JP5657366B2 - LIGHT EMITTING ELEMENT DRIVE CIRCUIT, LIGHT EMITTING DEVICE USING THE SAME, AND ELECTRONIC DEVICE - Google Patents

Description

  The present invention relates to a driving technique for a light emitting element.

  In recent years, light-emitting devices using light-emitting elements such as LEDs (light-emitting diodes) have been used as backlights and lighting devices for liquid crystal panels. FIG. 1 is a circuit diagram illustrating a configuration example of a light-emitting device according to a comparative technique. The light emitting device 1003 includes a plurality of LED strings 1006_1 to 1006_n, a switching power supply 1004, and a current driving circuit 1008.

  Each LED string 1006 includes a plurality of LEDs connected in series. The switching power supply 1004 boosts the input voltage Vin and supplies the drive voltage Vout to one end of the LED strings 1006_1 to 1006_n.

The current driving circuit 1008 includes current sources CS _{1 to} CS _n provided for the LED strings 1006_1 to 1006_n. Each current source CS supplies the corresponding LED string 1006 with a drive current I _LED corresponding to the target luminance.

The switching power supply 1004 includes an output circuit 1102 and a control IC 1100. The output circuit 1102 includes an inductor L1, a switching transistor M1, a rectifier diode D1, and an output capacitor C1. Control IC1100, like the lowest one of the LED strings 1006_1~1006_n respective voltage generated to the cathode terminal (the detection voltage _{of) V LED1 ~V LEDn} approaches the target voltage Vref, the ON of the switching transistor M1, the duty ratio of the off Feedback control. As a result, the output voltage Vout of the switching power supply 1004 is stabilized at (Vref + Vf). Vf is a forward voltage (voltage drop) of the LED string 1006.

JP 2006-114324 A

In such a light emitting device 1003, in order to adjust the luminance of the LED string 1006, the drive current I _LED may be subjected to PWM (Pulse Width Modulation) control. Specifically, the PWM controller 1009 of the current drive circuit 1008 generates burst dimming pulses PWM _{1 to} PWM _n having a duty ratio according to the luminance, and performs switching control of the current sources CS _{1 to} CS _n corresponding to each of them. To do. Such control is also referred to as burst dimming or burst control.

As a result of studying such a light emitting device, the present inventors have recognized the following problems.
During a period in which the current source CS is turned off, that is, a period during which the LED string 1006 is turned off, the detection voltage V _LED is indefinite, and feedback control based on the detection voltage V _LED cannot be performed. Therefore, the control IC 1100 adjusts the ON / OFF duty ratio of the switching transistor M1 based on the detection voltage V _LED during the period when the current source CS is turned on, that is, the lighting period of the LED string 1006.

  Here, when the lighting period of the LED string 1006 is shortened, the period during which the feedback control is effective is shortened. When the lighting period becomes as short as the switching pulse of the switching transistor M1 of the switching power supply, feedback by the error amplifier cannot follow, the drive voltage Vout decreases, and the luminance of the LED string 6 decreases or emits light during the lighting period. Disappear.

  It should be noted that this recognition should not be taken as a range of common general knowledge in the field of the present invention. Furthermore, the above-mentioned examination itself has been conceived for the first time by the present applicant.

  The present invention has been made in view of these problems, and one of exemplary purposes of an embodiment thereof is to provide a control circuit capable of suppressing fluctuations in output voltage when the lighting time of burst dimming is short. .

  One embodiment of the present invention relates to a driver circuit for a light-emitting element. The drive circuit is connected to the switching power supply that supplies the drive voltage to the first terminal of the light emitting element to be driven, and is connected to the second terminal of the light emitting element, and supplies the drive current to the light emitting element during the period when the burst dimming pulse is asserted A current driver. The switching power supply includes a capacitor having a fixed potential at one end, an error amplifier that supplies a current corresponding to an error between a detection voltage generated at the second terminal of the light emitting element and a predetermined reference voltage, and an output terminal of the error amplifier; A switch that is provided between the capacitors and is turned on while the burst dimming pulse is asserted; a pulse generator that receives a feedback voltage generated in the capacitor and generates a switching pulse signal having a duty ratio corresponding to the feedback voltage; and a switching pulse A driver for driving the switching element of the switching power supply based on the signal, and a feedback voltage adjusting circuit configured to be able to switch between the on and off states according to the pulse width of the burst dimming pulse, and supplying a current to the capacitor in the on state; .

According to this aspect, when the pulse width of the burst dimming pulse is short, even if the current supply to the capacitor is insufficient due to a delay in the response of the error amplifier, the capacitor is charged by the current supply from the feedback adjustment circuit, and the feedback The voltage rises. As a result, the light emitting element can be reliably turned on by instantaneously increasing the pulse width of the switching pulse signal and increasing the output voltage.
Further, when the feedback adjustment circuit continues to be turned on and the current supply to the capacitor continues, the feedback voltage continues to rise and the output voltage rises. Therefore, when the pulse width of the burst dimming pulse is short, an excessive increase in the output voltage can be suppressed by turning off the feedback voltage adjustment circuit at an appropriate timing.

  The feedback voltage adjustment circuit is turned on when the pulse width of the burst dimming pulse is longer than a certain threshold value, and is turned on for the period when the burst dimming pulse is asserted when the pulse width of the burst dimming pulse is shorter than the threshold value. The state may then be turned off.

  The driving circuit according to an aspect may further include a short detection comparator that generates a short detection signal that is asserted when the detection voltage is higher than a predetermined threshold voltage. The feedback voltage adjustment circuit may be turned off when the short detection signal is asserted at the timing when the burst dimming pulse is negated.

  The feedback voltage adjustment circuit may include a flip-flop in which a short detection signal is input to its input terminal and an inverted signal of the burst dimming pulse is input to its clock terminal. The feedback voltage adjustment circuit may be switched between an on state and an off state according to the output signal of the flip-flop.

  The feedback voltage adjustment circuit may be turned on when the short detection signal is asserted during the period in which the burst dimming pulse is negated.

  The feedback voltage adjustment circuit includes a NAND gate that receives a burst dimming pulse and an inverted signal of the short detection signal, a short detection signal is input to its input terminal, and an inverted signal of the burst dimming pulse is input to its clock terminal. A flip-flop in which the output signal of the NAND gate is input to the reset terminal may be included. The feedback voltage adjustment circuit may be switched between an on state and an off state according to the output signal of the flip-flop.

  Another embodiment of the present invention is a light-emitting device. This device includes a light-emitting element and the drive circuit according to any one of the above-described modes for driving the light-emitting element.

  Yet another embodiment of the present invention is an electronic device. This electronic device includes a liquid crystal panel and the above-described light emitting device provided as a backlight of the liquid crystal panel.

  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 can be stabilized when the lighting time of burst dimming is short.

It is a circuit diagram which shows the structural example of the light-emitting device which concerns on a comparison technique. It is a circuit diagram which shows the structure of an electronic device provided with the light-emitting device which concerns on embodiment. It is a circuit diagram which shows the structural example of a feedback voltage adjustment circuit. It is a time chart which shows operation | movement of control IC of FIG. It is a time chart which shows operation | movement of control IC 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 in addition to the case where the member A and the member B are physically directly connected. It includes the case of being indirectly connected through another member that does not affect the connection state.
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 an electrical condition. It includes the case of being indirectly connected through another member that does not affect the connection state.

  FIG. 2 is a circuit diagram illustrating a configuration of an electronic device including the light-emitting device according to the embodiment.

  The electronic device 2 is a battery-driven device such as a notebook PC, a digital camera, a digital video camera, a mobile phone terminal, or a PDA (Personal Digital Assistant), and includes a light emitting device 3 and an LCD (Liquid Crystal Display) panel 5. The light emitting device 3 is provided as a backlight of the LCD panel 5.

  The light emitting device 3 includes LED strings 6_1 to 6_n that are light emitting elements, a current driving circuit 8, and a switching power supply 4. The current drive circuit 8 and the switching power supply 4 constitute a light emission string drive circuit.

  Each LED string 6 includes a plurality of LEDs connected in series. The switching power supply 4 is a step-up DC / DC converter that boosts an input voltage (for example, battery voltage) Vin input to the input terminal P1 and outputs an output voltage (drive voltage) Vout from the output terminal P2. One end (anode) of each of the plurality of LED strings 6_1 to 6_n is commonly connected to the output terminal P2.

  The switching power supply 4 includes a control IC 100 and an output circuit 102. The output circuit 102 includes an inductor L1, a rectifier diode D1, a switching transistor M1, and an output capacitor C1. Since the topology of the output circuit 102 is general, description thereof is omitted. In addition, it is understood by those skilled in the art that there are various variations in the topology, and is not limited in the present invention.

  The switching terminal P4 of the control IC 100 is connected to the gate of the switching transistor M1. The control IC 100 adjusts the ON / OFF duty ratio of the switching transistor M1 by feedback so that the output voltage Vout necessary for lighting the LED string 6 is obtained. The switching transistor M1 may be built in the control IC 100.

The current drive circuit 8 is connected to the other ends (cathodes) of the plurality of LED strings 6_1 to 6_n. Current drive circuit 8, each LED string 6_1～6_N, supplies intermittent drive current _I LED1 _{~I LEDn} corresponding to the target luminance. Specifically, the current drive circuit 8 includes a plurality of current sources CS _{1 to} CS _n provided for each of the LED strings 6_1 to 6_n, and a PWM controller 9. The i-th current source CS _i is connected to the cathode of the corresponding i-th LED string 6 — i. Current source CS _i, depending on the burst dimming pulse PWM _i output from the PWM controller 9, the operation of outputting the drive current _{I LEDi} and (active) state phi _ON, stopped phi _OFF to stop the drive current _{I LEDi} Is configured to be switchable. The PWM controller 9 generates burst dimming pulses PWM _{i to} PWM _n having a duty ratio corresponding to the target luminance, and outputs them to the current sources CS _{1 to} CS _n . Period burst dimming pulse PWM _i is asserted (e.g., high level) (lighting period _{T ON),} the corresponding current source CS _i is the operating state phi _ON next, LED string 6_i lights up. During a period in which the burst dimming pulse PWM _i is negated (for example, at a low level) (light extinction period T _OFF ), the corresponding current source CS _i is in the stop state φ _OFF and the LED string 6 — i is extinguished. By controlling the time ratio between the lighting period T _ON and the extinguishing period T _OFF , the effective value (time average value) of the drive current I _LED flowing in the LED string 6 — i is controlled, and the luminance can be adjusted. The frequency of PWM drive by the current drive circuit 8 is several tens to several hundreds Hz. Hereinafter, the burst dimming pulses PWM _{1 to} PWM _n are assumed to transition at the same timing, and are collectively referred to as a burst dimming pulse PWM.

  The control IC 100 and the current driving circuit 8 may be integrated on a single semiconductor chip, or may be integrated on separate chips. They may constitute a single package (module) or separate packages.

The above is the overall configuration of the light emitting device 3. Next, the configuration of the control IC 100 will be described. The control IC 100 includes LED terminals LED ₁ to LED _n provided for the LED strings 6_1 to 6_n. Each LED terminal LED _i is connected to the cathode terminal of the corresponding LED string 6_i. The LED string need not be plural, and may be one.

The control IC 100 mainly includes an error amplifier 22, a first switch SW10a, a pulse generator 20, a driver 28, short detection circuits 60 _{1 to} 60 _n , and feedback circuits 70 ₁ to 70 _n .

  A phase compensating resistor R7 and a phase compensating capacitor C3 are provided between the FB terminal and an external fixed voltage terminal (ground terminal).

The feedback circuits 70 ₁ to 70 _n are provided for each LED terminal (channel). The i-th feedback circuit 70 _i outputs a voltage V _LEDi ′ corresponding to the detection voltage V _LEDi at the corresponding LED terminal to the error amplifier 22. Specifically, the feedback circuit 70 is a voltage dividing circuit including resistors R11 and R12, and _divides the detection voltage V _LEDi by a voltage dividing ratio K1. The first switch SW11 is turned on during the period when the burst dimming signal PWM _i of the corresponding channel is asserted (lighting period) and turned off during the period when it is negated (light-out period). The first switch SW11 of the i-th channel is turned off when the channel is excluded from the feedback target. For example, the first switch SW11 is an N-channel MOSFET that is controlled according to the burst dimming signal PWM _i . The second switch SW12 is turned on when the channel is to be excluded from the feedback target, and pulls up the detection voltage V _LEDi ′ to, for example, the power supply voltage V _DD . As a result, the detection voltage V _LEDi ′ of that channel can be made higher than the detection voltage V _LEDj ′ (j ≠ i) of other channels, and can be excluded from feedback. Since the divided voltage of the detection voltage is not an essential process, V _LED ′ and V _LED are not distinguished in the following description unless particularly necessary. For example, the second switch SW12 is a P-channel MOSFET that is controlled according to the burst dimming signal PWM _i .

The error amplifier 22 is a so-called gm (transconductance) amplifier, and generates a current corresponding to an error between the detection voltage V _LED and the reference voltage Vref during the lighting period of the LED string 6 and supplies the current to the FB terminal. A feedback voltage V _FB corresponding to an error between the detection voltage V _LED and the reference voltage Vref is generated at the FB terminal.

Specifically, the error amplifier 22 has a plurality of inverting input terminals (−) and one non-inverting input terminal (+). The detection voltages V _{LED1 to} V _LEDn are input to the plurality of inverting input terminals, and the reference voltage Vref is input to the non-inverting input terminal. The error amplifier 22 outputs a current corresponding to the error between the lowest detection voltage V _LED and the reference voltage Vref.

The first switch SW10a is provided between the output terminal of the error amplifier 22 and the FB terminal. The first switch SW10a is a period during which the burst dimming pulse PWM is asserted, that is turned to the lighting period _{T ON,} OFF period, i.e. turn-off period _{T OFF} to be negated. If multiple current sources _CS 1 to CS _n burst dimming pulse _PWM 1 ～PWM _n to the phase is shifted, the period in which at least one of the burst dimming pulse PWM is asserted, the first switch SW10a is turned on.

The pulse generator 20 is, for example, a pulse width modulator, receives the voltage V _FB generated at the FB terminal, and generates a switching pulse signal Spwm having a duty ratio corresponding thereto. Specifically, the duty ratio of the switching pulse signal Spwm increases as the feedback voltage V _FB increases.
The pulse generator 20 includes an oscillator 24 and a PWM comparator 26. The oscillator 24 generates a periodic voltage Vosc of a triangular wave or a sawtooth wave.

The PWM comparator 26 compares the feedback voltage V _FB with the periodic voltage Vosc, and generates a PWM signal Spwm having a level corresponding to the comparison result. A pulse frequency modulator or the like may be used as the pulse generator 20. The frequency of the PWM signal Spwm is sufficiently higher than the frequency of PWM drive by the current drive circuit 8 and is several hundred kHz (for example, 600 kHz).

  The driver 28 drives the switching transistor M1 of the switching power supply 4 based on the switching pulse signal Spwm.

The short detection circuits 60 _{1 to} 60 _n are provided for the respective channels of the LED strings 6_1 to 6_n, and are configured similarly. Short-circuit detecting circuit 60 _i is the detected voltage _{V LEDi} of LED terminals in the lighting period _{T ON} generates a short-circuit detection signal LSPiCH which is asserted when higher than the predetermined threshold voltage _{V TH.} In the extinguishing period T _OFF , the short detection is invalidated.

The short detection circuit 60 _i includes a short detection comparator 62, resistors R 1 and R 2, and a transistor 63.
The detection voltage V _{LEDi at the} LED terminal is _divided by resistors R1 and R2. When R1 = 2.4 MΩ and R2 = 0.6 MΩ, the voltage dividing ratio β = 1/5. Transistor 63 is controlled synchronously with the burst dimming pulse PWM _i, off-on, the turn-off period _{T OFF} in the lighting period _{T ON.} Short detection comparator 62, the lighting period _{T ON,} the resistor R1, 'the threshold voltage _{V TH'} the divided detection voltage _{V LEDi} by R2 compared to _the high level when _{V LEDi '> V TH'} The short detection signal LSPiCH that becomes (asserted) is output. V _TH '= V _TH × β is established.

  The feedback voltage adjustment circuit 50 is configured to be able to switch between an on state and an off state according to the pulse width of the burst dimming pulse PWM, and supplies the current Ic to the phase compensation capacitor C3 in the on state, and for phase compensation in the off state. The current supply to the capacitor C3 is stopped.

  When the pulse width of the burst dimming signal PWM is longer than a certain threshold value, the feedback voltage adjustment circuit 50 is turned on in both the lighting period and the extinguishing period. Further, the feedback voltage adjustment circuit 50 is turned off after the lighting period ends when the pulse width of the burst dimming signal PWM is shorter than the threshold value.

By injecting the current Ic while the feedback voltage adjustment circuit 50 is in the on state, the feedback voltage V _FB is changed so that the on-time of the switching transistor M1 becomes longer. Specifically, the feedback voltage adjustment circuit 50 increases the ON time of the switching transistor M1 by increasing the feedback voltage Vfb in the ON state.

  The injected current Ic is preferably smaller than the source current and sink current of the error amplifier 22. For example, when the source current and sink current are 100 μA at the maximum, the injected current Ic of the feedback voltage adjusting circuit 50 is preferably about 1 μA.

Specifically, the feedback voltage adjustment circuit 50 transitions from on to off when the following condition is satisfied. _Assume that the detection voltage V _LEDi of the i-th channel is fed back. At this time, the feedback voltage adjustment circuit 50 is turned off when the short detection signal LSPiCH is asserted at the timing when the burst dimming pulse PWM _i transitions from assertion to negation.

  Thereafter, the feedback voltage adjustment circuit 50 is turned on when the short detection signal LSPiCH is asserted during the period in which the burst dimming pulse PWM is negated.

  FIG. 3 is a circuit diagram illustrating a configuration example of the feedback voltage adjustment circuit 50. The feedback voltage adjustment circuit 50 includes a flip-flop 52, a NAND gate 54, a current source 56, a switch 58, and an OR gate 59.

The current source 56 generates a current Ic to be supplied to the phase compensation capacitor C3. The current Ic is, for example, about 1 μA. The switch 58 is provided on the path of the current Ic, and on / off of the switch 58 is associated with on / off of the feedback voltage adjustment circuit 50. As the current Ic flows into the phase compensation capacitor C3, the feedback voltage _VFB increases.

  The flip-flop 52 and the NAND gate 54 are provided for each channel of the LED string 6. The short detection signal LSPiCH is input to the input terminal D of the i-th flip-flop 52, and the inverted signal PWM # of the burst dimming pulse PWM is input to the clock terminal. In the drawing, the logic inversion is indicated by a bar.

  The NAND gate 54 generates a negative logical product (NAND) of the inverted signal of the burst dimming pulse PWM and the short detection signal LSPiCH #. The output signal of the NAND gate 54 is input to the reset terminal of the flip-flop 52.

The OR gate 59 generates a logical sum of the output signals Q _{1 to} Q _n of the flip-flops 52 of each channel and supplies the logical sum to the switch 58 according to the logical sum. The switch 58 is turned on when the output signal of the OR gate 59 is at a low level, and turned off when the output signal of the OR gate 59 is at a high level.

  The above is the configuration of the control IC 100. Next, the operation will be described. FIG. 4 shows a time chart when the pulse width of the burst dimming pulse PWM is long to some extent, and FIG. 5 shows a time chart when the pulse width of the burst dimming pulse PWM is short.

  Reference is first made to FIG. Now, a burst dimming pulse PWM having a somewhat long pulse width is repeatedly generated. For the sake of easy understanding and simplification of description, the description will be given focusing only on the first channel.

Before time t0, the current source CS ₁ from burst dimming pulse PWM ₁ is at the low level is off, LED strings 6_1 is turned off. At this time, the transistor 63 is turned off, the short detection is invalidated, and the detection voltage V _LED1 ′ is pulled down to a low level (ground voltage), so that LSP1CH is at a low level.

When the burst dimming pulse PWM ₁ transitions high level at time t0, the current source CS ₁ is turned on, starting the drive current to the LED string 6_1 flows, the voltage drop Vf of the LED strings 6_1 gradually increases from zero. The detection voltage V _LED1 is
V _LED1 = Vout-Vf
Because it is given by, it gradually decreases with time. Immediately after the burst dimming pulse PWM ₁ transitions to the high level, V _LED1 ′> V _{TH ′} is established, and thus the short detection signal LSP1CH becomes the high level. When the detection voltage V _LED1 ′ becomes lower than the threshold voltage V _TH ′ at time t1, the short detection signal LSP1CH transitions to the low level, and then maintains the low level.

At the timing when the burst dimming signal PWM transitions to the low level at time t2, since the inverted short detection signal LSP1CH # is at the low level, the output signal Q1 of the flip-flop 52 is at the low level, and then the burst dimming is performed. During the extinguishing period T _OFF until time t3 when the signal PWM changes to the high level, the output signal Q1 becomes the low level.

  Since the operation from time t0 to t3 is repeated and the control signal of the switch 58 is maintained at the low level, the switch 58, that is, the feedback voltage adjustment circuit 50 is kept on, and the injection current Ic is supplied to the phase compensation capacitor C3. to continue. As described above, when the pulse width of the burst dimming signal PWM is long to some extent, the feedback voltage adjustment circuit 50 is turned on. Since the current capability of the error amplifier 22 is sufficiently larger than the injection current Ic of the feedback voltage adjustment circuit 50, there is almost no influence of the injection current Ic.

Next, referring to FIG. Time transition burst dimming pulse PWM ₁ is a high level t0, the detection voltage _{V LED1} gradually reduced with time. When the pulse width of the burst dimming signal PWM is reduced, before the detection voltage _{V LED1} is lower than the threshold voltage _{V TH,} that is, before the short-circuit detection signal LSP1CH changes to the low level, the burst dimming signal PWM low Transition to the level (time t1). Therefore, the output signal Q1 of the flip-flop 52 becomes high level.

Here, in order to clarify the effect of the control IC 100 of FIG. 2, an operation when the feedback voltage adjustment circuit 50 is not present will be described.
If the pulse width of the burst dimming pulse PWM ₁ is short, the response of the error amplifier 22 is delayed, so that current supply from the error amplifier 22 to the phase compensation capacitor C3 becomes insufficient, and the feedback voltage V _FB decreases. As a result, the ON time of the switching pulse signal Spwm is shortened and the drive voltage Vout is lowered. When the drive voltage Vout decreases, the LED string 6 does not emit light.

Next, the operation when the feedback voltage adjustment circuit 50 is provided will be described. Even when the response of the error amplifier 22 is delayed and the current supply from the error amplifier 22 to the phase compensation capacitor C3 is insufficient, the injection current Ic is supplied from the feedback voltage adjustment circuit 50 to the phase compensation capacitor C3. Therefore, the decrease in the feedback voltage V _FB can be suppressed or increased, and the ON time of the switching pulse signal Spwm becomes longer. As a result, a decrease in the drive voltage Vout can be suppressed and the LED string 6 can emit light.

However, if the current Ic continues to be supplied to the phase compensation capacitor C3 in the subsequent extinguishing period T _OFF , the feedback voltage _VFB continues to rise and the output voltage Vout becomes too high. When the pulse width of the burst dimming pulse PWM is short, an increase in the output voltage Vout can be suppressed by cutting off the current Ic after the transition to the extinguishing period _TOFF .

  As described above, according to the control IC 100 according to the embodiment, it is possible to suppress a decrease in the output voltage due to a delay in the response speed of the error amplifier 22, and it is possible to cause the LED string 6 to emit light.

  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, a non-insulated switching power supply using an inductor has been described, but the present invention can also be applied to an insulating switching power supply using a transformer.

  In the embodiment, the electronic apparatus has been described as an application of the light emitting device 3, but the application is not particularly limited and can be used for lighting or the like.

  In this embodiment, the setting of logic signals of high level, low level, assert, and negate is an example, and can be freely changed by appropriately inverting it with an inverter or the like.

  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 2 ... Electronic device, 3 ... Light-emitting device, 4 ... Switching power supply, 5 ... LCD panel, 6 ... LED string, 8 ... Current drive circuit, 9 ... PWM controller, 100 ... Control IC, 102 ... Output circuit, 19 ... Pulse modulation 20 ... pulse width modulator, 22 ... error amplifier, 24 ... oscillator, 26 ... PWM comparator, 28 ... driver, C3 ... phase compensation capacitor, R7 ... phase compensation resistor, SW10a ... first switch, 40 ... software OFF circuit, C4 ... capacitor for soft-off, 42 ... discharge circuit, 44 ... current source, M6 ... switch, C2 ... capacitor, M2, M3 ... transistor, L1 ... inductor, C1 ... output capacitor, D1 ... rectifier diode, M1 ... Switching transistor 50 ... Feedback voltage adjustment circuit 52 ... Flip-flop 54 ... N ND gate, 56 ... current source, 58 ... switch, 59 ... OR gate, 60 ... short-circuit detecting circuit, 62 ... short detection comparator.

Claims (8)

A switching power supply for supplying a driving voltage to the first terminal of the light emitting element to be driven;
A current driver connected to the second terminal of the light emitting element and supplying a driving current to the light emitting element during a period in which a burst dimming pulse is asserted;
With
The switching power supply is
A capacitor with a fixed potential at one end;
An error amplifier for supplying a current corresponding to an error between a detection voltage generated at the second terminal of the light emitting element and a predetermined reference voltage to the capacitor;
A switch that is provided between the output terminal of the error amplifier and the capacitor and that is turned on during a period when the burst dimming pulse is asserted;
A pulse generator that receives a feedback voltage generated in the capacitor and generates a switching pulse signal having a duty ratio according to the feedback voltage;
Based on the switching pulse signal, a driver for driving a switching element of the switching power supply,
A feedback voltage adjusting circuit configured to switch between an on state and an off state according to a pulse width of the burst dimming pulse, and supplying a current to the capacitor in the on state;
With
The feedback voltage adjustment circuit is turned on when the pulse width of the burst dimming pulse is longer than a threshold value, and the burst dimming pulse is asserted when the pulse width of the burst dimming pulse is shorter than the threshold value. driving circuit of the light emitting element you characterized in that period on the result, then turned off to be. A switching power supply for supplying a driving voltage to the first terminal of the light emitting element to be driven;
A current driver connected to the second terminal of the light emitting element and supplying a driving current to the light emitting element during a period in which a burst dimming pulse is asserted;
With
The switching power supply is
A capacitor with a fixed potential at one end;
An error amplifier for supplying a current corresponding to an error between a detection voltage generated at the second terminal of the light emitting element and a predetermined reference voltage to the capacitor;
A switch that is provided between the output terminal of the error amplifier and the capacitor and that is turned on during a period when the burst dimming pulse is asserted;
A pulse generator that receives a feedback voltage generated in the capacitor and generates a switching pulse signal having a duty ratio according to the feedback voltage;
Based on the switching pulse signal, a driver for driving a switching element of the switching power supply,
A feedback voltage adjusting circuit configured to switch between an on state and an off state according to a pulse width of the burst dimming pulse, and supplying a current to the capacitor in the on state;
A short detection comparator that generates a short detection signal that is asserted when the detection voltage is higher than a predetermined threshold voltage ;
With
The feedback voltage control circuit, at the timing when pre Symbol burst dimming pulse is negated, when the short detection signal is asserted, driving dynamic circuit of the light emitting element you characterized in that off. The feedback voltage adjustment circuit includes:
The short detection signal is input to the input terminal thereof, and includes a flip-flop to which the inverted signal of the burst dimming pulse is input to the clock terminal,
3. The drive circuit according to claim 2 , wherein the on / off state is switched according to an output signal of the flip-flop. 4. The drive circuit according to claim 2, wherein the feedback voltage adjustment circuit is turned on when the short detection signal is asserted during a period in which the burst dimming pulse is negated. The feedback voltage adjustment circuit includes:
A NAND gate for receiving the burst dimming pulse and an inverted signal of the short detection signal;
The short detection signal is input to the input terminal, the inverted signal of the burst dimming pulse is input to the clock terminal, and a flip-flop in which the output signal of the NAND gate is input to the reset terminal,
3. The drive circuit according to claim 2 , wherein the on / off state is switched according to an output signal of the flip-flop. The feedback voltage control circuit, drive circuit according to any one of claims 1 to 5, characterized in that it comprises a current source for supplying a current to the capacitor in the on-state. A light emitting element;
The drive circuit according to any one of claims 1 to 6 , which drives the light emitting element;
A light emitting device comprising: LCD panel,
The light emitting device according to claim 7 provided as a backlight of the liquid crystal panel;
An electronic device comprising:

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