JP6163061B2 - LIGHT EMITTING ELEMENT DRIVE CIRCUIT, ITS CONTROL CIRCUIT, CONTROL METHOD, AND LIGHT EMITTING DEVICE AND ELECTRONIC DEVICE USING THE SAME - Google Patents

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 of a light emitting device 100r investigated by the present inventors. The light emitting device 100r includes an LED string 102 and a drive circuit 200r. The LED string 102 includes a plurality of LEDs connected in series. The drive circuit 200r supplies the drive current I _LED to the LED string 102, and controls the brightness according to the amount of the drive current I _LED (analog light control). Further, the drive circuit 200r performs an intermittent operation that alternately repeats the lighting section and the extinguishing section, and controls the luminance of the LED string 102 according to the duty ratio (PWM dimming).

  The drive circuit 200r includes a DC / DC converter 202, resistors R1 and R2, a PWM dimming transistor (dimming switch) M2, a current detection resistor Rs2, and a control circuit 300r.

A dimming switch M2 and a current detection resistor Rs2 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, and the dimming switch M2 is turned on in the lighting section, and the dimming switch M2 is turned off in the unlit section. The current detection resistor Rs2 in the lighting period, a voltage drop proportional to the drive current _{I LED} (current detection signal Vs2) is generated. The current detection signal Vs2 is fed back to the current detection (ISENSE) terminal of the control circuit 300r.

The DC / DC converter 202 includes an inductor L1, a rectifier diode D1, a switching transistor M1, an output capacitor C1, and a current detection resistor Rs1, and constitutes a step-up DC / DC converter together with the control circuit 300r. An input voltage _VIN is input to the input line 204, and an anode of the LED string 102 is connected to the output line 206. The control circuit 300r supplies the boosted output voltage _VOUT to the anode of the LED string 102 by switching the switching transistor M1. In the current detection resistor Rs1, a voltage drop (current detection signal) Vs1 proportional to the coil current _IL1 flowing in the inductor L1 is generated in the ON period of the switching transistor M1. The current detection signal Vs1 is fed back to the current detection (CS) terminal of the control circuit 300r.

The resistors R1 and R2 divide the output voltage V _OUT and generate a voltage detection signal V _OVP . The voltage detection signal V _OVP is input to the overvoltage protection (OVP) terminal of the control circuit 300r.

  The control circuit 300r includes an error amplifier 302, a switch 304, a duty controller 306, a gate driver 308, a dimming driver 310, a feedback voltage holding unit 312, a threshold voltage generation unit 314, and a comparator 316.

  The control circuit 300r generates a dimming pulse signal PWM that is pulse width modulated in accordance with the target luminance. For example, the high level of the dimming pulse signal PWM corresponds to the lighting interval, and the low level corresponds to the extinction interval. The dimming driver 310 switches the dimming switch M2 based on the dimming pulse signal PWM.

The error amplifier 302 is a gm amplifier, which amplifies an error between the current detection signal Vs2 and the dimming control voltage V _ADIM and generates a current corresponding to the error. A phase compensation resistor R _FB and a capacitor C _FB are connected to the feedback (FB) terminal. The switch 304 is turned on when the dimming pulse signal PWM is at a high level and turned off when the light control pulse signal PWM is at a low level. Therefore, during the lighting period in which the dimming pulse signal PWM is at a high level, the feedback voltage V _FB generated at the FB terminal is adjusted so that the error between the current detection signal Vs2 and the dimming control voltage V _ADIM becomes zero.

The duty controller 306 generates a gate pulse signal S1. The duty controller 306 adjusts the duty ratio of the gate pulse signal S1 based on at least the feedback voltage _VFB during the lighting period. The duty controller 306 is a current mode pulse modulator, for example, and may reflect the current detection signal Vs1, that is, the coil current _IL1 , in the duty ratio of the gate pulse signal S1.

  The gate driver 308 switches the switching transistor M1 based on the gate pulse signal S1.

If the switching of the switching transistor M1 is completely stopped in the extinguishing section, the output voltage _VOUT may be lowered. Therefore, the control circuit 300r maintains the output voltage _VOUT at a constant level by switching the switching transistor M1 during the extinction period. This is called a voltage holding function. The feedback voltage holding unit 312, the threshold voltage generation unit 314, and the comparator 316 are provided for the voltage holding function.

The feedback voltage holding unit 312 samples and holds the feedback voltage _VFB when the dimming pulse signal PWM transits to a low level, that is, transitions from the lighting period to the extinguishing period. The threshold voltage generation unit 314 samples and holds the voltage detection signal V _OVP when the dimming pulse signal PWM transits to a low level, that is, transitions from the lighting interval to the extinction interval.

During the extinguishing period, the duty controller 306 generates a gate pulse signal S1 having a duty ratio corresponding to the feedback voltage V _{FB_H} sampled and held by the feedback voltage holding unit 312.

The threshold voltage generation unit 314 outputs a threshold voltage V _TH corresponding to the sampled voltage detection signal V _OVP during the extinguishing period. The comparator 316 compares the voltage detection signal V _OVP with the threshold voltage V _TH during the turn-off period, and generates a comparison signal S2. The gate driver 308 switches the switching transistor M1 according to the gate pulse signal S1 when V _OVP <V _TH during the extinguishing period, and stops switching of the switching transistor M1 when V _OVP > V _TH .

JP 2008-186668 A JP2013-105628A

  As a result of studying the light emitting device 100r, the present inventor has recognized the following problems.

FIG. 2 is an operation waveform diagram of the light emitting device 100r of FIG. FIG. 2 shows an operation when the duty ratio of the dimming pulse signal PWM gradually decreases.
Consider a case where the threshold voltage V _TH is set to be equal to or lower than the voltage detection signal V _OVP sampled immediately before. In this case, the threshold voltage V _TH decreases for each cycle of the dimming pulse signal PWM, and the output voltage _VOUT decreases accordingly. That is, the voltage holding function is disabled. When the output voltage _VOUT decreases, the drive voltage for the LED string 102 becomes insufficient, the luminance of the LED string 102 becomes smaller than the target value, and there is a possibility that it will become impossible to light up.

Further, it was confirmed that even when the threshold voltage V _TH is set higher than the voltage detection signal V _OVP sampled immediately before, the situation where the output voltage V _OUT decreases can occur.

The present invention has been made in view of the above problems, and one of the exemplary purposes of one aspect thereof is that the output voltage _VOUT can be effectively maintained even in a situation where the luminance of PWM dimming is decreased with time. The provision of control circuits.

  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 interval, and turns off the dimming switch when the dimming pulse signal indicates the extinction interval, and the voltage drop of the current detection resistor An error amplifier that generates a feedback voltage corresponding to an error between the first current detection signal corresponding to the light intensity and the dimming voltage that indicates the target luminance of the light emitting element, and the previous lighting period when transitioning from the lighting period to the extinction period When the voltage is longer than the minimum time, a threshold voltage generator that samples and holds the voltage detection signal according to the output voltage and generates a threshold voltage according to the sampled and held value, and a duty ratio according to the feedback voltage A duty controller for generating a gate pulse signal having a comparator, a comparator for comparing a threshold voltage with a voltage detection signal in the extinguishing period, and (i) point In the lamp section, the DC / DC converter is switched according to the gate pulse signal. (Ii) In the extinguishing section, the switching state and the stop state are changed according to the comparison result of the comparator. (Ii-1) In the switching state And (ii-2) a gate driver that stops switching of the DC / DC converter in a stopped state.

  According to this aspect, in a situation where the duty ratio of the dimming pulse signal gradually decreases, when the duty ratio decreases to a certain value, the threshold voltage generator stops sampling the voltage detection signal, and the threshold voltage Is maintained at a level corresponding to the last sampled voltage detection signal. Thereby, it is possible to prevent the threshold voltage from continuing to decrease, and to maintain the output voltage at an appropriate level.

The minimum time may be a predetermined number of times the switching period of the DC / DC converter.
In this case, since the minimum time can be determined in units of the switching cycle of the DC / DC converter, the output voltage can be maintained at an appropriate level in a wide switching cycle (frequency).

  The dimming pulse signal may take an off level in the extinguishing section and an on level in the lighting section. The threshold voltage generation unit includes a permission signal generation unit that generates a permission signal that is asserted when the on-level length of the dimming pulse signal is longer than the minimum time, and the dimming in a state where the permission signal is asserted. And a sampling control unit that asserts a sampling signal when the pulse signal transitions from an on level to an off level. The threshold voltage generator may sample the voltage detection signal when the sampling signal is asserted.

  The permission signal generation unit starts measurement over time when the dimming pulse signal transitions from the off level to the on level, and when the minimum time elapses before the dimming pulse signal transitions from the on level to the off level, the permission signal May be asserted.

  The permission signal generation unit starts counting when the dimming pulse signal transitions from the off level to the on level, is reset when the dimming pulse signal transitions from the on level to the off level, and the count value becomes a value corresponding to the minimum time. A counter may be included that asserts a grant signal when reached.

The counter may count a clock signal corresponding to the switching period of the DC / DC converter.
In this case, since the minimum time during which the threshold voltage is not updated can be determined in units of the switching period of the DC / DC converter, a wide range of switching periods (frequency) can be handled. In addition, since the oscillator used by the duty controller can be used for time measurement by a counter, it is not necessary to provide a separate oscillator, and the circuit scale can be reduced.

  The dimming pulse signal may take a low level during the extinguishing period and a high level during the lighting period. A high level of the permission signal may be associated with assertion. The sampling control unit may include an inverter that inverts the dimming pulse signal, and an AND gate that takes a logical product of the output of the inverter and the permission signal and generates a sampling signal.

  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 (i) adjusts the duty ratio of the gate pulse signal according to the feedback voltage during the lighting period, and (ii) adjusts the duty ratio of the gate pulse signal according to the feedback voltage held by the feedback voltage holding unit during the extinction period. The duty ratio may be adjusted.

  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.

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 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. And N current sources configured to be switched between an on state that generates a predetermined drive current and an off state that stops the drive current. The control circuit generates N dimming pulse signals corresponding to the N light emitting elements, switches the ON state and the OFF state of the N current sources, and adjusts the first number of the N light emitting elements. An error amplifier that generates a feedback voltage corresponding to an error between the lowest voltage of the two terminals and a predetermined reference voltage;
When at least one of the N dimming pulse signals transitions from the lighting section instructing the on state to the extinguishing section in which all of the N dimming pulse signals indicate the off state, the immediately preceding lighting section is the minimum time. A threshold voltage generator that samples and holds a voltage detection signal according to the output voltage and generates a threshold voltage according to the sampled and held value, and a gate having a duty ratio according to the feedback voltage. A duty controller that generates a pulse signal; a comparator that compares a threshold voltage with a voltage detection signal in the extinction period; and (i) a DC / DC converter that switches in response to the gate pulse signal in the lighting period; ii) In the light-off period, the switching state and the stop state are changed according to the comparison result of the comparator, and (i i-1) switching a DC / DC converter according to a gate pulse signal in a switching state, and (ii-2) a gate driver that stops switching of the DC / DC converter in a stop state.

  According to this aspect, in a situation where the lighting section is gradually shortened, when the lighting section is shortened to the minimum time, the threshold voltage generator no longer samples the voltage detection signal, and the threshold voltage is finally sampled. The level is maintained according to the voltage detection signal. Thereby, it is possible to prevent the threshold voltage from continuing to decrease, and to maintain the output voltage at an appropriate level.

The minimum time may be a predetermined number of times the switching period of the DC / DC converter.
In this case, since the minimum time during which the threshold voltage is not updated can be determined in units of the switching period of the DC / DC converter, the output voltage can be maintained at an appropriate level in a wide switching period (frequency).

  The dimming control unit may generate an all-channel off signal that is asserted when N dimming pulse signals indicate an off state and is negated when at least one indicates an on state. The threshold voltage generation unit generates a permission signal that is asserted when the all channel off signal is negated longer than the minimum time, and the all channel off signal in a state where the permission signal is asserted. And a sampling control unit that asserts a sampling signal. The threshold voltage generator may sample the voltage detection signal when the sampling signal is asserted.

  The permission signal generation unit may start measurement over time when the all channel off signal is negated, and may assert the permission signal when the minimum time elapses before the all channel off signal is asserted.

  The enable signal generator starts counting when the all channel off signal is negated, resets when the all channel off signal is asserted, and asserts the enable signal when the count value reaches a value corresponding to the minimum time. May be included.

The counter may count a clock signal corresponding to the switching period of the DC / DC converter.
In this case, since the minimum time can be determined with the switching cycle of the DC / DC converter as a unit, the output voltage can be maintained at an appropriate level in a wide switching cycle (frequency). In addition, since the oscillator used by the duty controller can be used for time measurement by a counter, it is not necessary to provide a separate oscillator, and the circuit scale can be reduced.

  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 (i) adjusts the duty ratio of the gate pulse signal according to the feedback voltage during the lighting period, and (ii) adjusts the duty ratio of the gate pulse signal according to the feedback voltage held by the feedback voltage holding unit during the extinction period. The duty ratio may be adjusted.

  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.

  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, the output voltage of the DC / DC converter can be effectively maintained even in a situation where the luminance of PWM dimming is decreased with time.

It is the circuit diagram 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. It is a circuit diagram which shows the structural example of a threshold voltage generation part. 5A and 5B are operation waveform diagrams of the threshold voltage generation unit of FIG. 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. FIG. 8 is a circuit diagram illustrating an electronic device including the light emitting device of FIG. 7. It is a perspective view which shows the electronic device of FIG.

  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 an LED string 102 that is a light emitting element and a drive circuit 200. The LED string 102 has a plurality of LEDs connected in series. The drive circuit 200 includes a dimming switch M2 and a current detection resistor Rs2 connected in series with the LED string 102, a DC / DC converter 202, and a control circuit 300, similarly to the drive circuit 200r of FIG. The DC / DC converter 202 supplies an output voltage _VOUT between both ends of the LED string 102, the dimming switch M2, and the current detection resistor Rs2. The control circuit 300 is a functional IC integrated on a single semiconductor substrate, and controls the dimming switch M2 and the DC / DC converter 202. “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.

  Similar to the control circuit 300r of FIG. 1, the control circuit 300 includes an error amplifier 302, a switch 304, a duty controller 306, a gate driver 308, a dimming driver 310, a feedback voltage holding unit 312, a threshold voltage generation unit 314, and a comparator. 316.

  The control circuit 300 switches the dimming switch M2 according to the dimming pulse signal PWM. The dimming pulse signal PWM takes an on level (for example, high level) in the lighting period and an off level (for example, low level) in the extinction period. The dimming pulse signal PWM may be generated inside the control circuit 300 or may be generated by an external processor.

  The dimming driver 310 turns on the dimming switch M2 when the dimming pulse signal PWM is on level, and turns off the dimming switch M2 when the dimming pulse signal PWM is off level.

The error amplifier 302 amplifies an error between the first current detection signal Vs1 corresponding to the voltage drop of the current detection resistor Rs2 and the dimming voltage V _ADIM indicating the target luminance of the light emitting element 102, and a feedback voltage V corresponding to the error. _FB is generated. The switch 304 is turned on when the lighting section, that is, the dimming pulse signal PWM is on level, and turned off when the lighting section, that is, the dimming pulse signal PWM is off level.

The feedback voltage holding unit 312 samples and holds the feedback voltage _VFB when transitioning from the lighting period to the extinguishing period. Specifically, the feedback voltage holding unit 312 may sample the feedback voltage _VFB using an edge (negative edge) at which the dimming pulse signal PWM transitions from an on level to an off level.

The duty controller 306 (i) generates a gate pulse signal S1 having a duty ratio corresponding to the feedback voltage _VFB in a lighting period, that is, a period in which the dimming pulse signal PWM is on level. During a period in which the pulse signal PWM is in the off level, the gate pulse signal S1 having a duty ratio corresponding to the feedback voltage V _{FB_H} sampled and held by the feedback voltage holding unit 312 is generated. The configuration and modulation method of the duty controller 306 are not particularly limited. For example, the duty controller 306 may include a peak current mode or average current mode pulse width modulator. In this case, the duty controller 306 may reflect the voltage drop Vs1 of the current detection resistor Rs1 in the duty ratio. Alternatively, duty controller 306 may include a voltage mode pulse modulator.

In the present embodiment, the configuration and operation of the threshold voltage generation unit 314 are different from the threshold voltage generation unit 314 of FIG. When the threshold voltage generation unit 314 in FIG. 3 transitions from the lighting period to the extinguishing period, that is, when the dimming pulse signal PWM transitions from the on level to the off level, the immediately preceding lighting period is longer than the minimum time _TMIN. In this case, the voltage detection signal V _OVP corresponding to the output voltage V _OUT is sampled and held. Further, when the threshold voltage generation unit 314 transitions from the lighting period to the extinction period, if the immediately previous lighting period is shorter than the minimum time _TMIN , the threshold voltage generation unit 314 does not newly sample the voltage detection signal V _OVP and has already sampled. Continue to hold the value. The threshold voltage generation unit 314 generates a threshold voltage V _TH according to the held voltage detection signal V _OVP .

The comparator 316 compares the threshold voltage V _TH with the voltage detection signal V _OVP in the extinguishing period, and generates a comparison signal S2 indicating the comparison result. The comparator 316 may be a hysteresis comparator.

  The gate driver 308 switches the switching transistor M1 of the DC / DC converter 202 according to the gate pulse signal S1 during (i) the lighting period. Further, the gate driver 308 (ii) in the light-off period, (ii-1) a switching state in which the DC / DC converter is switched in accordance with the comparison signal S2 indicating the comparison result of the comparator 316, and (ii) -2) Repeat the stop state to stop switching.

FIG. 4 is a circuit diagram illustrating a configuration example of the threshold voltage generation unit 314.
The threshold voltage generation unit 314 includes a sampling signal generation unit 320 and a sample hold circuit 340.

The sampling signal generator 320 is configured to change the sampling signal when the previous lighting period is longer than the minimum time _TMIN when the lighting period transitions from the lighting period to the extinction period, that is, when the dimming pulse signal PWM transitions from the on level to the off level. Assert S4 (for example, high level).

  The sampling signal generation unit 320 includes a permission signal generation unit 322 and a sampling control unit 328.

The permission signal generation unit 322 asserts the permission signal S3 when the length of the ON level (high level) of the dimming pulse signal PWM is longer than the minimum time _TMIN . When the dimming pulse signal PWM transitions from the off level (low level) to the on level (high level), that is, when a positive edge occurs, the permission signal generation unit 322 in FIG. 4 starts measurement over time. The permission signal generation unit 322 includes the minimum time T before the dimming pulse signal PWM transitions from the on level (high level) to the off level (low level), that is, before the negative edge of the dimming pulse signal PWM. _{When MIN} elapses, the permission signal S3 is asserted.

Specifically, permission signal generation unit 322 includes an oscillator 324 and a counter 326. The oscillator 324 generates a clock signal CK having a predetermined cycle. The counter 326 starts counting the clock signal CK when the dimming pulse signal PWM transitions from an off level (low level) to an on level (high level), that is, when a positive edge of the dimming pulse signal PWM occurs. The counter 326 is reset when the dimming pulse signal PWM transitions from an on level (high level) to an off level (low level), that is, when a negative edge of the dimming pulse signal PWM occurs. When the count value reaches a value corresponding to the minimum time _TMIN , the counter 326 asserts the permission signal S3. The configuration of the permission signal generation unit 322 is not limited to that of FIG. The permission signal generation unit 322 may be configured using an analog timer.

  The sampling control unit 328 changes the dimming pulse signal PWM from the on level (high level) to the off level (low level) with the permission signal S3 asserted, that is, a negative edge of the dimming pulse signal PWM is generated. Then, the sampling signal S4 is asserted (for example, high level). The configuration of the sampling control unit 328 is not particularly limited. For example, the sampling control unit 328 includes an inverter 330 that inverts the dimming pulse signal PWM, an output #PWM (# indicates logic inversion) of the inverter 330, and a permission signal S3. And an AND gate 332 that outputs a sampling signal S4.

5A and 5B are operation waveform diagrams of the threshold voltage generation unit 314 of FIG. 5A shows a case where the pulse width τ of the dimming pulse signal PWM is longer than the minimum time T _MIN , and FIG. 5B shows a case where the pulse width τ of the dimming pulse signal PWM is shorter than the minimum time T _MIN. Show.

Preferably, the minimum time T _MIN is a predetermined number of times the switching period of the DC / DC converter 202. The current mode or voltage mode duty controller 306 includes an oscillator that generates a periodic signal having a switching period. Therefore, the threshold voltage generation unit 314 may measure the minimum time T _MIN using the periodic signal generated by the duty controller 306 (or a periodic signal having a frequency division / multiplication relationship therewith) as the clock signal CK. .

  When the clock signal CK and the periodic signal of the duty controller 306 are separate signals, the oscillator 324 is required separately from the oscillator of the duty controller 306, but if the clock signal CK and the periodic signal are the same signal, The oscillator 324 of the permission signal generation unit 322 can be omitted.

In addition, when the clock signal CK and the periodic signal of the duty controller 306 are separate signals that are independent of each other, the minimum time T _MIN is constant without depending on the switching period (frequency) of the DC / DC converter 202. . On the other hand, for the designer of the light emitting device 100, the design may be easier if the minimum time _TMIN is linked to the switching period. That is, if the clock signal CK and the periodic signal of the duty controller 306 are the same signal, the minimum time T _MIN is always N cycles (N of switching of the DC / DC converter, regardless of the frequency of the DC / DC converter). Is the length of the counter 326). The same applies to the case where the clock signal CK and the periodic signal of the duty controller 306 have a multiplication or division relationship. Therefore, the designer of the light emitting device 100 can manage the minimum time T _MIN in units of switching periods.

When the sampling signal S4 is asserted, the sample hold circuit 340 samples the voltage detection signal V _OVP and generates a threshold voltage V _TH corresponding to the sampled voltage.

The sample hold circuit 340 in FIG. 4 includes an A / D converter 342, a register 344, and a D / A converter 346. When the sampling signal S4 is asserted, the A / D converter 342 samples the voltage detection signal V _OVP and converts it to a digital value S5. The digital value S5 is written into the register 344 and held. The D / A converter 346 converts the digital value S5 of the register 344 into an analog voltage and outputs a threshold voltage _VTH .

When the A / D converter 342 and the D / A converter 346 are configured with high accuracy, the circuit scale increases. Therefore, the A / D converter 342 and the D / A converter 346 may be configured with an accuracy that 1LSB is about 0.1V. In this case, since there is a quantization error that cannot be ignored in each case, it is difficult to completely match V _OVP and V _TH , but the sample hold circuit 340 is configured to generate V _TH as close as possible to V _OVP. The For example, the A / D converter 342 may generate digital data S5 indicating a maximum value that does not exceed V _OVP , and the D / A converter 346 may convert the digital data S5 into an analog voltage _VTH . In this case, the relationship V _TH ≦ V _OVP is established.

The sample and hold circuit 340 may be an analog sample and hold circuit including a capacitor and a switch. In this case also, V _TH ≦ V _OVP is established. Note that a level shift circuit may be incorporated in the sample hold circuit 340 to generate the threshold voltage V _TH so that V _TH > V _OVP .

  The above is the configuration of the threshold voltage generation unit 314. Next, the operation of the light emitting device 100 in FIG. 3 will be described.

  FIG. 6 is an operation waveform diagram of the light emitting device 100 of FIG. As in FIG. 2, the duty ratio of the dimming pulse signal PWM decreases with time, that is, the lighting interval decreases with time.

The first dimming pulse signal PWM pulse width (lighting period) .tau.1 is longer than the minimum time _{T MIN.} Therefore, at the timing of the negative edge of the dimming pulse signal PWM at time t1, the threshold voltage generation unit 314 samples and holds the voltage detection signal V _OVP and updates the threshold voltage V _TH .

2 th of the dimming pulse signal PWM pulse width τ2 is shorter than the minimum time _{T MIN.} Accordingly, at the timing t2 of the subsequent negative edge of the dimming pulse signal PWM, the sampling signal S4 is not asserted, and the voltage detection signal V _OVP is not sampled by the threshold voltage generation unit 314. As a result, the previous value V _TH1 is maintained as the threshold voltage V _TH . Then, 3 th, 4 th dimming pulse signal PWM pulse width .tau.3, shorter than the minimum time is also _{T MIN} .tau.4. Therefore, the sampling signal S4 is not asserted at the negative edge timings t3 and t4 of the dimming pulse signals PWM, and the threshold voltage generator 314 does not sample the voltage detection signal _VOVP . As a result, the original value V _TH1 is maintained as the threshold voltage V _TH .

As described above, according to the light emitting device 100 of FIG. 3, even when the duty ratio of the dimming pulse signal PWM is gradually decreased, the threshold voltage V _TH can be prevented from decreasing. As a result, the voltage detection signal V _{OVP, in} other words, the output voltage _VOUT can be maintained at a voltage level at which the LED string 102 can emit light with the target luminance.

(Second Embodiment)
In the first embodiment, the drive circuit 200 that drives the single LED string 102 has been described. In the second embodiment, a driving circuit 200a that drives the LED strings 102 of a plurality of channels will be described focusing on differences from the first embodiment.

  FIG. 7 is a circuit diagram of the light emitting device 100a according to the second embodiment. The light emitting device 100a includes one or a plurality of LED strings 102_1 to 102_N and a driving circuit 200a.

  The drive circuit 200a includes a DC / DC converter 202 and a control circuit 300a. In the present embodiment, a plurality of current sources CS1 to CSN are built in the control circuit 300a, but some or all of them may be externally attached to the control circuit 300a.

The DC / DC converter 202 supplies the output voltage _VOUT to one end (anode) commonly connected to the plurality of LED strings 102_1 to 102_N.

The plurality of current sources CS1 to CSN are associated with the plurality of LED strings 102_1 to 102_N. Each current source CS_i (1 ≦ i ≦ N) is connected to the second terminal (cathode) of the corresponding LED string _{102_i} to generate an on state that generates a predetermined drive current I _LEDi and an off state that stops the drive current I _LEDi And can be switched.

  The control circuit 300a includes a dimming control unit 350 in addition to the control circuit 300 of FIG. 350 generates a plurality of dimming pulse signals PWM1 to PWMN corresponding to the plurality of LED strings 102_1 to 102_N, respectively. Then, the dimming control unit 350 switches between the on state and the off state of the plurality of current sources CS1 to CSN according to the plurality of dimming pulse signals PWM1 to PWMN.

  The dimming control unit 350 is asserted (for example, high level) when all of the dimming pulse signals PWM1 to PMWN indicate an off state, and negated (for example, low level) when at least one of the dimming control signals 350 indicates an on state. All channel off signal PWM_ALL_L to be generated is generated. The all channel off signal PWM_ALL_L may be generated by a NOR gate that generates a negative logical sum of a plurality of dimming pulse signals PWM1 to PWMN.

The error amplifier 302, a feedback voltage corresponding to the error of the lowest voltage and the predetermined reference voltage _{V REF} of the second terminal (cathode) voltage LED1~LEDN _V LED1 _{~V LEDN} plurality of LED strings 102_1~102_N _{V FB} Is generated.

  In the present embodiment, a section in which at least one of the plurality of dimming pulse signals PWM1 to PWMN indicates an on state is referred to as a lighting section. A state in which all of the plurality of dimming pulse signals PWM1 to PWMN indicate an off state is referred to as an extinguishing section. The all-channel off signal PWM_ALL_L is negated in the lighting section, and the all-channel off signal PWM_ALL_L is asserted in the unlit section.

The feedback voltage holding unit 312a samples and holds the feedback voltage _VFB when transitioning from the lighting period to the extinguishing period, that is, when the all-channel off signal PWM_ALL_L is asserted.

The threshold voltage generator 314a samples and holds the voltage detection signal V _OVP corresponding to the output voltage _VOUT when the previous lighting period is longer than the minimum time _TMIN when the lighting period transitions from the lighting period. A threshold voltage V _TH corresponding to the sampled and held value is generated. For example, the threshold voltage generation unit 314a can perform this process using the all-channel off signal PWM_ALL_L.

More specifically, the threshold voltage generation unit 314a can be configured similarly to FIG. That enable signal generating unit 322, when all channels off signal PWM_ALL_L is negated to begin time measurement, prior to all channels off signal PWM_ALL_L is asserted, the minimum time _{T MIN} has elapsed, asserts a permission signal S3 do it.

The drive circuit 200a according to the second embodiment also reduces the threshold voltage _VTH even when the duty ratio of the dimming pulse signal PWM is gradually reduced, as in the first embodiment. Can be prevented. As a result, the voltage detection signal V _{OVP, in} other words, the output voltage _VOUT can be maintained at a voltage level at which the LED string 102 can emit light with the target luminance.

Lastly, an electronic device using the light emitting device according to the embodiment will be described.
FIG. 8 is a circuit diagram illustrating an electronic apparatus 500 including the light emitting device 100a 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. 9 is a perspective view showing the electronic apparatus 500 of FIG. The electronic device 500 in FIG. 9 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 shown in FIG.

  Note that the backlight using the light emitting device may be (i) the edge light type shown in FIG. 8, 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 100 in FIG. 3 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.

  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.

  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.

  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, 302 ... Error amplifier, 304 ... Switch, 306 ... Duty controller 308, gate driver, 310, dimming driver, 312 ... feedback voltage holding unit, 314 ... threshold voltage generation unit, 316 ... comparator, 320 ... sampling signal generation unit, 322 ... permission signal generation unit, 324 ... oscillator, 326... Counter 328 Sampling control unit 330 Inverter 332 AND gate 340 Sample hold circuit 342 A / D converter 344 Register 346 D / A converter 350 Dimming control unit PWM: Dimming pulse signal, M2: Dimming Switch, Rs2 ... current detection resistor, S1 ... gate pulse signal, S2 ... comparison signal, S3 ... permission signal, S4 ... sampling signal, L1 ... inductor, C1 ... output capacitor, D1 ... rectifier diode, M1 ... switching transistor.

Claims (25)

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 interval, and turns off the dimming switch when the dimming pulse signal indicates an extinguishing interval;
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;
When transitioning from the lighting period to the extinguishing period, if the previous lighting period is longer than the minimum time, the voltage detection signal corresponding to the output voltage is sampled and held, and the threshold voltage corresponding to the sampled and held value is set. A threshold voltage generator to generate;
A duty controller that generates a gate pulse signal having a duty ratio according to the feedback voltage;
A comparator that compares the threshold voltage with the voltage detection signal in the extinction period;
(I) switching the DC / DC converter according to the gate pulse signal in the lighting period, and (ii) transitioning between a switching state and a stop state according to the comparison result of the comparator in the extinguishing period, (Ii-1) switching the DC / DC converter according to the gate pulse signal in the switching state, and (ii-2) a gate driver that stops switching of the DC / DC converter in the stop state;
A control circuit comprising:   The control circuit according to claim 1, wherein the minimum time is a predetermined number of times a switching period of the DC / DC converter. The dimming pulse signal takes off level in the extinguishing section, on level in the lighting section,
The threshold voltage generator is
A permission signal generator that generates a permission signal that is asserted when the length of the on-level of the dimming pulse signal is longer than the minimum time;
When the dimming pulse signal transitions from the on level to the off level in a state where the permission signal is asserted, a sampling control unit that asserts a sampling signal;
Including
The control circuit according to claim 1, wherein the threshold voltage generation unit samples the voltage detection signal when the sampling signal is asserted.   The permission signal generation unit starts measurement over time when the dimming pulse signal transitions from the off level to the on level, and before the dimming pulse signal transitions from the on level to the off level, 4. The control circuit according to claim 3, wherein the permission signal is asserted when a minimum time has elapsed. The permission signal generator is
Counting starts when the dimming pulse signal transitions from the off level to the on level, resets when the dimming pulse signal transitions from the on level to the off level, and the count value is a value corresponding to the minimum time. 5. The control circuit according to claim 3, further comprising a counter that asserts the permission signal when reaching.   The control circuit according to claim 5, wherein the counter counts a clock signal corresponding to a switching period of the DC / DC converter. The dimming pulse signal takes a low level in the extinguishing section and takes a high level in the lighting section,
A high level of the permission signal is associated with assertion,
The sampling controller is
An inverter for inverting the dimming pulse signal;
An AND gate that takes the logical product of the output of the inverter and the permission signal and generates the sampling signal;
The control circuit according to claim 3, further comprising: A feedback voltage holding unit that samples and holds the feedback voltage when transitioning from the lighting interval to the extinction interval;
The duty controller (i) adjusts the duty ratio of the gate pulse signal in accordance with the feedback voltage in the lighting period, and (ii) sets the feedback voltage held by the feedback voltage holding unit in the extinction period. 8. The control circuit according to claim 1, wherein the duty ratio of the gate pulse signal is adjusted accordingly.   9. The control circuit according to claim 1, 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 1 to 9, 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 1 to 9, 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 1 to 9, wherein the dimming switch is switched and the DC / DC converter is controlled.
An electronic device comprising: 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;
When at least one of the N dimming pulse signals transitions from the lighting section instructing the on state to the extinguishing section in which all of the N dimming pulse signals indicate the off state, the previous lighting is performed. A threshold voltage generation unit that samples and holds a voltage detection signal corresponding to the output voltage when a section is longer than a minimum time, and generates a threshold voltage corresponding to the sampled and held value;
A duty controller that generates a gate pulse signal having a duty ratio according to the feedback voltage;
A comparator that compares the threshold voltage with the voltage detection signal in the extinction period;
(I) In the lighting period, the DC / DC converter is switched in accordance with the gate pulse signal, and (ii) in the extinguishing period, the switching state and the stop state are changed according to the comparison result of the comparator, (ii -1) a gate driver that switches the DC / DC converter according to the gate pulse signal in the switching state, and (ii-2) that stops the switching of the DC / DC converter in the stop state;
A control circuit comprising:   The control circuit according to claim 13, wherein the minimum time is a predetermined number of times a switching period of the DC / DC converter. The dimming control unit 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;
The threshold voltage generator is
A permission signal generator for generating a permission signal that is asserted when the all channel off signal is negated for longer than the minimum time;
A sampling control unit that asserts a sampling signal when the all-channel off signal is asserted in a state where the permission signal is asserted;
Including
The control circuit according to claim 13 or 14, wherein the threshold voltage generation unit samples the voltage detection signal when the sampling signal is asserted.   The permission signal generation unit starts measurement over time when the all channel off signal is negated, and asserts the permission signal when the minimum time elapses before the all channel off signal is asserted. The control circuit according to claim 15. The permission signal generator is
A counter that starts counting when the all channel off signal is negated, resets when the all channel off signal is asserted, and asserts the enable signal when the count value reaches a value corresponding to the minimum time The control circuit according to claim 15 or 16, characterized in that   The control circuit according to claim 17, wherein the counter counts a clock signal corresponding to a switching period of the DC / DC converter. A feedback voltage holding unit that samples and holds the feedback voltage when transitioning from the lighting interval to the extinction interval;
The duty controller (i) adjusts the duty ratio of the gate pulse signal in accordance with the feedback voltage in the lighting period, and (ii) sets the feedback voltage held by the feedback voltage holding unit in the extinction period. 19. The control circuit according to claim 13, wherein a duty ratio of the gate pulse signal is adjusted accordingly.   The control circuit according to claim 13, 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 13 to 20, 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 13 to 20, 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 13 to 20, which controls the N current sources and the DC / DC converter;
An electronic device 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 the dimming pulse signal indicates a lighting interval, and turning off the dimming switch when the dimming pulse signal indicates an extinguishing interval;
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;
Sample-holding the feedback voltage when transitioning from the lighting interval to the extinction interval;
When transitioning from the lighting period to the extinguishing period, if the previous lighting period is longer than the minimum time, the voltage detection signal corresponding to the output voltage is sampled and held, and the threshold voltage corresponding to the sampled and held value is set. Generating step;
Generating a gate pulse signal having a duty ratio according to the feedback voltage in the lighting section;
Generating a gate pulse signal having a duty ratio according to the held feedback voltage in the extinction period;
Comparing the threshold voltage with the voltage detection signal in the extinction section, and generating a comparison signal indicating a comparison result;
Switching the DC / DC converter according to the gate pulse signal in the lighting section;
Switching the DC / DC converter intermittently according to the comparison signal and according to the gate pulse signal in the extinction section;
A control method 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;
When at least one of the N dimming pulse signals transitions from the lighting section instructing the on state to the extinguishing section in which all of the N dimming pulse signals indicate the off state, the feedback voltage is changed. A sample and hold step;
When transitioning from the lighting period to the extinguishing period, if the previous lighting period is longer than the minimum time, the voltage detection signal corresponding to the output voltage is sampled and held, and the threshold voltage corresponding to the sampled and held value is set. Generating step;
Generating a gate pulse signal having a duty ratio according to the feedback voltage in the lighting section;
Generating a gate pulse signal having a duty ratio according to the held feedback voltage in the extinction period;
Comparing the threshold voltage with the voltage detection signal in the extinction section, and generating a comparison signal indicating a comparison result;
Switching the DC / DC converter according to the gate pulse signal in the lighting section;
Switching the DC / DC converter intermittently according to the comparison signal and according to the gate pulse signal in the extinction section;
A control method comprising:

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