shows the operation during the blanking period, and the period during which the synchronization signal SYNC is “L” is the blanking period. In FIG. 23, a gamma characteristic curve is shown, and the reference voltage is proportional to the 2.2th power of the input video signal level.
That is, in FIG. 23, the reference voltage VREF ₀ is the input video signal level VIN ₀ (that is, the upper (nk) bits of the n-bit digital signal are “00... 00” and the lower k bits are “00... 00”. ). The reference voltage VREF ₁ corresponds to the input video signal level VIN ₂ (that is, the upper (nk) bits are “00... 01” and the lower k bits are “00... 00”). The reference voltage VREF ₂ corresponds to the input video signal level VIN ₄ (that is, the upper (nk) bits are “00... 10” and the lower k bits are “00... 00”). Similarly, the reference voltage VREF _m−1 is set to the input video signal level VIN _2m−2 (that is, the upper (nk) bits are “11... 11” and the lower k bits are “00... 00”). Correspond. The reference voltage VREF _m corresponds to the input video signal level VIN _2m−1 (that is, the upper (nk) bits are “11... 11” and the lower k bits are “11... 11”). Therefore, this gamma curve is divided into 2 ^ (n−k) (= m) ranges, and the reference voltage is composed of (m + 1) VREF ₀ ,..., VREF _m .

In this light emitting element driving circuit, at the temperature TP1, as shown in FIG. 22, at the time T ₁ after the blanking period starts, the input signal switching circuit 41 receives the upper bits (D _k ,..., D _n of the input digital signal. _-1 ) and the lower bits (D ₀ ,..., D _k-1 ) all output “0”. In addition, the timing signal generation circuit 47B turns off the control signal CNT4. Therefore, the selection circuit 51 of the current output circuit 42A selects the control voltage VM ₀ , the main current source 52 outputs the current I _M (= I _0.1 ) corresponding to the control voltage VM ₀ , and the sub current source 54 outputs the current. No adjustment is made, and the adjustment current source 56 does not output current. For this reason, the current output circuit 42A outputs the drive current I _LD (= I _0.1 ) and supplies it to the laser diode 43. The laser diode 43 emits light with a light intensity P ₀ corresponding to the driving current I _0.1 at this time. The photodiode 44 outputs a detection current I _PD0 corresponding to the light intensity P ₀ to the current / voltage conversion circuit 45, and the detection voltage DET from the current / voltage conversion circuit 45 is the inverting input (−) of the differential amplifier 46. To be supplied. In the selection circuit 49A, the reference voltage VREF ₀ is selected based on the control signal CNT3 and supplied to the non-inverting input (+) of the differential amplifier 46. In the differential amplifier 46, the differential voltage CM of the reference voltage VREF ₀ and the voltage value of the detection voltage DET is outputted.

In the sample-hold circuit 48A, the differential voltage CM is outputted as a control voltage VM ₀ is sampled. Therefore, at time T ₁ , as the APC loop, “control voltage VM ₀ → main current source 52 → laser diode 43 → photodiode 44 → current / voltage conversion circuit 45 → detection voltage DET → differential amplifier 46 → differential voltage CM → The sample hold circuit 48A → control voltage VM ₀ ″ is formed, and the voltage value of the control voltage VM ₀ is determined so that the laser diode 43 emits light with the light intensity P ₀ corresponding to the reference voltage VREF ₀ .

At time T ₂ , the input signal switching circuit 41 sets all “0” and lower bits (D ₀ ,..., D _k−1 ) to the upper bits (D _k ,..., D _n−1 ) of the input digital signal. All “1” is output. The timing signal generation circuit 47B turns on the control signal CNT4. Therefore, the main current source 52 of the current output circuit 42A outputs a current I _M (= I _0.1 ) corresponding to the control voltage VM ₀ , and the sub current source 54 is a current I _S (= I corresponding to the control voltage VS _{0). SALL1} = I _S1.1 ) and the adjustment current source 56 outputs a current I _ADJ (= I _S1.1LSB ) corresponding to the control voltage VS ₀ , and the current adder 55A outputs the current I _M and the current I. _S and current I _ADJ are added. As a result, the drive current I _LD (= I _2.1 ) is output from the current output circuit 42A and supplied to the laser diode 43. The laser diode 43 emits light with a light intensity P ₂ corresponding to the driving current I _LD (= I _2.1 ) at this time. The photodiode 44 outputs a detection current I _PD2 corresponding to the light intensity P ₂ to the current / voltage conversion circuit 45, and the detection voltage DET from the current / voltage conversion circuit 45 is the inverting input (−) of the differential amplifier 46. To be supplied. In the selection circuit 49A, the reference voltage VREF ₁ is selected based on the control signal CNT3 and supplied to the non-inverting input (+) of the differential amplifier 46. The differential amplifier 46 outputs a differential voltage CM between the voltage value of the detection voltage DET and the reference voltage VREF ₁ .

In the sample-hold circuit 48A, the differential voltage CM is outputted as a control voltage VS ₀ is sampled. Therefore, at time T ₂ , as the APC loop, “control voltage VS ₀ → sub-current source 54 + adjusting current source 56 → laser diode 43 → photodiode 44 → current / voltage conversion circuit 45 → detection voltage DET → differential amplifier 46 → differential voltage CM → sample hold circuit 48A → control voltage VS ₀ ″ is formed, and the voltage value of the control voltage VS ₀ is determined so that the laser diode 43 emits light with the light intensity P ₂ corresponding to the reference voltage VREF ₁ .

At time T ₃ , the input signal switching circuit 41 sets “00... 01” and lower bits (D ₀ ,..., D _k−1 ) to the upper bits (D _k ,..., D _n−1 ) of the input digital signal. All “0” is output to. The timing signal generation circuit 47B turns off the control signal CNT4. Therefore, the main current source 52 of the current output circuit 42A outputs a current I _M (= I _2.1 ) corresponding to the control voltage VM ₁ , the sub current source 54 does not output current, and the adjustment current source 56 does not output current. Is not output. As a result, the drive current I _LD (= I _2.1 ) is output from the current output circuit 42A and supplied to the laser diode 43. The laser diode 43 emits light with a light intensity P ₂ corresponding to the driving current I _LD (= I _2.1 ) at this time. The photodiode 44 outputs a detection current I _PD2 corresponding to the light intensity P ₂ to the current / voltage conversion circuit 45, and the detection voltage DET from the current / voltage conversion circuit 45 is the inverting input (−) of the differential amplifier 46. To be supplied. In the selection circuit 49A, the reference voltage VREF ₁ is selected based on the control signal CNT3 and supplied to the non-inverting input (+) of the differential amplifier 46. The differential amplifier 46 outputs a differential voltage CM between the voltage value of the detection voltage DET and the reference voltage VREF ₁ .

In the sample-hold circuit 48A, the differential voltage CM is outputted as a control voltage VM ₁ is sampled. Therefore, at time T ₃ , as the APC loop, “control voltage VM ₁ → main current source 52 → laser diode 43 → photodiode 44 → current / voltage conversion circuit 45 → detection voltage DET → differential amplifier 46 → differential voltage CM → The sample hold circuit 48A → control voltage VM ₁ ″ is formed, and the voltage value of the control voltage VM ₁ is determined so that the laser diode 43 emits light with the light intensity P ₂ corresponding to the reference voltage VREF ₁ .

At time T ₄ , the input signal switching circuit 41 sets “00... 01” and lower bits (D ₀ ,..., D _k−1 ) to the upper bits (D _k ,..., D _n−1 ) of the input digital signal. All “1” is output. The timing signal generation circuit 47B turns on the control signal CNT4. Therefore, the current I _M (= I _2.1 ) corresponding to the control voltage VM ₁ is output from the main current source 52 of the current output circuit 42A, and the current I _S (= I _SALL1) corresponding to the control voltage VS ₁ is output from the sub current source 54. = I _S3.1 ) is output, the current I _ADJ (= I _S3.1LSB ) corresponding to the control voltage VS ₁ is output from the adjustment current source 56, and the current I _M and the current I _S are The current I _ADJ is added. As a result, the drive current I _LD (= I _4.1 ) is output from the current output circuit 42 A and supplied to the laser diode 43. The laser diode 43 emits light with a light intensity P ₄ corresponding to the driving current I _4.1 at this time. The photodiode 44 outputs a detection current I _PD4 corresponding to the light intensity P ₄ to the current / voltage conversion circuit 45, and the detection voltage DET from the current / voltage conversion circuit 45 is the inverting input (−) of the differential amplifier 46. To be supplied. In the selection circuit 49A, the reference voltage VREF ₂ is selected based on the control signal CNT3 and supplied to the non-inverting input (+) of the differential amplifier 46. In the differential amplifier 46, differential voltage CM voltage value of the detection voltage DET and the reference voltage VREF ₂ is outputted.

In the sample-hold circuit 48A, the differential voltage CM is outputted as a control voltage VS ₁ is sampled. Therefore, at time T ₄ , as the APC loop, “control voltage VS ₁ → sub-current source 54 + adjusting current source 56 → laser diode 43 → photodiode 44 → current / voltage conversion circuit 45 → detection voltage DET → differential amplifier 46 → differential voltage CM → sample hold circuit 48A → control voltage VS ₁ ″ is formed, and the voltage value of the control voltage VS ₁ is determined so that the laser diode 43 emits light with the light intensity P ₄ corresponding to the reference voltage VREF ₂ .

Hereinafter, the upper bits (D _k ,..., D _n-1 ) and the lower bits (D ₀ ,..., D _k-1 ) of the input digital signal output from the input signal switching circuit 41 and the output from the timing signal generation circuit 47B. The reference voltage VREF ₀ ,..., VS _m−1 is switched by switching the control signal CNT4 and the sampling outputs (control voltages) VM ₀ ,..., VM _m−1 , VS ₀ ,. ..., control voltages VM ₀ , ..., VM _m-1 and control voltages VS ₀ , ..., VS _m-1 so that the laser diode 43 emits light at light intensities P ₀ , ..., P _2m-1 corresponding to VREF _m. Is determined. As a result, the output current I _LD of the current output circuit 42A has the characteristics shown in FIG. 14 with respect to the input video signal level. Further, since the reference voltages VREF ₀ ,..., VREF _m are proportional to the 2.2th power of the input video signal level, the output light intensity of the laser diode 43 is 2 with respect to the input video signal level as shown in FIG. It has a gamma characteristic proportional to the square.

Further, the light-emitting element driving circuit, be varied from the temperature TP1 TP2, a reference voltage VREF _0, ..., VREF _m light intensity P ₀ corresponding to, ..., the laser diode 43 emits light with P _2m-1 control voltage VM ₀ as, ..., VM _m-1 and the control voltage _{VS 0, ..., VS m-} 1 operates as determined.

In the operation of this embodiment, as in the first embodiment, the same blanking control in the ranking period voltage _{VM 0, ..., VM m-} 1 and the control voltage VS _0, ..., for all VS _m-1 APC Although control is performed, individually controlled voltage VM ₀ at a plurality of different blanking period, ..., VM _m-1 and the control voltage VS _0, ..., may be performed APC control of VS _m-1. Further, although the timing for switching the output of the input signal switching circuit 41 and the timing for switching the sampling output of the sample hold circuit 48A are performed every clock, they may be performed every plural clocks or synchronized with the clock CLK. You don't have to. In this embodiment, the timing of switching the input signal (reference voltage) VREF ₀ ,..., VREF _m of the selection circuit 49A and the timing of turning on the output of the adjustment current source 56 are performed every two clocks. It may be performed every plural clocks or may not be synchronized with the clocks.

As described above, in the third embodiment, the light emitting element driving circuit operates so that the output light intensity of the laser diode 43 is always constant with respect to a certain input level even when the temperature changes. Since the output light intensity of the laser diode 43 operates so as to have a gamma characteristic proportional to the power of 2.2 with respect to the input video signal level, gamma correction is accurately performed as in the second embodiment. Further, since the adjustment current source 56 is provided, it is only necessary to set (m + 1) reference voltages VREF ₀ ,..., VREF _m, and the hardware configuration for setting the reference voltage is reduced.

FIG. 24 is a block diagram showing the main part of a video display apparatus according to the fourth embodiment of the present invention.
As shown in FIG. 34, the video display device includes an image processing circuit 71, a light emitting element driving circuit 72, a collimator lens 73, a reflection mirror 74, scanning mirrors 75 and 76, and a projection screen 77. ing. The light emitting element driving circuit 72 has the same configuration as the light emitting element driving circuit of the third embodiment, for example, and includes a laser diode 43 and a photodiode 44.

  In this video display device, if the input video signal Vin is analog, it is input to the image processing circuit 71 and subjected to predetermined signal processing such as A / D (analog / digital) conversion, and then the light emitting element driving circuit 72. As an input signal. In the light emitting element driving circuit 72, the same operation as that of the light emitting element driving circuit of the third embodiment is performed, and a driving current is supplied to the laser diode 43. The laser diode 43 emits laser light having a light intensity corresponding to the drive current. A part of the laser light emitted from the laser diode 43 is reflected by the mirror 74 and received by the photodiode 44, and the photodiode 44 outputs a detection current corresponding to the received light intensity. The laser light from the laser diode 43 is collimated by the collimator lens 73. The collimated laser light is scanned by the scanning mirrors 75 and 76, and an image having a two-dimensional spread is projected on the projection screen 77 with accurate gamma correction.

  In each of the above embodiments, the light intensity detection signal, each reference signal, and the difference signal are the light intensity detection voltage, the reference voltage, and the difference voltage corresponding to each, but the hardware configuration that can correspond to the current mode is used. If it exists, the current corresponding to each signal may be used.

  The present invention can be applied to video display apparatuses such as projectors and image forming apparatuses such as printers and copiers, and is particularly effective when performing APC (Auto Power Control) control and gamma correction. . The present invention can also be applied as a drive circuit for a light emitting element when another light emitting element having the same function and characteristics as a laser diode is manufactured.

It is a block diagram which shows the electrical constitution of the principal part of the light emitting element drive circuit for demonstrating the basic principle of this invention. It is a characteristic view which shows the relationship between an input video signal level and a reference voltage. FIG. 6 is a characteristic diagram showing the relationship between the drive current I _LD of the light emitting element 32 and the output light intensity. It is a characteristic view which shows the relationship between the light reception illumination intensity of the light intensity detection part 33, and the detection voltage DET. FIG. 6 is a characteristic diagram illustrating a relationship between an input video signal level and a driving current I _LD when the driving unit 31 is at a temperature TP1. FIG. 6 is a characteristic diagram showing a relationship between an input video signal level and a drive current I _LD when the drive unit 31 is at a temperature TP2. 6 is a characteristic diagram showing a relationship between an input video signal level and output light intensity of the light emitting element 32. FIG. 1 is a block diagram showing an electrical configuration of a main part of a light emitting element driving circuit according to a first embodiment of the present invention. It is a block diagram which shows the electrical constitution of the principal part of the current output circuit 42 in FIG. It is a time chart explaining operation | movement of the light emitting element drive circuit of FIG. It is a characteristic view which shows the relationship between an input video signal level and a reference voltage. FIG. 6 is a characteristic diagram showing the relationship between the drive current I _LD of the laser diode 43 and the output light intensity. It is a characteristic view which shows the relationship between the light reception illumination intensity of a photodiode (PD), and an output current. FIG. 6 is a characteristic diagram showing a relationship between an input video signal level and a drive current I _LD at a temperature TP1 of the current output circuit FIG. 7 is a characteristic diagram showing a relationship between an input video signal level and a drive current I _LD at a temperature TP2 of the current output circuit 6 is a characteristic diagram showing a relationship between an input video signal level and output light intensity of a laser diode 43. FIG. It is a figure which shows the relationship between an input video signal level and digital data. It is a block diagram which shows the electric constitution of the principal part of the light emitting element drive circuit which is 2nd Example of this invention. It is a time chart explaining operation | movement of the light emitting element drive circuit of FIG. It is a block diagram which shows the electric constitution of the principal part of the light emitting element drive circuit which is the 3rd Example of this invention. It is a block diagram which shows the electrical constitution of the principal part of the current output circuit 42A in FIG. FIG. 21 is a time chart illustrating an operation of the light emitting element driving circuit of FIG. 20. It is a characteristic view which shows the relationship between an input video signal level and a reference voltage. It is a block diagram which shows the principal part of the video display apparatus which is 4th Example of this invention. It is a circuit diagram which shows the electrical constitution of the conventional light emitting element drive circuit. FIG. 11 is a block diagram showing an electrical configuration of a light emitting element driving circuit described in Patent Document 1.

Explanation of symbols

41 Input signal switching circuit 42, 42A Current output circuit (part of light intensity control means)
43 Laser diode (LD) (light emitting device)
_{44,44 1, 44 2, ...,} 44 2m photodiode (PD) (part of the light intensity detecting means, the light detecting element)
45, 45 ₁ , 45 ₂ ,..., 45 _2m current / voltage conversion circuit (part of light intensity detection means)
46, 46 ₁ , 46 ₂ ,..., _462m differential amplifier (difference signal generating means)
47, 47A, 47B Timing signal generation circuit 48, 48A Sample hold circuit (S / H) (difference signal holding means)
49, 49A selection circuit (reference voltage selection circuit)
51 selection circuit (part of current output circuit 42, first selection circuit)
52 Main current source (part of current output circuit 42)
53 selection circuit (part of current output circuit 42, second selection circuit)
54 Sub-current source (part of current output circuit 42)
55 Current adder (part of current output circuit 42)
55A current adder (part of current output circuit 42A)
56 Current source for adjustment (part of current output circuit 42A)
71 Image processing circuit (part of video display device)
72 Light emitting element drive circuit (part of video display device)
73 Collimator lens (part of video display device)
74 Reflection mirror (part of video display device)
75,76 Scanning mirror (part of video display device)
77 Projection screen (part of video display device)

Claims (11)

A light emitting element driving circuit for modulating and driving a light emitting element based on a level of an input signal,
Light intensity detection means for detecting the light intensity of the light emitting element and generating a light intensity detection signal based on the detection result;
A plurality of reference signals having a predetermined non-linear relationship with respect to the level of the input signal are set, and each of the reference signals and the light intensity detection corresponding to a change from the minimum value to the maximum value of the level of the input signal Differential signal generating means for generating a differential signal with the signal;
Difference signal holding means for holding each difference signal generated by the difference signal generating means;
Light intensity control means for driving the light emitting element based on a difference signal corresponding to the current level of the input signal among the difference signals held in the difference signal holding means. A light-emitting element driving circuit. The light intensity detecting means is
At least one photodetecting element that receives the output light of the light emitting element and outputs a detection current corresponding to the light intensity of the output light;
Comprising at least one current / voltage conversion circuit for converting the detection current output from the light detection element into a light intensity detection voltage;
The differential signal generating means
A plurality of reference voltages are set as the plurality of reference signals, a difference between the light intensity detection voltage and each reference voltage is taken, and a differential amplifier that outputs a plurality of difference voltages as the difference signal is provided.
The differential signal holding means is
A sample and hold circuit that samples and holds each differential voltage and outputs a plurality of hold voltages,
The light intensity control means includes
A current output circuit is provided that selects a hold voltage corresponding to the level of the input signal from the plurality of hold voltages and outputs a drive current based on the selected hold voltage to the light emitting element. Item 2. A light-emitting element driving circuit according to Item 1. The light emitting element is
It is composed of a laser diode that emits light spontaneously when the drive current is less than or equal to a predetermined threshold current, while emitting light when the drive current is greater than or equal to the threshold current,
The current output circuit is
The light emission according to claim 2, wherein the driving current is output so that the minimum output light intensity of the laser diode is equal to or less than the light intensity when the laser diode starts the laser emission. Element drive circuit. The current output circuit is
An n-bit (n: natural number) digital signal corresponding to a change from the minimum value to the maximum value of the level of the input signal is input, and based on the upper (nk) bits (k: natural number) of the digital signal. A configuration in which the range of the input signal is divided into 2 ^ (n−k) regions, and the output current of the current output circuit increases and decreases linearly and linearly based on the lower k bits of the digital signal in each region. 4. The light-emitting element driving circuit according to claim 2, wherein the light-emitting element driving circuit is provided. The current output circuit is
A first selection circuit that selects one of the plurality of hold voltages based on upper (n−k) bits of the digital signal and outputs it as a first control voltage;
A main current source that outputs a first current according to the first control voltage;
Based on the upper (n−k) bits of the digital signal, a hold voltage output next to the hold voltage corresponding to the first control voltage is selected from the plurality of hold voltages as a second control voltage. A second selection circuit for outputting;
A secondary current source that outputs a second current that linearly increases or decreases in a binary manner by the lower k bits of the digital signal, and that adjusts the second current based on the second control voltage;
5. The light emitting element drive circuit according to claim 4, comprising a current adder for adding the first current of the main current source and the second current of the sub current source. The current output circuit is
A third current having a current value corresponding to the least significant bit of the lower k bits of the digital signal is output, and the third current is output in accordance with the second control voltage selected by the second selection circuit. 6. The light emitting element drive circuit according to claim 5, further comprising an adjustment current source that is variable and that controls on / off of the third current based on a given control signal. The differential signal generating means
3. The light emitting element driving circuit according to claim 2, further comprising a reference voltage selection circuit that selects one reference voltage from the plurality of reference voltages in a time division manner and sends the reference voltage to the differential amplifier.   The digital signal is sent to the current output circuit during the non-display period of an input video signal having a display period and a non-display period in one frame period, while the input video signal is directly output to the current output circuit during the display period. 7. The light emitting element driving circuit according to claim 4, further comprising an input signal switching circuit for sending the input signal as an input signal. The plurality of reference voltages are:
9. The light emitting element driving circuit according to claim 8, wherein the light emitting element driving circuit is set in a relationship proportional to the level of the level of the input video signal to the power of 2.2. A light-emitting element driving method used for a light-emitting element driving circuit that modulates and drives a light-emitting element based on a level of an input signal,
A light intensity detection process for detecting the light intensity of the light emitting element and generating a light intensity detection signal based on the detection result;
A plurality of reference signals having a predetermined non-linear relationship with respect to the level of the input signal are set, and each reference signal and the light intensity detection are performed in response to a change from the minimum value to the maximum value of the level of the input signal. Differential signal generation processing for generating a differential signal with the signal;
A difference signal holding process for holding the generated difference signals;
And a light intensity control process for driving the light emitting element based on a difference signal corresponding to a current level of the input signal among the held difference signals.   An image display device comprising the light emitting element driving circuit according to claim 1.

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