JP2009118698A - Power circuit for led driving, and method of driving the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power circuit for LED driving that reduces the voltage resistance and size of the circuit by reducing an input voltage to a switch at charging and prevent a current path from going through a step-down regulator at discharging, thus lessening a voltage drop of output caused by the resistance of a transistor of the step-down regulator, and to provide a method of driving the same. <P>SOLUTION: The power circuit includes the step-down regulator 2, and a step-up circuit 3 which charges up with an output voltage VREG from the step-up circuit 3 and supplies a step-up voltage generated by discharging with a power voltage VIN, to an output terminal 4. The step-up circuit 3 has switches SW1 and SW2 connected in series between the output terminal 4 and the step-down regulator 2, switches SW3 and SW4 connected in series between a power supply and the ground, and a capacitor C. The power circuit also includes the current path connecting a power terminal 1 in series to the output terminal 4 via a switch 7 made up of a single MOS transistor (P1). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an LED driving power supply circuit using a booster circuit that reduces a voltage drop due to a resistance component included in a step-down regulator.

  As a conventional LED driving power supply circuit, Patent Document 1 discloses a power supply circuit that suppresses an inrush current and current consumption flowing in the circuit. This LED drive power supply circuit has a configuration in which a step-down regulator is connected in front of the charge pump circuit, and the step-up rate control circuit sets the step-up rate of the charge pump circuit to 1 to realize a short mode. When this is done, the internal path of the charge pump circuit is short-circuited and the first transistor is completely turned on. As a result, an inrush current is generated from the battery voltage of the lithium ion battery to the charge pump circuit. Therefore, the constant current circuit is operated so as to turn on the first transistor slowly. Further, the operation of the oscillation circuit and the operational amplifier is stopped when the short mode is realized.

  FIG. 8 illustrates a configuration in which a step-down regulator is connected to the front stage of a charge pump circuit (see Patent Document 1) in a conventional power supply circuit for driving an LED. The power supply circuit includes a charge pump circuit 102 that boosts the input voltage VIN supplied from the power supply 101 at a set boost rate, a step-down regulator 103 for making the input voltage VIN to the charge pump circuit 102 constant, and a voltage dividing resistor A capacitor C for charging R1, R2 and the output voltage is provided. The step-down regulator 103 includes an operational amplifier 121 that compares the reference voltage (Vref) 105 from the reference voltage source 105 and the feedback voltage from the output terminal 104, and a MOS transistor 122 that has the output of the operational amplifier 121 connected to the gate. ing. A first input (+) of the operational amplifier 121 is connected between voltage dividing resistors R1 and R2 connected between the output terminal 104 and the ground (GND), and a feedback voltage is supplied therefrom. The reference voltage Vref is supplied to the second input (−) of the operational amplifier 121. The input voltage VIN is connected to one end of the MOS transistor 122, the input of the charge pump circuit unit 102 is connected to the other end, and the output of the charge pump circuit unit 102 is connected to the output terminal 104. The capacitor C has one end connected to a connection point between the output of the charge pump circuit 102 and the output terminal 104, and the other end connected to a connection point between the voltage dividing resistors R 1 and R 2 and the first input of the operational amplifier 121. It is connected.

The charge pump circuit unit 102 selectively charges and discharges a boosting capacitor by an on / off operation of a switch provided therein, boosts the input voltage VIN at a set boosting rate, and outputs an output voltage Vout This is a booster circuit. The operational amplifier 121 compares the reference voltage Vref from the reference voltage source 105 with the feedback voltage from the output terminal, and controls the MOS transistor 122 according to the error voltage. Thereby, the step-down regulator 121 adjusts the input voltage VIN, stabilizes it to the target voltage value, and supplies it to the charge pump circuit unit 122. Thus, the difference between the feedback voltage from the output terminal and the reference voltage Vref can be controlled to be zero, and the charge pump circuit unit 122 obtains the output voltage Vout by making the input voltage VIN constant to the target voltage value.
In the LED drive power supply circuit shown in FIG. 8, a voltage drop occurs due to the on-resistance of the charging / discharging switching element, the transistor resistance of the circuit (step-down regulator) connected in the previous stage, and the set step-up ratio. However, there is a problem that a desired output voltage cannot be obtained. In addition, the conventional power supply circuit has a problem that the circuit scale increases as the size of the switching element increases.
JP 2005-318786 A

In the known example shown in Patent Document 1, a step-down voltage is used using a step-down regulator both during charging and discharging. Further, when the boosting rate is 1, it is a short mode in which the power supply voltage is used as it is. In this case, a step-down regulator is used, and the step-down voltage is increased by 1.0 times. When the step-up rate is 1.5 times, the power supply voltage is regulated and the step-down voltage is made 1.5 times. In either case, the inrush current flowing in the circuit and the current consumption are suppressed, but the suppression is not sufficient because it passes through the transistor of the step-down regulator.
The present invention has been made in order to solve the above-described problems. An LED driving power supply circuit in which output impedance is reduced by reducing switching elements (transistors constituting a switch) existing in a discharge path and driving thereof. A method is provided.

  The LED drive power supply circuit of the present invention uses a power supply terminal to which an external power supply is connected, a step-down regulator for stepping down a power supply voltage input from the power supply terminal, and charge / discharge of a capacitive element from the output voltage of the step-down regulator And a booster circuit for generating a boost voltage for driving the LED, and a current path for connecting the power supply terminal and the output terminal of the booster circuit in series via a switch composed of one MOS transistor. It is characterized by. In the step-up mode, the power supply voltage is supplied to the output terminal through the current path by turning on the switch. In the step-up mode, one end of the capacitor is connected to the output end of the step-down regulator. And connecting the other end to the low potential side to charge the capacitive element, connecting one end to the power supply terminal and connecting the other end to the output terminal to discharge the capacitive element. A boosted voltage may be output from the output terminal.

  The LED driving power supply circuit according to the present invention includes a power supply terminal to which an external power supply is connected, a step-down regulator that steps down a power supply voltage input from the power supply terminal, and an output terminal and an output terminal of the step-down regulator. First and second switches connected in series, third and fourth switches connected in series between the external power source and ground, a connection point of the first switch and the third switch A booster circuit for boosting the step-down output voltage of the step-down regulator, the power supply terminal, and the output terminal of the booster circuit in series. And a current path connected via a fifth switch composed of two MOS transistors. In the step-up mode, the power supply voltage is supplied to the output terminal via the current path by turning on the fifth switch. In the step-up mode, the first and fourth switches are turned on. Thus, the capacitor is charged by the step-down regulator output, the capacitor is discharged by turning on the second and third switches, and the boosted voltage is output from the output terminal by this discharge. Also good.

A first control switch is provided between the MOS transistor and the power supply terminal, and a second control switch is provided between the MOS transistor and the output terminal, and the MOS transistor is provided by the first and second control switches. The substrate voltage may be controlled.
In the driving method of the LED drive power supply circuit according to the present invention, when the LED drive power supply circuit is driven, a current flowing from the power supply terminal to the output terminal via the booster circuit is passed, It is characterized in that a current flows through the current path after a lapse of time.

  Since the present invention uses the current path that passes through the power supply terminal, one MOS transistor switch, and the output terminal that do not pass through the step-down regulator in the 1.0 times mode, the number of intervening switching elements is small and the output impedance is lowered. In addition, since the current circuit passing through the booster circuit is also used, the output impedance can be further reduced. In the boost mode, the circuit scale can be reduced by not passing the current path through the power supply terminal, MOS transistor switch and output terminal, and by reducing the input voltage of the switching element and reducing the withstand voltage and size compared to the conventional case during charging. During discharging, the current path does not pass through the step-down regulator, and the output voltage drop due to the resistance of the switching element included in the step-down regulator can be reduced.

The present invention has a basic configuration in which a step-down regulator is connected in front of the booster circuit, and the LED drive power supply circuit boosts the input voltage supplied from the power supply terminal at a set boosting rate, A step-down regulator for making the input voltage to the step-up circuit constant, a voltage dividing resistor, and a capacitor for charging the output voltage are provided. In addition to these components, the power supply terminal and the output terminal of the step-up circuit are connected in series. It is characterized in that a current path connected through a switch composed of one MOS transistor is added. With such a configuration, the present invention makes it possible to reduce the output impedance by reducing the switching elements present in the discharge path.
Hereinafter, embodiments of the invention will be described with reference to examples.

First, Embodiment 1 will be described with reference to FIG.
FIG. 1 is a circuit diagram of an LED driving power supply circuit. The LED drive power supply circuit of this embodiment uses a step-down regulator 2 that steps down the input voltage VIN supplied from the power supply 1 according to the reference voltage Vref and the feedback voltage from the output terminal 4, and the output voltage VREG of the step-down regulator 2. The booster circuit unit 3 supplies the boosted voltage VOUT generated by charging and discharging by the input voltage VIN to the output terminal 4, and the power source 1 and the output terminal 4 are connected in series via a switch 7 constituted by one MOS transistor. And a current path connected to each other. That is, the LED driving power supply circuit boosts the input voltage VIN supplied from the power supply 1 at a set boosting rate, and makes the input voltage VIN to the boosting circuit section 3 constant. Are provided with a step-down regulator 2, a voltage dividing resistor R1, R2, a capacitor C for charging the output voltage VOUT, and a switch 7 for cutting off a current path formed between the power source 1 and the output terminal 4.

The step-down regulator 2 includes an operational amplifier 21 and a MOS transistor 22 whose output is connected to the gate. The operational amplifier 21 compares the reference voltage Vref from the reference voltage source 5 with the feedback voltage from the output terminal 4 and controls the MOS transistor 22 according to the error voltage. The first input (+) of the operational amplifier 21 is connected to a connection point between the voltage dividing resistors R1 and R2 connected in series between the output terminal 4 and the ground (GND), from which a feedback voltage is supplied. The A reference voltage Vref is supplied to the second input (−) of the operational amplifier 21. One end of the MOS transistor 22 is connected to the input voltage VIN, and the other end is connected to the first input of the booster circuit 2. The output of the booster circuit unit 3 is connected to the output terminal 4 of the power supply circuit.
The capacitor C has one end connected to a connection point between the output of the booster circuit unit 3 and the output terminal 4 and the other end connected in series to the connection point between the voltage dividing resistor R1 and the voltage dividing resistor R2 and the operational amplifier 21. Connected to a connection point between the first inputs (+) of the two. The step-up circuit unit 3 selectively charges and discharges a step-up capacitor by an on / off operation of a switch provided therein, and steps up the input voltage VIN at a set step-up rate to output the output voltage VOUT. Circuit.

  The step-up circuit unit 3 includes a first switch SW1 and a second switch SW2 connected in series between the output terminal 4 and the output terminal of the P-channel MOS transistor 22 constituting the step-down regulator 2, the power supply 1, and the ground. The third switch SW3 and the fourth switch SW4 connected in series with the end, one end is connected to the connection point of the first switch SW1 and the second switch SW2, and the other end is the third switch The boosting capacitor C1 is connected to the connection point between the switch SW3 and the fourth switch SW4. In the embodiment, the first to fourth switches SW1 to SW4 are composed of MOS transistors. The first input terminal of the booster circuit unit 3 to which the output voltage VREG of the step-down regulator 2 is input is connected to the first switch SW1, and the second input terminal to which the input voltage VIN supplied from the power supply 1 is input. Is connected to the third switch SW3. The operational amplifier 21 constituting the step-down regulator 2 compares the reference voltage Vref from the reference voltage source 5 with the feedback voltage from the output terminal 4 and controls the MOS transistor 22 according to the difference voltage. Thereby, the step-down regulator 2 adjusts the input voltage VIN, stabilizes it to the target voltage value, and supplies it to the step-up circuit unit 3. As a result, the difference between the feedback voltage from the output terminal 4 and the reference voltage Vref can be controlled to be zero, and the booster circuit unit 3 obtains the output voltage VOUT by making the input voltage VIN constant to the target voltage value.

In this way, the step-down regulator 2 for stepping down the input voltage VIN supplied from the power supply 1 according to the feedback voltage from the output terminal 4 is connected to the previous stage of the step-up circuit unit 3, and the output voltage VREG of the step-down regulator 2 is only charged. Is supplied to the boosting capacitor C1 of the boosting circuit unit 3.
The capacitor C1 included in the step-up circuit unit 3 is charged when the output voltage VREG of the step-down regulator 2 is applied to one end, and is discharged when the input voltage VIN from the power supply 1 is applied to the other end. At the time of charging, the output voltage VREG from the output terminal of the MOS transistor 22 constituting the step-down regulator 2 is charged from the first input terminal to the boosting capacitor C1 through the first switch SW1 in the ON state. At this time, the second switch SW2 and the third switch SW3 are in an off state, and the fourth switch SW4 is in an on state. At the time of discharging, the input voltage VIN supplied from the power source 1 passes through the third switch SW3 in the on state from the second input terminal, and the charging voltage VREG of the boosting capacitor C1 is added to turn on the second switch SW2 in the on state. And output from the output terminal 4 as an output voltage VOUT. This output is used, for example, as a power source for causing a white LED to emit light. At this time, the first switch SW1 and the fourth switch SW4 are in the off state.

  For example, when a lithium battery is used, the power source 1 has a power of about 3 to 4.2V. Since the white LED is driven at about 4.5 to 5 V, the power supply circuit boosts the power supply voltage as necessary to generate the output voltage VOUT that drives the white LED. Even when the power supply is consumed, an appropriate output voltage VOUT is output using this booster circuit. In the step-up circuit unit 3, the output when the step-up ratio is double is VREG + VIN. When the output of the booster circuit unit 3 increases as the input voltage VIN increases, the output voltage VREG of the step-down regulator 2 decreases. Therefore, the output voltage VOUT is constant with respect to the boost ratio regardless of the input voltage VIN.

In the step-up operation in this embodiment, charging is performed with the output voltage VREG of the step-down regulator connected to the previous stage of the step-up circuit section, so that the input voltage of the switching element is smaller than in the conventional example. Therefore, the breakdown voltage and size can be reduced, and the circuit scale can be reduced. Since the current path during discharging does not pass through the step-down regulator, the output voltage drop due to the resistance of the switching element included in the step-down regulator can be reduced.
Next, the operation in the 1.0 times mode will be described. This embodiment is characterized in that a current path for connecting the power supply terminal and the output terminal of the booster circuit in series via a switch composed of one MOS transistor is added. That is, the power supply voltage is supplied to the output terminal through the first current path of VIN → P1 → VOUT. Normally, two or more MOS transistors are arranged in the current path for supplying the voltage in the 1.0 times mode to the output terminal by passing through a regulator, etc. In the first current path of this embodiment, There is only one MOS transistor. As a result, a low output impedance was realized. In addition to the first current path described above, a second current path of VIN → VREG → SW1 → SW2 → VOUT can be set. These first current path and second current path current circuit Can be used at the same time to further reduce the output impedance.

Next, Embodiment 2 will be described with reference to FIGS.
FIG. 2 is a circuit diagram of an LED driving power supply circuit, FIGS. 3 to 5 are schematic circuit diagrams for explaining a charging / discharging method of the LED driving power supply circuit, and ON / OFF of transistors used in the power supply circuit. It is a characteristic view explaining a state.
This embodiment is different from the first embodiment in the configuration of the booster circuit section. The LED drive power supply circuit of this embodiment uses a step-down regulator 2 that steps down the input voltage VIN supplied from the power supply 1 according to the reference voltage Vref and the feedback voltage from the output terminal 4, and the output voltage VREG of the step-down regulator 2. A booster circuit unit 6 for supplying a boosted voltage (output voltage) VOUT generated by charging and discharging by the input voltage VIN to the output terminal 4, a power supply 1 and an output terminal (VOUT) 4 in series with one MOS transistor. And a current path connected via a switch 8 configured.
The LED driving power supply circuit includes a boosting circuit unit 6 that boosts the input voltage VIN supplied from the power supply 1 at a set boosting rate, a step-down regulator 2 for making the input voltage VIN to the boosting circuit unit 6 constant, A voltage dividing resistor R1, R2, a capacitor C for charging the output voltage VOUT, and a switch 8 for cutting off a current path formed between the power source 1 and the output terminal 4 are provided.

  The step-down regulator 2, the capacitor C, and the switch 8 have the same configuration as in the first embodiment. As in the first embodiment, the booster circuit unit 6 selectively charges and discharges the booster capacitors C1 and C2 by the on / off operation of the switches (SW1 to SW9) provided therein, and the input voltage VIN is set. The booster circuit outputs the output voltage VOUT after boosting at the boosting rate. The booster circuit unit 6 is composed of a plurality of booster circuits 61 and 62 connected in parallel (using two circuits in this embodiment) between the power supply 1 and the output terminal 4, and a switch is provided between the booster circuits 61 and 62. (In this embodiment, the fifth switch SW5 is used).

  The first booster circuit 61 includes a first switch SW1 and a second switch SW2 connected in series between the output terminal 4 and the output terminal of the MOS transistor 22 constituting the step-down regulator 2, the power supply 1, and the ground. The third switch SW3 and the fourth switch SW4 connected in series with the end, one end is connected to the connection point of the first switch SW1 and the second switch SW2, and the other end is the third switch The boosting capacitor C1 is connected to the connection point between the switch SW3 and the fourth switch SW4. These first to fourth switches SW1 to SW4 are composed of, for example, MOS transistors. The switches SW1 to SW3, SW7 and SW8 are composed of PMOS transistors and are indicated by P3 to P5, P6 and P2, respectively. The switches SW5 and SW9 are composed of NMOS transistors and are represented by N2 and N1, respectively.

The third switch SW3 (P3) is connected to the second input terminal 64 of the first booster circuit 61 to which the input voltage VIN supplied from the power supply 1 is input, and the output voltage VREG of the step-down regulator 2 is input. The first input SW 63 is connected to the first switch SW1 (P4). The second booster circuit 62 includes a sixth switch SW6 and a seventh switch SW7 (P6) connected in series between the output terminal 4 and the output terminal of the MOS transistor 22, and the power source 1 and the ground terminal. Between the eighth switch SW8 (P2) and the ninth switch SW9 (N1) connected in series between them, one end is connected to the connection point of the sixth switch SW6 and the seventh switch SW7 (P6). The other end is composed of a boosting capacitor C2 connected to a connection point between the eighth switch SW8 (P2) and the ninth switch SW9 (N1). An eighth switch SW8 (P2) is connected to the second input terminal 66 of the second booster circuit 62 to which the input voltage VIN is input, and the first input terminal 65 to which the output voltage VREG is input is Connected to the sixth switch SW6.
In this way, the step-down regulator 2 that steps down the input voltage VIN supplied from the power supply 1 according to the feedback voltage from the output terminal 4 is connected to the previous stage of the step-up circuit unit 6, and the output voltage VREG of the step-down regulator 2 is only charged. Is supplied to the boosting capacitor C1 or C2 or both of the boosting circuit unit 6.

  Next, a charge / discharge method of the LED drive power supply circuit of this embodiment will be described. For example, when two booster circuits are connected in parallel to the booster circuit unit, outputs of 1 × (VREG), 1.5 × (1/2 VREG + VIN), etc. are controlled by ON / OFF control of the first to ninth switches. Obtainable. As shown in FIG. 3, when the output is in the 1 × (VREG) mode, the output (VREG) of the step-down regulator 2 is set as the output voltage VOUT.

In this LED drive power supply circuit, the first switch in the first current path, booster circuit 6 in which the power supply 1 and the output terminal 4 are connected in series via a switch 8 composed of one MOS transistor. There is a second current path from the output of the step-down regulator 2 to the output terminal 3 where P4 and the second switch P5 are turned on and the other switches are controlled to be turned off. The first current path is discharged through VIN → P1 → VOUT, and the second current path is discharged through the path VIN → VREG → P4 → P5 → VOUT (see arrow in FIG. 3). In this embodiment, as shown in FIG. 3, P1 is on, P4 and P5 are on, and P2, P3, N1, and N2 are off. At this time, the output terminal 4 is output.
When the output is in the 1 × mode, the step-down regulator 2 simply acts as a switch (for example, when VIN is 2.7 to 4.6V and VOUT is set to 4.9V). In this case, since there is no switching, no ripple occurs in the output voltage VOUT.

Next, a charging method and a discharging method in the 1.5 times (1/2 VREG + VIN) mode will be described with reference to FIGS.
FIG. 4 is a schematic circuit diagram illustrating a charging method in which the output is 1.5 times (1/2 VREG + VIN) mode, and FIG. 5 is a schematic circuit diagram illustrating a discharging method in which the output is 1.5 times mode. . In the charging path, the first switch P4, the fifth switch N2, and the ninth switch N1 are turned on, and the other switches (P2, P3, P5, P6) of the booster circuit 6 are turned off. At this time, the switch 8 is always turned off in order to turn off the first current path of FIG. With this charging method, the capacitors C1 and C2 for charging are charged up to the voltage of the output voltage VREG of the step-down regulator 2. As a result, each of the capacitors C1 and C2 is charged with a voltage of ½ VREG. The second switch P5 and the seventh switch P6 are turned off, and current is supplied only by the charge of the capacitor C3. Therefore, the voltage at the output terminal 4 is reduced by the load current (see FIG. 4).

  In the discharge path, the second switch P5, the third switch P3, the seventh switch P6, and the eighth switch P2 are turned on, and the other switches of the booster circuit 6 are turned off. According to this charging method, the first current path (VIN → P3 → C2 → P5 → VOUT) passing through the second switch P5 and the third switch P3 between the power source (VIN) and the output terminal 4 and the seventh The second path (VIN → P2 → C1 → P6 → VOUT) passing through the second switch P6 and the eighth switch P2 is formed. Capacitors C1 and C2 are charged up to a voltage of VREG, and a voltage of 1/2 VREG is applied (see FIG. 5).

  First, the inventor reduced the input voltage of the switching element during charging to reduce the withstand voltage and size, and prevented the current path from passing through the step-down regulator during discharging to prevent the resistance of the switching element included in the step-down regulator. Has developed a charge pump type power supply circuit capable of reducing the voltage drop of the output due to the above (Japanese Patent Application No. 2007-220606). This power supply circuit is used for LED driving, and is a prior art of the power supply circuit described in the first and second embodiments shown in FIGS. This prior art LED drive power circuit is charged by the step-down regulator 2 that steps down the power supply voltage in accordance with the reference voltage Vref and the feedback voltage from the output terminal 4, and the output voltage VREG of the step-down regulator, and is discharged by the power supply voltage VIN. And a booster circuit unit 3 for supplying the boosted voltage generated by the operation to the output terminal. The step-up circuit unit includes a switch SW1 and a switch SW2 connected in series between the output terminal and the step-down regulator, a switch SW3 and a switch SW4 connected in series between the power source and the ground, and a first switch SW1. , SW2 and a capacitor C connected between the second switches SW3 and SW4.

  That is, the power supply circuit according to the first and second embodiments is configured by adding a current path formed by connecting a power supply terminal and an output terminal via a switch made of a MOS transistor to the above-described prior art. . Further, in the second embodiment, the power supply circuit can be driven without using the switch 4 and the switch 6. Therefore, as shown in FIGS. 3 to 5, the wiring including the switch 6 from the output terminal of the MOS transistor 22 to the switch SW7 can be eliminated, and the wiring including the switch SW4 from the switch SW3 to GND can be eliminated. As a result, the output impedance at the time of discharging can be further reduced and the transistor can be omitted. As a result, the chip on which the power supply circuit is mounted is further reduced, which contributes to resource saving.

  As described above, in this embodiment, charging is performed with the output voltage VREG of the step-down regulator connected to the previous stage of the step-up circuit section, so that the input voltage of the switching element is smaller than in the conventional example. Therefore, the withstand voltage and size can be reduced, and the circuit scale can be reduced. Since the current path during discharge does not pass through the step-down regulator, the output voltage drop due to the resistance of the switching element included in the regulator can be reduced, which can reduce the output voltage drop due to the resistance of the switching element included in the step-down regulator. It becomes possible. In addition, since the voltage drop at the time of discharge is avoided, the output voltage drop can be reduced even if the voltage boost ratio is variable.

Next, Example 3 will be described with reference to FIG.
FIG. 6 is a schematic circuit diagram for explaining the soft start of the discharging method in which the output is in the 1.5 times mode. In the driving method of this embodiment, when a power supply circuit is driven, a current flowing from the power supply terminal to the output terminal via the booster circuit is passed, and after a predetermined time has elapsed, the power supply terminal (VIN) and the output terminal ( The current flows through the current path including the switch of the MOS transistor.
In order to suppress the inrush current, after the reference voltage is stabilized (62 μsec after turning on EN in the embodiment), first, a current is passed through the second current path (VIN → VREG → P4 → P5 → VOUT), and then (In the embodiment, after 500 μsec) The switch 8 (P1) is turned on to flow a current into the first current path. By such a method, as shown in FIG. 6B, the output from the output terminal (VOUT) gradually reaches a predetermined steady voltage. The soft start operation described in this embodiment can be applied to the power supply circuits of the first and second embodiments.

Next, Example 4 will be described with reference to FIG.
FIG. 7 is a schematic circuit diagram of a switch composed of a MOS transistor that switches between a power supply terminal and an output terminal constituting the LED drive power supply circuit described in this embodiment. The power supply circuit for RED driving of this embodiment includes a current path that connects a power supply terminal (VIN) and an output terminal VOUT in series via a switch 9 composed of a PMOS transistor (P1). In addition to the PMOS transistor (P1) having a switching action, the switch 9 is provided with control switches 10 and 11 for controlling the substrate voltage of the transistor. The control switches 10 and 11 are composed of a pair of PMOS transistors each having a gate and a substrate connected to each other. The control switch 10 has one end connected to a power supply terminal (VIN) and the other end connected to a PMOS transistor (P1). The control switch 11 has one end connected to the substrate of the PMOS transistor (P1) and the other end connected to the output terminal (VOUT).

  The substrate voltage is controlled by ON / OFF control of the control switches 10 and 11. During standby, the PMOS transistor (P1) is turned off, the control switch 10 is turned on, and the control switch 11 is turned off. During the 1.0 × mode operation, the PMOS transistor (P1) is turned on, the control switch 10 is turned on, and the control switch 11 is turned off. At the time of 1.5 times mode operation, the PMOS transistor (P1) is turned off, the control switch 10 is turned off, and the control switch 11 is turned on. In this embodiment, the influence of the substrate voltage of the switch can be reduced.

FIG. 3 is a circuit diagram of a power supply circuit for driving an LED according to the first embodiment. FIG. 5 is a circuit diagram of an LED driving power supply circuit according to a second embodiment. FIG. 3 is a schematic circuit diagram illustrating a 1.0 × mode discharge method of the LED drive power supply circuit of FIG. 2 and a characteristic diagram illustrating an on / off state of a transistor used in the power supply circuit. FIG. 3 is a schematic circuit diagram for explaining a 1.5 × mode charging method of the LED drive power supply circuit of FIG. 2 and a characteristic diagram for explaining an on / off state of a transistor used in the power supply circuit. FIG. 3 is a schematic circuit diagram illustrating a 1.5 × mode discharge method of the LED drive power supply circuit of FIG. 2 and a characteristic diagram illustrating an on / off state of a transistor used in the power supply circuit. FIG. 10 is a schematic circuit diagram illustrating soft start of a discharge method in which the output of the LED drive power supply circuit according to the third embodiment is in a 1.5 times mode. FIG. 10 is a schematic circuit diagram of a current path having a switch formed of a MOS transistor that switches between a power supply terminal and an output terminal that constitute an LED drive power supply circuit according to Embodiment 4; The circuit diagram of the conventional power supply circuit for LED drive.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Power source 2 ... Buck regulator 3, 6 ... Boost circuit part 4 ... Output terminal 5 ... Reference voltage source 7, 8, 9 ... Switch (MOS transistor)
DESCRIPTION OF SYMBOLS 10, 11 ... Control switch 21 ... Operational amplifier 22 ... MOS transistor 61, 62 ... Booster circuit (charge pump circuit)
63-66 ... Input terminal

Claims (6)

A power supply terminal to which an external power supply is connected, a step-down regulator for stepping down a power supply voltage input from the power supply terminal, and an LED drive step-up voltage from the output voltage of the step-down regulator using charge / discharge of a capacitor element A power supply circuit for driving an LED, comprising: a booster circuit; and a current path for connecting the power supply terminal and the output terminal of the booster circuit in series via a switch composed of one MOS transistor. In the step-up mode, the power supply voltage is supplied to the output terminal through the current path by turning on the switch. In the step-up mode, one end of the capacitor is connected to the output end of the step-down regulator. And connecting the other end to the low potential side to charge the capacitive element, connecting one end to the power supply terminal and connecting the other end to the output terminal to discharge the capacitive element. The LED driving power supply circuit according to claim 1, wherein the boosted voltage is output from the output terminal. A power supply terminal to which an external power supply is connected, a step-down regulator for stepping down a power supply voltage input from the power supply terminal, and a first and a second connected in series between an output terminal and the output terminal of the step-down regulator A switch, a third and a fourth switch connected in series between the power supply terminal and the ground, a connection point of the first switch and the second switch, the third switch and the fourth switch A booster circuit for boosting the step-down output voltage of the step-down regulator, and a power supply terminal and an output terminal of the booster circuit are configured in series with one MOS transistor. And a current path connected via a fifth switch. An LED driving power supply circuit. In the step-up mode, the power supply voltage is supplied to the output terminal via the current path by turning on the fifth switch. In the step-up mode, the first and fourth switches are turned on. Thus, the capacitor is charged by the output of the step-down regulator, the capacitor is discharged by turning on the second and third switches, and the boosted voltage is output from the output terminal by this discharge. The LED driving power supply circuit according to claim 3. A first control switch is provided between the MOS transistor and the power supply terminal, and a second control switch is provided between the MOS transistor and the output terminal, and the MOS transistor is provided by the first and second control switches. 5. The LED driving power supply circuit according to claim 1, wherein the substrate voltage is controlled. 6. When driving the LED drive power supply circuit according to claim 1, a current flowing from the power supply terminal to the output terminal through the booster circuit is allowed to flow for a predetermined time. A driving method of an LED driving power supply circuit, wherein a current is passed through the current path after elapse of time.

Images