Drive circuit of light-emitting element and light-emitting device using it, electronic apparatus

The present invention relates to a technique for driving 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 showing 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 drive 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 drive circuit 1008 includes a current source CS provided for each LED string 1006_1 to 1006_n._１~ CS_nTo be equipped. Each current source CS has a corresponding LED string 1006 and a drive current I according to the target brightness._LEDTo supply.

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. The control IC 1100 has a voltage (referred to as a detection voltage) V generated at each cathode terminal of the LED strings 1006_1 to 1006_n._LED1~ V_LEDnThe on / off duty ratio of the switching transistor M1 is feedback-controlled so that the lowest one of them approaches the target voltage Vref. As a result, the output voltage Vout of the switching power supply 1004 is stabilized to (Vref + Vf). Vf is the forward voltage (voltage drop) of the LED string 1006.

<p><patcit num="1"><text>Japanese Unexamined Patent Publication No. 2006-114324</text></patcit></p>

<p num="0007"> In such a light emitting device 1003, in order to adjust the brightness of the LED string 1006, the drive current I_LEDMay be controlled by PWM (Pulse Width Modulation). Specifically, the PWM controller 1009 of the current drive circuit 1008 has a burst dimming pulse PWM having a duty ratio according to the brightness._１~ PWM_nAnd the corresponding current source CS for each_１~ CS_nSwitching control. Such control is also referred to as burst dimming or burst control.</p><p num="0008"> As a result of studying such a light emitting device, the present inventors have come to recognize the following problems. The detection voltage V is the period during which the current source CS is turned off, that is, the period during which the LED string 1006 is turned off._LEDBecomes indefinite and the detection voltage V_LEDFeedback control based on this cannot be performed. Therefore, the control IC 1100 has a detection voltage V during the period when the current source CS is turned on, that is, the lighting period of the LED string 1006._LEDBased on this, the on / off duty ratio of the switching transistor M1 is adjusted.</p><p num="0009"> 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 is shortened to the same extent as the switching pulse of the switching transistor M1 of the switching power supply, the feedback by the error amplifier cannot follow, the drive voltage Vout decreases, and the brightness of the LED string 6 decreases or emits light during the lighting period. It disappears.</p><p num="0010"> This recognition should not be regarded as a range of common general knowledge in the field of the present invention. Furthermore, the above examination itself was conceived by the applicant for the first time.</p><p num="0011"> The present invention has been made in view of these problems, and one of the exemplary purposes of the embodiment is to provide a control circuit capable of suppressing fluctuations in output voltage when the lighting time of burst dimming is short. ..</p>

<p num="0012"> One aspect of the present invention relates to a drive circuit of a light emitting element. The drive circuit is connected to a switching power supply that supplies a drive voltage to the first terminal of the light emitting element to be driven and a second terminal of the light emitting element, and supplies a drive current to the light emitting element during the period when the burst dimming pulse is asserted. It is equipped with a current driver. The switching power supply includes a capacitor with a fixed potential at one end, an error amplifier that supplies a current corresponding to the error between the detection voltage generated at the second terminal of the light emitting element and a predetermined reference voltage to the capacitor, and an output terminal of the error amplifier. A switch provided between capacitors that is turned on during the period when the burst dimming pulse is asserted, a pulse generator that receives the 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 that drives the switching element of a switching power supply based on a signal, and a feedback voltage adjustment circuit that is configured to switch on and off according to the pulse width of the burst dimming pulse and supplies current to the capacitor in the on state. , Equipped with.</p><p num="0013"> 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 the delay in the response of the error amplifier, the capacitor is charged by the current supply from the feedback adjustment circuit and feedback is performed. The voltage rises. As a result, the pulse width of the switching pulse signal is instantaneously increased and the output voltage is increased, so that the light emitting element can be reliably lit. If the feedback adjustment circuit continues to turn 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, the feedback voltage adjustment circuit can be turned off at an appropriate timing to suppress an excessive increase in the output voltage.</p><p num="0014"> 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 during which the burst dimming pulse is asserted when the pulse width of the burst dimming pulse is shorter than the threshold value. It may be in a state and then turned off.</p><p num="0015"> In some embodiments, the drive circuit may further include a short detection comparator that produces a short detection signal that is asserted when the detection voltage is higher than a predetermined threshold voltage. The feedback voltage control circuit may be turned off when the short detection signal is asserted at the timing when the burst dimming pulse is negated.</p><p num="0016"> The feedback voltage adjustment circuit may include a flip-flop in which a short-circuit detection signal is input to its input terminal and an inversion signal of a burst dimming pulse is input to its clock terminal. The feedback voltage adjustment circuit may be switched on and off according to the output signal of the flip-flop.</p><p num="0017"> The feedback voltage control circuit may be turned on if the short detection signal is asserted during the period when the burst dimming pulse is negated.</p><p num="0018"> The feedback voltage adjustment circuit is a NAND gate that receives the inverted signal of the burst dimming pulse and the short-circuit detection signal, the short-circuit detection signal is input to the input terminal, and the inverted signal of the burst dimming pulse is input to the 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 on and off according to the output signal of the flip-flop.</p><p num="0019"> Another aspect of the present invention is a light emitting device. The device comprises a light emitting element and a drive circuit of any of the above embodiments for driving the light emitting element.</p><p num="0020"> Yet another aspect of the present invention is an electronic device. This electronic device includes a liquid crystal panel and the above-mentioned light emitting device provided as a backlight of the liquid crystal panel.</p><p num="0021"> It should be noted that any combination of the above components or components and expressions of the present invention that are mutually replaced between methods, devices, systems, and the like are also effective as aspects of the present invention.</p>

<p num="0022"> According to an aspect of the present invention, the output voltage can be stabilized when the lighting time of burst dimming is short.</p>

<figref num="1">It is a circuit diagram which shows the structural example of the light emitting device which concerns on a comparative technique.</figref><figref num="2">It is a circuit diagram which shows the structure of the electronic device which comprises the light emitting device which concerns on embodiment.</figref><figref num="3">It is a circuit diagram which shows the structural example of the feedback voltage adjustment circuit.</figref><figref num="4">It is a time chart which shows the operation of the control IC of FIG.</figref><figref num="5">It is a time chart which shows the operation of the control IC of FIG.</figref>

Hereinafter, the present invention will be described with reference to the drawings based on preferred embodiments. The same or equivalent components, members, and processes shown in the drawings shall be designated by the same reference numerals, and redundant description will be omitted as appropriate. Further, the embodiment is not limited to the invention but is an example, and all the features and combinations thereof described in the embodiment are not necessarily essential to the invention.

In the present specification, the "state in which the member A is connected to the member B" means that the member A and the member B are physically directly connected, and the member A and the member B are electrically connected to each other. It also includes the case of being indirectly connected via another member that does not affect the connection state. Similarly, "a state in which the member C is provided between the member A and the member B" means that the member A and the member C, or the member B and the member C are directly connected, and also electrically. It also includes the case of being indirectly connected via another member that does not affect the connection state.

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

The electronic device 2 is a battery-powered device such as a notebook PC, a digital camera, a digital video camera, a mobile phone terminal, and 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 which are light emitting elements, a current drive circuit 8, and a switching power supply 4. The current drive circuit 8 and the switching power supply 4 form a drive circuit for the light emitting string.

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 the input voltage (for example, battery voltage) Vin input to the input terminal P1 and outputs the 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, the description thereof will be omitted. Further, those skilled in the art understand that there are various variations in the topology, and the present invention is not limited to this.

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 required for lighting the LED string 6 can be obtained. The switching transistor M1 may be built in the control IC 100.

The current drive circuit 8 is connected to the other end (cathode) of the plurality of LED strings 6_1 to 6_n. In the current drive circuit 8, the intermittent drive current I according to the target brightness is applied to each of the LED strings 6_1 to 6_n._LED1~ I_LEDnTo supply. Specifically, the current drive circuit 8 includes a plurality of current sources CS provided for each of the LED strings 6_1 to 6_n._１~ CS_nAnd the PWM controller 9. i-th current source CS_iIs connected to the cathode of the corresponding i-th LED string 6_i. Current source CS_iIs the burst dimming pulse PWM output from the PWM controller 9._iDepending on the drive current I_LEDiOperation (active) state φ_ONAnd the drive current I_LEDiStop state φ_OFFIs configured to be switchable. The PWM controller 9 has a burst dimming pulse PWM having a duty ratio according to the target brightness._i~ PWM_nGenerates a current source CS_１~ CS_nOutput to. Burst dimming pulse PWM_iIs asserted (for example, high level) (lighting period T)_ON), Corresponding current source CS_iIs the operating state φ_ONThen, the LED string 6_i lights up. Burst dimming pulse PWM_iIs negated (for example, low level) (light-off period T)_OFF), Corresponding current source CS_iIs stopped φ_OFFThen, the LED string 6_i is turned off. Lighting period T_ONAnd turn-off period T_OFFDrive current I flowing through the LED string 6_i by controlling the time ratio of_LEDThe effective value (time average value) of is controlled, and the brightness can be adjusted. The frequency of PWM drive by the current drive circuit 8 is several tens to several hundreds Hz. Below, burst dimming pulse PWM_１~ PWM_nAre transitioned at the same timing, and they are collectively referred to as burst dimming pulse PWM.

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

The above is the configuration of the entire light emitting device 3. Subsequently, the configuration of the control IC 100 will be described. The control IC 100 is an LED terminal LED provided for each LED string 6_1 to 6_n._１~ LED_nTo be equipped. Each LED terminal LED_iIs connected to the cathode terminal of the corresponding LED string 6_i. The number of LED strings does not have to 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, and a short-circuit detection circuit 60._１〜６０_n, Feedback circuit 70_１〜７０_nTo be equipped.

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

Feedback circuit 70_１〜７０_nAre provided for each LED terminal (channel). i-th feedback circuit 70_iIs the detection voltage V of the corresponding LED terminal_LEDiVoltage V according to_LEDi'Is output to the error amplifier 22. Specifically, the feedback circuit 70 is a voltage dividing circuit including resistors R11 and R12, and the detection voltage V_LEDiIs divided by the voltage dividing ratio K1. The first switch SW11 is a burst dimming signal PWM of the corresponding channel._iIs asserted (lighting period) and turned off during the negated period (lighting period). Further, 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 has a burst dimming signal PWM._iIt is an N-channel MOSFET controlled according to. The second switch SW12 is turned on when the channel should be excluded from the feedback target, and the detection voltage V_LEDi'For example, power supply voltage V_DDPull up to. As a result, the detection voltage V of that channel_LEDi'The detection voltage V of other channels_LEDjIt can be higher than'(j i) and can be excluded from feedback. Note that the voltage division of the detected voltage is not an essential process, so in the following description, unless otherwise required, V_LED'And V_LEDDo not distinguish. For example, the second switch SW12 has a burst dimming signal PWM._iIt is a P-channel MOSFET controlled according to.

The error amplifier 22 is a so-called gm (transconductance) amplifier, and has a detection voltage V during the lighting period of the LED string 6._LEDAnd a current corresponding to the error of the reference voltage Vref is generated and supplied to the FB terminal. Detection voltage V at the FB terminal_LEDAnd the feedback voltage V according to the error of the reference voltage Vref_FBOccurs.

Specifically, the error amplifier 22 has a plurality of inverting input terminals () and one non-inverting input terminal (+). Detection voltage V for each of the multiple inverting input terminals_LED1~ V_LEDnIs input, and the reference voltage Vref is input to the non-inverting input terminal. The error amplifier 22 has the lowest detection voltage V._LEDAnd the current corresponding to the error of the reference voltage Vref is output.

The first switch SW10a is provided between the output terminal of the error amplifier 22 and the FB terminal. The first switch SW10a has a period during which the burst dimming pulse PWM is asserted, that is, a lighting period T._ONOn and negated period, that is, off period T_OFFTurn off. Multiple current sources CS_１~ CS_nBurst dimming pulse PWM for_１~ PWM_nWhen the phase of is shifted, the first switch SW10a is turned on while at least one burst dimming pulse PWM is asserted.

The pulse generation unit 20 is, for example, a pulse width modulator, and the voltage V generated at the FB terminal._FBTo generate a switching pulse signal Spwm having a duty ratio corresponding to the response. Specifically, the feedback voltage V_FBThe higher the value, the larger the duty ratio of the switching pulse signal Spwm. The pulse generation unit 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 has a feedback voltage V._FBIs compared with the periodic voltage Vosc to generate 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 generation unit 20. The frequency of the PWM signal Spwm is sufficiently higher than the frequency of PWM driving 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.

Short detection circuit 60_１〜６０_nIs provided for each channel of the LED strings 6_1 to 6_n, and is similarly configured. Short detection circuit 60_iIs the lighting period T_ONDetection voltage V of the LED terminal in_LEDiHowever, the predetermined threshold voltage V_THGenerates a short detection signal LSPiCH that is asserted at higher levels. Off period T_OFFWill disable short circuit detection.

Short detection circuit 60_iIncludes a short detection comparator 62, resistors R1, R2, and a transistor 63. LED terminal detection voltage V_LEDiIs divided by the resistors R1 and R2. When R1 = 2.4 MΩ and R2 = 0.6 MΩ, the voltage division ratio β = 1/5. The transistor 63 is a burst dimming pulse PWM._iSynchronized control with, lighting period T_ONOn, off period T_OFFTurn off at. The short detection comparator 62 has a lighting period T._ONThe detection voltage V divided by the resistors R1 and R2_LEDi', Threshold voltage V_THCompare with', V_LEDi'&gt; V_THOutputs the short detection signal LSPiCH, which becomes a high level (assert) when'. V_TH'= V_TH× β holds.

The feedback voltage adjustment circuit 50 is configured so that the on / off state can be switched according to the pulse width of the burst dimming pulse PWM. The current Ic is supplied to the phase compensation capacitor C3 in the on state, and the phase compensation circuit 50 is 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 adjusting circuit 50 is turned on for both the lighting period and the extinguishing period. Further, when the pulse width of the burst dimming signal PWM is shorter than the threshold value, the feedback voltage adjusting circuit 50 is turned off after the end of the lighting period.

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

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

Specifically, the feedback voltage adjusting circuit 50 transitions from on to off when the following conditions are satisfied. Detection voltage V of the i-th channel_LEDiIs being fed back. At this time, the feedback voltage adjustment circuit 50 uses the burst dimming pulse PWM._iTurns off when the short detection signal LSPiCH is asserted at the timing of transition from assert to negate.

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

FIG. 3 is a circuit diagram showing a configuration example of the feedback voltage adjusting circuit 50. The feedback voltage adjusting 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 the on / off of the switch 58 is associated with the on / off of the feedback voltage adjusting circuit 50. The feedback voltage V is caused by the current Ic flowing into the phase compensation capacitor C3._FBRise.

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

The NAND gate 54 generates a negative logical product (NAND) of the burst dimming pulse PWM and the inverted signal of 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 is the output signal Q of the flip-flop 52 of each channel._１~ Q_nIs generated and supplied to the switch 58 according to the OR. The switch 58 turns on when the output signal of the OR gate 59 is low level and turns off when the output signal of the OR gate 59 is 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.

First, refer to FIG. Now, burst dimming pulse PWM having a somewhat long pulse width is repeatedly generated. For ease of understanding and simplification of explanation, only the first channel will be focused on.

Burst dimming pulse PWM before time t0_１Is low level, so the current source CS_１Is off, and the LED string 6_1 is off. At this time, the transistor 63 is turned off, the short circuit detection is invalidated, and the detection voltage V_LED1Since'is pulled down to the low level (ground voltage), LSP1CH is at the low level.

Burst dimming pulse PWM at time t0_１When transitions to a high level, the current source CS_１Turns on, a drive current begins to flow in the LED string 6_1, and the voltage drop Vf of the LED string 6_1 gradually increases from zero. Detection voltage V_LED1Is V_LED1= Vout-Vf Since it is given in, it gradually decreases over time. Burst dimming pulse PWM_１Immediately after the transition to the high level, V_LED1'&gt; V_TH'Therefore, the short detection signal LSP1CH becomes a high level. At time t1, the detection voltage V_LED1'Is the threshold voltage V_THWhen it becomes lower than', the short detection signal LSP1CH transitions to the low level and then maintains the low level.

Since the inverted short detection signal LSP1CH # is at the low level at the timing when the burst dimming signal PWM transitions to the low level at time t2, the output signal Q1 of the flip-flop 52 is at the low level, and then the burst dimming. Off period T until time t3 when signal PWM transitions to high level_OFF, The output signal Q1 has a low level.

Since the operations at times t0 to t3 are repeated and the control signal of the switch 58 maintains a low level, the switch 58, that is, the feedback voltage adjustment circuit 50 keeps 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 adjusting circuit 50 is turned on. Since the current capacity 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.

Then, refer to FIG. Burst dimming pulse PWM at time t0_１Transitions to a high level and the detection voltage V_LED1Gradually declines over time. When the pulse width of the burst dimming signal PWM becomes short, the detection voltage V_LED1Is the threshold voltage V_THThe burst dimming signal PWM transitions to the low level before it becomes lower, that is, before the short detection signal LSP1CH transitions to the low level (time t1). Therefore, the output signal Q1 of the flip-flop 52 becomes a high level.

Here, in order to clarify the effect of the control IC 100 of FIG. 2, the operation when the feedback voltage adjusting circuit 50 does not exist will be described. Burst dimming pulse PWM_１If the pulse width of is short, the response of the error amplifier 22 is delayed, so that the current supply from the error amplifier 22 to the phase compensation capacitor C3 becomes insufficient, and the feedback voltage V_FBDecreases. 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 drops, the LED string 6 stops emitting light.

Subsequently, the operation when the feedback voltage adjusting circuit 50 is provided will be described. Even if 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 feedback voltage V_FBThe decrease in the switching pulse signal Spwm can be suppressed or increased, and the on-time of the switching pulse signal Spwm becomes long. As a result, the decrease in the drive voltage Vout can be suppressed, and the LED string 6 can be made to emit light.

However, the subsequent extinguishing period T_OFFWhen the current Ic continues to be supplied to the phase compensation capacitor C3, the feedback voltage V_FBContinues to rise and the output voltage Vout becomes too high. Burst dimming pulse When the pulse width of PWM is short, the extinguishing period T_OFFBy shutting off the current Ic after the transition to, the increase in the output voltage Vout can be suppressed.

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 the LED string 6 can be made to emit light.

The present invention has been described above based on the embodiments. This embodiment is an example, and there may be various variations in each of these components, each processing process, and their combination. Hereinafter, such a modification will be described.

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

In the embodiment, the electronic device 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 and the like.

Further, in the present embodiment, the setting of the high level, low level, assert, and negate logic signals 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 merely indicate the principles and applications of the present invention, and the embodiments are defined in the claims. Many modifications and arrangement changes are permitted without departing from the ideas of the present invention.

2 ... Electronic equipment, 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 Instrument, 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 ... Soft Off circuit, C4 ... soft-off capacitor, 42 ... discharge circuit, 44 ... current source, M6 ... switch, C2 ... capacitor, M2, M3 ... transistor, L1 ... inductor, C1 ... output capacitor, D1 ... rectifying diode, M1 ... Switching transistor, 50 ... feedback voltage adjustment circuit, 52 ... flip flop, 54 ... NAND gate, 56 ... current source, 58 ... switch, 59 ... OR gate, 60 ... short detection circuit, 62 ... short detection comparator.