JP5039372B2 - Switching power supply control circuit, power supply device using the same, and electronic equipment - Google Patents

Description

  The present invention relates to a switching power supply, and more particularly to a circuit protection technique thereof.

  In information terminals such as cellular phones and PDAs (Personal Digital Assistance) in recent years, devices that require a voltage higher or lower than the battery output voltage are used. In this way, when a voltage higher or lower than the battery voltage is required inside the information terminal, the battery voltage is boosted or lowered using a power supply device using a switching regulator, etc., and supplied to each device. Proper voltage to be generated. For example, Patent Document 1 describes a related technique.

JP 2004-166428 A

  The switching regulator includes a switching transistor and a control circuit that controls an on / off state of the switching transistor. A power supply voltage is supplied to the control circuit, and a drive signal having a binary value of a high level corresponding to the power supply voltage or a low level corresponding to the ground voltage is generated and supplied to the gate or base of the switching transistor.

  Here, when the battery voltage is supplied as the power supply voltage of the control circuit, a situation in which the high level and the low level of the drive signal become unstable can occur when the battery voltage decreases. In such a situation, if the switching transistor continues to be turned on without a switching operation, the reliability of the switching transistor and other circuit components may be impaired.

  The present invention has been made in view of these problems, and a comprehensive object thereof is to provide a circuit protection technique when the power supply voltage is lowered.

  According to an embodiment of the present invention, there is provided a control circuit for a switching power supply that controls an on / off state of a switching transistor having a fixed potential at one end. This control circuit compares a power supply voltage supplied to the control circuit itself with a predetermined threshold voltage, and outputs a low voltage detection signal of a predetermined level when the power supply voltage is lower than the threshold voltage. And a driver circuit that generates a drive signal to be supplied to the control terminal of the switching transistor based on a pulse signal that defines the ON time of the switching transistor, and is provided between the driver circuit and the control terminal of the switching transistor to control the drive signal. A level indefinite prevention circuit capable of switching between a first state that can be transmitted to the terminal and a second state in which the voltage of the control terminal of the switching transistor is fixed to a level at which the switching transistor is turned off.

The switching power supply refers to a step-up and step-down switching regulator, a DC / AC converter, a charge pump circuit, a charging-type high voltage generation circuit that repeatedly charges a capacitor by switching, and the like. Further, “the potential at one end is fixed” means a state in which a DC voltage such as a battery voltage or a ground voltage is applied.
According to this embodiment, when the power supply voltage supplied to the control circuit is low, the on / off state of the switching transistor can be prevented from becoming indefinite, and circuit protection can be achieved.

When the low voltage detection signal is at a predetermined level, the level indefinite prevention circuit is provided between a voltage fixing element that fixes the control terminal of the switching transistor to a voltage level at which the switching transistor is turned off, and between the driver circuit and the control terminal of the switching transistor. A switch that is provided and can be switched between an on state in which the drive signal can be transmitted to the control terminal and an off state in which the output is substantially open may be included. The switch may be turned off when the low voltage detection signal is at a predetermined level and turned on when the low voltage detection signal is at a level complementary to the predetermined level.
The switch may be a transfer gate.

  The switch includes an input terminal connected to the driver circuit, an output terminal connected to the control terminal of the switching transistor, an inverter, and an N-channel MOSFET (one end connected to the input terminal and the other end connected to the output terminal). A first transistor of a metal oxide semiconductor field effect transistor) and a second transistor of a P-channel MOSFET having one end connected to the input terminal and the other end connected to the output terminal may be included. The inverter may include a power supply voltage terminal fixed to the power supply voltage and an inverting transistor and a resistor connected in series between the ground terminal fixed to the ground voltage, and may invert the low voltage detection signal. The low voltage detection signal is input to one gate of the first transistor and the second transistor, and the output signal of the inverter is input to the other gate. When the low voltage detection signal is at a predetermined level, the switch The second transistor may be turned off.

  The predetermined level is a low level, and the inverting transistor of the inverter is a P-channel MOSFET whose source is connected to the power supply voltage terminal and the low voltage detection signal is input to the gate, and one end of the resistance of the inverter is connected to the ground terminal The other end is connected to the drain of the inverting transistor, the low voltage detection signal is input to the gate of the first transistor, and the output signal of the inverter is input to the gate of the second transistor.

  When the predetermined level is the high level, the inverter of the inverter is an N-channel MOSFET whose source is connected to the ground voltage terminal and whose gate is supplied with the low voltage detection signal, and one end of the resistance of the inverter is connected to the power supply voltage terminal. The other end may be connected to the drain of the inverting transistor.

  When the predetermined level is high, a low voltage detection signal may be input to the gate of the second transistor, and an output signal of the inverter may be input to the gate of the first transistor. That is, a configuration in which the first and second transistors are interchanged with the case where the predetermined level is the low level may be employed.

The voltage fixing element may be a resistor provided between a control terminal of the switching transistor and a fixed voltage terminal fixed at a voltage level at which the switching transistor is turned off.
When the switching transistor is an N-channel MOSFET or an NPN bipolar transistor, the resistor may be a pull-down resistor provided between the control terminal of the switching transistor and the ground terminal.
When the switching transistor is a P-channel MOSFET or a PNP bipolar transistor having one end connected to the input terminal to which the input voltage of the switching power supply is applied, the resistor is provided between the control terminal and the input terminal of the switching transistor. A pull-up resistor may be used.
Since the above-described switch has a high impedance in the off state, the level of the control terminal of the switching transistor can be easily fixed by using a resistor as a voltage fixing element.

The voltage fixing element may be a transistor that is turned on when a signal corresponding to the low voltage detection signal is input to the control terminal and the power supply voltage is lower than the threshold voltage.
When a transistor is used, the level of the control terminal of the switching transistor can be reliably fixed to the level to be turned off even when the switch OFF state is incomplete and the drive signal leaks to the control terminal of the switching transistor. it can.

  In one embodiment, the control circuit is provided in parallel with an oscillator that generates a pulse signal having a predetermined duty ratio and a switching transistor, and is turned on / off based on an output signal of the oscillator when the low voltage detection signal is at a predetermined level. And an auxiliary switching transistor.

  The control circuit may be integrated on a single semiconductor substrate. “Integrated integration” includes the case where all of the circuit components are formed on a semiconductor substrate and the case where the main components of the circuit are integrated. A resistor, a capacitor, or the like may be provided outside the semiconductor substrate. By integrating the control circuit as one LSI, the circuit area can be reduced.

  An embodiment of the present invention relates to a power supply apparatus. This power supply device includes an output circuit including an inductor and a capacitor, and the above-described control circuit that drives a switching transistor connected to the inductor.

  The switching transistor may be an N-channel MOSFET or an NPN bipolar transistor having one end grounded and the other end connected to an inductor, and the power supply device may be a step-up switching regulator.

  As the power supply voltage of the control circuit, a boost type switching regulator output voltage may be used. In this case, circuit protection functions effectively in a state where the input voltage is low, and a stable operation can be realized because the power supply voltage increases as the boosting operation proceeds.

  The switching transistor may be a step-down switching regulator, which is a P-channel MOSFET or a PNP-type bipolar transistor having one end applied with an input voltage and the other end connected to an inductor.

One embodiment of the present invention is an electronic device. This electronic device includes a battery and the above-described power supply device that stabilizes and outputs the voltage of the battery.
According to this aspect, even when the voltage of the battery is lowered, the power supply device can be stably operated.

  It should be noted that any combination of the above-described constituent elements, and those obtained by replacing constituent elements and expressions of the present invention with each other among methods, apparatuses, systems, etc. are also effective as embodiments of the present invention.

  With the control circuit according to the present invention, circuit protection is realized.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

  FIG. 1 shows a configuration of a power supply apparatus 200 according to an embodiment of the present invention. The power supply apparatus 200 according to the present embodiment is a step-up DC / DC converter configured to include two blocks of a control circuit 100 and a switching regulator output circuit (hereinafter simply referred to as an output circuit) 110. The power supply device 200 includes an input terminal 202 and an output terminal 204, and voltages applied to or appearing at the terminals are referred to as an input voltage Vin and an output voltage Vout, respectively. As the input voltage Vin, for example, a battery voltage output from a battery (not shown) is supplied. The power supply apparatus 200 boosts the input voltage Vin so that the output voltage Vout approaches the target value.

FIG. 2 is a block diagram illustrating a configuration of an electronic device 300 on which the power supply device 200 of FIG. 1 or the power supply device 200a of FIG. 4 is mounted. The electronic device 300 is a battery-driven small information terminal such as a mobile phone terminal, a digital camera, or a mobile game device. Electronic device 300 includes power supply device 200, load circuit 210, and battery 220. The battery 220 is a lithium ion battery or the like, and outputs a battery voltage Vbat of about 3V to 4V and outputs it to the input terminal 202 of the power supply apparatus 200.
The load circuit 210 is an LSI such as a CPU that integrally controls the entire electronic device 300, an LED (Light Emitting Diode), or the like, and operates by receiving a predetermined power supply voltage. The power supply terminal of the load circuit 210 is connected as a load to the output terminal 204 of the power supply apparatus 200, and the output voltage Vout of the power supply apparatus 200 is supplied as the power supply voltage. Hereinafter, the configuration of the power supply apparatus 200 will be described in detail.

Returning to FIG. The output circuit 110 includes a first terminal 111, a second terminal 112, a rectifier diode D1, an output inductor L1, and an output capacitor C1. One end of the output inductor L 1 is connected to the input terminal 202, and the other end is connected to the first terminal 111. The anode of the rectifier diode D1 is connected to the first terminal 111, and the cathode is connected to the second terminal 112. The output capacitor C1 is provided between the second terminal 112 and the ground terminal. The first terminal 111 is connected to the switching terminal 102 of the control circuit 100, and the second terminal 112 is connected to the output terminal 204.
Note that the topology of the output circuit 110 is not limited to that of FIG. 1, and can be changed as appropriate according to the type of a power supply device such as a step-down switching regulator, a DC / AC inverter, or a capacitor charging circuit.

  The control circuit 100 includes a switching terminal 102, a feedback terminal 104, and a power supply voltage terminal 106. A power supply voltage Vdd is supplied to the power supply voltage terminal 106. In the present embodiment, the power supply voltage terminal 106 is connected to the output terminal 204, and the control circuit 100 uses the boosted output voltage Vout of the power supply device 200 as the power supply voltage Vdd, not the battery voltage of the battery (not shown). To do. A feedback voltage Vfb obtained by dividing the output voltage Vout of the output terminal 204 by the first feedback resistor R10 and the second feedback resistor R11 is input to the feedback terminal 104.

  The control circuit 100 includes a pulse width modulator 10, a level indefinite prevention circuit 20, a driver circuit 30, a low voltage detection circuit 40, an oscillator 50, a switching transistor M1, and an auxiliary transistor Q1.

  The switching transistor M1 is an N-channel MOSFET and functions as a switching element that is switched on and off by a pulse width modulated drive signal Sd applied to the gate. The source of the switching transistor M1 is grounded, and the drain is connected to the first terminal 111 of the output circuit 110 via the switching terminal 102.

In the output circuit 110, when the switching transistor M1 is on, a current flows from the input terminal 202 via the output inductor L1 and the switching transistor M1, and energy is stored in the output inductor L1.
When the switching transistor M1 is turned off, the output inductor L1 keeps flowing the current that has been flowing during the period in which the switching transistor M1 is on, so that the current flows through the rectifier diode D1. At this time, the output capacitor C1 is charged by the current flowing through the rectifier diode D1.

  Thus, by repeatedly turning on and off the switching transistor M1, energy is converted between the output inductor L1 and the output capacitor C1, the input voltage Vin is boosted, and the output terminal 204 is smoothed by the output capacitor C1. The output voltage Vout is output.

  The control circuit 100 is a circuit that outputs a drive signal Sd to the gate terminal of the switching transistor M1 and controls the switching operation. In the present embodiment, the switching transistor M1 is built in the control circuit 100, but may be externally attached.

  The drive signal Sd is a pulse width modulation signal in which a high level and a low level are alternately repeated. The on / off time of the switching transistor M1 is controlled according to the low level period and the high level period, and the output voltage Vout is adjusted.

A feedback voltage Vfb is input to the pulse width modulator 10. The pulse width modulator 10 generates a pulse width modulation signal (hereinafter referred to as a PWM signal Sp) whose duty ratio is adjusted so that the feedback voltage Vfb matches a predetermined reference voltage Vref.
The pulse width modulator 10 includes an error amplifier 12, a PWM comparator 14, and an oscillator 16. A feedback voltage Vfb proportional to the output voltage Vout is input to the inverting input terminal of the error amplifier 12, and a predetermined reference voltage Vref is input to the non-inverting input terminal. The error amplifier 12 amplifies an error between the feedback voltage Vfb and the reference voltage Vref. Feedback is applied by the error amplifier 12 so that the error between the feedback voltage Vfb and the reference voltage Vref becomes 0V, and an error voltage Verr is generated.

  The oscillator 16 generates a sawtooth or triangular wave periodic voltage Vosc having a constant frequency. The error voltage Verr output from the error amplifier 12 is input to the inverting input terminal of the PWM comparator 14, and the periodic voltage Vosc output from the oscillator 16 is input to the non-inverting input terminal. The PWM comparator 14 generates a PWM signal Sp that is at a high level when Verr> Vosc and at a low level when Verr <Vosc. The duty ratio of the PWM signal Sp is controlled based on the error voltage Verr.

  The PWM signal Sp is a pulse signal that defines the ON time of the switching transistor M1. Based on the PWM signal Sp, the driver circuit 30 generates a drive signal Sd to be supplied to the gate that is the control terminal of the switching transistor M1. Driver circuit 30 includes an inverter having a size sufficient to charge and discharge the gate capacitance of switching transistor M1.

The low voltage detection circuit 40 compares the power supply voltage Vdd supplied to the control circuit 100 with a predetermined threshold voltage. Resistors R20 and R21 divide the power supply voltage Vdd. The low voltage detection circuit 40 includes a comparator. The threshold voltage Vth1 is input to the inverting input terminal of the comparator, and the divided power supply voltage Vdd ′ is input to the non-inverting input terminal. The low voltage detection circuit 40 outputs a low voltage detection signal S_UVLO having a predetermined level (hereinafter referred to as a low level) when Vdd ′ <Vth1.
In this embodiment, since Vdd = Vout, the low voltage detection circuit 40 compares the feedback voltage Vfb proportional to the output voltage Vout with the threshold voltage Vth1 to thereby compare the power supply voltage Vdd and the threshold voltage. The voltages may be compared indirectly.

  The level indefinite prevention circuit 20 is provided between the driver circuit 30 and the gate of the switching transistor M1. The level indefinite prevention circuit 20 is configured to be switchable between a first state and a second state. In the first state, the level indefinite prevention circuit 20 transmits the drive signal Sd to the gate of the switching transistor M1. In the second state, the level indefinite prevention circuit 20 fixes the voltage of the gate of the switching transistor M1 to a level at which the switching transistor M1 is turned off (a ground voltage that is a low level in the present embodiment).

FIG. 3 is a circuit diagram showing a configuration of the level indefinite prevention circuit 20 of FIG. The level indefinite prevention circuit 20 includes an input terminal 21 that receives the drive signal Sd from the driver circuit 30, and an output terminal 22 that is connected to the gate of the switching transistor M1.
Level indefinite prevention circuit 20 includes a pull-down resistor Rpd1 and a switch 24.

  The pull-down resistor Rpd1 is provided between the output terminal 22 connected to the gate of the switching transistor M1 and the ground terminal. The ground voltage applied to the ground terminal is a voltage that can turn off the switching transistor M1.

The switch 24 includes a transfer gate and is provided between an input terminal 21 connected to the driver circuit 30 and an output terminal 22 connected to the gate of the switching transistor M1. The switch 24 can switch between an on state in which the drive signal Sd supplied to the input terminal 21 can be transmitted to the output terminal 22 that is the gate of the switching transistor M1, and an off state in which the output terminal 22 is substantially open. It has become.
The switch 24 is turned on when the low voltage detection signal S_UVLO is at a high level, and is turned off when it is at a low level.

  When the switch 24 is turned off when the low voltage detection signal S_UVLO is at low level, the output becomes high impedance. Therefore, the pull-down resistor Rpd1 fixes the gate of the switching transistor M1 at low level and turns off the switching transistor M1. Functions as a voltage fixing element.

The switch 24 includes an inverter 26, a first transistor M10, and a second transistor M12.
Inverter 26 includes an inverting transistor M14 and a resistor R20 connected in series between power supply voltage terminal 106 and the ground terminal. Specifically, the source of the inverting transistor M14 is connected to the power supply voltage terminal 106, and the low voltage detection signal S_UVLO is input to the gate thereof. The resistor R20 has one end grounded and the other end connected to the drain of the inverting transistor M14. The inverter 26 inverts and outputs the low voltage detection signal S_UVLO.

The second transistor M12 is an N-channel MOSFET, and one end is connected to the input terminal 21 and the other end is connected to the output terminal 22. Further, the back gate of the first transistor M10 is connected to the ground terminal, and the low voltage detection signal S_UVLO is input to the gate. The second transistor M12 is a P-channel MOSFET, and one end is connected to the input terminal 21 and the other end is connected to the output terminal 22. Further, the back gate of the second transistor M12 is connected to the power supply voltage terminal 106, and the output signal of the inverter 26, that is, the inverted low voltage detection signal * S_UVLO is input to the gate. Here, * represents logical inversion.
In the switch 24, when the low voltage detection signal S_UVLO is at a low level, the first transistor M10 and the second transistor M12 are turned off and set to the above-described off state. In the off state, the output of the switch 24 is in a high impedance state that is not affected by the signal level on the input terminal 21 side.

  Returning to FIG. The oscillator 50 includes an enable terminal * EN to which the low voltage detection signal S_UVLO output from the low voltage detection circuit 40 is input. The oscillator 50 becomes active when the low voltage detection signal S_UVLO is at a low level, generates a pulse signal Sp2 having a predetermined duty ratio, and sets the pulse signal Sp2 to a low level when it is at a high level.

  The auxiliary transistor Q1 is provided in parallel with the switching transistor M1. The auxiliary transistor Q1 is an NPN bipolar transistor, and a pulse signal Sp2 output from the oscillator 50 is input to the base thereof. Therefore, the auxiliary transistor Q1 is repeatedly turned on and off based on the pulse signal Sp2 from the oscillator 50 when the low voltage detection signal S_UVLO is at a low level, and is turned off when the low voltage detection signal S_UVLO is at a high level.

The operation of the control circuit 100 configured as described above will be described.
Attention is paid to the state before starting the boosting operation when the power supply apparatus 200 is activated. It is assumed that pulse width modulator 10 and driver circuit 30 in control circuit 100 require a certain voltage Vth2 or higher, for example, Vth2 = 1.8 V or higher, as power supply voltage Vdd for stable operation. Actually, Vth2 = 2V is set in consideration of the margin.
Now, when an input voltage Vin of 0.9 V or less is applied from a battery (not shown), the output voltage Vout (and thus the power supply voltage Vdd) before the start of the boosting operation is 0.9 V or less. The level indefinite prevention circuit 20 does not operate normally, and the signal levels of the PWM signal Sp and the drive signal Sd are not determined to be high and low, and become indefinite.

The low voltage detection circuit 40 described above detects that the power supply voltage Vdd ′ is lower than the threshold voltage Vth1. Therefore, the values of resistors R20 and R21 and threshold voltage Vth1 are determined so that Vth1 = Vth2 × R21 / (R20 + R21).
When the power supply voltage Vdd is lower than the threshold voltage Vth2 at which the pulse width modulator 10 and the driver circuit 30 operate stably, the low voltage detection signal S_UVLO is at a low level.

  At this time, the switch 24 in the level indefinite prevention circuit 20 is turned off, and the drive signal Sd does not appear on the output terminal 22 side, and becomes high impedance. As a result, the signal level of the output terminal 22 is fixed to the ground voltage by the pull-down resistor Rpd1 that is a voltage fixing element, and the switching transistor M1 is forcibly turned off.

  At this time, the oscillator 50 becomes active, the auxiliary transistor Q1 performs a switching operation, and a boosting operation is performed. As a result of the boosting operation, when the output voltage Vout rises and the power supply voltage Vdd becomes 2V or higher, the low voltage detection signal S_UVLO becomes high level, the oscillator 50 becomes inactive, and the auxiliary transistor Q1 is turned off. At this time, the switch 24 of the level indeterminate prevention circuit 20 is turned on, the switching transistor M1 is switched by the drive signal Sd based on the PWM signal Sp, the boosting operation is continued, and the output voltage Vout corresponds to the reference voltage Vref. Stabilized to a certain level.

  When the level indefinite prevention circuit 20 is not provided, if the driving signal Sd is maintained higher than the threshold voltage Vt between the gate and source of the switching transistor M1 due to unstable operation of the driver circuit 30, switching is performed by the driving signal Sd. Since the transistor M1 is turned on, a current constantly flows through the output inductor L1 and the switching transistor M1, which may affect the reliability of the circuit.

  On the other hand, in the control circuit 100 according to the present embodiment, the switch 24 of the level indeterminacy prevention circuit 20 prevents the drive signal Sd from being transmitted to the switching transistor M1, and the gate of the switching transistor M1 by the pull-down resistor Rpd1. The voltage is fixed to a logic level (ground voltage) at which the switching transistor M1 is turned off. As a result, in a low voltage state where the power supply voltage Vdd is lower than the threshold voltage Vth2, it is possible to prevent the switching transistor M1 from being turned on steadily and to improve the reliability of the circuit.

  Further, in the level indefinite prevention circuit 20 of FIG. 3, an inverter using a combination of a resistor and a transistor is used without using an inverter using a complementary MOSFET (so-called CMOS inverter). When the CMOS inverter is used, the low-voltage detection signal S_UVLO having a small amplitude in the low-voltage state does not stabilize the logic level of the gate of the second transistor M12, and the state of the inverter 26 becomes indefinite, and the input terminal 21 side is driven. The signal Sd may leak to the output terminal 22 side. On the other hand, since the inverter 26 includes the resistor R20 and the inverting transistor M14, the inverting transistor M14 can be reliably switched on and off even when the amplitude of the low voltage detection signal S_UVLO is small. In the state, the second transistor M12 can be reliably turned off.

  The embodiments are exemplifications, and it will be understood by those skilled in the art that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are within the scope of the present invention. .

  In the embodiment, the low voltage detection signal S_UVLO is in the low voltage state. As a result, even when the voltage comparison by the low voltage detection circuit 40 is not stable immediately after the power is turned on, the low voltage detection signal S_UVLO does not exceed the power supply voltage Vdd, so that the low voltage detection signal S_UVLO is at a low level. There are advantages. However, the low voltage detection signal S_UVLO may be at a high level in a low voltage state. In this case, for example, the oscillator 50 may be set to active low. Furthermore, the N channel and the P channel of the first transistor M10 and the second transistor M12 may be switched.

In the level indefinite prevention circuit 20 of FIG. 3, the pull-down resistor Rpd1 is used as voltage fixation prevention, but the present invention is not limited to this. As an alternative element to the pull-down resistor Rpd1, a transistor can be used. For example, a MOSFET or a bipolar transistor may be used. In this case, a signal corresponding to the low voltage detection signal S_UVLO may be input to the gate or base of the transistor so that the transistor is turned on in the low voltage state.
That is, the setting of high level and low level logical values in this embodiment is an example, and can be freely changed by appropriately inverting it with an inverter or the like.

  In the embodiment, the case where the power supply device 200 is a step-up switching regulator has been described. However, the present invention is not limited to this, and a step-down switching regulator or a switching regulator using a transformer instead of an inductor It can be widely used in switching power supplies having switching transistors such as a capacitor charging circuit for generating a high voltage for strobe light emission and a DC / AC converter.

  FIG. 4 is a circuit diagram showing a part of the configuration of a power supply apparatus 200a according to a modification. In this modification, the power supply device 200a of FIG. 4 is a step-down switching regulator. In FIG. 4, components equivalent to those in FIG. 1 are omitted. In the step-down switching regulator, the topology of the output circuit 110a is different from that of FIG. The output circuit 110a includes a rectifier diode D2, an output inductor L2, and an output capacitor C2. The rectifier diode D2 has an anode grounded and a cathode connected to the switching terminal 102. The output inductor L2 has one end connected to the switching terminal 102 and the other end connected to the output terminal 204. The output capacitor C2 has one end connected to the output terminal 204 and the other end grounded. In the modification of FIG. 4, the control circuit 100 a uses not the output voltage Vout but the input voltage Vin output from the battery (not shown) supplied to the power supply voltage terminal 106 as the power supply voltage Vdd.

  The switching transistor M1a is a P-channel MOSFET (or a PNP type bipolar transistor). In this case, since the gate may be fixed to the power supply voltage Vdd so that the P-channel MOSFET is turned off in the low voltage state, it is understood that the pull-up resistor Rpu1 may be used instead of the pull-down resistor Rpd1 as the voltage fixing element. . The pull-up resistor Rpu1 may be provided between the gate of the transistor and the power supply voltage terminal 106. Alternatively, a transistor may be used instead of the pull-up resistor Rpu1. The configuration of the level indefinite prevention circuit 20a is the same as that of the level indefinite prevention circuit 20 in FIG.

  According to the modification of FIG. 4, even when the drive signal Sd is fixed at a low level in the low voltage state, the gate voltage of the switching transistor M1a is set to the source voltage (that is, the input voltage Vin) by the level indefinite prevention circuit 20a. ) To be equal. Therefore, it is possible to prevent the switching transistor M1a from being constantly turned on.

  In the case of a synchronous rectification type switching regulator instead of the diode rectification type, the level indefinite prevention circuit 20 may be provided for both the high-side and low-side transistors.

It is a figure which shows the structure of the power supply device which concerns on embodiment of this invention. It is a block diagram which shows the structure of the electronic device carrying the power supply device of FIG. 1 or FIG. FIG. 2 is a circuit diagram showing a configuration of a level indefinite prevention circuit of FIG. 1. It is a circuit diagram which shows a part of structure of the power supply device which concerns on a modification.

Explanation of symbols

  C1 output capacitor, D1 rectifier diode, L1 output inductor, C2 output capacitor, D2 rectifier diode, L2 output inductor, M1 switching transistor, Q1 auxiliary transistor, R10 first feedback resistor, R11 second feedback resistor, 10 pulse width modulator, 12 error amplifier, 14 PWM comparator, 16 oscillator, 20 level indefinite prevention circuit, 21 input terminal, 22 output terminal, 24 switch, 26 inverter, Rpd1 pull-down resistor, R20 resistor, M10 first transistor, M12 second transistor, M14 inversion Transistor, 30 driver circuit, 40 low voltage detection circuit, 50 oscillator, 100 control circuit, 102 switching terminal, 104 feedback terminal, 106 power supply voltage terminal, 110 output circuit, 111 first terminal, 112 second terminal, 200 power supply device, 202 input terminal, 204 output terminal, 210 load circuit, 220 battery, 300 electronic device.

Claims (14)

A switching power supply control circuit for controlling the on / off state of a switching transistor having a fixed potential at one end,
A low voltage detection circuit that compares a power supply voltage supplied to the control circuit with a predetermined threshold voltage, and outputs a low voltage detection signal of a predetermined level when the power supply voltage is lower than the threshold voltage;
A driver circuit that generates a drive signal to be supplied to a control terminal of the switching transistor based on a pulse signal that defines an on-time of the switching transistor;
A first state provided between the driver circuit and the control terminal of the switching transistor and capable of transmitting the drive signal to the control terminal; and a voltage at the control terminal of the switching transistor at a level at which the switching transistor is turned off. A second state to be fixed, and a level indefinite prevention circuit that can be switched;
Equipped with a,
The level indefinite prevention circuit is
A voltage fixing element that fixes a control terminal of the switching transistor at a voltage level at which the switching transistor is turned off when the low voltage detection signal is at the predetermined level;
A switch that is provided between the driver circuit and the control terminal of the switching transistor and that can be switched between an on state in which the drive signal can be transmitted to the control terminal and an off state in which the output is substantially open; Including
The control circuit is characterized in that the switch is turned off when the low voltage detection signal is at the predetermined level and turned on when the low voltage detection signal is at a level complementary to the predetermined level . The switch is
An input terminal connected to the driver circuit;
An output terminal connected to a control terminal of the switching transistor;
A power supply voltage terminal fixed to the power supply voltage; an inverter that is connected in series between a ground terminal fixed to the ground voltage; and an inverter, and an inverter that inverts the low voltage detection signal;
A first transistor of an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor) having one end connected to the input terminal and the other end connected to the output terminal;
A P-channel MOSFET second transistor having one end connected to the input terminal and the other end connected to the output terminal;
Including
When the low voltage detection signal is input to one gate of the first transistor and the second transistor and the output signal of the inverter is input to the other gate, the switch has the low voltage detection signal at a predetermined level. , the control circuit according to claim 1, wherein the first transistor and the second transistor, characterized in that the turned off. The predetermined level is a low level;
The inversion transistor of the inverter is a P-channel MOSFET having a source connected to the power supply voltage terminal and a gate to which the low voltage detection signal is input.
One end of the resistor of the inverter is connected to the ground terminal, and the other end is connected to the drain of the inverting transistor.
3. The control circuit according to claim 2 , wherein the low voltage detection signal is input to the gate of the first transistor, and the output signal of the inverter is input to the gate of the second transistor. 2. The control according to claim 1 , wherein the voltage fixing element is a resistor provided between a control terminal of the switching transistor and a fixed voltage terminal fixed at a voltage level at which the switching transistor is turned off. circuit. When the switching transistor is an N-channel MOSFET or NPN-type bipolar transistor, the resistance, as claimed in claim 4, wherein a pull-down resistor provided between the control terminal and the ground terminal of the switching transistor Control circuit. When the switching transistor is a P-channel MOSFET or a PNP-type bipolar transistor having one end connected to an input terminal to which an input voltage of the switching power supply is applied.
The control circuit according to claim 4 , wherein the resistor is a pull-up resistor provided between a control terminal of the switching transistor and the input terminal. 2. The transistor according to claim 1 , wherein the voltage fixing element is a transistor that is turned on when a signal corresponding to the low voltage detection signal is input to a control terminal and the power supply voltage is lower than the threshold voltage. Control circuit. An oscillator that generates a pulse signal having a predetermined duty ratio;
An auxiliary switching transistor that is provided in parallel with the switching transistor and is turned on and off based on an output signal of the oscillator when the low voltage detection signal is at a predetermined level;
Control circuit according to any one of claims 1 to 7, characterized by further comprising a.   9. The control circuit according to claim 1, wherein the control circuit is integrated on a single semiconductor substrate. An output circuit including an inductor and a capacitor;
The control circuit according to any one of claims 1 to 9 , which drives a switching transistor connected to the inductor;
A power supply apparatus comprising: The switching transistor is an N-channel MOSFET or an NPN-type bipolar transistor having one end grounded and the other end connected to the inductor,
The power supply device according to claim 10 , wherein the power supply device is a step-up type switching regulator. 12. The power supply apparatus according to claim 11 , wherein the step-up switching regulator output voltage is used as the power supply voltage of the control circuit. The switching transistor is a P-channel MOSFET or a PNP-type bipolar transistor in which an input voltage is applied to one end and the other end is connected to the inductor,
The power supply device according to claim 10 , wherein the power supply device is a step-down switching regulator. Battery,
The power supply device according to any one of claims 10 to 13, wherein the battery voltage is stabilized and output.
An electronic device comprising:

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