JP2009231644A - Light-emitting element drive circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light-emitting element drive circuit that can shorten delay time from the input of a light emitting signal to sufficient light emission by the light emitting element. <P>SOLUTION: A power supply circuit 2 is brought into operation in advance so that so small a current (a primary current) as not to cause a Light Emitting Diode 3 (LED 3) to emit light may flow through the LED 3, and by entering a light emitting signal SL, the current running through the LED 3 is changed to a current (a secondary current) with a current value required for the LED 3 to emit light. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a drive circuit for driving a light emitting element such as an LED (Light Emitting Diode).

  A light emitting element such as an LED has a large variation in forward voltage, and in the case of a driving circuit that applies a constant voltage assuming the maximum value of the forward voltage in a light emitting element to be driven, power consumption increases. Occurs. Therefore, in order to control the amount of light emission while absorbing variations in the forward voltage, a method of controlling the current flowing through the light emitting element by adjusting the voltage applied to the light emitting element is frequently used.

  FIG. 15 shows a conventional light emitting element driving circuit (see, for example, Patent Document 1). The light emitting element driving circuit includes a booster circuit 102 that boosts an input DC voltage Vin from an input DC power supply 101 to generate a DC power supply voltage Vo, and a DC power supply voltage Vo from the booster circuit 102 is supplied to an anode via a terminal A. LED 103 to be applied, voltage value control circuit 104 for controlling the value of DC power supply voltage Vo applied to LED 103 by controlling booster circuit 102, and current flowing to LED 103 via terminal K to which the cathode of LED 103 is connected The voltage value control circuit 104 adjusts the value of the DC power supply voltage Vo so that the current ILED detected by the current detection circuit 105 becomes a set value.

  The current detection circuit 105 includes a detection resistor 106 that detects the current ILED, a reference voltage source 107, and a comparator 108 that compares the value of the voltage generated in the detection resistor 106 with a reference voltage value set in the reference voltage source 107. The voltage value control circuit 104 is configured so that the voltage value generated in the detection resistor 106 is equal to the reference voltage value set in the reference voltage source 107 based on the comparison result by the comparator 108. The value of the DC power supply voltage Vo is adjusted. By controlling the DC power supply voltage Vo in this way, a constant current set in the LED 103 flows.

  According to this light emitting element driving circuit, when the forward voltage of the LED 103 is VF and the voltage value set in the reference voltage source 107 is Vr, the value of the DC power supply voltage Vo when the LED 103 emits light is “VF + Vr”. Therefore, the efficiency of this circuit is determined only by the efficiency of the booster circuit 102 and the loss caused by the current detection circuit 105.

Thus, conventionally, by controlling the DC power supply voltage Vo applied to the LED 103 so that a constant current set in the LED 103 flows, a constant voltage assuming the maximum value of the forward voltage in the LED to be driven is obtained. Compared with the case where it is applied to the LED, the power loss in the entire drive system is suppressed.
JP 2003-152224 A

  However, the above-described conventional configuration has a problem that a light emission loss occurs due to a delay time from the start of a power supply circuit such as a booster circuit until the light emitting element emits light. Such a light emission loss becomes more noticeable in an imaging system in which a light emitting element as a light source emits light instantaneously, such as a flash of a digital still camera, because the light emission time is short.

  FIG. 16 is a startup waveform diagram of a digital still camera using the above-described conventional light emitting element driving circuit. From the top, the waveform of the light emission signal, the waveform of the DC power supply voltage Vo, the waveform of the current ILED flowing through the LED 103, and An imaging operation is shown.

  As shown in FIG. 16, the boosting circuit 102 starts operating when the light emission signal rises, and the DC power supply voltage Vo equal to the input DC voltage Vin rises to “VF + Vr”, which is a current value sufficient for the LED 103 to emit light. The period until the current flows through the LED 103 is the delay time Tr. As described above, in the conventional light emitting element driving circuit, the delay time Tr corresponding to the time required to start up the booster circuit 102 (startup delay time) is generated. The light emission loss occurs at a rate of “(Tr / T) × 100 [%]” when the light emission time is T, but the startup delay time of the booster circuit is usually about several hundred μsec. When the time T is 5 ms and the delay time Tr is 100 μs, a light emission loss of 20% occurs and an appropriate light emission amount cannot be obtained.

  The present invention solves the above-described conventional problems, and can reduce a delay time from the input of a light emission signal until a current having a current value sufficient for light emission of the light emitting element flows to the light emitting element. An object of the present invention is to provide a light emitting element driving circuit that can be used.

  The light emitting element driving circuit according to claim 1 of the present invention has a first circuit for supplying a controllable DC power supply voltage to the light emitting element, and a current value lower than a current value emitted by the light emitting element according to a light emission signal. And a second circuit for controlling the DC power supply voltage so that one of the set first current and the second current set to the current value emitted from the light emitting element flows through the light emitting element. It is characterized by.

  The light emitting element driving circuit according to claim 2 of the present invention is the light emitting element driving circuit according to claim 1, wherein the second circuit is arranged between the first and second currents in the light emitting element. A current supply circuit for passing one of the currents selected in step (b), a selection circuit for selecting a current to be supplied to the light emitting element by the current supply circuit between the first and second currents according to the light emission signal; And a control circuit that controls the DC power supply voltage so that the first or second current selected by the selection circuit flows through the light emitting element.

  The light emitting element drive circuit according to claim 3 of the present invention is the light emitting element drive circuit according to claim 2, wherein the current supply circuit is set so that the first current flows through the light emitting element. The light emitting element is connected in series, and is configured to be able to select a resistance value between the first resistance value and the second resistance value set so that the second current flows through the light emitting element. A resistance circuit, wherein the selection circuit selects a resistance value of the resistance circuit according to the light emission signal, and the control circuit generates a current from the light emitting element in the resistance circuit by flowing through the resistance circuit. The DC power supply voltage is controlled according to the potential.

  The light emitting element driving circuit according to claim 4 of the present invention is the light emitting element driving circuit according to claim 3, wherein the current supply circuit further causes a current from the light emitting element to flow through the resistance circuit. And an error amplifier that generates a signal level signal corresponding to a voltage difference between a feedback potential generated in the resistor circuit and a reference potential, and the control circuit is configured to respond to a signal level of the signal generated by the error amplifier. The DC power supply voltage is controlled so that the feedback potential is equal to the reference potential.

  The light-emitting element drive circuit according to claim 5 of the present invention is the light-emitting element drive circuit according to claim 3, wherein the resistance circuit is set to the first resistance value connected in series to the light-emitting element. One selected between the first resistance unit that is configured, the second resistance unit that is set in series with the light-emitting element and that is set to the second resistance value, and the first and second resistance units And a switch section for passing a current from the light emitting element, wherein the selection circuit is configured to cause the current from the light emitting element to flow to one of the first and second resistance sections according to the light emission signal. It is characterized by controlling the part.

  The light-emitting element drive circuit according to claim 6 of the present invention is the light-emitting element drive circuit according to claim 4, wherein the resistance circuit is set to the first resistance value connected in series to the light-emitting element. Between the first and second resistance portions and the light emitting element, the second resistance portion set to the second resistance value connected in series to the light emitting element, and the second resistance portion. And a switch unit for passing a current from the light emitting element to one selected between the first and second resistance units, and the selection circuit is configured to remove the light emitting element from the light emitting element according to the light emission signal. And the error amplifier is controlled by the selection circuit so that the current from the light emitting element flows through the first or second resistance unit. Connection portion between the second resistance portion and the switch portion And generates a signal level of a signal corresponding to the voltage difference between the feedback voltage and the reference potential generated.

  The light-emitting element drive circuit according to claim 7 of the present invention is the light-emitting element drive circuit according to claim 4, wherein the resistance circuit is set to the first resistance value connected in series to the light-emitting element. The first resistance section, the second resistance section set to the second resistance value connected in series to the light emitting element, and the light emitting element side of the first and second resistance sections A switch unit that is connected to an end on the opposite side and that allows the current from the light emitting element to flow through one selected between the first and second resistor units, The switch unit is controlled so that a current from the light emitting element flows to one of the first and second resistance units, and the error amplifier is configured to allow a current from the light emitting element to flow by the selection circuit. The first or second resistance portion and the light emitting element And generates a signal level of a signal corresponding to the voltage difference between the feedback voltage and the reference potential generated in connection part.

  The light-emitting element drive circuit according to claim 8 of the present invention is the light-emitting element drive circuit according to claim 3, wherein the resistor circuit includes a third resistor portion having one end connected in series to the light-emitting element. And a fourth resistor connected in series to the other end of the third resistor, and a switch that short-circuits the fourth resistor, the fourth resistor being in a non-short-circuited state The first resistance value, and the fourth resistance portion becomes the second resistance value when the fourth resistance portion is in a short-circuited state. The switch unit is controlled so as to be in a short circuit state.

  The light-emitting element drive circuit according to claim 9 of the present invention is the light-emitting element drive circuit according to claim 4, wherein the resistor circuit includes a third resistor portion having one end connected in series to the light-emitting element. And a fourth resistor connected in series to the other end of the third resistor, and a switch that short-circuits the fourth resistor, the fourth resistor being in a non-short-circuited state The first resistance value, and the fourth resistance portion becomes the second resistance value when the fourth resistance portion is in a short-circuited state. The switch unit is controlled so as to be in a short-circuit state, and the error amplifier is a signal corresponding to a voltage difference between the feedback potential and the reference potential generated at a connection portion between the third resistor unit and the light emitting element. A level signal is generated.

  The light emitting element drive circuit according to claim 10 of the present invention is the light emitting element drive circuit according to claim 2, wherein the current supply circuit is connected to the light emitting element between the first and second currents. A current source circuit connected in series with the light emitting element, configured to select a current to flow; and the selection circuit selects a current that the current source circuit passes through the light emitting element according to the light emission signal. The control circuit controls the DC power supply voltage in accordance with a potential generated at a connection portion between the current source circuit and the light emitting element.

  The light-emitting element drive circuit according to claim 11 of the present invention is the light-emitting element drive circuit according to claim 10, wherein the current supply circuit is further provided at a connection portion between the current source circuit and the light-emitting element. An error amplifier that generates a signal having a signal level corresponding to a difference voltage between the generated feedback potential and a reference potential, and the control circuit determines the feedback potential according to the signal level of the signal generated by the error amplifier. The DC power supply voltage is controlled so as to be equal to the reference potential.

  The light emitting element driving circuit according to claim 12 of the present invention is the light emitting element driving circuit according to claim 10, wherein the current source circuit is connected in series to the light emitting element to supply the first current. A first current source for flowing, and a second current source for connecting the light emitting element in series to flow the second current, and the selection circuit includes the first and the first according to the light emission signal. One of the two current source units is activated.

  The light-emitting element drive circuit according to claim 13 of the present invention is the light-emitting element drive circuit according to claim 11, wherein the current source circuit is connected in series to the light-emitting element to supply the first current. A first current source for flowing, and a second current source for connecting the light emitting element in series to flow the second current, and the selection circuit includes the first and the first according to the light emission signal. One of the two current source units is activated, and the error amplifier has the feedback potential generated at a connection portion between the first or second current source unit activated by the selection circuit and the light emitting element. A signal having a signal level corresponding to a voltage difference from the reference potential is generated.

  A light-emitting element drive circuit according to claim 14 of the present invention is the light-emitting element drive circuit according to any one of claims 1 to 13, wherein the first circuit has a voltage value higher than an input voltage. It is a booster circuit that generates a DC power supply voltage.

  A light emitting element drive circuit according to claim 15 of the present invention is the light emitting element drive circuit according to any one of claims 1 to 13, wherein the first circuit has a voltage value lower than an input voltage. The step-down circuit generates a DC power supply voltage.

  According to a preferred embodiment of the present invention, the first circuit is not activated by the light emission signal, but by a separately provided activation signal, the minute current (the first current is set to such a value that the light emitting element does not emit light). The first circuit is operated in advance so that a current is supplied to the light emitting element, and the current flowing through the light emitting element by the light emission signal is a current value sufficient for light emission of the light emitting element from the minute current. Can be switched to the light emission current (second current) set to, so that the start-up delay time of the first circuit (power supply circuit) does not occur, and it is sufficient for light emission of the light emitting element from the input of the light emission signal The delay time until the current having the current value flows to the light emitting element can be shortened.

(Embodiment 1)
Hereinafter, a light-emitting element driving circuit according to Embodiment 1 of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an example of a circuit configuration of a light emitting element driving circuit according to Embodiment 1 of the present invention.

  This light emitting element driving circuit has a current lower than the current value emitted by the LED 3 in accordance with the power supply circuit 2 which is a first circuit for supplying a controllable DC power supply voltage Vo to the LED (light emitting element) 3 and the light emission signal SL. And a second circuit 4 that controls the DC power supply voltage Vo so that one of the first current Id1 set to a value and the second current Id2 set to a current value emitted by the LED 3 flows to the LED 3. .

  Here, the second circuit 4 includes a current supply circuit 5 for causing the LED 3 to pass one of the currents selected between the first current Id1 and the second current Id2, and the current supply circuit 5 according to the light emission signal SL. The logic circuit 6 which is a selection circuit that selects the current flowing through the LED 3 between the first current Id1 and the second current Id2, and the first current Id1 or the second current Id2 selected by the logic circuit 6 And a control circuit 7 that controls the DC power supply voltage Vo so as to flow through the LED 3.

  Further, here, the current supply circuit 5 includes a resistance circuit 8 connected in series to the LED 3. The resistance circuit 8 has a resistance value between a first resistance value set so that the first current Id1 flows in the LED 3 and a second resistance value set so that the second current Id2 flows in the LED 3. It is configured to allow selection. The logic circuit 6 realizes the above-described function by selecting the resistance value of the resistor circuit 8 in accordance with the light emission signal SL. The control circuit 7 realizes the above-described function by controlling the DC power supply voltage Vo according to the potential (voltage) generated in the resistance circuit 8 when the current from the LED 3 flows to the resistance circuit 8.

  Further, here, the current supply circuit 5 is a signal corresponding to the voltage difference between the feedback potential (feedback voltage) generated in the resistor circuit 8 and the reference potential (reference voltage Vr) when the current from the LED 3 flows to the resistor circuit 8. An error amplifier 9 for generating a level (voltage value) error signal is provided, and the control circuit 7 makes the feedback potential generated in the resistor circuit 8 equal to the reference potential in accordance with the signal level of the error signal. As described above, the above-described function is realized by controlling the DC power supply voltage Vo.

  Hereinafter, details of the light emitting element driving circuit according to the first embodiment will be described. In FIG. 1, an input DC power supply 1 generates an input DC voltage Vin. The power supply circuit 2 that is the first circuit generates a DC power supply voltage Vo from the input DC voltage Vin. In FIG. 1, as an example of a power supply circuit 2, an inductor 10 having one end connected to an input DC power supply 1, a main switch (N-type power MOS transistor) 11 having a drain connected to the other end of the inductor 10, an inductor 10 and a main switch 11 is a step-up DC-DC converter comprising a diode 12 having an anode connected to a connection point 11 and a cathode connected to a terminal A, and a capacitor 13 having one end connected to the connection point between the diode 12 and the terminal A and the other end grounded. Booster circuit). This step-up DC-DC converter generates a voltage Vo having a higher voltage value from the input DC voltage Vin. Further, here, the control circuit 7 is connected to the gate of the main switch 11, and the control circuit 7 controls the on / off operation of the main switch 11 to adjust the DC power supply voltage Vo.

  The step-up operation of the step-up DC-DC converter will be described in detail. The inductor 10 repeatedly stores and discharges energy as the main switch 11 is repeatedly turned on and off. The energy release of the inductor 10 charges the capacitor 13 via the diode 12. The voltage (DC power supply voltage) Vo generated in the capacitor 13 is a boost expressed as “Vo = Vi / (1−δ)”, where the duty ratio δ is the ratio of the ON time in one switching cycle of the main switch 11. This is a DC voltage, and this voltage Vo is supplied to the LED 3 as a load via the terminal A. As is apparent from the above equation, the DC power supply voltage Vo can be controlled by adjusting the duty ratio δ.

  In the LED 3 that is a light emitting element, the DC power supply voltage Vo from the power supply circuit 2 is applied to the anode via the terminal A. The cathode of LED 3 is connected to the drains of NMOS transistor 14 and NMOS transistor 15 via terminal K. The source of the NMOS transistor 14 is grounded via a resistor 16, and the source of the NMOS transistor 15 is grounded via a resistor 17.

  Here, the NMOS transistors 14 and 15 connected in series to the LED 3, the resistor (first resistance unit) 16 connected in series to the LED 3 via the NMOS transistor 14, and the series connected to the LED 3 via the NMOS transistor 15. The resistor (second resistor unit) 17 constitutes the resistor circuit 8. In addition, the NMOS transistor 14 and the NMOS transistor 15 constitute a switch unit that allows the current from the LED 3 to flow through one selected between the resistor 16 and the resistor 17. Here, the switch part is arrange | positioned between the resistor 16 and the resistor 17, and LED3.

  In the resistor circuit 8 connected in series to the LED 3, the resistor 16 has a resistance value (first resistance value) such that a current (first current Id 1) having a minute current value that does not cause the LED 3 to emit light flows through the LED 3. R16 is set, and the resistor 17 has a resistance value (second resistance value) R17 so that a current (second current Id2) having a current value necessary for the light emission amount required for the LED 3 flows to the LED 3. Is set. That is, R17 <R16.

  The resistance circuit 8 is configured to be able to select a resistance value between the resistance value R16 and the resistance value R17 by controlling on / off of the NMOS transistors 14 and 15 (switch unit).

  The logic circuit 6 serving as a selection circuit includes an XOR gate 18 and an AND gate 19, and receives a logic level start signal SS and a light emission signal SL. The XOR gate 18 calculates an exclusive OR of the activation signal SS and the light emission signal SL. The calculated value is applied to the gate of the NMOS transistor 14. The AND gate 19 calculates the logical product of the activation signal SS and the light emission signal SL. The calculated value is applied to the gate of the NMOS transistor 15.

  When the activation signal SS is at a high level and the light emission signal SL is at a low level, the logic circuit 6 turns on the NMOS transistor 14 and turns off the NMOS transistor 15, and the current from the LED 3 flows to the resistor 16 through the NMOS transistor 14. When both the start signal SS and the light emission signal SL are at the high level, the NMOS transistor 14 is turned off and the NMOS transistor 15 is turned on so that the current from the LED 3 flows to the resistor 17 via the NMOS transistor 15. That is, the logic circuit 6 controls the NMOS transistors 14 and 15 (switch unit) so that the current from the LED 3 flows to one of the resistor 16 and the resistor 17 in accordance with the light emission signal SL. As described above, the logic circuit 6 selects the resistance value of the resistor circuit 8 by controlling the NMOS transistors 14 and 15 (switch unit) according to the light emission signal SL.

  In the error amplifier 9, voltages generated in the resistor 16 and the resistor 17 are applied to the negative input terminal (feedback terminal), respectively, and the higher voltage (feedback voltage) of them and the reference voltage Vr generated by the reference voltage source 20. An error signal is generated by amplifying the difference voltage between the two. The activation signal SS is input to the error amplifier 9 as an enable signal. That is, the error amplifier 9 operates when the activation signal SS is at a high level.

  The control circuit 7 controls the on / off operation of the main switch 11 of the power supply circuit 2 so that the feedback voltage and the reference voltage Vr become equal to each other in accordance with the voltage value of the error signal from the error amplifier 9. The voltage Vo is adjusted so that the first current Id1 or the second current Id2 flows through the LED 3. Specifically, as described above, the DC power supply voltage Vo can be controlled by adjusting the duty ratio δ, and the control circuit 7 sets the duty ratio δ according to the voltage value of the error signal from the error amplifier 9. Thus, the on / off operation of the main switch 11 is controlled. Further, the activation signal SS is input to the control circuit 7 as an enable signal. That is, the control circuit 7 operates when the activation signal SS is at a high level.

  Next, the operation of the light emitting element driving circuit according to the first embodiment will be described. FIG. 2 is an operation waveform diagram for explaining an example of the operation of the light emitting element driving circuit according to the first embodiment of the present invention. From the top, the start signal SS, the light emission signal SL, the gate voltage of the NMOS transistor 14, The gate voltage of the NMOS transistor 15, the voltage applied to the feedback terminal of the error amplifier 9 from the connection between the resistor 16 and the NMOS transistor 14 (voltage generated at the resistor 16), and the error from the connection between the resistor 17 and the NMOS transistor 15. The voltage applied to the feedback terminal of the amplifier 9 (voltage generated in the resistor 17), the DC power supply voltage Vo, and the activation waveforms of the current ILED flowing through the LED 3 are shown.

  Before the time t0, since the activation signal SS is at a low level, the operation of the control circuit 7 and the error amplifier 9 is stopped, and the light emission signal SL is also at a low level, so that both the NMOS transistors 14 and 15 are off. is doing.

  When the activation signal SS is input (at a high level) at time t0, the logic circuit 6 turns on the NMOS transistor 14 and the power supply circuit 2 is activated in a state where the resistor 16 is connected to the cathode of the LED 3. The power supply voltage Vo increases. When the DC power supply voltage Vo rises, the current ILED flowing through the LED 3 flows to the resistor 16 via the NMOS transistor 14, and the voltage generated at the resistor 16 rises from the ground potential. The increase continues until time t1 when it becomes equal to the reference voltage Vr. The error amplifier 9 is an error signal having a voltage value corresponding to the difference voltage between the feedback voltage generated at the connection between the resistor 16 and the NMOS transistor 14 through which the current from the LED 3 flows by the logic circuit 6 and the NMOS transistor 14. Is generated. The control circuit 7 controls the on / off operation of the main switch 11 by adjusting the duty ratio δ so that the feedback voltage becomes equal to the reference voltage Vr.

  At time t1, when the voltage generated in the resistor 16 having the resistance value R16 becomes equal to the reference voltage Vr, the first current Id1 flows through the LED 3. The current value of the first current Id1 is “Vr / R16”. As described above, the resistance value R16 is set so that the first current Id1 is at a minute level that does not cause the LED 3 to emit light. When the forward voltage of the LED 3 at the first current Id1 is VF1, the DC power supply voltage Vo is adjusted to a voltage “VF1 + Vr” obtained by adding the reference voltage Vr to the forward voltage VF1. At this time, only a very small current that does not cause the LED 3 to emit light flows, and thus the LED 3 does not emit light.

  Thereafter, when the light emission signal SL is input at time t2 (when it becomes high level), the logic circuit 6 turns off the NMOS transistor 14 and turns on the NMOS transistor 15 to change the resistance connected to the LED 3 from the resistance 16 to the resistance 16. Switch to 17. At this time, the voltage generated in the resistor 16 is lowered from the reference voltage Vr and fixed to the ground potential because the NMOS switch 14 is turned off. On the other hand, since the resistance value R17 of the resistor 17 is smaller than the resistance value R16 of the resistor 16, the voltage generated in the resistor 17 is lower than the reference voltage Vr. Since the voltages at the two negative input terminals (feedback terminals) of the error amplifier 9 are both lower than the reference voltage Vr, the control circuit 7 controls the main switch 11 to be turned on / off with a larger duty ratio, and the DC power supply voltage Vo. To raise. The increase continues until time t3 when the voltage generated in the resistor 17 becomes equal to the reference voltage Vr. The error amplifier 9 generates an error signal having a voltage value corresponding to the difference voltage between the feedback voltage generated at the connection between the resistor 17 and the NMOS transistor 15 through which the current from the LED 3 flows by the logic circuit 6 and the NMOS transistor 15 and the reference voltage Vr. Generate. The control circuit 7 controls the on / off operation of the main switch 11 by adjusting the duty ratio δ so that the feedback voltage becomes equal to the reference voltage Vr.

  When the voltage generated in the resistor 17 having the resistance value R17 becomes equal to the reference voltage Vr at time t3, the second current Id2 flows through the LED3. The current value of the second current Id2 is “Vr / R17”. As described above, the resistance value R17 is set so that the second current Id2 is at a level at which the LED 3 can sufficiently emit light. When the forward voltage of the LED 3 at the second current Id2 (forward voltage when the LED 3 emits light) is VF2, the DC power supply voltage Vo is adjusted to “VF2 + Vr”.

  Here, when the operation waveform of the light emitting element driving circuit according to the first embodiment and the operation waveform of the conventional light emitting element driving circuit described with reference to FIG. 15 are compared, as shown in FIG. In the light emitting element driving circuit according to FIG. 1, the time t1 to the time t2 is the delay time Tr1 from when the light emission signal SL is input until the LED emits light, whereas in the conventional light emitting element driving circuit, the time is The delay time Tr2 is from t1 to time t3. This is because, in the light emitting element driving circuit according to the first embodiment, the boosting operation when the light emission signal SL is input is performed by the forward voltage VF1 at the first current Id1 and the forward voltage VF2 at the second current Id2. However, in the conventional light emitting element driving circuit, the voltage is increased from the input DC voltage Vin to the voltage “VF1 + Vr” obtained by adding the reference voltage Vr to the forward voltage VF2. This is necessary.

  According to the first embodiment, the delay time from when the light emission signal is input to when the LED emits light can be shortened to several tens of microseconds, and the light emission loss can be reduced. In the conventional light emitting element driving circuit, the delay time is usually about several hundreds of microseconds. For example, when the light emission time T is 5 ms and the delay time Tr2 is 100 μs, a light emission loss of 20% occurs. On the other hand, in the light emitting element driving circuit according to the first embodiment, when the light emission time is 5 ms and the delay time Tr1 is 40 μs, the light emission loss is reduced to 8%.

  Here, the case where the power supply circuit 2 is a booster circuit has been described. This is because it is assumed that the battery voltage is lower than the forward voltage of the light emitting element. In the case of a battery voltage sufficiently higher than the forward voltage of the light emitting element, a step-down circuit is used. FIG. 4 shows an example of a circuit configuration of a light-emitting element driving circuit using a step-down power supply circuit. In FIG. 4, as an example of the power supply circuit (voltage stepdown circuit) 2, a main switch (P-type power MOS transistor) 21 whose source is connected to the input DC power supply 1, a cathode is connected to the drain of the main switch 21, and an anode is grounded. A diode 22 having one end connected to the connection point between the main switch 21 and the diode 22 and the other end connected to the terminal A, and a capacitor having one end connected to the connection point between the inductor 23 and the terminal A and the other end grounded A step-down DC-DC converter composed of 24 is shown. This step-down DC-DC converter generates a voltage (DC power supply voltage) Vo having a voltage value lower than the input DC voltage Vin.

  Although the case where the logic circuit 6 is configured by the XOR gate 18 and the AND gate 19 has been described here, of course, the configuration of the logic circuit 6 is not limited to this, and the NMOS transistor 14, Any configuration may be used as long as one of 15 is turned on. For example, as shown in FIG. 5, an AND gate 19 that inputs the activation signal SS and the light emission signal SL and a composite AND gate 25 that inputs the signal (NOT signal) obtained by inverting the activation signal SS and the light emission signal SL are configured. May be.

(Embodiment 2)
Hereinafter, a light-emitting element driving circuit according to Embodiment 2 of the present invention will be described with reference to the drawings. FIG. 6 is a diagram showing an example of a circuit configuration of the light emitting element driving circuit according to Embodiment 2 of the present invention. However, members corresponding to those described in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  This light emitting element driving circuit is different from the light emitting element driving circuit according to the first embodiment described above only in the configuration of the current supply circuit 5, and other configurations and operations are the same as those of the light emitting element driving circuit according to the first embodiment described above. It is the same.

  Here, the current supply circuit 5 includes a current source circuit 26 connected in series to the LED 3. The current source circuit 26 is configured to generate a current flowing through the LED 3 between the first current Id1 set to a lower current value than the current value emitted from the LED 3 and the second current Id2 set to the current value emitted from the LED 3. It is configured to allow selection. The logic circuit 6 selects the current that the current source circuit 26 flows to the LED 3 in accordance with the light emission signal SL, so that the current that is supplied to the LED 3 by the current supply circuit 5 according to the light emission signal SL described in the first embodiment. Is selected between the first current Id1 and the second current Id2. The control circuit 7 controls the DC power supply voltage Vo in accordance with the potential generated at the connection portion (terminal K) between the current source circuit 26 and the LED 3, so that the logic circuit 6 described in the first embodiment is used. A function of controlling the DC power supply voltage Vo so that the selected first current Id1 or second current Id2 flows through the LED 3 is realized.

  The error amplifier 9 included in the current supply circuit 5 has a difference voltage between a feedback potential (feedback voltage) generated at a connection portion (terminal K) between the current source circuit 26 and the LED 3 and a reference potential (reference voltage Vr). An error signal having a corresponding signal level (voltage value) is generated. Here, the error amplifier 9 has a negative input terminal (feedback terminal) reduced to one as compared with the first embodiment described above.

  Hereinafter, details of the light emitting element driving circuit according to the second embodiment will be described. In FIG. 6, the cathode of the LED 3 that is a light emitting element is connected to a constant current source 27 and a constant current source 28 via a terminal K.

  Here, the LED 3 is connected in series, and a constant current source (first current source unit) 27 for supplying a current (first current Id1) with a minute current value that does not cause the LED 3 to emit light, and the LED 3 are connected in series. Thus, a constant current source (first current source unit) 28 that supplies a current (second current Id2) having a current value necessary for the amount of light emission required for the LED 3 constitutes a current source circuit 26.

  The constant current source 27 is turned on / off according to an exclusive OR operation result of the XOR gate 18 included in the logic circuit 6. The constant current source 28 is turned on / off according to the result of the logical product of the AND gate 19 included in the logic circuit 6.

  When the activation signal SS is at a high level and the light emission signal SL is at a low level, the logic circuit 6 turns on the constant current source 27 and turns off the constant current source 28 to set the current flowing through the LED 3 to the first current Id1. Further, when both the activation signal SS and the light emission signal SL are at a high level, the constant current source 27 is turned off and the constant current source 28 is turned on, so that the current flowing through the LED 3 becomes the second current Id2. That is, the logic circuit 6 activates one of the constant current source 27 and the constant current source 28 in accordance with the light emission signal SL. Thus, the logic circuit 6 selects the current that the current source circuit 26 passes through the LED 3 in accordance with the light emission signal SL.

  In the error amplifier 9, a voltage (feedback voltage) generated at a connection portion between the terminal K, the constant current source 27 and the constant current source 28, that is, a voltage at the terminal K is applied to a negative input terminal (feedback terminal). An error signal is generated by amplifying the difference voltage with respect to the reference voltage Vr generated by 20.

  As described above, in the second embodiment, switching between the first current (micro current) Id1 and the second current (light emission current) Id2 is performed by the constant current sources 27 and 28, so that the error amplifier The number of negative input terminals is reduced to one.

  Subsequently, an operation of the light emitting element driving circuit according to the second embodiment will be described. FIG. 7 is an operation waveform diagram for explaining an example of the operation of the light emitting element driving circuit according to the second embodiment of the present invention. In order from the top, the start signal SS, the light emission signal SL, and the calculated values of the XOR gate 18 ( XOR output), the calculated value of the AND gate 19 (AND output), the voltage generated at the connection between the terminal K, the constant current source 27 and the constant current source 28 (feedback voltage of the error amplifier 9), the DC power supply voltage Vo, and the LED 3 The starting waveform of each flowing current ILED is shown.

  Prior to time t0, since the activation signal SS is at a low level, the operation of the control circuit 7 and the error amplifier 9 is stopped, and the light emission signal SL is also at a low level. Inactive state (off state).

  When the activation signal SS is input (at a high level) at time t0, the logic circuit 6 turns on the constant current source 27 and the power supply circuit 2 is activated in a state where the constant current source 27 is activated. The power supply voltage Vo increases. This increase in the DC power supply voltage Vo continues until time t1 when the feedback voltage of the error amplifier 9 rises from the ground potential and becomes equal to the reference voltage Vr. The error amplifier 9 generates an error signal having a voltage value corresponding to the difference voltage between the feedback voltage generated at the connection portion (terminal K) between the constant current source 27 activated by the logic circuit 6 and the LED 3 (terminal K) and the reference voltage Vr. To do. The control circuit 7 controls the on / off operation of the main switch 11 by adjusting the duty ratio δ so that the feedback voltage becomes equal to the reference voltage Vr.

  When the feedback voltage of the error amplifier 9 becomes equal to the reference voltage Vr at time t1, the first current Id1 flows through the LED 3. As described above, the current value of the first current Id1 is set to a minute level that does not cause the LED 3 to emit light. When the forward voltage of the LED 3 at the first current Id1 is VF1, the DC power supply voltage Vo is adjusted to a voltage “VF1 + Vr” obtained by adding the reference voltage Vr to the forward voltage VF1. At this time, only a very small current that does not cause the LED 3 to emit light flows, and thus the LED 3 does not emit light.

  Thereafter, when the light emission signal SL is input (at a high level) at time t2, the logic circuit turns off the constant current source 27 and turns on the constant current source 28 to activate the constant current source 28. At that time, since the forward voltage VF of the LED 3 becomes large, the feedback voltage of the error amplifier 9 becomes lower than the reference voltage Vr, and the control circuit 7 controls the main switch 11 to be turned on / off with a larger duty ratio, and the direct current The power supply voltage Vo is increased. The increase continues until time t3 when the feedback voltage of the error amplifier 9 becomes equal to the reference voltage Vr. The error amplifier 9 generates an error signal having a voltage value corresponding to the difference voltage between the feedback voltage generated at the connection portion (terminal K) between the constant current source 28 activated by the logic circuit 6 and the LED 3 (terminal K) and the reference voltage Vr. To do. The control circuit 7 controls the on / off operation of the main switch 11 by adjusting the duty ratio δ so that the feedback voltage becomes equal to the reference voltage Vr.

  When the feedback voltage of the error amplifier 9 becomes equal to the reference voltage Vr at time t3, the second current Id2 flows through the LED3. As described above, the current value of the second current Id2 is set to a level that causes the LED 3 to emit light sufficiently. When the forward voltage of the LED 3 at the second current Id2 is VF2, the DC power supply voltage Vo is adjusted to “VF2 + Vr”.

  According to the second embodiment, as in the first embodiment described above, the step-up operation when the light emission signal SL is input is performed by the forward voltage VF1 at the first current Id1 and the forward current at the second current Id2. Since only the voltage difference dVF = VF2−VF1 with respect to the direction voltage VF2 is required, the delay time Tr from when the light emission signal is input to when the LED emits light can be shortened. Therefore, a section where an appropriate light emission amount cannot be obtained can be shortened, and a light emission loss can be reduced.

  Although the case where the power supply circuit 2 is a booster circuit has been described here, a step-down circuit may be used as in the first embodiment. FIG. 8 illustrates an example of a circuit configuration of a light-emitting element driving circuit using a step-down power supply circuit. The step-down DC-DC converter shown in FIG. 8 has the same configuration as the step-down DC-DC converter shown in FIG.

  Although the case where the logic circuit 6 is configured by the XOR gate 18 and the AND gate 19 has been described here, of course, the configuration of the logic circuit 6 is not limited to this, and the constant current source 27 is set according to the light emission signal SL. , 28 may be activated. For example, as described in the first embodiment, the AND gate that inputs the activation signal SS and the light emission signal SL, and the composite AND that inputs the signal (NOT signal) obtained by inverting the activation signal SS and the light emission signal SL. You may comprise by a gate etc.

(Embodiment 3)
Hereinafter, a light-emitting element driving circuit according to Embodiment 3 of the present invention will be described with reference to the drawings. FIG. 9 is a diagram showing an example of a circuit configuration of the light emitting element driving circuit according to Embodiment 3 of the present invention. However, members corresponding to those described in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  This light emitting element driving circuit is different from the light emitting element driving circuit according to the first embodiment described above only in the configuration of the current supply circuit 5, and other configurations and operations are the same as those of the light emitting element driving circuit according to the first embodiment described above. It is the same.

  That is, the NMOS transistor 14 and the NMOS transistor 15 constituting the switch unit for passing the current from the LED 3 to one selected between the resistor 16 and the resistor 17 are the ends of the resistor 16 and the resistor 17 on the side opposite to the LED 3 side. Is arranged. Further, the error amplifier 9 has a signal level (voltage value) corresponding to a difference voltage between a feedback potential (feedback voltage) generated at a connection portion (terminal K) between the resistor circuit 8 and the LED 3 and a reference potential (reference voltage Vr). The error signal is generated. Here, the error amplifier 9 has a negative input terminal (feedback terminal) reduced to one as compared with the first embodiment described above.

  Hereinafter, details of the light emitting element driving circuit according to the third embodiment will be described. In FIG. 9, the cathode of the LED 3 is connected to a resistor (first resistor portion) 16 and a resistor (second resistor portion) 17 via a terminal K. The drain of the NMOS transistor 14 is connected to the other end of the resistor 16 whose one end is connected to the LED 3, and the source of the NMOS transistor 14 is grounded. The other end of the resistor 17 whose one end is connected to the LED 3 is connected to the drain of the NMOS transistor 15, and the source of the NMOS transistor 15 is grounded. As in the first embodiment described above, the resistor 16 has a resistance value (first resistance value) such that a current having a very small current value (first current Id1) that does not cause the LED 3 to emit light flows through the LED 3. R16 is set, and the resistor 17 has a resistance value (second resistance value) R17 so that a current (second current Id2) having a current value necessary for the light emission amount required for the LED 3 flows to the LED 3. Is set.

  In the error amplifier 9, a voltage (feedback voltage) generated at a connection portion between the terminal K, the resistor 16 and the resistor 17, that is, a voltage generated at the terminal K is applied to the negative input terminal (feedback terminal). An error signal is generated by amplifying the difference voltage from the generated reference voltage Vr.

  As described above, in the third embodiment, the number of the negative input terminals of the error amplifier 9 is reduced to one by changing the arrangement of the NMOS transistors 14 and 15 (switch unit) that switches the resistance value of the resistance circuit 8. ing.

  Next, the operation of the light emitting element driving circuit according to Embodiment 3 will be described. FIG. 10 is an operation waveform diagram for explaining an example of the operation of the light-emitting element driving circuit according to Embodiment 3 of the present invention. Starting signal SS, light-emitting signal SL, gate voltage of NMOS transistor 14, The respective waveforms of the gate voltage of the NMOS transistor 15, the voltage generated at the connection between the terminal K, the resistor 16 and the resistor 17 (feedback voltage of the error amplifier 9), the DC power supply voltage Vo, and the current ILED flowing through the LED 3 are shown.

  Before the time t0, since the activation signal SS is at a low level, the operation of the control circuit 7 and the error amplifier 9 is stopped, and the light emission signal SL is also at a low level, so that both the NMOS transistors 14 and 15 are off. is doing.

  When the activation signal SS is input (at a high level) at time t0, the logic circuit 6 turns on the NMOS transistor 14 and the power supply circuit 2 is activated in a state where the LED 3 is grounded via the resistor 16, and the DC circuit is activated. The power supply voltage Vo increases. When the DC power supply voltage Vo rises, the current ILED flowing through the LED 3 flows through the resistor 16, and the voltage generated at the resistor 16 rises from the ground potential. That is, the feedback voltage of the error amplifier 9 (the voltage at the terminal K) rises from the ground potential. The increase continues until time t1 when it becomes equal to the reference voltage Vr. The error amplifier 9 has a voltage value corresponding to the difference voltage between the feedback voltage generated at the connection portion (terminal K) between the resistor 16 and the LED 3 through which the current from the LED 3 flows by the logic circuit 6 and the reference voltage Vr. An error signal is generated. The control circuit 7 controls the on / off operation of the main switch 11 by adjusting the duty ratio δ so that the feedback voltage becomes equal to the reference voltage Vr.

  At time t1, when the voltage generated in the resistor 16 having the resistance value R16, that is, the feedback voltage of the error amplifier 9, becomes equal to the reference voltage Vr, the first current Id1 flows through the LED 3. The current value of the first current Id1 is “Vr / R16”. As described above, the resistance value R16 is set so that the first current Id1 is at a minute level that does not cause the LED 3 to emit light. When the forward voltage of the LED 3 at the first current Id1 is VF1, the DC power supply voltage Vo is adjusted to a voltage “VF1 + Vr” obtained by adding the reference voltage Vr to the forward voltage VF1. At this time, only a very small current that does not cause the LED 3 to emit light flows, and thus the LED 3 does not emit light.

  Thereafter, when the light emission signal SL is input (at a high level) at time t2, the logic circuit 6 turns off the NMOS transistor 14 and turns on the NMOS transistor 15 so that the resistance through which the current ILED from the LED 3 flows is the resistance 16. To resistor 17. At this time, since the resistance value R17 of the resistor 17 is smaller than the resistance value R16 of the resistor 16, the voltage generated in the resistor 17 becomes lower than the reference voltage Vr. That is, since the feedback voltage of the error amplifier 9 becomes lower than the reference voltage Vr, the control circuit 7 controls the main switch 11 to be turned on / off with a larger duty ratio to increase the DC power supply voltage Vo. The increase continues until time t3 when the feedback voltage of the error amplifier 9 (the voltage at the terminal K) becomes equal to the reference voltage Vr. The error amplifier 9 has a voltage value corresponding to the difference voltage between the feedback voltage generated at the connection portion (terminal K) between the resistor 17 and the LED 3 through which the current from the LED 3 flows by the logic circuit 6 and the reference voltage Vr. An error signal is generated. The control circuit 7 controls the on / off operation of the main switch 11 by adjusting the duty ratio δ so that the feedback voltage becomes equal to the reference voltage Vr.

  When the feedback voltage of the error amplifier 9 becomes equal to the reference voltage Vr at time t3, the second current Id2 flows through the LED3. The current value of the second current Id2 is “Vr / R17”. As described above, the resistance value R17 is set so that the second current Id2 is at a level at which the LED 3 can sufficiently emit light. When the forward voltage of the LED 3 at the second current Id2 is VF2, the DC power supply voltage Vo is adjusted to “VF2 + Vr”.

  According to the third embodiment, as in the first embodiment described above, the step-up operation when the light emission signal SL is input is performed in the forward voltage VF1 at the first current Id1 and the forward current at the second current Id2. Since only the voltage difference dVF = VF2−VF1 with respect to the direction voltage VF2 is required, the delay time Tr from when the light emission signal is input to when the LED emits light can be shortened. Therefore, a section where an appropriate light emission amount cannot be obtained can be shortened, and a light emission loss can be reduced.

  Although the case where the power supply circuit 2 is a booster circuit has been described here, a step-down circuit may be used as in the first embodiment. FIG. 11 shows an example of a circuit configuration of a light-emitting element driving circuit using a step-down power supply circuit. The step-down DC-DC converter shown in FIG. 11 has the same configuration as the step-down DC-DC converter shown in FIG.

  Although the case where the logic circuit 6 is configured by the XOR gate 18 and the AND gate 19 has been described here, of course, the configuration of the logic circuit 6 is not limited to this, and the NMOS transistor 14, Any configuration may be used as long as one of 15 is turned on. For example, as described in the first embodiment, the AND gate that inputs the activation signal SS and the light emission signal SL, and the composite AND that inputs the signal (NOT signal) obtained by inverting the activation signal SS and the light emission signal SL. You may comprise by a gate etc.

(Embodiment 4)
Hereinafter, a light-emitting element driving circuit according to Embodiment 4 of the present invention will be described with reference to the drawings. FIG. 12 is a diagram showing an example of a circuit configuration of the light emitting element driving circuit according to Embodiment 4 of the present invention. However, members corresponding to those described in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  This light emitting element driving circuit is different from the light emitting element driving circuit according to the first embodiment described above only in the configuration of the current supply circuit 5 and the logic circuit 6, and other configurations and operations are the light emitting elements according to the first embodiment described above. This is the same as the element drive circuit.

  That is, as shown in FIG. 12, the resistance circuit 8 includes a resistor (third resistor portion) 29 having one end connected to the cathode of the LED 3 via the terminal K, and one end connected to the other end of the resistor 29. A resistor (fourth resistor unit) 30 whose end is grounded and an NMOS transistor 31 as a switch unit that short-circuits the resistor 30 are provided. In the resistor circuit 8, the resistance value (first resistance value) when the resistor 30 is in a non-short-circuit state, that is, the combined resistance value of the resistor 29 and the resistor 30, is a current having a very small current value such that the LED 3 does not emit light. 1 current Id1) is set to flow through the LED3. In addition, the resistance value (second resistance value) when the resistor 30 is in a short-circuit state, that is, the resistance value of the resistor 29 is a current value (second current Id2) required for the light emission amount required for the LED 3. It is set to flow through the LED 3.

  The logic circuit 6 as a selection circuit is composed of only the AND gate 32. The AND gate 32 calculates the logical product of the activation signal SS and the light emission signal SL. The calculated value is applied to the gate of the NMOS transistor 31.

  When the start signal SS is at a high level and the light emission signal SL is at a low level, the logic circuit 6 turns off the NMOS transistor 31 and puts the resistor 30 into a non-short circuit state, and when both the start signal SS and the light emission signal SL are at a high level. Then, the NMOS transistor 31 is turned on to make the resistor 30 short-circuited. That is, the logic circuit 6 controls the NMOS transistor 31 (switch unit) so that the resistor 30 is in a non-short circuit state or a short circuit state in accordance with the light emission signal SL. As described above, the logic circuit 6 selects the resistance value of the resistance circuit 8 by controlling the NMOS transistor 31 (switch unit) in accordance with the light emission signal SL.

  The error amplifier 9 has a difference voltage between a feedback potential (feedback voltage) generated at a connection portion between the resistor circuit 8 and the LED 3, that is, a connection portion (terminal K) between the resistor 29 and the LED 3 and a reference potential (reference voltage Vr). An error signal having a corresponding signal level (voltage value) is generated. Here, the error amplifier 9 has a negative input terminal (feedback terminal) reduced to one as compared with the first embodiment described above.

  As described above, in the fourth embodiment, the number of NMOS switches for switching the resistance value of the resistance circuit 8 is reduced to one. Further, the logic circuit 6 is composed of only AND gates, and XOR gates are reduced. Further, the number of the negative input terminals of the error amplifier 9 is reduced to one.

  Subsequently, an operation of the light emitting element driving circuit according to the fourth embodiment will be described. FIG. 13 is an operation waveform diagram for explaining an example of the operation of the light emitting element driving circuit according to Embodiment 4 of the present invention. In order from the top, the start signal SS, the light emission signal SL, the gate voltage of the NMOS transistor 31, The voltage generated at the connection between the terminal K and the resistor 29 (feedback voltage of the error amplifier 9), the DC power supply voltage Vo, and the activation waveforms of the current ILED flowing through the LED 3 are shown.

  Prior to time t0, since the activation signal SS is at a low level, the operation of the control circuit 7 and the error amplifier 9 is stopped, and the NMOS transistor 31 is off.

  When the activation signal SS is input (at a high level) at time t0, the power supply circuit 2 is activated in a state where the resistor 30 is not short-circuited (a state where the resistor 29 is grounded via the resistor 30). The voltage Vo increases. When the DC power supply voltage Vo increases, the current ILED flowing through the LED 3 flows through the resistor 29 and the resistor 30, and the voltage generated at the connection portion between the resistor 29 and the LED 3 increases from the ground potential. That is, the feedback voltage of the error amplifier 9 (the voltage at the terminal K) rises from the ground potential. The increase continues until time t1 when it becomes equal to the reference voltage Vr. The error amplifier 9 generates an error signal having a voltage value corresponding to the difference voltage between the feedback voltage generated at the connection between the resistor 29 and the LED 3 and the reference voltage Vr. The control circuit 7 controls the on / off operation of the main switch 11 by adjusting the duty ratio δ so that the feedback voltage becomes equal to the reference voltage Vr.

  When the voltage at the terminal K becomes equal to the reference voltage Vr at time t1, the first current Id1 flows through the LED 3. The current value of the first current Id1 is “Vr / (R29 + R30)” where the resistance value of the resistor 29 is R29 and the resistance value of the resistor 30 is R30. As described above, the combined resistance value “R29 + R30” of the resistor 29 and the resistor 30 is set so that the first current Id1 is at a minute level that does not cause the LED 3 to emit light. When the forward voltage of the LED 3 at the first current Id1 is VF1, the DC power supply voltage Vo is adjusted to a voltage “VF1 + Vr” obtained by adding the reference voltage Vr to the forward voltage VF1. At this time, only a very small current that does not cause the LED 3 to emit light flows, and thus the LED 3 does not emit light.

  Thereafter, when the light emission signal SL is input at time t <b> 2 (when it becomes high level), the logic circuit 6 turns on the NMOS transistor 31 to short-circuit the resistor 30. That is, the resistor 29 is grounded via the NMOS transistor 31. At this time, since the resistance value R29 of the resistor 29 is smaller than the combined resistance value “R29 + R30” of the resistors 29 and 30, the feedback voltage of the error amplifier 9 becomes lower than the reference voltage Vr. On / off control is performed with a larger duty ratio to increase the DC power supply voltage Vo. The increase continues until time t3 when the voltage at the terminal K becomes equal to the reference voltage Vr.

  When the voltage at the terminal K becomes equal to the reference voltage Vr at time t3, the second current Id2 flows through the LED3. The current value of the second current Id2 is “Vr / R29”. As described above, the resistance value R29 is set so that the second current Id2 is at a level that causes the LED 3 to emit light sufficiently. When the forward voltage of the LED 3 at the second current Id2 is VF2, the DC power supply voltage Vo is adjusted to “VF2 + Vr”.

  According to the fourth embodiment, as in the first embodiment described above, the step-up operation when the light emission signal SL is input is performed by the forward voltage VF1 at the first current Id1 and the forward current at the second current Id2. Since only the voltage difference dVF = VF2−VF1 with respect to the direction voltage VF2 is required, the delay time Tr from when the light emission signal is input to when the LED emits light can be shortened. Therefore, a section where an appropriate light emission amount cannot be obtained can be shortened, and a light emission loss can be reduced.

  Although the case where the power supply circuit 2 is a booster circuit has been described here, a step-down circuit may be used as in the first embodiment. FIG. 14 shows an example of a circuit configuration of a light-emitting element driving circuit using a step-down power supply circuit. The step-down DC-DC converter shown in FIG. 14 has the same configuration as the step-down DC-DC converter shown in FIG.

  The light emitting element driving circuit according to the present invention reduces the light emission loss by shortening the delay time from the input of the light emission signal until the current having a current value sufficient for light emission of the light emitting element flows into the light emitting element. It is useful for various electronic devices such as a digital still camera that performs imaging by using an LED as a light source by flashing light instantaneously.

The figure which shows an example of the circuit structure of the light emitting element drive circuit which concerns on Embodiment 1 of this invention. Operation waveform diagram for explaining an example of the operation of the light emitting element driving circuit according to the first embodiment of the present invention. Operational comparison diagram between light emitting element driving circuit according to Embodiment 1 of the present invention and a conventional light emitting element driving circuit The figure which shows the other example of the circuit structure of the light emitting element drive circuit which concerns on Embodiment 1 of this invention. The figure which shows the other example of the circuit structure of the light emitting element drive circuit which concerns on Embodiment 1 of this invention. The figure which shows an example of the circuit structure of the light emitting element drive circuit which concerns on Embodiment 2 of this invention. Operation waveform diagram for explaining an example of the operation of the light emitting element driving circuit according to the second embodiment of the present invention. The figure which shows the other example of the circuit structure of the light emitting element drive circuit which concerns on Embodiment 2 of this invention. The figure which shows an example of the circuit structure of the light emitting element drive circuit which concerns on Embodiment 3 of this invention. Operation waveform diagram for explaining an example of the operation of the light emitting element driving circuit according to Embodiment 3 of the present invention. The figure which shows the other example of the circuit structure of the light emitting element drive circuit which concerns on Embodiment 3 of this invention. The figure which shows an example of the circuit structure of the light emitting element drive circuit which concerns on Embodiment 4 of this invention. Operation waveform diagram for explaining an example of the operation of the light emitting element driving circuit according to Embodiment 4 of the present invention. The figure which shows the other example of the circuit structure of the light emitting element drive circuit which concerns on Embodiment 4 of this invention. Circuit diagram of conventional light-emitting element drive circuit Start-up waveform diagram of a digital still camera using a conventional light emitting element driving circuit

Explanation of symbols

1 Input DC power supply 2 Power supply circuit (first circuit)
3 LED (light emitting element)
4 Second Circuit 5 Current Supply Circuit 6 Logic Circuit 7 Control Circuit 8 Resistor Circuit 9 Error Amplifier 10 Inductor 11 Main Switch 12 Diode 13 Capacitor 14, 15 NMOS Transistor 16, 17 Resistor 18 XOR Gate 19 AND Gate 20 Reference Voltage Source 21 Main switch 22 Diode 23 Inductor 24 Capacitor 25 Composite AND gate 26 Current source circuit 27, 28 Constant current source 29, 30 Resistor 31 NMOS transistor 32 AND gate 101 Input DC power supply 102 Boost circuit 103 LED
104 voltage value control circuit 105 current detection circuit 106 detection resistor 107 reference voltage source 108 comparator

Claims (15)

  A first circuit for supplying a controllable DC power supply voltage to the light emitting element, and the first current set to a current value lower than the current value emitted by the light emitting element and the light emitting element emit light according to the light emission signal. And a second circuit that controls the DC power supply voltage so that one of the second currents set to a current value flows through the light emitting element. The second circuit includes:
A current supply circuit configured to flow one current selected between the first and second currents to the light emitting element;
A selection circuit for selecting, between the first and second currents, a current passed through the light emitting element by the current supply circuit according to the light emission signal;
A control circuit for controlling the DC power supply voltage so that the first or second current selected by the selection circuit flows through the light emitting element;
The light emitting element drive circuit according to claim 1, comprising: The current supply circuit has a first resistance value set so that the first current flows in the light emitting element and a second resistance value set so that the second current flows in the light emitting element. Comprising a resistance circuit connected in series to the light emitting element, the resistance value being configured to be selected between,
The selection circuit selects a resistance value of the resistance circuit according to the light emission signal,
3. The light emitting element driving circuit according to claim 2, wherein the control circuit controls the DC power supply voltage in accordance with a potential generated in the resistance circuit when a current from the light emitting element flows in the resistance circuit. . The current supply circuit further includes an error amplifier that generates a signal having a signal level corresponding to a voltage difference between a feedback potential generated in the resistor circuit and a reference potential when a current from the light emitting element flows in the resistor circuit. Equipped,
4. The light emitting device according to claim 3, wherein the control circuit controls the DC power supply voltage so that the feedback potential and the reference potential are equal to each other in accordance with a signal level of a signal generated by the error amplifier. Element drive circuit. The resistance circuit includes a first resistance portion set to the first resistance value connected in series to the light emitting element, and a second resistance value set to the second resistance value connected in series to the light emitting element. And a switch section for passing a current from the light emitting element in one selected between the first and second resistance sections,
4. The light emitting element according to claim 3, wherein the selection circuit controls the switch unit so that a current from the light emitting element flows to one of the first and second resistance units according to the light emission signal. Driving circuit. The resistance circuit includes a first resistance portion set to the first resistance value connected in series to the light emitting element, and a second resistance value set to the second resistance value connected in series to the light emitting element. Current flowing from the light emitting element to one selected between the first resistance part and the first and second resistance parts, arranged between the first and second resistance parts and the light emitting element A switch part,
The selection circuit controls the switch unit so that a current from the light emitting element flows to one of the first and second resistance units according to the light emission signal,
The error amplifier is configured such that a current from the light emitting element is allowed to flow by the selection circuit, and the feedback potential generated at a connection portion between the first or second resistance portion and the switch portion and the reference potential. 5. The light emitting element driving circuit according to claim 4, wherein a signal level signal corresponding to the differential voltage is generated. The resistance circuit includes a first resistance portion set to the first resistance value connected in series to the light emitting element, and a second resistance value set to the second resistance value connected in series to the light emitting element. The light-emitting element is connected to one end selected from the first and second resistance parts and connected to the end of the first and second resistance parts opposite to the light-emitting element side. And a switch part for passing a current of
The selection circuit controls the switch unit so that a current from the light emitting element flows to one of the first and second resistance units according to the light emission signal,
The error amplifier is configured such that the feedback potential generated at a connection portion between the first or second resistance unit and the light emitting element, in which a current from the light emitting element is allowed to flow by the selection circuit, and the reference potential. 5. The light emitting element driving circuit according to claim 4, wherein a signal level signal corresponding to the differential voltage is generated. The resistor circuit includes a third resistor unit having one end connected in series to the light emitting element, a fourth resistor unit connected in series to the other end of the third resistor unit, and the fourth resistor A switch portion that short-circuits the first resistance value when the fourth resistance portion is in a non-short-circuited state, and the second resistance value when the fourth resistance portion is in a short-circuited state,
4. The light emitting element drive circuit according to claim 3, wherein the selection circuit controls the switch unit so that the fourth resistance unit is in a non-short circuit state or a short circuit state in accordance with the light emission signal. The resistor circuit includes a third resistor unit having one end connected in series to the light emitting element, a fourth resistor unit connected in series to the other end of the third resistor unit, and the fourth resistor A switch portion that short-circuits the first resistance value when the fourth resistance portion is in a non-short-circuited state, and the second resistance value when the fourth resistance portion is in a short-circuited state,
The selection circuit controls the switch unit according to the light emission signal so that the fourth resistance unit is in a non-short circuit state or a short circuit state,
The said error amplifier produces | generates the signal of the signal level according to the voltage difference between the said feedback electric potential and the said reference electric potential which generate | occur | produce in the connection part of a said 3rd resistance part and the said light emitting element. 5. The light emitting element drive circuit according to 4. The current supply circuit includes a current source circuit connected in series to the light emitting element, configured to be able to select a current to flow through the light emitting element between the first and second currents,
The selection circuit selects a current that the current source circuit passes through the light emitting element according to the light emission signal,
3. The light emitting element driving circuit according to claim 2, wherein the control circuit controls the DC power supply voltage according to a potential generated at a connection portion between the current source circuit and the light emitting element. The current supply circuit further includes an error amplifier that generates a signal having a signal level corresponding to a difference voltage between a feedback potential generated at a connection portion between the current source circuit and the light emitting element and a reference potential.
11. The light emitting device according to claim 10, wherein the control circuit controls the DC power supply voltage so that the feedback potential and the reference potential are equal to each other according to a signal level of a signal generated by the error amplifier. Element drive circuit. The current source circuit is:
A first current source unit that is connected in series to the light emitting element to flow the first current; and a second current source unit that is connected to the light emitting element in series to flow the second current. ,
11. The light emitting element driving circuit according to claim 10, wherein the selection circuit activates one of the first and second current source units according to the light emission signal. The current source circuit is:
A first current source unit that is connected in series to the light emitting element to flow the first current; and a second current source unit that is connected to the light emitting element in series to flow the second current. ,
The selection circuit activates one of the first and second current source units according to the light emission signal,
The error amplifier has a signal level corresponding to a voltage difference between the feedback potential and the reference potential generated at a connection portion between the first or second current source unit activated by the selection circuit and the light emitting element. The light emitting element driving circuit according to claim 11, wherein the signal is generated.   The light emitting element driving circuit according to claim 1, wherein the first circuit is a booster circuit that generates a DC power supply voltage having a voltage value higher than an input voltage.   The light emitting element drive circuit according to claim 1, wherein the first circuit is a step-down circuit that generates a DC power supply voltage having a voltage value lower than an input voltage.

Images