JP2009240025A - Step-up dc-dc converter and semiconductor integrated circuit for driving power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent current from continuously flowing from an output terminal, when circuit operation is stopped in a step-up DC-DC converter and to prevent liquid spill from a battery, in a step-up DC-DC converter that uses the battery as an input power supply. <P>SOLUTION: In the step-up DC-DC converter, an inductor and a rectifying diode are connected in series. The DC-DC converter includes: a driver element (SW1) that is connected with the connecting node between the inductor and the rectifying diode and passes current through the inductor; and a control circuit (10) that generates a signal for controlling the driver element according to the feedback voltage of output and outputs this signal. The converter is provided with a current cut-off switch element (SW2), which is in series with the rectifying diode. The control circuit includes a low voltage detection circuit (16) that monitors input voltage. When the low voltage detection circuit detects that input voltage has dropped to a predetermined potential or lower, driving of the driver element (step-up operation) is stopped and further, the current cut-off switching element is turned off. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a step-up DC-DC converter, and more particularly to a technique that is effective for use in a DC-DC converter using a battery as an input power source and a power source driving semiconductor integrated circuit constituting the DC-DC converter.

  In portable electronic devices, batteries (primary batteries, secondary batteries) are used as a power source. In such portable electronic devices, a voltage higher than the battery voltage may be required in order to rotate a motor or drive an electronic component. In such a case, a boost type DC-DC converter is generally used.

  Conventionally, for example, a switching regulator as shown in FIG. 6 is known as a step-up DC-DC converter. In this switching regulator, the switch transistor M1 is turned on to allow current to flow through the coil L1 to accumulate energy, and the M1 is turned off to release energy from the coil, which is rectified by the diode D1 and connected to the output terminal. A charge is supplied to the capacitor to generate a boosted voltage.

Furthermore, the output voltage Vout is divided by resistors R3 and R4 and fed back to a control circuit having a PWM (pulse width modulation) comparator. When the output voltage Vout decreases, the control pulse widens the pulse width of the control pulse and M1 is turned on. When the output voltage Vout increases, the pulse width of the control pulse is narrowed to shorten the time during which M1 is turned on, thereby making the output voltage Vout constant, which is several times the voltage of the battery. The output voltage Vout can be obtained.
JP 2005-217944 A

  If the battery is excessively discharged, the primary battery will cause liquid leakage, and the secondary battery will have characteristics that deteriorate.Therefore, the battery voltage is monitored to reduce the input voltage to some extent. In this case, it is desirable to stop turning on the switch transistor M1.

  However, since the switching regulator as shown in FIG. 6 has a current path of coil L1-diode D1-load, when the voltage of the primary battery is equal to or higher than the forward voltage of diode D1, switch transistor M1 is turned off. However, if the diode D1 is turned on and the current continues to flow to the load through the current path, and the battery is excessively discharged, the battery may leak or the characteristics may be deteriorated.

  In addition, in an LED drive power supply circuit comprising a DC-DC converter that boosts a DC voltage and outputs a WLED drive current, the current continues to flow to the LED when the boost operation is stopped, and the power consumption increases or the LED is lit weakly. In order to avoid this, an invention has been proposed in which a switch transistor is provided between an LED and a ground point, and the current is cut off by turning it off (see Patent Document 1).

  In the invention disclosed in Patent Document 1, a sense resistor is connected in series with an LED which is a load, and the voltage is fed back to a control circuit to perform PWM driving. When the switch inserted in series is turned off, the current flowing through the sense resistor is also cut off. However, in the configuration in which the sense resistor is connected in series with the LED that is the load, when the resistance value of the sense resistor is reduced, the detection accuracy is lowered, whereas when the resistance value of the sense resistor is increased, the loss in the resistor is increased. At the same time, the voltage that can be applied to the load is reduced.

  Further, since the power supply device disclosed in Patent Document 1 performs constant current control so that a predetermined drive current flows through the WLED, a sense resistor is connected in series with the LED as a load, and constant voltage control is performed. For the DC-DC converter that outputs a predetermined voltage, a system that feeds back the voltage divided by the resistors R3 and R4 connected between the output terminal and the ground as shown in FIG. 6 is suitable. However, in the power supply system in which the voltage dividing resistors R3 and R4 for generating the feedback voltage are provided in parallel with the load as shown in FIG. 6 for constant voltage control, even if the current flowing through the load is interrupted, the voltage dividing resistor R3 , R4 cannot be prevented from continuing to flow through R4.

  The present invention has been made under the background as described above, and an object of the present invention is to provide a step-up DC-DC converter capable of preventing current from continuing to flow from the output terminal when the operation of the circuit is stopped. It is to provide.

  Another object of the present invention is to make it difficult for a battery to leak or to deteriorate characteristics in a step-up DC-DC converter using the battery as an input power source.

  In order to achieve the above object, the present invention provides an inductor and a rectifier element connected in series between a voltage input terminal to which a DC voltage from a battery is input and an output terminal to which a load is connected. A step-up DC-DC comprising a driving element coupled to a connection node with a rectifying element and causing a current to flow through the inductor, and a control circuit for generating and outputting a signal for controlling the driving element in accordance with an output feedback voltage In the converter, a switching element for interrupting current is provided so as to form a series form with the rectifying element, and the control circuit is provided with a low voltage detection circuit for monitoring the input voltage, and the input voltage has dropped below a predetermined potential. When the low voltage detection circuit detects the driving element, the driving element is stopped and the current interrupting switching element is turned off. A.

  According to the above configuration, when the battery voltage as the input voltage decreases, the converter automatically stops the boosting operation and can block the current that may continue to flow from the output terminal via the rectifier element. As a result, wasteful consumption of the battery can be prevented.

  Preferably, the control circuit has a function of stopping driving of the driving element based on an input of an external control signal, and when the control signal instructing driving stop is input or an input voltage is When the low voltage detection circuit detects that the voltage has dropped below a predetermined potential, the drive element is stopped and the current interrupting switch element is turned off. As a result, even when the control signal instructing to stop driving is input from the outside, the boosting operation can be stopped and the current that may continue to flow from the output terminal via the rectifying element can be cut off. .

  More preferably, the current interrupting switch element is constituted by a P-channel field effect transistor connected between the rectifying element and the output terminal. As a result, the current flowing out from the output terminal when driving is stopped can be completely cut off, and the current flowing through the load can be cut off when a load without a switch for cutting off the current path is connected.

  Preferably, an output voltage voltage dividing circuit comprising a series resistor connected in parallel with the load is provided between the output terminal and the ground point, and the voltage generated by the voltage dividing circuit is fed back to the control circuit as a feedback voltage. The current interrupting switch element is configured to be connected in series with the voltage dividing circuit between the output terminal and a ground point. As a result, constant voltage control of the output voltage becomes possible, and current flowing out from the output terminal through the voltage dividing circuit can be cut off. Here, it is preferable that the current interrupting switch element is constituted by an N-channel field effect transistor.

  According to the present invention, a step-up DC-DC converter that can prevent current from continuously flowing from the output terminal when the operation of the circuit is stopped is realized. Further, in the step-up DC-DC converter using the battery as an input power source, there is an effect that the battery can hardly cause liquid leakage or characteristic deterioration.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.

  FIG. 1 shows an embodiment of a DC-DC converter to which the present invention is applied.

  The DC-DC converter of this embodiment includes a coil (inductor) L1 and a rectifying diode D1, which are connected in series between a voltage input terminal IN to which a DC voltage Vin from the battery 20 is input and an output terminal OUT. A driving switching transistor SW1 composed of an N-channel MOSFET (insulated gate field effect transistor) connected between a connection node between the coil L1 and the diode D1 and a ground point GND, and the driving switching transistor SW1 is turned on / off. The switching control circuit 10 to be controlled, the smoothing capacitor C2 connected between the output terminal OUT and the ground point, and the like are configured as a step-up type switching regulator.

  Further, a resistor voltage dividing circuit composed of resistors R3 and R4 in series is connected in parallel with the smoothing capacitor C2 between the output terminal OUT and the ground point, and the voltage divided by the resistors R3 and R4 is The switching control circuit 10 is configured to be applied to the feedback terminal FB. Furthermore, in this embodiment, a current cutoff switch SW2 made of a P-channel MOSFET is connected in series with D1 between the rectifying diode D1 and the output terminal OUT.

  Although not particularly limited, in this embodiment, the switching control circuit 10 includes a driving switching transistor SW1 and a current cutoff switch SW2 on a semiconductor integrated circuit (hereinafter referred to as a power source driving circuit) on one semiconductor chip. The coil L1, the rectifying diode D1, and the smoothing capacitor C1 are formed of discrete components and are connected to the power supply driving IC as external elements.

  The switching control circuit 10 includes a constant voltage circuit 11 including a constant current source CS and a Zener diode Dz in series, and a constant voltage Vz generated by the constant voltage circuit 11 is applied to a non-inverting input terminal so that the feedback terminal FB An error amplifier 12 that outputs a voltage corresponding to the potential difference between the feedback voltage and the constant voltage Vz when a voltage is applied to the inverting input terminal, and PWM (pulse width modulation) in which the output of the error amplifier 12 is input to a non-inverting input terminal A comparator 13 and a drive circuit (driver) 15 including an AND gate that generates and outputs an on / off drive signal of the drive switching transistor SW 1 according to the output of the PWM comparator 13.

  A waveform signal from a waveform generation circuit 14 that generates a waveform signal (RAMP signal) such as a triangular wave or sawtooth wave having a predetermined frequency is input to the inverting input terminal of the PWM comparator 13 and is fed to a feedback voltage. Accordingly, control is performed such that the pulse width of the output drive pulse is narrowed when the output voltage is high and the pulse width is widened when the feedback voltage is low. The pulse signal generated by the PWM comparator 13 is supplied to the drive circuit 15.

  Based on this pulse, the drive circuit 15 turns on the switch transistor SW1 to pass current through the coil L1 to accumulate energy, and turns off SW1 to release energy from the coil, rectifies it with the diode D1, and outputs it to the output terminal. Electric charge is supplied to the connected capacitor C2 to generate an output voltage Vout obtained by boosting the input voltage from the battery. Further, the feedback control based on the voltage from the voltage dividing resistors R3 and R4 can keep the output voltage Vout constant even when the load fluctuates.

  Further, in the present embodiment, the switching control circuit 10 includes a resistance voltage dividing circuit including resistors R1 and R2 connected between the voltage input terminal IN and the ground point and dividing the input voltage Vin, and resistors R1 and R1. A low voltage detection circuit 16 comprising a comparator that compares the voltage divided by R2 with the constant voltage Vz generated by the constant voltage circuit 11 to detect whether or not the input voltage Vin has fallen below a predetermined level; An AND gate circuit 17 that receives the output of the low voltage detection circuit 16 and an operation control signal CE supplied from the outside of the chip, and an inverter INV1 that inverts the output of the AND gate circuit 17 are provided.

  The operation control signal CE permits the operation of the switching control circuit 10 from the outside of the chip or stops the operation, and the operation of the chip is stopped when the level is low. The output of the AND gate circuit 17 is supplied to the drive circuit 15 to control its operation, and the output of the inverter INV1 is supplied to the gate terminal of the current cut-off switch SW2 so as to be turned on / off. ing.

  Next, the operation of the DC-DC converter of FIG. 1 will be described with reference to FIGS. 2 shows the case where the operation of the chip is stopped by the operation control signal CE in a state where the battery voltage is sufficiently high, and FIG. 3 shows the case where the operation of the chip is stopped by the detection signal of the low voltage detection circuit 16. It is.

  In the control circuit without the inverter INV1 and the current cut-off switch SW2 in the DC-DC converter of FIG. 1, when the operation control signal CE is changed to a low level, the drive circuit 15 does not output a drive pulse, so that the drive switching transistor SW1 Are turned off continuously. Here, when the voltage of the battery 20 is higher than the forward voltage VF of the rectifying diode D1, the current continues to flow through the current path of the coil L1-diode D1-load (and the voltage dividing resistors R3, R4). At this time, the output voltage Vout is Vin-VF. FIG. 2A shows state changes of the input voltage Vin, the operation control signal CE, and the output voltage Vout at this time. Even after the operation stop command is given to the chip, the current may continue to flow from the output terminal.

  On the other hand, in the DC-DC converter of FIG. 1, when the operation control signal CE is changed to a low level, the driving switching transistor SW1 is turned off, and between the rectifying diode D1 and the output terminal OUT. The current cutoff switch SW2 is turned off. As a result, the current path of the coil L1-diode D1-load (and the voltage dividing resistors R3, R4) disappears, the current flowing out from the output terminal becomes zero, and the output voltage Vout is 0 V as shown in FIG. To fall. As a result, there is no possibility that the battery is wasted.

  Next, in the control circuit without the inverter INV1 and the current cutoff switch SW2 in the DC-DC converter of FIG. 1, when the voltage of the battery 20 (input voltage Vin) becomes lower than a predetermined determination level UVLO, the low voltage detection circuit 16 Since the output of the AND gate circuit 17 becomes low level and the drive circuit 15 stops outputting the drive pulse, the drive switching transistor SW1 is continuously turned off. Here, when the voltage of the battery 20 is higher than the forward voltage VF of the rectifying diode D1, the current continues to flow through a current path of the coil L1-diode D1-load (and the voltage dividing resistors R3, R4). Therefore, especially when the internal resistance of the load is small, the current continues to flow from the output terminal, and as shown in FIG. 3A, the input voltage Vin and the output voltage Vout gradually decrease, and the battery is excessively discharged. This may cause liquid leakage or deterioration of characteristics.

  On the other hand, in the DC-DC converter of FIG. 1, when the battery voltage decreases and becomes lower than the determination level UVLO of the low voltage detection circuit 16, the output of the detection circuit changes to a low level and the driving switching transistor SW1 continues. And the current cutoff switch SW2 between the rectifying diode D1 and the output terminal OUT is turned off. As a result, the current path of the coil L1-diode D1-load (and the voltage dividing resistors R3, R4) disappears, the current flowing out from the output terminal becomes zero, and the output voltage Vout is 0 V as shown in FIG. To fall.

  As a result, the input voltage Vin does not drop greatly, and the battery can be prevented from leaking. In FIG. 3B, the input voltage Vin gradually decreases even after the output of the detection circuit changes to the low level because of the consumption current of the switching control circuit 10. The DC-DC converter of this embodiment is particularly effective when using a load that always has an internal current path. When a switch (SW0 in FIG. 5) that cuts off the current path is provided inside the load 30 and is configured to be turned off by the control signal CE, the current cut-off switch SW2 is connected to the output terminal and the voltage dividing resistor R3. You may provide between.

  In the embodiment of FIG. 1, the P-channel MOSFET is used as the current cutoff switch SW2. The N-channel MOSFET can be turned on by setting the gate voltage to a high level. This is because even if the control circuit (inverter INV1) that operates at is tried to be turned on, the gate voltage cannot be raised to Vin or higher and cannot be turned on. When a P-channel MOSFET is used as the current cutoff switch SW2, it can be turned on by applying a low level (ground potential) to its gate.

  FIG. 4 shows a modification of the DC-DC converter. In this modification, the current cutoff switch SW2 is provided not between the diode D1 and the output terminal OUT but between the voltage dividing resistor R4 and the ground point, and instead of the P-channel MOSFET as the current cutoff switch SW2. An N-channel MOSFET is used.

  Even when configured as in this modification, substantially the same effect as the DC-DC converter of the above-described embodiment can be obtained. The load 30 and the capacitor C2 are connected between the output terminal OUT and a connection node of the voltage dividing resistor R4 and the current cutoff switch SW2. As a result, the current can be cut off even when a load having a constant current path is used. The capacitor C2 may be connected between the output terminal OUT and the ground point.

  FIG. 5 shows a second modification of the DC-DC converter. In this modification, a current cutoff switch SW2 is provided between the voltage dividing resistor R4 and the ground point, and the load 30 and the capacitor C2 are connected between the output terminal OUT and the ground point. is there. This modification is effective when a switch SW0 that cuts off the current path is provided in the load 30. In conjunction with the switch SW0 in the load 30 being turned off by the operation control signal CE, the operation of the chip is stopped by the signal CE and the current cutoff switch SW2 is turned off. -The same effect as a DC converter can be obtained.

  Although the invention made by the present inventor has been specifically described based on the embodiment, the present invention is not limited to the above embodiment. For example, in the DC-DC converter shown in FIG. 1, the low voltage detection circuit 16 and the AND gate circuit 17 are provided to cut off the current when the input voltage drops or when an operation stop command is inputted from the outside. Although the switch configured to turn off the switch SW2 has been described, only one of a function that turns off when the input voltage drops or a function that turns off when an operation stop command is input from the outside is provided. It may be configured.

  In the above embodiment, the PWM comparator 13 is provided after the error amplifier 12, but a PFM (pulse frequency modulation) comparator may be provided instead of the PWM comparator.

  Further, in the above-described embodiment, the driving switching transistor SW1 and the rectifying diode D1 are formed in the IC chip on which the control circuit 10 is formed. However, the driving switching transistor SW1 and the rectifying diode D1 are connected to the IC as external elements. It may be configured. Further, instead of using external elements as the resistors R3 and R4 that divide the output voltage, these resistors may be formed inside the chip.

  In the above description, the example in which the present invention is applied to the step-up DC-DC converter has been described. However, the present invention is not limited to the present invention. The battery voltage is used as the input voltage, and the current is output from the output terminal when the converter is stopped. It can be widely used for power supply devices that have a path through which current flows.

It is a block block diagram which shows one Embodiment of the DC-DC converter to which this invention is applied. It is a time chart which shows the relationship between the input voltage Vin, the control signal CE, and the output voltage Vout in the DC-DC converter which has the operation stop function by the external control signal input, and the DC-DC converter which does not have such a function. The relationship between the input voltage Vin, the low voltage detection signal UVLO, and the output voltage Vout in the DC-DC converter having the operation stop function by the detection signal of the low voltage detection circuit and the DC-DC converter not having such a function is shown. It is a time chart. It is a block block diagram which shows the modification of the DC-DC converter to which this invention is applied. It is a block block diagram which shows the other modification of the DC-DC converter to which this invention is applied. It is a block block diagram which shows an example of the conventional step-up type DC-DC converter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Switching control circuit 11 Constant voltage circuit 12 Error amplifier 13 PWM comparator 14 Waveform generation circuit 15 Drive circuit (driver)
16 Low voltage detection circuit 17 AND gate circuit 20 Secondary battery (lithium ion battery)
30 Load L1 Coil (Inductor)
C1 Smoothing capacitor SW1 Coil drive switching transistor SW2 Current cutoff switch R3, R4 Output voltage dividing resistor

Claims (6)

An inductor and a rectifier element are connected in series between a voltage input terminal to which a DC voltage from the battery is input and an output terminal to which a load is connected, and coupled to a connection node between the inductor and the rectifier element. A step-up DC-DC converter comprising: a drive element for passing a current to the control circuit; and a control circuit for generating and outputting a signal for controlling the drive element in accordance with an output feedback voltage,
A switch element for current interruption is provided so as to form a series form with the rectifying element,
The control circuit includes a low voltage detection circuit that monitors an input voltage, and stops driving the drive element when the low voltage detection circuit detects that the input voltage has dropped below a predetermined potential. A step-up DC-DC converter characterized in that the switch element for interrupting current is turned off.   The control circuit has a function of stopping driving of the driving element based on an input of an external control signal, and when the control signal instructing driving stop is input or an input voltage is equal to or lower than a predetermined potential. 2. The apparatus according to claim 1, wherein when the low voltage detection circuit detects that the voltage has dropped, the driving of the driving element is stopped and the switch element for cutting off the current is turned off. The step-up DC-DC converter described in 1.   3. The step-up DC-DC according to claim 2, wherein the current interrupting switch element is configured by a P-channel field effect transistor connected between the rectifying element and the output terminal. converter.   An output voltage voltage dividing circuit comprising a series resistor connected in parallel with the load is provided between the output terminal and the ground point, and the voltage generated by the voltage dividing circuit is supplied to the control circuit as a feedback voltage. The step-up type according to claim 2, wherein the current-cutting switch element is connected in series with the voltage dividing circuit between the output terminal and a ground point. DC-DC converter.   5. The step-up DC-DC according to claim 4, wherein the current interrupting switch element is configured by an N-channel field effect transistor connected between the voltage dividing circuit and a ground point. converter. A voltage input terminal to which a DC voltage is input, an output terminal to which a load is connected, a first external element connection terminal to which one terminal of the inductor is connected, the other terminal of the inductor and an anode terminal of the rectifier element; A second external element connection terminal to be connected; a third external element connection terminal to which the cathode terminal of the rectifying element is connected; and a drive element that draws a current into an inductor connected to the second external element connection terminal; A control circuit for generating and outputting a signal for controlling the drive element in accordance with an output feedback voltage; and a P-channel field effect for current interruption provided between the third external element connection terminal and the output terminal A power source driving semiconductor integrated circuit comprising a transistor,
The control circuit includes a low voltage detection circuit that monitors an input voltage, and has a function of stopping driving of the drive element based on an input of an external control signal, and the control signal instructing drive stop is When the low voltage detection circuit detects that the input voltage or the input voltage has dropped below a predetermined potential, driving of the drive element is stopped and the current blocking P-channel field effect transistor is turned off. A semiconductor integrated circuit for driving a power supply, characterized in that:

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