JP2006339458A - Current drive circuit, light-emitting element driver, and mobile apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a current drive circuit capable of realizing a slope function causing less noise, a light-emitting element driver provided with the current drive circuit, and a mobile apparatus employing the same. <P>SOLUTION: A slope controller 13 sets a slope time included in a time change of a reference current outputted from a slope DAC 15 to generate data for switching the switch of the slope DAC 15 and decode the data in, e.g. 6 bits. The slope DAC 15, adopting a structure of applying current mirror processing to a reference current Iref by using 2<SP>n</SP>-1 sets of reference elements, carries out switching processing on the basis of the data from the slope controller 13, and provides an output current as a sum of mirror current values I1+I2+...+I(2<SP>n</SP>-1) to a driver 16, as the reference current. The driver 16 makes the reference current Ioutref multipled by a fixed number of C-fold and drives an LED 20 as a driven object by the multiplied current. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a current drive circuit that supplies a reference current to a current output type driver circuit that monitors a reference current, a light-emitting element drive device including the current drive circuit, and a portable device using the same.

In a portable device such as a cellular phone, for example, a light emitting diode (e.g., a plurality of light emitting elements having different emission colors) is used to perform light emission as a backlight of an image display device including a liquid crystal display device (LCD) or display an incoming call. Light Emitting Diode (hereinafter referred to as LED) is provided.
In current portable devices, a red (R) LED, a green (G) LED, a blue (B) LED, and a white (W) LED are generally provided.

In recent years, portable devices having a function of lighting these LEDs or modulating the luminance with time by gradation control have been put into practical use.
As a method for adjusting the lighting brightness of the LED, there is a method for realizing the lighting brightness / darkness and the slope function by PWM control as shown in FIG. 1 (refer to Patent Document 1 for PWM control).
The slope function is a function that makes the lighting of the LED gradually brighter or darker with respect to time.

In the gradation display method as shown in FIG. 1, the lighting and extinguishing of the LEDs are alternately repeated, and the repetition rate is one time within the unit period of display, and the relative display depends on the gradation. The percentage changes over time.
In this method, the output current from the LED driver is PWM (discrete), and is always on (ON) and off (OFF).
When the LED driver IC is aimed at low power consumption and the anode-cathode voltage of the LED is monitored and fed back to the power supply voltage, the anode-cathode voltage of the LED must not vary discretely.
Japanese Patent Laid-Open No. 2-1812

However, in the method of controlling by ON / OFF of the pulse, first, the ON / OFF of the pulse itself becomes a noise source. This is a factor that reduces the accuracy of gradation control.
Second, the potential at the LED cathode end varies greatly with time according to ON / OFF of the pulse. For example, in a circuit configuration in which LED cathode voltage is monitored and feedback is applied, the cathode terminal needs to be DC (stable potential).

  An object of the present invention is to provide a current driving circuit capable of realizing a slope function with less noise, a light emitting element driving device provided with the current driving circuit, and a portable device using the same.

  A first aspect of the present invention is a current driving circuit having a slope function that gradually increases or decreases gradually with respect to a time for driving a driving target, and includes a switching unit, which is used as a reference current of a reference current source. The current output is a current value set for a predetermined gradation with a predetermined ratio, and adds a current value selected by the switching process of the switching unit according to a predetermined digital signal to obtain a reference current output A slope time included in a time change of a reference current value output from the digital / analog converter and a digital / analog converter, and switching the switching unit of the digital / analog converter based on the slope time A controller for generating the digital signal and an output base of the digital / analog converter. The current based on the current and a driver for supplying to the drive target.

  According to a second aspect of the present invention, there is provided a light emitting element driving device having a slope function that gradually increases or decreases gradually with respect to a time for driving a light emitting element that is a driving target, and includes a switching unit and a reference current. A current value set for a predetermined gradation with a predetermined ratio with respect to the reference current of the source, and adding the current value selected by the switching process of the switching unit in accordance with a predetermined digital signal to add the reference current A current output type digital / analog converter for obtaining an output, and a slope time included in a time change of a reference current value output from the digital / analog converter is set, and the switching of the digital / analog converter is performed based on the slope time. A controller for generating the digital signal for switching the unit, and the digital / analog The current based on the converter output reference current and a driver for supplying to the light emitting element.

  Preferably, the current output type digital / analog converter has a current mirror structure having a reference element connected to the reference current source and a plurality of current mirror elements, and each mirror current by the current mirror element is added. To obtain the reference current output.

  Preferably, a switching element switchable by the digital signal is connected to each of the mirror mirror elements.

  Preferably, a switching element in an on state is connected to the reference element.

  Preferably, the digital signal generated by the controller includes a gray code.

  Preferably, the power supply circuit supplies a drive voltage to the light emitting element, and the power supply circuit monitors the terminal level of the current output terminal of the driver to set the drive voltage to the light emitting element.

  Preferably, the driver multiplies a reference current by the current output type digital / analog converter by a constant and supplies the reference current to the drive target.

  According to a third aspect of the present invention, there is provided a light emitting element driving apparatus in which a plurality of light emitting elements having different drive voltages required for light emission are connected in parallel and drives one or a plurality of light emitting elements. And a plurality of current driving circuits connected to the corresponding light emitting elements of the plurality of light emitting elements and driving the corresponding light emitting elements with brightness based on a set value, and based on each driving state of the plurality of driving circuits, A power supply circuit that determines a maximum driving voltage value among one or a plurality of light emitting elements that are driven to emit light, and supplies at least the determined driving voltage to the plurality of light emitting elements, and each of the current drives The circuit has a slope function that gradually brightens or darkens gradually with respect to the driving time of the light emitting element to be driven, has a switching unit, and has a predetermined ratio to the reference current of the reference current source. Current output type digital / analog converter for obtaining a reference current output by adding the current values set by a predetermined gradation and adding the current values selected by the switching processing of the switching unit according to a predetermined digital signal And a slope time included in the time change of the reference current value output from the digital / analog converter, and the digital signal for switching the switching unit of the digital / analog converter based on the slope time. And a driver for supplying a current based on an output reference current of the digital / analog converter to the light emitting element.

  A fourth aspect of the present invention is a portable device having a battery as a power supply voltage source, a plurality of light emitting elements having different driving voltages required for light emission, and at least one illuminated portion illuminated by the light emitting elements. A plurality of light emitting elements connected in parallel and driving one or a plurality of light emitting elements among the plurality of light emitting elements, and the light emitting element driving apparatus includes the plurality of light emitting elements. A plurality of current drive circuits that are connected to corresponding light emitting elements of the elements and drive the corresponding light emitting elements with brightness based on a set value, and one of the plurality of drive circuits that is driven to emit light. Or a power supply circuit that determines a maximum driving voltage value among the plurality of light emitting elements and supplies at least the determined driving voltage to the plurality of light emitting elements, and each of the current driving circuits is a driving target. It has a slope function that gradually increases or decreases with respect to the driving time of the light emitting element, has a switching unit, and is set for a predetermined gradation with a predetermined ratio with respect to the reference current of the reference current source. A current output type digital / analog converter that obtains a reference current output by adding the current values selected by the switching process of the switching unit according to a predetermined digital signal, and the digital / analog converter A controller configured to set a slope time included in a time change of the output reference current value and generate the digital signal for switching the switching unit of the digital / analog converter based on the slope time; A current based on the output reference current of the analog converter is supplied to the light emitting element. Including a driver for, the.

According to the present invention, the accuracy of the output current value is determined only by the accuracy of the current DAC, and it is possible to improve the accuracy of the DAC by overcoming problems such as switching noise and current mirror ratio. The output noise from the driver stage is suppressed.
In addition, by controlling the reference current value of the driver stage by the DAC, it is possible to light up the light emitting element of 2 ⁿ gradations regardless of the setting value of the driver.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 2 is a block diagram illustrating a configuration example of a light emitting element driving apparatus including a current driving circuit according to the present embodiment. FIG. 3 is a block diagram showing a current drive circuit that realizes the slope function in FIG. FIG. 4 is a diagram showing a time change of the output current value including the DAC slope period according to the present embodiment.

The light-emitting element driving device 10 of the present embodiment is configured to be able to realize a slope function with a lower number of noises, for example, 64 gradations, while performing current control using a DAC.
As shown in FIG. 2, the light emitting element driving device 10 includes an oscillator 11, a counter 12, a slope controller (Slope CTRL) 13, a level shift circuit (LSFT) 14, and a current output digital / analog converter with a slope function including a switching portion ( (Hereinafter referred to as slope DAC) 15 and driver 16. Further, reference numeral 17 in FIG. 3 denotes a reference current source.

  The oscillator 11 supplies a reference clock CLK having a frequency fr [Hz] to the counter 12.

  The counter 12 counts the reference clock CLK from the oscillator 11, in other words, supplies the frequency-divided divided clock to the slope controller 13.

The slope controller 13 receives a slope mode rush signal STRAT and a slope time setting signal SLPTM from a control system (not shown), and is included in the time change of the reference current value output from the slope DAC 15 based on the output of the counter 12, for example, FIG. A slope time (Tr or Tf in FIG. 4) as shown in FIG. 4 is set and output to the level shift circuit 14.
The slope controller 13 receives the output signal of the counter 12, generates data for switching the switching portion of the slope DAC 15 as a gray code, decodes it to 6 bits, for example, and outputs it to the level shift circuit 14.

  For example, the level shift circuit 14 shifts the output signal (data) of the 3V system slope controller 13 to the 4.5V system and supplies it to the slope DAC 15.

As shown in FIG. 3, the slope DAC 15 has a structure in which the reference current Iref is subjected to current mirror processing with 2 ⁿ −1 reference elements. The value of each mirror current I 1 + I 2 +... + I (2 ⁿ −1) becomes the output value of the slope DAC 15, and the output current of the slope DAC 15 is input as the reference current of the driver 16.

  The driver 16 monitors the reference current Ioutref by the slope DAC 15 and drives the LED 20 as a driving target by multiplying the reference current Ioutref by a certain constant C.

  Hereinafter, the basic operation according to the above configuration, the reason for using the Gray code instead of the binary code, and the specific configurations and functions of the slope DAC 15 and the driver 16 will be described in order.

In the present embodiment, the slope time (Tr or Tf in FIG. 5) of the slope DAC 15 is set by the slope controller 13.
A counter 12 circuit is required for determining the slope time. The slope time is determined by the CLK frequency f [Hz] output from the counter 12. For example, in 64 gradation (6 bits) control, the rising slope time Tr and the falling slope time Tf are 64 / f [second (s)]. ].
In the slope controller 13, the output signal from the counter 12 is converted into a gray code, decoded into 6 bits (6 bits), and input to the switching portion of the slope DAC 15 via the level shift circuit 14.
Thus, one of the characteristic elements of this embodiment is that the code is converted into a gray code in advance when decoding into 6 bits.

In the slope DAC 15, the reference current Iref is current mirrored by 2 ⁿ -1 reference elements, and the value Ioutref of each mirror current I1 + I2 + ... + I (2 ⁿ -1) becomes the output value of the slope DAC15, which is the driver 16 reference currents are input.
That is, during the slope, the reference current value supplied to the driver 16 is increased (during the upward slope) or decreased (during the downward slope) in time, which is one of the characteristic elements of this embodiment. .

The driver 16 drives the LED 20 by outputting the reference current supplied from the slope DAC 15 by a certain constant C times.
That is, when the mirror ratio of the driver 16 in the slope is constant, the following equation is established when the output current of the slope DAC 15 is Ioutref and the output current of the driver stage is Iout.

(Equation 1)
Iout (t) = C x Ioutref (t) (1)

  Therefore, even if the function is the same Ioutref (t), the slope time is the same by changing the setting of the mirror ratio C in the driver stage (C1, C2, C3 in FIG. 5) as shown in FIG. Thus, it is possible to perform slope lighting with different final current values (that is, LED brightness).

  On the other hand, since the current value is controlled in an analog manner, for example, noise such as whiskers mixed in the signal on the reference current side in the DAC portion may directly become noise in the output current.

  Next, in this embodiment, the reason why the gray code is used instead of the binary code in the current DAC method will be described.

In general, in the current DAC method, there is a method of obtaining an analog current value by switching a switch using a binary binary code as shown in FIGS.
FIG. 6 shows a case where the 2-bit signal (01) is switched to (10) as an example.
As shown in FIG. 6C, when a time lag occurs in the switching of bits, noise called a glitch is generated. In addition, since the DAC is weighted, there is a possibility that the deviation at a high bit becomes large. That is, the DAC output is not linear.

  One method for solving this problem is to use a thermometer code.

FIG. 7 is a diagram illustrating a correspondence relationship between the binary code and the thermometer code. FIG. 8 is a diagram illustrating a configuration example of a DAC when a binary code is converted into a thermometer code. FIGS. 9A to 9G are diagrams showing a state of bit switching in the binary code.
FIG. 10 is a diagram showing the correspondence between the binary code and the thermometer code. FIGS. 11A to 11G are diagrams showing a state of bit switching in the Gray code.

As shown in FIG. 7, in the thermometer code, as the value of the binary code increases, the bit that outputs “1” monotonously increases bit by bit. Thus, as shown in FIG. 8, the binary-binary signal of n bits will be decoded by ON one by one 2 ⁿ -1 pieces of switches also in the thermometer code of 2 ⁿ -1. In FIG. 8, 1 indicates a DAC, and 2 indicates a thermometer code conversion circuit.
At this time, however, there is another noise source. It is a mustache that can occur when decoding binary code.
As shown in FIGS. 9A to 9G, there is a timing at which bit signals are simultaneously switched in the binary code. When these signals are input to the logic gate, for example, a beard as shown in FIG. As described above, the noise present in the signal decoded into the thermometer code causes the generation of glitch noise in the DAC.

Therefore, in order to eliminate the influence on the DAC accuracy, a binary code is not used for a signal before decoding, and a gray code is used instead.
As shown in FIGS. 11 and 10, unlike the binary code, the gray code does not have a timing for switching two or more bits simultaneously. Therefore, the beard at the time of bit switching can be reduced. As shown in FIG. 10, the decoded signal uses the thermometer code.

As described above, in this embodiment, the basic unit is simply added by the thermometer code without performing the DAC weighting, thereby improving the output accuracy and maintaining the linearity.
At the same time, by decoding binary gray code instead of binary binary code (converting it to thermometer code), the noise of DAC is further reduced.

In the present embodiment, the output current Ioutref from the slope DAC 15 is supplied as a reference current for the post-stage driver 16. In the circuit configuration of the present embodiment, the reference current of the driver is made variable, and the luminance value can be modulated by changing the current value for driving the LED by multiplying the constant mirror ratio of the driver stage. That is, as shown in FIG. 5, the final arrival brightness of the slope is variable depending on the mirror ratio of the driver stage.
Compared with the PWM control method (FIG. 1) used as a means for modulating the brightness of other LEDs, the current addition DAC method employed in this embodiment always provides a driver current as a continuous DC signal even during the slope. Is a big difference. This is an indispensable element in a configuration that feeds back the output. A configuration that feeds back the output will be described in detail later.

  Next, specific configurations and functions of the slope DAC 15 and the driver 16 will be described.

<First configuration example of slope DAC>
FIG. 12 is a circuit diagram showing a first configuration example of the slope DAC according to the present embodiment.

The slope DAC 15A in FIG. 12 includes p-channel MOS (PMOS) transistors PT0 to PT63, switches SW1-1 to SW1-63, switches SW2-1 to SW2-63, and inverters INV1 to INV63.
The driver 16 includes n-channel MOS (NMOS) transistors NT0 to NTx (x is a positive integer).

In the slope DAC 15A, the drain and gate of the PMOS transistor PT0 as the reference element are connected in common to the reference current source 17, and the source is connected to the supply line 18 of the power supply voltage VCC. Further, the sources of the PMOS transistors PT1 to PT63 as current mirror elements are connected in common to the supply line 18 of the power supply voltage VCC, the drains are connected (wired OR connection), and the drain common connection point ND15 A node ND15I for the reference current output of the slope DAC 15 is formed.
Further, a pair of switches SW1-1 and SW2-1,..., SW63-1 and SW63− are provided between the supply line 18 of the power supply voltage VCC and the connection line 19 between the drain and gate of the PMOS transistor PT0. 2 are connected in parallel, and the connection points of the switches in each of the series switch groups SWG1 to SWG63 are respectively connected to the gates of the PMOS transistors PT1 to PT63 as the corresponding current mirror elements. .
The two switches of the series switch groups SWG1 to SWG63 are complementarily turned on and off through the inverters INV1 to INV63 by corresponding decode signals.
The size ratio of the PMOS transistor PT0 as the reference element and the size of the other PMOS transistors PT1 to PT63 as the current mirror elements is set to 8: 1, for example.

In the driver 16, the drain and gate of the NMOS transistor NT0 are commonly connected to the current output node ND15I of the slope DAC 15 and the gates of the other NMOS transistors NT1 to NRx as current mirror elements.
The sources of the NMOS transistors NT0 to NTx are grounded, and the drains of the NMOS transistors NT1 to NTx are connected to the cathode of the LED 20 to be driven.

In the slope DAC 15A of FIG. 12, the switches are simplified and indicated by symbols, but actually, for example, a PMOS switch is used.
The current output of the slope DAC 15A becomes the reference current of the driver stage.
In the present embodiment, 6-bit gradation is realized, and one of the switches SWG1 to SW63 in series of switches SW1-1 and SW2-1,..., SW63-1 and SW63-2 is a thermometer. As the current is sequentially turned ON / OFF by the code, 63 PMOS transistors PT1 to PT63 as current mirror elements and 63 × 2 switches are required.
When the current mirror element is ON, the lower switch (SWx-2) connected to the gate is turned ON, and the signal in the opposite phase is input to the upper switch (SWx-1).
If only the lower switch is present, the gate voltage of the current mirror element that is turned off becomes indefinite, and the voltage is held by parasitic capacitance, so it cannot be turned off. Therefore, when the switch is OFF, the upper switch is turned ON to connect the gate of the current mirror element to the power supply potential VCC for stable operation.

A feature of the circuit configuration of FIG. 12 for switching the gate is that one end of the switch is connected to a switch corresponding to the gate of the PMOS transistor PT0 as a reference element or another current mirror element.
In other words, in such a structure, a switching signal of a certain bit may cause the gate voltage of all other current mirror elements to fluctuate via the gate capacitance of the switch, resulting in an error in the output current.

<Second configuration example of the slope DAC>
FIG. 13 is a circuit diagram showing a second configuration example of the slope DAC according to the present embodiment.
In FIG. 13, the same components as those of the circuit of FIG.

  The slope DAC 15B in FIG. 13 has a circuit configuration that takes into consideration that the switching signal does not interfere with the gates of the other current mirror elements (PMOS transistors PT1 to PT63).

In the configuration of FIG. 12, there is a risk that noise to the switch causes other gate voltages to fluctuate through the gate capacitance, resulting in output fluctuation. However, in the configuration of FIG. Switches SW1 to SW63 are inserted.
As a result, the PMOS transistors PT1 to PT63 as the current mirror elements and the switches SW1 to SW63 for turning them on / off are independent of the noise, and the noise to the switches SW1 to SW63 is the other current mirror element. As a result, there is little influence on the PMOS transistors PT1 to PT63. Therefore, it can be suppressed to a minimum error.

<Third configuration example of the slope DAC>
FIG. 14 is a circuit diagram showing a third configuration example of the slope DAC according to the present embodiment.
In FIG. 14, the same components as those in the circuit of FIG.

A slope DAC 15C in FIG. 14 is an improvement of the circuit in FIG.
As shown in FIG. 13, although noise immunity is enhanced by inserting the switches SW1 to SW63 on the source side of the PMOS transistors PT1 to PT63 as current mirror elements, the voltage drop due to the ON resistance of this switch causes a mismatch in the mirror ratio. There is a risk of causing it. That is, while the source potential of the PMOS transistor PT0 as the reference element is the power supply voltage VCC), the source potential VS on the current mirror side is as follows.

(Equation 2)
VS = VCC- (I / gm) (2)

Here, I is the mirror current of each bit t, and gm is the conductance of the switch.
When PMOS is used as a switch in FIG. 13, this gm is given by the following equation.

(Equation 3)
gm = √ (2 μpCox (W / L) ID) = 2 ID / (Vgs−Vth) (3)

  Furthermore, it can be expressed as follows.

(Equation 4)
gm = μpCox (W / L) (Vgs−Vth) (4)

Here, in order to solve the problem of deviation in mirror ratio due to the difference in source potential between the reference element (PMOS transistor PT0) side and the current mirror element (PMOS transistors PT1 to PT63) side, drop on the current mirror element side is performed. It is conceivable that the conductance of the switch is increased to a negligible level, or the source potential on the reference element side is dropped by the same amount as that on the current mirror element side to be matched.
However, in order to realize the former, it is necessary to increase the mirror current from the equations (3) and (4) or increase the transistor size W (channel width). Increasing the current value increases the current consumption, and increasing the size increases the layout area (considering that there are 2 ^n-1 switches per channel, the high bit becomes strict).
Therefore, the slope DAC 15C of FIG. 14 employs a method of dropping the source voltage on the reference element side as well.

In the slope DAC 15C of FIG. 14, the switch between the reference element side and the current mirror element side is inserted by inserting a switch SW0 that is always ON (the gate is grounded using a PMOS switch) on the source side of the PMOS transistor PT0 that is the reference element. Match the source-gate voltage.
However this time, the mirror ratio in the reference device side and the current mirror element side is 8: must match the conductance g _m of the switch by considering that 1. That is, since eight times the current flows to the reference element side, it is necessary to make the conductance on the reference side eight times from Equation (3). For this, the channel length L of the switch on the reference element side may be increased by eight times after making the channel length L the same (the threshold value Vth is the same) from the equation (4).

From the above, in this embodiment, in order to eliminate the influence of noise as much as possible, to match the mirror ratio, and to supply a highly linear DAC output to the reference current of the driver stage with a thermometer code, it is preferable It is desirable to adopt the configuration diagram of FIG.
The slope DAC in FIG. 15 corresponds to the circuit configuration in FIG.

  As described above, according to the present embodiment, by using the current DAC method and supplying the output current of the DAC to the reference current of the driver, the following effects are brought about compared with the PWM method gradation control.

The accuracy of the output current value is determined only by the accuracy of the current DAC, and it is possible to improve the accuracy of the DAC by overcoming problems such as switching noise and current mirror ratio. As a result, the output from the driver stage Noise is suppressed.
In the PWM method, the slope gradation becomes low when the driver setting value is low, but by controlling the reference current value of the driver stage with the DAC as in this embodiment, ²ⁿ gradations are used regardless of the driver setting value. LED lighting can be realized.

  It is useful in analog circuits that the output is DC. For example, by monitoring the output value and applying feedback, it is possible to light the LED with the minimum necessary drive voltage. This circuit configuration is designed to operate with the consideration that LEDs are driven with high efficiency by a step-up DC-DC converter, and is a slope function implementation circuit that is indispensable for reducing the power consumption of ICs such as portable devices. is there.

  FIG. 16 is a circuit diagram illustrating a configuration example of an LED driving circuit that employs feedback control to the DC-DC converter according to the present embodiment and drives a plurality of light emitting elements (LEDs).

As shown in FIG. 16, the LED driving apparatus 100 includes a plurality of n (n is a positive integer) LEDs 20-1 to 20-n having different driving voltages, that is, forward voltages Vf, that are required to emit light in parallel. The LEDs 20-1 to 20-n are each driven with an arbitrary luminance (drive current).
In the example of FIG. 16, the case where seven LEDs 20-1 to 20-7 are provided in parallel is shown as an example.
Of the seven LEDs, four LEDs 20-1 to 20-4 are white LEDs, one LED 20-5 is a red LED, one LED 20-6 is a green LED, and one LED 20-7 is a blue LED. is there.

The LED driving device 100 uses an optimum voltage (for example, the lowest voltage) that can be driven by the set current of the LED having the maximum forward voltage Vf among the plurality of LEDs 20-1 to 0-7 connected in parallel. Output from the terminal TVO to the anodes of the LEDs 20-1 to 20-7.
The LED drive device 100 is supplied with the power supply voltage DVCC via a terminal TVI by a power supply voltage source such as a battery.

In the LED driving device 100, the current driving circuit 10 employing the slope DAC described in association with FIG. 2 and the like is provided corresponding to the red LED 20-5, the green LED 20-6, and the blue LED 20-7.
In FIG. 16, in order to facilitate understanding, the same components as those in FIG. 2 and the like are denoted by the same reference numerals as those in FIG.

  16 includes an oscillator 11, a counter 12, an RGB current drive circuit 101, a W drive circuit 102, a serial / parallel conversion circuit (S / P) 103, a luminance setting circuit 104, a level shift circuit 105, and a DC. A DC converter 106, a PMOS transistor 107, an NMOS transistor 108, an external inductor L101, and capacitors C101 to C106.

The RGB current drive circuit 101 includes an R slope controller 13R, a G slope controller 13G, a B slope controller 13B, an R level shift circuit 14R, a G level shift circuit 14G, and a B level shift circuit 14B, R. A slope DAC 15R for G, a slope DAC 15G for G, a slope DAC 15B for B, a driver 16R for R, a driver 16G for G, and a driver 16B for B.
Since these functions are the same as the functions described in association with FIG. 2 etc., the description thereof is omitted here.
The output of the R driver 16R is connected to the cathode of the LED 20-5 via the terminal TR, the output of the G driver 16G is connected to the cathode of the LED 20-6 via the terminal TG, and the output of the B driver 16B is connected to the terminal It is connected to the cathode of the LED 20-7 via TB.

The W drive circuit 102 has only four drivers 1021 to 1024, the output of the driver 1021 is connected to the cathode of the LED 20-1 via the terminal TW1, and the output of the driver 1022 is connected to the LED 20-2 via the terminal TW2. The output of the driver 2023 is connected to the cathode of the LED 20-3 via the terminal TW3, and the output of the driver 2024 is connected to the cathode of the LED 20-4 via the terminal TW4.
The drivers 1021 to 1024 drive the corresponding white LEDs 20-1 to 20-4 according to the luminance setting signal level-shifted by the level shift circuit 105.

  The drivers 1021 to 1024 are connected to the ground through the terminal TG1. Similarly, the drivers 16R, 16G, and 16B of the RGB current drive circuit 101 are connected to the ground through the terminal TG2.

  Further, the outputs of the drivers 16R, 16G, and 16B of the RGB current drive circuit 101 and the outputs of the drivers 1021 to 1024 (connection levels with the cathodes of the LEDs 20-1 to 20-7) are fed back to the DC-DC converter 106. The

The serial / parallel conversion circuit 103 receives, as parallel data, digital serial data SDAT relating to a current (luminance) value for driving the LEDs 20-1 to 20-n supplied by a host device such as a CPU (not shown) input via a terminal TDI. The digital data ID1 to IDn related to the converted current (luminance) values are output to the corresponding luminance setting circuit 104.
Further, the serial / parallel conversion circuit 103 receives digital serial data SDAT relating to a current (luminance) value for driving the LEDs 20-1 to 20-n supplied by a host device such as a CPU (not shown) input via the terminal TDI. The data is converted into parallel data, and the slope mode entry signal START and the slope time setting signal SLPTM are generated and supplied to the slope controllers 13R, 13G, and 13B of the current drive circuit 101.

The luminance setting circuit 104 converts the data relating to the drive current (luminance) value supplied from the serial / parallel conversion circuit 103 into a current setting signal LEDout and supplies it to the level shift circuit 105.
The current setting signal LEDout is a signal for setting the luminance of the LEDs 20-1 to 20-7, and specifically, a signal for setting the mirror ratio of the driver stage. In the slope operation, the set value here becomes the slope final reached luminance. It can also be set for each channel.

  The level shift circuit 105 converts the current setting signal from the luminance setting circuit 104 from, for example, the 3V system to the 4.5V system and supplies the converted signal to the W and RGB driving circuits.

The DC-DC converter 106 is supplied with the reference clock CLK from the oscillator 11 as an operation clock, and monitors the cathode voltages Vf1 to Vf3 of the LEDs 20-1 to 20-7, particularly the LEDs 20-5 to 20-7, via the bus 109. When the power supply voltage VCC decreases and any one of the cathode voltages Vf (1 to 3) falls below a certain threshold value, the power supply voltage is increased to increase the power supply voltage from the terminal TVO (seven in FIG. 16). The drive voltage VDCDC is supplied in parallel to the LEDs 20-1 to 20-7.
In the DC-DC converter 106, the DC-DC voltage is controlled so that the cathode voltage value (= VDCDC-Vf (1-3)) of the LED becomes a minimum necessary value. In other words, the LED is driven with high efficiency. In order to realize this function, Vf1 to Vf3 must not swing in an alternating current (ac) manner. In other words, the PWM method is not suitable. Therefore, a current DAC type slope DAC is required.

The source of the PMOS transistor 107 is connected to the terminal TVO2 connected to the supply line of the drive voltage VDCDC, the drain is connected to the drain of the NMOS transistor 108, and the source of the NMOS transistor 108 is connected to the ground via the terminals TG3 and TG4. Has been.
Each drain of the PMOS transistor 107 and the NMOS transistor 108 is connected to one end side of the inductor L101 via terminals TD1 and TD2.
Control potentials of the gates of the PMOS transistor 107 and the NMOS transistor 108 are controlled by the DC-DC converter 106 to form a desired drive voltage VDCDC.

  When the threshold voltage of the DC-DC converter 106 is 1V, the LED driving device 100 according to the present embodiment is a terminal voltage to which the cathode of the LED having the maximum forward voltage Vf is connected, in other words, a driver. Feedback is applied to the boost power supply so that the drain voltages of the NMOS transistors NT1 to NTx of the 16R, 16G, and 16B become 1V.

  Next, the operation according to the above configuration will be described.

For example, digital serial data relating to current (luminance) values for the LEDs 20-5 to 20-7 to be driven according to the operation mode is input from the host device to the serial-parallel conversion circuit 103 via the terminal TDI.
In the serial / parallel conversion circuit 103, the digital serial data relating to the current (luminance) value to drive the supplied LEDs 20-1 to 20-n is converted into parallel data.
In the serial / parallel conversion circuit 103, a slope mode entry signal START and a slope time setting signal SLPTM are generated and supplied to the slope controllers 13R, 13G and 13B of the current drive circuit 101.
The digital data ID1 to IDn also includes information that does not drive the corresponding LED.

In the slope controllers 13R, 13G, and 13B, the slope time (Tr or Tf in FIG. 4) included in the time change of the reference current value output from the slope DAC 15 based on the output of the counter 12, for example, as shown in FIG. ) Is set and output to the level shift circuits 14R, 14G, and 14B.
In the slope controllers 13R, 13G, and 13B, the output frequency division signal of the counter 12 is received, and data for switching the switching portion of the slope DACs 15R, 15G, and 15B is generated as a gray code, and is decoded into, for example, 6 bits and level The signals are supplied to the slope DACs 15R, 15G, and 15B through the shift circuits 14R, 14G, and 14B.
As shown in FIG. 3, the slope DACs 15R, 15G, and 15B have a structure in which the reference current Iref is subjected to current mirror processing with 2 ⁿ −1 reference elements, and each mirror current I1 + I2 +... + I (2 ⁿ −1) Becomes the output values of the slope DACs 15R, 15G, and 15B, and the output currents of the slope DACs 15R, 15G, and 15B are input as the reference currents of the drivers 16R, 16G, and 16B.
In the drivers 16R, 16G, and 16B, the reference current Ioutref by the slope DACs 15R, 15G, and 15B is monitored, and the LEDs 20-5 to 20-7 to be driven are driven to emit light by multiplying the reference current Ioutref by a certain constant C. .

  In FIG. 16, as an example, the cathode voltages Vf1 to Vf3 of the 3ch LEDs 20-5 to 20-7 are monitored, the power supply voltage VCC is lowered, and any one of the voltages Vf1 to Vf3 is present. If the voltage is lower than that, the DC-DC converter 106 operates to boost the power supply voltage. At this time, the DCDC voltage is controlled so that the cathode voltages (= VDCDC-Vf1 to Vf3) of the LEDs 20-5 to 20-7 become the minimum necessary values. That is, the LEDs 20-5 to 20-7 are driven with high efficiency.

The specific operation of the DC-DC converter 106 as a step-up power supply for light emission (lighting) of each color LED is as follows.
Here, it is assumed that the LEDs 20-1 to 20-n include one or a plurality of red (R) LEDs, green (G) LEDs, blue (B) LEDs, and white LEDs.
The forward voltage of each color LED is as follows.
The forward voltage Vfr of the red LED is set to 1.9V, the forward voltages Vfg and Vfb of the green and blue LEDs are set to about 3.1V, and the forward voltage Vfw of the white LED is set to 3.5V.
Further, assuming that the power supply voltage source is a lithium ion battery, the power supply voltage V _CC is used in the range of 3.2V to 4.2V.
Further, the minimum operating voltage α required for the LED current drive circuit 101 is set to 0.5V.
In the following description, “through” means that the PMOS transistor 107 constituting the output stage of the boost power supply is turned on and the NMOS transistor 108 is turned off (the DC-DC converter operates with 100% duty). It shows that.

In order to light a white LED whose forward voltage Vfw is 3.5V, a voltage required as the drive voltage VDRV is 4V (= 3.5V + 0.5V).
The operation of the boost power supply 15 in this case is “through” when the power supply voltage V _CC is in the range of 4.0 V <V _CC <4.2 V.
On the other hand, when the power supply voltage V _CC is in the range of 3.2 V <V _CC <4.0 V, for example, the operation is to “boost” 4 V.

In order to turn on the green LED and the blue LED whose forward voltages Vfg and Vfb are 3.1 V, the voltage required as the drive voltage VDRV is 3.6 V (= 3.1 V + 0.5 V).
The operation of the boost power supply 15 in this case is “through” when the power supply voltage V _CC is in the range of 3.6 V <V _CC <4.2 V.
On the other hand, when the power supply voltage V _CC is in the range of 3.2 V <V _CC <3.6 V, the operation is to “boost” to 3.6 V, for example.

In order to light a red LED having a forward voltage Vfr of 1.9V, a voltage required as the drive voltage VDRV is 2.4V (= 1.9V + 0.5V).
The operation of the step-up power supply 15 in this case assumes that the power supply voltage V _CC is in the range of 3.2 V <V _CC <4.2 V. In this embodiment, since the step-down power supply is not included, “Through” in range.

  That is, the operation of the boost power supply is “through” when the output of the boost power supply expected by applying feedback is equal to or lower than the power supply voltage (battery voltage) VCC, and thus the LED is directly driven by the power supply voltage VCC. It will be.

  As described above, according to the LED drive device of FIG. 16, it is possible to light the LED with the minimum required drive voltage by monitoring the output values of the current drive circuits 101 and 102 and applying feedback. . This circuit configuration is designed to operate with the high-efficiency LED driven by the step-up DC-DC converter 106, and is a slope function implementation circuit that is indispensable for reducing the power consumption of ICs such as portable devices. It is.

  FIG. 17 is a block diagram illustrating a configuration example of a portable device (terminal) to which the LED driving device of FIG. 16 described above can be applied.

The mobile device 200 is composed of, for example, a mobile phone device, and as shown in FIG. 17, a CPU 201, a first image display device 202, a second image display device 203, an input device 204, an incoming call display unit 205, a synchronization signal supply, It has the circuit 206 and the LED drive device 207 which has the structure of FIG.
And the 1st image display apparatus 202, the 2nd image display apparatus 203, the input device 204, and the incoming call display part 205 comprise the to-be-illuminated part illuminated with LED.

  The CPU 201 controls the operation of the device based on the input data from the input device 204, the display control of the first image display device 202 and the second image display device 203 when the power is turned on, the drive control of the synchronization signal supply circuit 206, and the operation mode. Control of supply to the LED driving device 207 such as current (brightness) setting data and blinking operation command data according to is performed.

The first image display device 202 functions as a main display unit of the mobile device 200 and is configured by a liquid crystal display device capable of color display.
In the vicinity of the first image display device 202, four white LEDs (LEDs 20-1 to 20-4 in the example of FIG. 16) connected in parallel to the LED driving device 207 are arranged as illumination backlights. Has been.
Under the control of the CPU 201, the first image display device 202 displays a radio wave reception state, an icon menu and various images, a destination telephone number and a message input by the input device 204 or received.

The second image display device 203 functions as a sub display unit of the portable device 200 and is configured by a liquid crystal display device.
In the vicinity of the second image display device 203, LEDs for three colors of red, green, and blue connected in parallel to the LED driving device 207 (for the example of FIG. 16, LEDs 20-5 to 20-7) are used for illumination. ) Is arranged.
The second image display device 203 displays time, date, and the like under the control of the CPU 201, and at the time of incoming call or transmission, the LED driving device 207 selects one of the three colors of LEDs or any two of them. Alternatively, all color LEDs are lit or blinked.

The input device 204 includes a power switch, a numeric keypad, and the like, and in the vicinity thereof, LEDs for three colors of red, green, and blue (LED 20 in the example of FIG. 16) are connected in parallel to the LED driving device 207 for illumination. -5 to 20-7) are arranged.
When the power is turned on under the control of the CPU 201, the input device 204 is illuminated by one of the three colors of LEDs, or any two of them, or all colors by the LED driving device 207.

The incoming call display unit 205 includes LEDs of three colors, red, green, and blue (LEDs 20-5 to 20-7 in the example of FIG. 16) connected in parallel to the LED driving device 207.
In the incoming call display unit 205, when the incoming call is received, the LED driving device 207 turns on or blinks one of the three colors of LEDs, or any of the two colors or all of the LEDs.

  The synchronization signal supply circuit 206 is configured by a sound source IC such as MIDI, and supplies a synchronization signal SYNC used for, for example, a blinking operation to the LED driving device 207 under the control of the CPU 201.

  For example, a lithium ion battery 208 is used as the power source of the portable device 200.

  In the mobile device 200 having such a configuration, the first part where the first image display device 202, the second image display device 203, the incoming call display unit 205 are arranged, and the second part where the input device 204 is arranged Is carried by the user in a folded state, for example, by the hinge mechanism.

When the user operates to open the first part and the second part, for example, when the power switch is turned on, the CPU 201 causes the LED driving device to illuminate the first image display device 202 with the backlight. Current (luminance) setting data for driving the white LED is supplied to 207.
Thereby, the white LED is driven by the LED driving device 207, and the first image display device 42 is illuminated brightly in white.
At this time, for example, the CPU 201 supplies current (luminance) setting data for driving the green LED to the LED driving device 207 so that the input device 207 is illuminated by the green LED.
Thereby, the green LED is driven by the LED driving device 207, and the input device 204 is lightly illuminated in green.

Further, for example, when there is an incoming call in a state where the first part and the second part are folded when the power is turned on, the CPU 201 is used to turn on or blink the incoming call display unit 205 or the second image display device 203 by, for example, a red LED. Thus, current (luminance) setting data for driving the red LED is supplied to the LED driving device 207. When the mode for blinking operation is set, the blinking operation instruction command data is output to the LED drive device 207 by the CPU 201, and the synchronization signal SYNC is supplied from the synchronization signal supply circuit 206 to the LED drive device 207. To be controlled.
Accordingly, the red LED is driven by the LED driving device 207, and the incoming call display unit 205 and the second image display device 203 are displayed in red or blinking.

The operation of the LED driving device 207 during each of the above operations is the value of the power supply voltage VCC so that the terminal voltage to which the cathode of the LED having the maximum forward voltage Vf is connected becomes the threshold voltage (reference voltage). And is output as a drive voltage VDCDC.
As a result, even if the brightness of the plurality of LEDs is individually adjusted or a plurality of LEDs having different forward voltages are simultaneously driven, the lowest voltage that always satisfies the driving condition is output.
Therefore, the luminous efficiency is high and the power loss is kept low.
Here, detailed operation of the LED driving device 207 is omitted.

  According to the portable device 200 of FIG. 17, the illumination LED can always output the lowest voltage that satisfies the driving conditions, the light emission efficiency can be improved, and the power loss can be reduced. There is an advantage that the battery life can be extended.

It is a figure which shows the implementation example of the LED slope function by PWM control. It is a block diagram which shows the structural example of the light emitting element drive device provided with the current drive circuit which concerns on this embodiment. It is a block diagram which shows the current drive circuit which implement | achieves the slope function in FIG. It is a figure which shows the time change of the output electric current value containing the slope period of DAC which concerns on this embodiment. It is a figure which shows the example of a slope output waveform from which the last reached value with the same slope time. It is a figure which shows an example of the glitch at the time of the bit change seen by a binary code system. It is a figure which shows the correspondence of a binary code and a thermometer code. It is a figure which shows the structural example of DAC at the time of converting a binary code into a thermometer code. It is a figure which shows the mode of the bit change in a binary code. It is a figure which shows the correspondence of a gray code and a thermometer code. It is a figure which shows the mode of the bit change in a gray code. It is a circuit diagram showing the 1st example of composition of slope DAC concerning this embodiment. It is a circuit diagram which shows the 2nd structural example of slope DAC which concerns on this embodiment. It is a circuit diagram which shows the 3rd structural example of slope DAC which concerns on this embodiment. It is a circuit diagram which shows the suitable structural example of slope DAC which concerns on this embodiment. It is a circuit diagram which shows the structural example of the LED drive circuit which employ | adopts the feedback control to the DC-DC converter which concerns on this embodiment, and drives a several light emitting element (LED). It is a block diagram which shows the structural example of the portable apparatus (terminal) which can apply the LED drive device of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Light emitting element drive device, 11 ... Oscillator, 12 ... Counter, 13, 13R, 13G, 13B ... Slope controller, 14, 14R, 14G, 14B ... Level shift circuit, 15, 15A to 15C, 15R, 15G, 15B ... current output digital / analog converter with a slope function including a switching portion (referred to as slope DAC), 16, 16R, 16G, 16B ... driver, 17 ... reference current source 20, 20-1 to 20-7... LED, 100... LED driving device, 101... Current driving circuit, 102... Driving circuit, 103. , 104... Brightness setting circuit, 105... Level shift circuit, 106... DC-DC converter, 200. DESCRIPTION OF SYMBOLS 1 ... CPU, 202 ... 1st image display apparatus, 203 ... 2nd image display apparatus, 204 ... Input device, 205 ... Incoming call display part, 206 ... Synchronization signal supply circuit, 207 ... LED driving device.

Claims (14)

A current driving circuit having a slope function of gradually increasing or decreasing gradually with respect to a time for driving a driving target;
A current value set for a predetermined gradation with a predetermined ratio with respect to a reference current of a reference current source, and selected by a switching process of the switching unit according to a predetermined digital signal A current output type digital / analog converter that adds a current value to obtain a reference current output; and
A slope time included in a time change of a reference current value output from the digital / analog converter is set, and the digital signal for switching the switching unit of the digital / analog converter is generated based on the slope time. A controller,
And a driver for supplying a current based on an output reference current of the digital / analog converter to the drive target. The current output type digital / analog converter has a current mirror structure having a reference element connected to the reference current source and a plurality of current mirror elements, and each of the mirror currents by the current mirror elements is added to the reference current source. The current drive circuit according to claim 1, wherein a current output is obtained. The current drive circuit according to claim 2, wherein a switching element that can be switched by the digital signal is connected to each of the mirror mirror elements. The current drive circuit according to claim 3, wherein a switching element in an on state is connected to the reference element. The current drive circuit according to claim 1, wherein the digital signal generated by the controller includes a Gray code. A light-emitting element driving device having a slope function of gradually brightening or gradually darkening with respect to time for driving a light-emitting element to be driven,
A current value set for a predetermined gradation with a predetermined ratio with respect to a reference current of a reference current source, and selected by a switching process of the switching unit according to a predetermined digital signal A current output type digital / analog converter that adds a current value to obtain a reference current output; and
A slope time included in a time change of a reference current value output from the digital / analog converter is set, and the digital signal for switching the switching unit of the digital / analog converter is generated based on the slope time. A controller,
A light-emitting element driving device comprising: a driver that supplies a current based on an output reference current of the digital / analog converter to the light-emitting element. The current output type digital / analog converter has a current mirror structure having a reference element connected to the reference current source and a plurality of current mirror elements, and each of the mirror currents by the current mirror elements is added to the reference current source. The light emitting element drive device according to claim 6, wherein a current output is obtained. The light emitting element drive device according to claim 7, wherein a switching element that can be switched by the digital signal is connected to each of the mirror mirror elements. The light emitting element drive device according to claim 8, wherein a switching element in an on state is connected to the reference element. The light-emitting element driving device according to claim 6, wherein the digital signal generated by the controller includes a gray code. A power supply circuit for supplying a driving voltage to the light emitting element;
The light emitting element driving device according to claim 6, wherein the power supply circuit monitors a terminal level of a current output terminal of the driver and sets a driving voltage to the light emitting element. The light-emitting element driving device according to claim 6, wherein the driver multiplies a reference current by the current output digital / analog converter by a constant and supplies the reference current to the light-emitting element. A plurality of light emitting elements having different driving voltages required for light emission are connected in parallel, and the light emitting element driving device drives one or a plurality of light emitting elements among the plurality of light emitting elements,
A plurality of current driving circuits connected to the corresponding light emitting elements of the plurality of light emitting elements and driving the corresponding light emitting elements with brightness based on a set value;
Based on each driving state of the plurality of driving circuits, the maximum driving voltage value among one or a plurality of light emitting elements driven to emit light is determined, and at least the determined driving voltage is supplied to the plurality of light emitting elements. A power supply circuit,
Each of the current drive circuits is
It has a slope function to gradually lighten or darken gradually with respect to the time to drive the light emitting element to be driven,
A current value set for a predetermined gradation with a predetermined ratio with respect to a reference current of a reference current source, and selected by a switching process of the switching unit according to a predetermined digital signal A current output type digital / analog converter that adds a current value to obtain a reference current output; and
A slope time included in a time change of a reference current value output from the digital / analog converter is set, and the digital signal for switching the switching unit of the digital / analog converter is generated based on the slope time. A controller,
And a driver for supplying a current based on an output reference current of the digital / analog converter to the light emitting element. A portable device having a battery as a power supply voltage source,
A plurality of light emitting elements having different driving voltages required for light emission;
At least one illuminated part illuminated by the light emitting element;
A plurality of light emitting elements connected in parallel and driving one or a plurality of light emitting elements among the plurality of light emitting elements;
Have
The light emitting element driving device is:
A plurality of current driving circuits connected to the corresponding light emitting elements of the plurality of light emitting elements and driving the corresponding light emitting elements with brightness based on a set value;
Based on each driving state of the plurality of driving circuits, the maximum driving voltage value among one or a plurality of light emitting elements driven to emit light is determined, and at least the determined driving voltage is supplied to the plurality of light emitting elements. A power supply circuit,
Each of the current drive circuits is
It has a slope function to gradually lighten or darken gradually with respect to the time to drive the light emitting element to be driven,
A current value set for a predetermined gradation with a predetermined ratio with respect to a reference current of a reference current source, and selected by a switching process of the switching unit according to a predetermined digital signal A current output type digital / analog converter that adds a current value to obtain a reference current output; and
A slope time included in a time change of a reference current value output from the digital / analog converter is set, and the digital signal for switching the switching unit of the digital / analog converter is generated based on the slope time. A controller,
And a driver for supplying a current based on an output reference current of the digital / analog converter to the light emitting element.

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