JP2015060822A - Drive circuit for light-emitting element, control circuit thereof, control method, and light-emitting device and electronic equipment using the drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress reduction in drive current at re-lighting in PWM dimming.SOLUTION: A dimming control part 350 switches between an on state and an off state of N current sources CS_1 to CS_N on the basis of N dimming pulse signals PWM_1 to PWM_N. A duty controller 306 generates a pulse modulation signal S1 having a duty ratio corresponding to a feedback voltage Vgenerated by an error amplifier 302. A gate driver 308 (i) switches a DC/DC converter 202 corresponding to the pulse modulation signal S1 in a lighting time period in which at least one of the N dimming pulse signals PWM_1 to PWM_N instructs the on state, and (ii) switches the DC/DC converter 202 at a predetermined number of times after transition from the lighting time period to an extinguishing time period in which all of the N dimming pulse signals instruct the off state, and stops the switching of the DC/DC converter 202 for the remaining extinguishing time period.

Description

  The present invention relates to a drive circuit for a light emitting element.

  In recent years, light-emitting elements such as LEDs (light-emitting diodes) have been used as backlights and lighting devices for liquid crystal panels. Patent Documents 1 and 2 disclose related technologies.

  FIG. 1 is a circuit diagram showing a configuration of a light emitting device studied by the present inventors. The light emitting device 100r includes N (N is a natural number) LED strings 102_1 to 102_N and a driving circuit 200r thereof.

Each LED string 102 includes a plurality of LEDs connected in series. The drive circuit 200r includes a DC / DC converter 202r and a control circuit 300r, boosts the input voltage _VIN of the input line 204, and drives the drive voltage to one end (anode) of the LED strings 102_1 to 102_N connected to the output line 206. V _OUT (also referred to as output voltage) is supplied. The drive circuit 200r controls the luminance of the LED string 102 by individually adjusting the drive current I _LED flowing through the LED string 102.

The DC / DC converter 202 includes an inductor L1, a switching transistor M1, a rectifier diode D1, and an output capacitor C1. The control circuit 300r adjusts the drive voltage _VOUT by controlling the on / off duty ratio of the switching transistor M1.

  The control circuit 300r includes current sources CS_1 to CS_N corresponding to the N LED strings 102_1 to 102_N, a dimming control unit 350, and a controller 301r of the DC / DC converter 202.

Each current source CS_i supplies the corresponding LED string 102_i with a drive current I _LEDi corresponding to the target luminance.

The controller 301r feedback-controls the duty ratio of the switching transistor M1 so that the lowest voltage among the cathode terminal voltages V _{LED1 to} V _LEDN of the LED strings 102_1 to 102_N of the plurality of channels matches the predetermined reference voltage V _REF. To do.

A phase compensation resistor R _FB and a capacitor C _FB are connected to the feedback terminal (also referred to as FB terminal). The error amplifier 302 is a transconductance amplifier (also referred to as LED terminal) cathode terminal and the lowest voltage of the voltages _V LED1 _{~V LEDn} of LED_1～LED_N, amplifies an error of a predetermined reference voltage _{V REF,} according to the error To generate a feedback voltage V _FB at the feedback terminal FB.

The duty controller 306 is a voltage mode pulse width modulator, and includes a PWM comparator 320 and an oscillator 322. The oscillator 322 generates a periodic signal V _OSC having a predetermined frequency. The periodic signal V _OSC has a ramp wave (sawtooth wave) or a triangular waveform. The PWM comparator 320 compares the feedback voltage V _FB with the periodic signal V _OSC and generates a pulse modulation signal S1 that has been subjected to pulse width modulation. The gate driver 308 applies a gate pulse signal S2 corresponding to the pulse modulation signal S1 to the gate of the switching transistor M1, and switches it.

JP 2008-186668 A JP2013-105628A

In such a light emitting device 100r, the drive current I _LED may be subjected to PWM (Pulse Width Modulation) control in order to adjust the luminance of the LED string 102. Specifically, the dimming control unit 350 generates dimming pulse signals PWM_1 to PWM_N having a duty ratio corresponding to the luminance of the LED strings 102_1 to 102_N of each channel. The current sources CS_1 to CS_N of each channel are subjected to switching control based on the corresponding dimming pulse signals PWM_1 to PWM_N. Thereby, the LED string 102_i of each channel is dimmed according to the duty ratio of the corresponding PWM_i (also referred to as PWM dimming).

When PWM dimming is performed, the potential V _LEDi of the cathode terminal of the LED string 102 of the channel is pulled up to a high level voltage and excluded from the feedback target during the period when the current source CS_i of the channel is turned off. . This is because the cathode terminal V _{LEDi of the} channel takes a level that is unrelated to the state of the load.

All channel off signal PWM_ALL_L (hereinafter referred to as PWM_ALL_L signal) is asserted (high level) while current sources CS_1 to CS_N of all channels are simultaneously turned off. When the PWM_ALL_L signal is asserted, the controller 301r stops switching of the switching transistor M1. Specifically, when the PWM_ALL_L signal is asserted, the output S2 of the gate driver 308 is fixed at a low level, and the switching transistor M1 is turned off. Further, during the period when the PWM_ALL_L signal is asserted, the switch 304 is turned off, the FB terminal becomes high impedance, and the feedback voltage V _FB is stored.

  FIG. 2 is an operation waveform diagram of the light emitting device 100r of FIG.

FIG. 2 is an operation waveform diagram of the light emitting device 100r of FIG. Here, for ease of understanding, the case of N = 1 channel will be considered. Dimming pulse signal PWM_1 in the period _{T 12} is asserted (high level), the current _{I LED1} to LED string 102_1 is supplied by the current source CS_1. During this time, the output voltage V _OUT is stabilized at a certain level.

When the dimming pulse signal PWM_1 is negated (low level) at time t2, the PWM_ALL_L signal is asserted, the gate pulse signal S2 is fixed at the low level, and the switching of the switching transistor M1 is stopped. Between the output line 206 of the DC / DC converter 202r and the ground line, resistors R1 and R2 for dividing the output voltage _VOUT , or a discharge resistor (not shown) are provided. Therefore during the period _{T 23} in which the switching of the DC / DC converter 202 is stopped, the discharging via their resistance, the output voltage _{V OUT} decreases with time. During the period _{T 23,} the feedback voltage _{V FB} is the value immediately before is held.

Thereafter, the dimming pulse signal PWM_1 is asserted again at time t3. At the same time as the dimming pulse signal PWM_1 is asserted, the current source CS_1 is turned on and the drive current I _LED1 starts to flow. However, since the lower than the level at the output voltage _{V OUT} period _{T 12} at this time, the drive current _{I LED1} will not reach the desired amount of current _{I REF,} insufficient brightness. Thereafter, when the output voltage _VOUT rises with the switching of the switching transistor M1, the drive current I _LED1 increases toward the desired current amount I _REF .

As described above, the light emitting device 100r of FIG. 1 has a problem in that when the PWM dimming is turned on again, the drive current I _LED is temporarily reduced due to the shortage of the output voltage _VOUT , and the dimming accuracy is lowered. It should be noted that the above consideration should not be regarded as a range of common general knowledge in the field of the present invention. Furthermore, the above-mentioned consideration itself is the first time the present applicant has conceived.

  The present invention has been made in view of these problems, and one of the exemplary purposes of one aspect thereof is to suppress a decrease in drive current when PWM dimming is re-lighted.

  Another embodiment of the present invention relates to a control circuit for a drive circuit that drives N (N is a natural number) light-emitting elements. The drive circuit is associated with a DC / DC converter that supplies an output voltage to a commonly connected first terminal of the N light emitting elements, and is associated with the N light emitting elements, each connected to a second terminal of the corresponding light emitting element. And N current sources configured to be switchable between an on state for generating a predetermined drive current and an off state for stopping the drive current. The control circuit generates N dimming pulse signals corresponding to each of the N light emitting elements, and performs dimming for switching the ON state and the OFF state of the N current sources in accordance with the N dimming pulse signals. A control unit; an error amplifier that generates a feedback voltage according to an error between the lowest voltage of the second terminals of the N light emitting elements and a predetermined reference voltage; and pulse modulation having a duty ratio according to the feedback voltage (I) switching the DC / DC converter in accordance with the pulse modulation signal in a lighting period in which at least one of the N dimming pulse signals indicates an ON state; and (ii) After the transition from the lighting period to the extinguishing period in which all of the N dimming pulse signals indicate the OFF state, the DC / DC converter is switched a predetermined number of times. It provided during subsequent remaining off period, a gate driver to stop the switching of the DC / DC converter, a.

  According to this aspect, the DC / DC converter switches for a predetermined number of times in a substantially no-load state each time the transition is made to the extinguishing period. As a result, the output voltage of the DC / DC converter increases by an increase width corresponding to the number of times of switching. In other words, by setting the number of times of switching so that the increase range of the output voltage and the decrease range of the output voltage after switching stops, it is possible to maintain a sufficient output voltage at the next transition to the lighting period. And a decrease in drive current can be suppressed.

  The control circuit according to an aspect may further include a feedback voltage holding unit that samples and holds the feedback voltage when transitioning from the lighting period to the extinguishing period. The duty controller generates a pulse modulation signal having a duty ratio corresponding to the feedback voltage held by the feedback voltage holding unit during the extinguishing period, and the gate driver generates a DC / DC converter according to the pulse modulation signal during the extinguishing period. May be switched a predetermined number of times.

  An aspect of the control circuit is a logic gate that generates an all-channel off signal that is asserted when all of the N dimming pulse signals indicate an off state and is negated when at least one indicates an on state; A counter that generates a mask signal that counts the number of switching of the DC / DC converter when the all channel off signal is asserted, is negated until the count value reaches a predetermined number of times, and is asserted after the count value is reached. You may prepare. The driver may switch the DC / DC converter based on the pulse modulation signal while the mask signal is negated, and stop the switching of the DC / DC converter while the mask signal is asserted.

  The counter may count clock signals synchronized with the switching of the DC / DC converter.

  Another embodiment of the present invention relates to a drive circuit for driving N light emitting elements. The driving circuit includes a DC / DC converter that supplies an output voltage to a commonly connected first terminal of the N light emitting elements, and a second terminal of the corresponding light emitting element, which is associated with the N light emitting elements. The N current sources, the N current sources, and the DC / DC converter that are connected and can be switched between an on state that generates a predetermined drive current and an off state that stops the drive current are controlled. Any one of the control circuits may be provided.

  Another embodiment of the present invention relates to a light emitting device. The light emitting device may include N light emitting elements and the above-described drive circuit that drives the N light emitting elements.

  Another embodiment of the present invention relates to an electronic device. The electronic apparatus may include a liquid crystal panel, N light emitting elements provided as a backlight of the liquid crystal panel, and the above-described driving circuit that drives the N light emitting elements.

One embodiment of the present invention relates to a control circuit of a driving circuit of a light emitting element. The drive circuit includes a dimming switch and a current detection resistor connected in series with the light emitting element, and a DC / DC converter that supplies an output voltage between both ends of the light emitting element, the dimming switch, and the current detection resistor. The control circuit turns on the dimming switch when the dimming pulse signal indicates the lighting period, and turns off the dimming switch when the dimming pulse signal indicates the extinguishing period, and the voltage drop of the current detection resistor An error amplifier that generates a feedback voltage according to an error between the first current detection signal according to the light intensity and a dimming voltage that indicates the target luminance of the light emitting element, and a pulse modulation signal that has a duty ratio according to the feedback voltage is generated And (i) switching the DC / DC converter according to the pulse modulation signal during the lighting period, and (ii) switching the DC / DC converter a predetermined number of times after transition from the lighting period to the extinguishing period. And a gate driver that stops switching of the DC / DC converter during the remaining light extinction period thereafter.

  According to this aspect, the DC / DC converter switches for a predetermined number of times in a substantially no-load state each time the transition is made to the extinguishing period. As a result, the output voltage of the DC / DC converter increases by an increase width corresponding to the number of times of switching. In other words, by setting the number of times of switching so that the increase range of the output voltage and the decrease range of the output voltage after switching stops, it is possible to maintain a sufficient output voltage at the next transition to the lighting period. And a decrease in drive current can be suppressed.

  In one aspect, the control circuit may further include a feedback voltage holding unit that samples and holds the feedback voltage when transitioning from the lighting period to the extinguishing period. The duty controller generates a pulse modulation signal having a duty ratio corresponding to the feedback voltage held by the feedback voltage holding unit during the extinguishing period, and the gate driver generates a DC / DC converter according to the pulse modulation signal during the extinguishing period. May be switched a predetermined number of times.

  In one aspect, the control circuit counts the number of switching of the DC / DC converter when transitioning to the extinguishing period, negates until the count value reaches a predetermined number of times, and generates a mask signal that is asserted after reaching the counter value. Further, it may be provided. The driver may switch the DC / DC converter based on the pulse modulation signal while the mask signal is negated, and stop the switching of the DC / DC converter while the mask signal is asserted.

  The counter may count clock signals synchronized with the switching of the DC / DC converter.

  Another embodiment of the present invention relates to a drive circuit for a light-emitting element. The drive circuit includes a dimming switch and a current detection resistor connected in series with the light emitting element, a DC / DC converter that supplies an output voltage between the light emitting element, the dimming switch, and the current detection resistor, and a dimming switch. One of the above-described control circuits that controls the DC / DC converter while switching may be provided.

  Another embodiment of the present invention relates to a light emitting device. The light emitting device may include a light emitting element and the above-described driving circuit that drives the light emitting element.

  Another embodiment of the present invention relates to an electronic device. The electronic device may include a liquid crystal panel, a light emitting element provided as a backlight of the liquid crystal panel, and the above-described driving circuit that drives the light emitting element.

  Note that any combination of the above-described constituent elements and the constituent elements and expressions of the present invention replaced with each other among methods, apparatuses, systems, and the like are also effective as an aspect of the present invention.

  According to an aspect of the present invention, it is possible to suppress a decrease in drive current at the time of PWM dimming relighting.

It is a circuit diagram which shows the structure of the light-emitting device which this inventor examined. It is an operation | movement waveform diagram of the light-emitting device of FIG. 1 is a circuit diagram of a light emitting device according to a first embodiment. FIG. 4 is an operation waveform diagram of the light emitting device of FIG. 3. It is a circuit diagram of the light-emitting device which concerns on 2nd Embodiment. It is a circuit diagram which shows an electronic device provided with the light-emitting device of FIG. FIG. 7 is a perspective view showing the electronic device of FIG. 6.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

In this specification, “the state in which the member A is connected to the member B” means that the member A and the member B are electrically connected to each other in addition to the case where the member A and the member B are physically directly connected. It includes cases where the connection is indirectly made through other members that do not substantially affect the general connection state, or that do not impair the functions and effects achieved by their combination.
Similarly, “the state in which the member C is provided between the member A and the member B” refers to the case where the member A and the member C or the member B and the member C are directly connected, as well as their electric It includes cases where the connection is indirectly made through other members that do not substantially affect the general connection state, or that do not impair the functions and effects achieved by their combination.

(First embodiment)
FIG. 3 is a circuit diagram of the light emitting device 100 according to the first embodiment. The light emitting device 100 includes N (N is a natural number) LED strings 102_1 to 102_N and a driving circuit 200. Hereinafter, the light emitting device 100 will be described focusing on differences from the light emitting device 100r of FIG.

  The drive circuit 200 includes a DC / DC converter 202 and a control circuit 300. The control circuit 300 includes a controller 301, a dimming control unit 350, and N current sources CS_1 to CS_N, and is integrated on a single semiconductor substrate. “Integrated integration” includes the case where all of the circuit components are formed on a semiconductor substrate and the case where the main components of the circuit are integrated. A resistor, a capacitor, or the like may be provided outside the semiconductor substrate.

The dimming control unit 350 and the current sources CS_1 to CS_N are the same as those in FIG.
The dimming control unit 350 generates N dimming pulse signals PWM_1 to PWM_N corresponding to the N LED strings 102_1 to 102_N, and N dimming pulse signals PWM_1 to PWM_N are generated according to the N dimming pulse signals PWM_1 to PWM_N. The current sources CS_1 to CS_N are switched between the on state and the off state.

  The controller 301 includes an additional pulse generation unit 330 and a feedback voltage holding unit 340 in addition to the error amplifier 302, the switch 304, the duty controller 306, and the gate driver 308 described above.

A phase compensation resistor R _FB and a capacitor C _FB are connected to the feedback terminal (also referred to as FB terminal). The error amplifier 302 is a transconductance amplifier (also referred to as LED terminal) N number of LED strings 102 each of the cathode terminal and the lowest voltage of the voltages _V LED1 _{~V LEDn} of LED_1～LED_N, the predetermined reference voltage _{V REF} The error is amplified, a current corresponding to the error is generated, and a feedback voltage _VFB is generated at the feedback terminal FB.

The duty controller 306 generates a pulse modulation signal S1 having a duty ratio corresponding to the feedback voltage _VFB . The duty controller 306 in FIG. 3 is a so-called voltage mode pulse width modulator, and includes a PWM comparator 320 and an oscillator 322.

  The logic gate 352 takes a negative OR of the plurality of dimming pulse signals PWM_1 to PWM_N and generates an all-channel off signal PWM_ALL_L (hereinafter referred to as a PWM_ALL_L signal). The PWM_ALL_L signal is asserted (for example, high level) during the extinguishing period when the current sources CS_1 to CS_N of all the channels are simultaneously turned off, and negated (low level) during the lighting period during which at least one current source CS is turned on.

During the period when the PWM_ALL_L signal is asserted, the switch 304 is turned off, the FB terminal becomes high impedance, and the feedback voltage V _FB is stored.

  The gate driver 308 switches the switching transistor of the DC / DC converter 202 according to the pulse modulation signal S1 during a lighting period in which at least one of the plurality of dimming pulse signals PWM indicates an ON state, that is, while PWM_ALL_L is negated. Switch M1.

  In FIG. 1, when the gate driver 308 transitions to the extinguishing period, that is, when the PWM_ALL_L signal is asserted, the gate driver 308 immediately stops switching of the switching transistor M1. On the other hand, the gate driver 308 in FIG. 3 (ii) immediately after the transition from the lighting period to the extinguishing period, switches the DC / DC converter 202 a predetermined number of times M (M is a natural number), and the remaining extinguishing periods thereafter. Meanwhile, the switching of the DC / DC converter 202 is stopped.

  The additional pulse generator 330 is provided for switching the DC / DC converter 202 a predetermined number of times M immediately after the transition in the extinguishing period.

  The additional pulse generator 330 includes a counter 332 and a mask circuit 334. When the PWM_ALL_L signal is asserted, the counter 332 starts counting the switching times of the DC / DC converter 202. The mask signal S3 is negated (low level) until the count value reaches the predetermined number M, and after reaching the predetermined number M, the mask signal S3 is asserted (high level). The count value of the counter 332 is reset at the positive edge or negative edge of the PWM_ALL_L signal, and the mask signal S3 is negated when the positive edge of the PWM_ALL_L signal occurs.

The counter 332 may count the clock signal CLK synchronized with the switching of the DC / DC converter 202. This clock signal CLK is synchronized with the periodic signal V _OSC generated by the oscillator 322. By using this clock signal CLK, the switching frequency of the DC / DC converter 202 can be counted.

  The mask circuit 334 passes the pulse modulation signal S1 to the gate driver 308 while the mask signal S3 is negated. When the mask signal S3 is asserted, the mask circuit 334 blocks the pulse modulation signal S1 and sets the input of the gate driver 308 low. Fix to level. Accordingly, the gate driver 308 switches the DC / DC converter 202 based on the pulse modulation signal S1 while the mask signal S3 is negated, and stops switching of the DC / DC converter 202 while the mask signal S3 is asserted. To do.

  In FIG. 3, the mask circuit 334 is configured by combining a plurality of logic gates 336, 338, and 340. The configuration corresponds to the high-level / low-level allocation of the pulse modulation signal S1 and the mask signal S3. Can be changed.

  The feedback voltage holding unit 340 is provided to determine the duty ratio of the pulse modulation signal S1 when switching the switching transistor M1 immediately after the transition to the extinguishing period.

The feedback voltage holding unit 340 samples the feedback voltage V _FB when it transitions from the lighting period to the extinguishing period, that is, at the timing (positive edge) when the PWM_ALL_L signal is asserted, and then holds it.

For example, feedback voltage holding unit 340 includes sample hold circuit 342 and selector 344. The sample hold circuit 342 samples the feedback voltage _VFB in response to the negative edge of the PWM_ALL_L signal. The selector 344 selects the output voltage V _FB of the error amplifier 302 during the lighting period (PWM_ALL_L signal is negated), and the feedback voltage V _{FB held} by the sample hold circuit 342 during the extinguishing period (PWM_ALL_L signal is asserted). Select.
The configuration of feedback voltage holding unit 340 is not limited to that shown in FIG.

The duty controller 306 generates a pulse modulation signal S1 having a duty ratio corresponding to the feedback voltage _VFB held by the feedback voltage holding unit 340 during the extinguishing period. The gate driver 308 switches the DC / DC converter 202 a predetermined number of times M in accordance with the pulse modulation signal S1 generated by the duty controller 306 during the extinguishing period.

The above is the configuration of the light-emitting device 100. Next, the operation will be described.
FIG. 4 is an operation waveform diagram of the light emitting device 100 of FIG. FIG. 4 shows a case where the predetermined number of times M = 4. Further, in order to simplify the explanation and facilitate understanding, a case where N = 1 will be described.

In the lighting period _{T 12,} is asserted dimming pulse signal PWM_1, the drive current _{I LED} flowing to the LED string 102_1. During this time, the output voltage V _OUT is stabilized at a certain level.

When the dimming pulse signal PWM_1 is negated (low level) at time t2, the PWM_ALL_L signal is asserted. Thereafter, the gate driver 308 switches the switching transistor M1 for a predetermined number of times M = 4 (T ₂₃ ). Due to this switching, the output voltage _VOUT rises.

When the number of switching times reaches M = 4 at time t3, the mask signal S3 is asserted and switching stops. During the period T ₃₄ when switching stops, the output voltage _VOUT of the DC / DC converter 202 decreases with time.

Thereafter, at time t4, the dimming pulse signal PWM_1 is asserted again. At the same time as the dimming pulse signal PWM_1 is asserted, the current source CS_1 is turned on and the drive current I _LED1 starts to flow. The output voltage _{V OUT} at this time are the level comparable to the period _{T 12,} it is possible to generate a driving current _{I LED1} desired current amount _{I REF.}

The above is the operation of the light emitting device 100.
According to the light emitting device 100, each time the transition from the lighting period _{T 12} in the extinction period _{T 24,} in a state of substantially no load, so that the DC / DC converter 202 is a predetermined number of times M, and switching. As a result, the output voltage _VOUT of the DC / DC converter 202 increases by an increase width ΔV1 corresponding to the number of times of switching M.

That is, as the decline ΔV2 of output voltage V _OUT after rise ΔV1 and the switching stop of the output voltage V _OUT is canceled, by determining the switching times M, when the transition to the lighting period to the next time t4 , it is possible to maintain a sufficient output voltage _{V OUT,} can suppress the reduction of the drive current _{I LED.}

(Second Embodiment)
In the first embodiment, the light emitting device 100 including the multi-channel LED strings 102_1 to 102_N has been described. In the second embodiment, a light emitting device 100a including a single channel LED string 102 will be described.

  FIG. 5 is a circuit diagram of a light emitting device 100a according to the second embodiment. The light emitting device 100a includes an LED string 102 and a drive circuit 200a. Hereinafter, the light emitting device 100a will be described focusing on differences from the light emitting device 100 of FIG.

  The drive circuit 200a includes a DC / DC converter 202, a PWM dimming transistor (dimming switch) M2, a current detection resistor Rs, and a control circuit 300a.

A dimming switch M2 and a current detection resistor Rs are provided in series between the cathode of the LED string 102 and the ground line. The dimming switch M2 is provided to perform PWM dimming. The dimming switch M2 is turned on during the lighting period, and the dimming switch M2 is turned off during the extinguishing period. During the lighting period, a voltage drop (current detection signal Vs) proportional to the drive current I _LED is generated in the current detection resistor Rs. The current detection signal Vs is fed back to the current detection (ISENSE) terminal of the control circuit 300a.

  The control circuit 300a includes a controller 301a and a dimming driver 310.

  The dimming driver 310 turns on the dimming switch M2 when the dimming pulse signal PWM indicates the lighting period (assert), and turns off the dimming switch M2 when the dimming pulse signal PWM indicates the extinguishing period (negate) To do.

The error amplifier 302 generates a feedback voltage V _FB corresponding to an error between the first current detection signal Vs corresponding to the voltage drop of the current detection resistor Rs and the dimming voltage V _ADIM indicating the target brightness of the LED string 102. . The switch 304 is turned on while the dimming pulse signal PWM is asserted, and is turned off while the dimming pulse signal PWM is negated.

The duty controller 306 generates a pulse signal S2 having a duty ratio corresponding to the feedback voltage _VFB .

  The gate driver 308 switches the DC / DC converter 202 in accordance with the pulse modulation signal S1 during (i) the lighting period. The gate driver 308 (ii) immediately after the transition from the lighting period to the extinguishing period, switches the DC / DC converter 202 a predetermined number of times M, and stops switching of the DC / DC converter 202 for the remaining extinguishing period thereafter. To do.

The above is the configuration of the light-emitting device 100a.
Similarly to the light-emitting device 100 of FIG. 3, the light-emitting device 100a of FIG. 5 switches the DC / DC converter 202 and raises the output voltage _VOUT immediately after the transition to the extinguishing period, thereby increasing the output voltage _VOUT . This can be maintained, and a decrease in the drive current I _LED can be suppressed.

Lastly, an electronic device using the light emitting device according to the embodiment will be described.
FIG. 6 is a circuit diagram illustrating an electronic apparatus 500 including the light emitting device 100 of FIG. The electronic device 500 is a device that operates by receiving power from a commercial AC power source, such as a notebook PC, a display device, or a television receiver.

  Electronic device 500 includes rectifier circuit 502, smoothing capacitor 504, DC / DC converter 506, and LCD panel 508 in addition to light emitting device 100. The light emitting device 100 is used as a backlight of the LCD panel 508.

Rectifier circuit 502 rectifies the commercial AC voltage _{V AC.} The smoothing capacitor 504 smoothes the rectified voltage and generates a DC voltage _VDC . The DC / DC converter 506 steps down the direct current voltage _VDC . The light emitting device 100 operates by receiving the direct current voltage _VIN from the DC / DC converter 506.

  FIG. 7 is a perspective view showing the electronic device 500 of FIG. 7 is an edge light type display device. A plurality of light guide plates 510_1 to 510_N are disposed on the back surface of the LCD panel 508. The 4-channel LED strings 102_1 to 102_4 are arranged to irradiate the corresponding light guide plates 510_1 to 510_N. In addition, the rectifier circuit 502, the smoothing capacitor 504, the DC / DC converter 506, the control circuit 300, and the like of FIG. 6 are built in the housing 512 of the electronic device 500, but are not illustrated in FIG.

  Note that the backlight using the light emitting device may be (i) the edge light type shown in FIG. 6, or (ii) the direct type in which the LED string 102 is disposed on the back surface of the LCD panel 508. . Alternatively, the light emitting device 100a in FIG. 5 may be used.

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and various modifications may exist in each of those constituent elements, each processing process, and a combination thereof. Hereinafter, such modifications will be described.

(Modification 1)
In the embodiment, the case has been described in which the switching duty ratio of the DC / DC converter 202 immediately after the transition to the extinguishing period is determined by the feedback voltage holding unit 340, but the present invention is not limited thereto. For example, the switching duty ratio of the DC / DC converter 202 immediately after the transition to the extinguishing period may be fixed to a predetermined value.

(Modification 2)
In the embodiment, the case where the predetermined number M is fixed has been described. However, the number M may be adaptively determined according to the duty ratio of the dimming pulse signal PWM, that is, the time ratio between the lighting period and the extinguishing period. . Specifically, the number M may be increased as the turn-off period becomes longer.

(Modification 3)
Although the case where the duty controller 306 is a voltage mode pulse width modulator has been described in the embodiment, the present invention is not limited thereto. The duty controller 306 may be another modulator such as a peak current mode, an average current mode, or a bottom detection on-time fixed method.

(Modification 4)
In the embodiment, the non-insulated DC / DC converter 202 using an inductor has been described. However, the present invention can also be applied to an isolated DC / DC converter using a transformer. Further, a synchronous rectification type including a transistor instead of the rectification diode D1 may be used. The switching transistor M1 may be a bipolar transistor.

(Modification 5)
In the embodiments, the case where the light emitting element is an LED has been described. However, the present invention is not limited to this, and other light sources that can be used now or in the future may be used.

(Modification 6)
In the embodiment, an electronic apparatus has been described as an application of the light emitting device 2, but the application is not particularly limited and can be used for lighting or the like.

(Modification 7)
Although FIG. 3 illustrates the case where the current source CS is integrated in the control circuit 300, the current source CS may be externally attached to the control circuit 300.

(Modification 8)
In the present embodiment, the setting of the high level and low level logic signals is merely an example, and can be freely changed by appropriately inverting it with an inverter or the like. For example, in a negative logic system, assertion may be assigned to a low level and negate may be assigned to a high level.

  Although the present invention has been described using specific terms based on the embodiments, the embodiments only illustrate the principles and applications of the present invention, and the embodiments are defined in the claims. Many variations and modifications of the arrangement are permitted without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 100 ... Light-emitting device, 102 ... LED string, 200 ... Drive circuit, 202 ... DC / DC converter, 204 ... Input line, 206 ... Output line, 300 ... Control circuit, 301 ... Controller, 302 ... Error amplifier, 304 ... Switch, 306 ... Duty controller, 308 ... Gate driver, 310 ... Dimming driver, 320 ... PWM comparator, 322 ... Oscillator, 330 ... Additional pulse generator, 332 ... Counter, 334 ... Mask circuit, 350 ... Dimming controller, 352 ... Logic gate 330 ... Addition pulse generator 340 ... Feedback voltage holding unit 342 ... Sample hold circuit 344 ... Selector PWM ... Dimming pulse signal Rs ... Current detection resistor S1 ... Pulse modulation signal S2 ... Gate pulse Signal, S3 ... Mask signal, L1 ... In Kuta, C1 ... output capacitor, D1 ... rectifier diode, M1 ... switching transistor, M2 ... dimmer switch.

Claims (18)

A drive circuit control circuit for driving N light emitting elements (N is a natural number),
The drive circuit is
A DC / DC converter for supplying an output voltage to the commonly connected first terminals of the N light emitting elements;
Corresponding to the N light emitting elements, each connected to the second terminal of the corresponding light emitting element, and configured to be able to switch between an on state that generates a predetermined driving current and an off state that stops the driving current N current sources,
With
The control circuit includes:
N dimming pulse signals corresponding to the N light emitting elements are generated, and dimming is performed to switch the on state and the off state of the N current sources according to the N dimming pulse signals. A control unit;
An error amplifier that generates a feedback voltage according to an error between the lowest voltage among the voltages of the second terminals of the N light emitting elements and a predetermined reference voltage;
A duty controller that generates a pulse modulation signal having a duty ratio according to the feedback voltage;
(I) switching the DC / DC converter according to the pulse modulation signal in a lighting period in which at least one of the N dimming pulse signals indicates the ON state; and (ii) from the lighting period The DC / DC converter is switched a predetermined number of times after all of the N dimming pulse signals have transitioned to the extinguishing period indicating the off state, and the DC / DC converter is switched during the remaining extinguishing period thereafter. A gate driver that stops switching,
A control circuit comprising: A feedback voltage holding unit that samples and holds the feedback voltage when transitioning from the lighting period to the extinguishing period;
The duty controller generates a pulse modulation signal having a duty ratio according to a feedback voltage held by the feedback voltage holding unit in the extinguishing period,
2. The control circuit according to claim 1, wherein the gate driver switches the DC / DC converter a predetermined number of times in accordance with the pulse modulation signal during the extinguishing period. A logic gate that generates an all channel off signal that is asserted when all of the N dimming pulse signals indicate an off state, and at least one is negated when indicating an on state;
When the all channel off signal is asserted, a counter that counts the number of switching of the DC / DC converter, negates until the count value reaches the predetermined number of times, and generates a mask signal that is asserted after reaching the predetermined value. ,
Further comprising
The driver switches the DC / DC converter based on the pulse modulation signal while the mask signal is negated, and stops switching the DC / DC converter while the mask signal is asserted. The control circuit according to claim 1, wherein the control circuit is characterized in that:   The control circuit according to claim 3, wherein the counter counts a clock signal synchronized with switching of the DC / DC converter.   The control circuit according to claim 1, wherein the control circuit is integrated on a single semiconductor substrate. A drive circuit for N light emitting elements,
A DC / DC converter for supplying an output voltage to the commonly connected first terminals of the N light emitting elements;
Corresponding to the N light emitting elements, each connected to the second terminal of the corresponding light emitting element, and configured to be able to switch between an on state that generates a predetermined driving current and an off state that stops the driving current N current sources,
The control circuit according to any one of claims 1 to 5, which controls the N current sources and the DC / DC converter;
A drive circuit comprising: N light emitting elements,
A drive circuit for driving the N light emitting elements;
With
The drive circuit is
A DC / DC converter for supplying an output voltage to the commonly connected first terminals of the N light emitting elements;
Corresponding to the N light emitting elements, each connected to the second terminal of the corresponding light emitting element, and configured to be able to switch between an on state that generates a predetermined driving current and an off state that stops the driving current N current sources,
The control circuit according to any one of claims 1 to 5, which controls the N current sources and the DC / DC converter;
A light emitting device comprising: LCD panel,
N light emitting elements provided as a backlight of the liquid crystal panel;
A drive circuit for driving the N light emitting elements;
With
The drive circuit is
A DC / DC converter for supplying an output voltage to the commonly connected first terminals of the N light emitting elements;
Corresponding to the N light emitting elements, each connected to the second terminal of the corresponding light emitting element, and configured to be able to switch between an on state that generates a predetermined driving current and an off state that stops the driving current N current sources,
The control circuit according to any one of claims 1 to 5, which controls the N current sources and the DC / DC converter;
An electronic device comprising: A control circuit for a drive circuit of a light emitting element,
The drive circuit is
A dimming switch and a current detection resistor connected in series with the light emitting element;
A DC / DC converter that supplies an output voltage between both ends of the light emitting element, the dimming switch, and the current detection resistor;
With
The control circuit includes:
A dimming driver that turns on the dimming switch when the dimming pulse signal indicates a lighting period, and turns off the dimming switch when the dimming pulse signal indicates a light-off period;
An error amplifier that generates a feedback voltage according to an error between a first current detection signal corresponding to a voltage drop of the current detection resistor and a dimming voltage indicating a target luminance of the light emitting element;
A duty controller that generates a pulse modulation signal having a duty ratio according to the feedback voltage;
(I) In the lighting period, the DC / DC converter is switched according to the pulse modulation signal. (Ii) After the transition from the lighting period to the extinguishing period, the DC / DC converter is switched a predetermined number of times. A gate driver for stopping the switching of the DC / DC converter during the remaining light extinction period thereafter;
A control circuit comprising: A feedback voltage holding unit that samples and holds the feedback voltage when transitioning from the lighting period to the extinguishing period;
The duty controller generates a pulse modulation signal having a duty ratio according to a feedback voltage held by the feedback voltage holding unit in the extinguishing period,
The control circuit according to claim 9, wherein the gate driver switches the DC / DC converter a predetermined number of times in accordance with the pulse modulation signal during the extinguishing period. A counter for generating a mask signal that counts the number of switching of the DC / DC converter when transitioning to the extinction period, is negated until the count value reaches the predetermined number of times, and is asserted after reaching the predetermined number of times;
The driver switches the DC / DC converter based on the pulse modulation signal while the mask signal is negated, and stops switching the DC / DC converter while the mask signal is asserted. The control circuit according to claim 9 or 10, characterized in that   The control circuit according to claim 11, wherein the counter counts a clock signal synchronized with switching of the DC / DC converter.   The control circuit according to claim 9, wherein the control circuit is integrated on a single semiconductor substrate. A driving circuit for a light emitting element,
A dimming switch and a current detection resistor connected in series with the light emitting element;
A DC / DC converter that supplies an output voltage between both ends of the light emitting element, the dimming switch, and the current detection resistor;
The control circuit according to any one of claims 9 to 13, wherein the dimming switch is switched and the DC / DC converter is controlled.
A drive circuit comprising: A light emitting element;
A drive circuit for driving the light emitting element;
With
The drive circuit is
A dimming switch and a current detection resistor connected in series with the light emitting element;
A DC / DC converter that supplies an output voltage between both ends of the light emitting element, the dimming switch, and the current detection resistor;
The control circuit according to any one of claims 9 to 13, wherein the dimming switch is switched and the DC / DC converter is controlled.
A light emitting device comprising: LCD panel,
A light emitting device provided as a backlight of the liquid crystal panel;
A drive circuit for driving the light emitting element;
With
The drive circuit is
A dimming switch and a current detection resistor connected in series with the light emitting element;
A DC / DC converter that supplies an output voltage between both ends of the light emitting element, the dimming switch, and the current detection resistor;
The control circuit according to any one of claims 9 to 13, wherein the dimming switch is switched and the DC / DC converter is controlled.
An electronic device comprising: A method of controlling a drive circuit for N light emitting elements,
The drive circuit is
A DC / DC converter for supplying an output voltage to the commonly connected first terminals of the N light emitting elements;
Corresponding to the N light emitting elements, each connected to the second terminal of the corresponding light emitting element, and configured to be able to switch between an on state that generates a predetermined driving current and an off state that stops the driving current N current sources,
With
The control method is:
Generating N dimming pulse signals corresponding to each of the N light emitting elements;
Switching the on-state and the off-state of the N current sources in response to the N dimming pulse signals;
Generating a feedback voltage according to an error between the lowest voltage among the voltages of the second terminals of the N light emitting elements and a predetermined reference voltage;
Generating a pulse modulation signal having a duty ratio according to the feedback voltage;
Switching the DC / DC converter in response to the pulse modulation signal in a lighting period in which at least one of the N dimming pulse signals indicates the ON state;
After all of the N dimming pulse signals from the lighting period transition to the extinguishing period indicating the off state, the DC / DC converter is switched a predetermined number of times, and during the remaining extinguishing period thereafter, Stopping switching of the DC / DC converter;
A control method comprising: A method for controlling a driving circuit of a light emitting element,
The drive circuit is
A dimming switch and a current detection resistor connected in series with the light emitting element;
A DC / DC converter that supplies an output voltage between both ends of the light emitting element, the dimming switch, and the current detection resistor;
With
The control method is:
Turning on the dimming switch when a dimming pulse signal indicates a lighting period, and turning off the dimming switch when the dimming pulse signal indicates a turn-off period;
Generating a feedback voltage according to an error between a first current detection signal corresponding to a voltage drop of the current detection resistor and a dimming voltage indicating a target luminance of the light emitting element;
Generating a pulse modulation signal having a duty ratio according to the feedback voltage;
Switching the DC / DC converter according to the pulse modulation signal in the lighting period;
After the transition from the lighting period to the extinguishing period, switching the DC / DC converter a predetermined number of times, and stopping the switching of the DC / DC converter during the remaining extinguishing period thereafter;
A control method comprising:

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