JP2013235469A - Constant voltage power supply circuit and overcurrent protection method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a constant voltage power supply circuit capable of highly accurately performing regulating and overcurrent protecting operation even when input voltage has large variations.SOLUTION: A constant voltage power supply circuit 1 includes a first output transistor M1 and a second output transistor Q2 which is controlled by drain current of the first output transistor M1. Overcurrent is detected based on difference between power supply voltage Vin, which is voltage across a current detection resistor R3, and short-circuit voltage Vshort. An overcurrent control signal Vcont is generated based on a result of comparison between first sum voltage, obtained by adding feedback voltage Vfb and second reference voltage Vref2, and second sum voltage obtained by adding overcurrent detection voltage Vsub and third reference voltage Vref3. An overcurrent control transistor M3 provided between the source of the first output transistor M1 and the ground G2 is turned off when the second sum voltage is equal to or greater than the first sum voltage.

Description

  The present invention relates to a constant voltage power supply circuit.

  In various electronic devices, a constant voltage power supply circuit that outputs a variable input voltage with a constant voltage is used.

  Patent Document 1 discloses a constant voltage power supply circuit having an overcurrent protection circuit that prevents an overcurrent exceeding a predetermined current value from being output from an output terminal.

  Patent Document 2 discloses an overcurrent control circuit that detects an overcurrent to a load and controls on / off of a switch element provided between the load and the overcurrent detection resistor, and a power supply side of the overcurrent detection resistor. A power supply circuit is disclosed that includes a detection circuit that detects the interruption between the terminal and the overcurrent control circuit, and the detection circuit turns off the switch element when the interruption is detected. As a result, overcurrent protection is enabled even when the power supply side of the overcurrent detection resistor and the overcurrent control circuit are interrupted.

  Patent Document 3 discloses a configuration in which a short-circuit protection circuit and a limit circuit are mounted on one circuit in an overcurrent protection circuit in a DC stabilized power supply circuit for the purpose of preventing the circuit configuration from becoming complicated.

  Patent Document 4 aims to realize a so-called U-shaped current cutoff characteristic in a stabilized power supply circuit having an overcurrent protection function against an output current without limiting the output voltage range. A configuration is disclosed in which the output current and output voltage are reduced when a current is detected.

JP 2006-309569 A JP 2009-240007 A JP 2003-186554 A Japanese Patent Laid-Open No. 2002-23868

  In recent years, the constant voltage power supply circuit is often used in an environment where the fluctuation of the input voltage is large. For example, the output voltage of a battery that supplies power to an in-vehicle electronic device varies greatly depending on whether it is discharging or charging. Usually, the output voltage of the battery varies from several volts during discharging to several tens of volts during charging.

  Usually, for a low input voltage of about several volts, an LDO (Low Dropout) type regulator circuit having a small input / output potential difference is used. On the other hand, for a high input voltage of several tens of volts, a regulator circuit that can take a high breakdown voltage, a large power loss and the like is used. When a large current output is required, power loss cannot be allowed inside the IC. Therefore, there is an LDO type regulator circuit that performs power control by a discrete element called a power device having a large element breakdown voltage and handles a control signal in the IC. Used.

  In addition, the constant voltage power supply circuit requires an overcurrent protection function in order to protect electronic devices such as microcomputers that are supplied with a constant voltage. An excellent overcurrent suppressing effect is obtained by a U-shaped current interruption characteristic as described in Patent Documents 1 and 4. The configuration, operation, and problems of the constant voltage power supply circuit according to Patent Document 1 will be described below. FIG. 5 shows a configuration of a constant voltage power supply circuit according to Patent Document 1.

  In FIG. 5, the constant voltage power supply circuit 101 generates a predetermined constant voltage from the input voltage Vin input to the input terminal IN, and outputs it from the output terminal OUT as the output voltage Vo. The constant voltage power supply circuit 101 includes a constant voltage circuit unit 102 and an overcurrent protection circuit unit 103. The constant voltage circuit unit 102 converts the input voltage Vin into a predetermined constant voltage and outputs it as an output voltage Vo from the output terminal OUT. When the output current io output from the output terminal OUT becomes a predetermined value or more, the overcurrent protection circuit unit 103 reduces the output current io while reducing the output voltage Vo with respect to the constant voltage circuit unit 102. Realize character characteristics.

  The constant voltage circuit unit 102 generates and outputs a predetermined reference voltage Vref, output voltage detection resistors R1 and R2 that divide the output voltage Vo to generate and output a divided voltage VFB, and gates An error amplifier that controls the operation of the output transistor M1 that is a PMOS transistor that controls the current io output from the output terminal OUT according to the input signal and the output transistor M1 so that the divided voltage VFB becomes the reference voltage Vref. Circuit A1.

  The overcurrent protection circuit unit 103 includes a differential amplifier circuit A2, PMOS transistors M2 and M3, NMOS transistor M4, and resistors R3 and R4. The differential amplifier circuit A2 includes PMOS transistors M5 and M6, NMOS transistors M7 and M8, and a constant current source 12. The error amplifier circuit A1, the circuit that generates the reference voltage Vref, and the resistors R1 and R2 form an output voltage control unit. The PMOS transistor M2 is a current detection transistor. The resistor R3 forms a current-voltage conversion circuit. The NMOS transistor M4 and the resistor R4 form an amplitude expansion circuit.

  In the constant voltage circuit unit 102, an output transistor M1 is connected between the input terminal IN and the output terminal OUT, and resistors R1 and R2 are connected in series between the output terminal OUT and the ground voltage. In the error amplifier circuit A1, the output terminal is connected to the gate of the PMOS transistor M1, the divided voltage VFB is input to the non-inverting input terminal, and the reference voltage Vref is input to the inverting input terminal.

  In the overcurrent protection circuit unit 103, the source of the PMOS transistor M2 is connected to the input terminal IN, and the gate of the PMOS transistor M2 is connected to the gate of the output transistor M1. A resistor R3 is connected between the drain of the PMOS transistor M2 and the ground voltage. The connection portion between the PMOS transistor M2 and the resistor R3 is connected to the gate of the PMOS transistor M6 that forms one input terminal of the differential amplifier circuit A2. A resistor R4 and an NMOS transistor M4 are connected in series between the input terminal IN and the ground voltage. The output terminal of the differential amplifier circuit A2, that is, the connection portion between the PMOS transistor M5 and the NMOS transistor M7 is connected to the gate of the NMOS transistor M4. The divided voltage VFB is input to the other input terminal of the differential amplifier circuit A2. A PMOS transistor M3 is connected between the input terminal IN and the gate of the output transistor M1. The gate of the PMOS transistor M3 is connected to the connection portion between the resistor R4 and the NMOS transistor M4.

  The sources of the PMOS transistors M5 and M6 forming the differential pair are connected to each other, and a constant current source 12 is connected between the connection portion and the input terminal. The NMOS transistors M7 and M8 form a current mirror circuit. The gates of the NMOS transistors M7 and M8 are connected to each other, and the connection is connected to the drain of the NMOS transistor M8. The sources of the NMOS transistors M7 and M8 are connected to the ground voltage. The drain of the NMOS transistor M7 is connected to the drain of the PMOS transistor M5, and the connection portion forms the output terminal of the differential amplifier circuit A2, and is connected to the gate of the NMOS transistor M4.

  The drain of the NMOS transistor M8 is connected to the drain of the PMOS transistor M6. A voltage Vof added to the source of the PMOS transistor M5 represents an offset voltage of the differential pair of the PMOS transistors M5 and M6.

  In such a configuration, the error amplifier circuit A1 controls the operation of the output transistor M1 so that the input divided voltage VFB becomes the reference voltage Vref.

  When the output current io is less than the predetermined overcurrent protection current value, the drain current of the current detection transistor M2 is small, and the voltage drop in the resistor R3 is smaller than the voltage obtained by adding the offset voltage Vof to the divided voltage VFB. The PMOS transistor M6 is turned on, the PMOS transistor M5 is turned off, and the drain voltage of the PMOS transistor M5 becomes almost the ground voltage. As a result, the NMOS transistor M4 is turned off, and the drain voltage of the NMOS transistor M4, that is, the gate voltage of the PMOS transistor M3 becomes substantially equal to the input voltage Vin. As a result, the PMOS transistor M3 is turned off and the overcurrent protection operation is not performed.

  On the other hand, when the output current io exceeds the overcurrent protection current value, the voltage drop of the resistor R3 becomes equal to the voltage obtained by adding the offset voltage Vof to the divided voltage VFB. Therefore, the drain voltage of the PMOS transistor M5 rises and the NMOS transistor M4 is turned on. Since the source of the NMOS transistor M4 is grounded, the drain voltage of the NMOS transistor M4 can be lowered to almost the ground voltage. Therefore, even when the input voltage Vin is small, the PMOS transistor M3 can be turned on. When the PMOS transistor M3 is turned on, the gate voltage of the output transistor M1 decreases, the increase in the output current io is suppressed, and the output voltage Vo decreases.

  When the output voltage Vo decreases, the gate voltage of the PMOS transistor M5 also decreases. Thereby, even if the voltage drop of the resistor R3 is small, the overcurrent protection function functions, the output voltage Vo decreases, and the output current io decreases. The gate voltage of the PMOS transistor M6 when the output terminal OUT is short-circuited to the ground voltage is the same voltage as the offset voltage Vof. The output current io when the output is short-circuited is a current value obtained by multiplying the current flowing through the resistor R3 by the current ratio between the output transistor M1 and the current detection transistor M2. That is, the magnitude of the short-circuit current can be set by the voltage of the offset voltage Vof and the resistance value of the resistor R3.

  The amplitude of the drain voltage of the PMOS transistor M5, which is the output voltage of the differential amplifier circuit A2, is expanded from the ground voltage to the input voltage Vin by the inverter circuit configured by the NMOS transistor M4 and the resistor R4 as described above. Therefore, even when the input voltage Vin is small, the on / off control of the PMOS transistor M3 is possible, and the overcurrent protection operation can be performed by controlling the output transistor M1.

  As described above, in the constant voltage power supply circuit according to Patent Document 1, the output voltage of the differential amplifier A2 having a small amplitude is input from the ground voltage by the amplitude expansion circuit configured by the inverter circuit including the NMOS transistor M4 and the resistor R4. The amplitude is expanded to the vicinity of the voltage Vin and is input to the gate of the PMOS transistor M3 that directly controls the output transistor M1. Thereby, even when the input voltage Vin is small, the on / off control of the PMOS transistor M3 can be performed, and the overcurrent protection operation can be performed by controlling the output transistor M1.

  Here, in the constant voltage power supply circuit according to Patent Document 1, when the overcurrent flows and the output voltage Vo decreases, the output of the error amplifier A1 decreases due to the feedback operation, the gate potential of the output transistor M1 decreases, and the output The action of increasing the voltage Vo works. At this time, the PMOS transistor M3 acts to increase the gate potential of the output transistor M1 and decrease the output current io. That is, in the state where the overcurrent is detected, the current limiting operation of the transistor M3 and the regulating operation by A1 are contradictory, and the output transistor M1 is controlled in the on / on state.

  Therefore, the operating point of the output transistor M1 is determined by the balance between the sink current of the error amplifier A1 and the current supply capability of the PMOS transistor M3. However, since the drain voltage and the gate voltage of the PMOS transistor M3 vary depending on the input voltage with respect to the error amplifier A1 that is less dependent on the power supply (input) voltage, the operating point of the output transistor M1 varies depending on the input voltage. That is, in the constant voltage power supply circuit according to Patent Document 1, when the potential difference between Vin and Vout is small, an overcurrent cutoff characteristic can be obtained with high accuracy, but when the potential difference is large, the operation of the output transistor M1 is achieved. Since the point is greatly deviated, there is a problem that the current detection accuracy is lowered and the accuracy of the overcurrent cutoff characteristic is lowered. For example, such a problem becomes conspicuous under the condition that Vin varies between 1.2 times and 10 times Vout.

  In one embodiment, the first output transistor controlled by the comparison result between the feedback voltage proportional to the output voltage and the first reference voltage, and the input voltage controlled by the drain current of the first output transistor are input. And a second output transistor for switching connection between the input terminal for outputting and the output terminal for outputting the output voltage. The occurrence of overcurrent is detected based on the difference between the input voltage, which is the voltage across the current detection resistor disposed between the input terminal and the second output transistor, and the short-circuit voltage. A first addition voltage obtained by adding the feedback voltage and a second reference voltage that is a divided voltage of the first reference voltage; an overcurrent detection voltage output when an overcurrent occurs; and a first reference voltage The overcurrent control signal is generated based on the comparison result with the second added voltage obtained by adding the third reference voltage that is the divided voltage. An overcurrent control transistor is provided between the source of the first output transistor and ground. The overcurrent control transistor is turned off when an overcurrent control signal indicating that the second addition voltage is equal to or higher than the first addition voltage is output.

  According to the above-described embodiment, it is possible to perform the regulation operation and the overcurrent protection operation with high accuracy even when the fluctuation of the input voltage is large.

1 is a circuit diagram illustrating a configuration of a constant voltage power supply circuit according to a first embodiment. It is a graph which shows the electric current interruption characteristic of a U-shape. FIG. 6 is a circuit diagram illustrating a configuration of a constant voltage power supply circuit according to a second embodiment. FIG. 6 is a circuit diagram illustrating a configuration of a constant voltage power supply circuit according to a third embodiment. It is a circuit diagram which shows the structure of the conventional constant voltage power supply circuit.

  Hereinafter, embodiments will be described with reference to the drawings. Since the drawings are simplified, the technical scope of the embodiments should not be narrowly interpreted based on the description of the drawings. Moreover, the same code | symbol is attached | subjected to the same element and the overlapping description is abbreviate | omitted.

  In the following embodiments, when it is necessary for the sake of convenience, the description will be divided into a plurality of sections or embodiments. However, unless otherwise specified, they are not irrelevant to each other. Are partly or entirely modified, application examples, detailed explanations, supplementary explanations, and the like. Also, in the following embodiments, when referring to the number of elements (including the number, numerical value, quantity, range, etc.), particularly when clearly indicated and when clearly limited to a specific number in principle, etc. Except, it is not limited to the specific number, and may be more or less than the specific number.

  Further, in the following embodiments, the constituent elements (including operation steps and the like) are not necessarily essential unless otherwise specified, or in principle considered to be clearly essential in principle. Similarly, in the following embodiments, when referring to the shape, positional relationship, etc., of components, etc., the shape is substantially the same unless otherwise specified or otherwise apparent in principle. And the like are included. The same applies to the above numbers and the like (including the number, numerical value, quantity, range, etc.).

Embodiment 1
FIG. 1 shows a configuration of a constant voltage power supply circuit 1 according to the first embodiment. The constant voltage power circuit 1 includes a regulator circuit 4, an overcurrent detection circuit 5, an overcurrent control signal generation circuit 6, and an overcurrent control circuit 7.

  The regulator circuit 4 adjusts the input voltage Vin input to the input terminal 2 to an output voltage Vout having a constant voltage value, and outputs it from the output terminal 3. The regulator circuit 4 includes a first output transistor M1 and a second output transistor Q1. The first output transistor M1 is controlled by a comparison result between the feedback voltage Vfb proportional to the output voltage Vout and the first reference voltage Vref1. The second output transistor Q1 is controlled by the drain current of the first output transistor M1, and switches the connection between the input terminal 2 and the output terminal 3.

  The overcurrent detection circuit 5 outputs an overcurrent detection voltage Vsub that is proportional to the short circuit voltage Vshort when the difference between the input voltage Vin, which is the voltage across the current detection resistor R3, and the short circuit voltage Vshort exceeds a predetermined value. The current detection resistor R3 is disposed between the input terminal 2 and the second output transistor Q1.

  The overcurrent control signal generation circuit 6 includes a first addition voltage obtained by adding the feedback voltage Vfb and the second reference voltage Vref2, and a second addition voltage obtained by adding the overcurrent detection voltage Vsub and the third reference voltage. The overcurrent control signal (voltage) Vcont, which is a comparison result with the above, is output. The second and third reference voltages Vref2 and Vref3 are voltages generated by dividing the first reference voltage Vref.

  The overcurrent control circuit 7 includes an overcurrent control transistor M3. The overcurrent control transistor M3 is provided between the source of the first output transistor M1 and the ground G2, and indicates that the second added voltage (Vsub + Vref3) is equal to or higher than the first added voltage (Vfb + Vref2). When the overcurrent control voltage Vcont is output, it is turned off.

  With the above configuration, when Vsub + Vref3 <Vfb + Vref2 (normal time), the overcurrent control transistor M3 is turned on, and secondly, the output transistor Q1 is controlled only by the first output transistor M1 to perform normal regulation operation. Is executed. On the other hand, when Vsub + Vref3 ≧ Vfb + Vref2 (when an overcurrent occurs), the output voltage Vout decreases, so the first output transistor M1 is turned on, but the overcurrent control transistor M3 is turned off. As a result, the base current of the second output transistor Q1 is reduced, the second output transistor Q1 is turned off, and an overcurrent suppressing effect is obtained.

  Hereinafter, the configuration of the constant voltage power supply circuit 1 will be described in more detail. In the present embodiment, the second output transistor Q1 and the current detection resistor R3 are discrete elements arranged outside the IC package of the constant voltage power supply circuit 1. This is because these elements Q1 and R3 generate a large amount of heat.

  The regulator circuit 4 includes a first output transistor M1, a second output transistor Q1, a reference power supply V11, resistors R20, R21, R22, resistors R1, R2, and an amplifier A1. The reference power supply V11 generates a first reference voltage Vref1. The resistors R20, R21, and R22 divide the first reference voltage Vref1 to generate a second reference voltage Vref2 and a third reference voltage Vref3. Resistors R1 and R2 divide the output voltage Vout to generate a feedback signal Vfb. The amplifier A1 compares the first reference voltage Vref1 and the feedback signal Vfb and amplifies the difference. In the present embodiment, the first output transistor M1 is an NMOS transistor, and the second output transistor Q1 is a PNP transistor. The first output transistor M1 converts the output signal of the amplifier A1 into a current, and controls the base current of the second output transistor Q1.

  The overcurrent detection circuit 5 includes an amplifier A2, feedback resistors R10, R11, R12, R13, and a transistor M2. The amplifier A2 amplifies the difference between the input voltage Vin and the short circuit voltage Vshort. The transistor M2 controls the output of the overcurrent detection voltage Vsub. The source of the transistor M2 is connected to the ground G1, the gate is connected to the output terminal of the amplifier A2, the drain is a terminal (inverting input terminal −) to which the short-circuit voltage of the amplifier A2 is input, and the transistor of the overcurrent control signal generation circuit 6 It is connected to the gate of M6.

  The overcurrent control signal generation circuit 6 includes a first differential pair 11, a second differential pair 12, a current mirror 13, a first current source I12, and a second current source I13. The first differential pair 11 includes a PMOS transistor M5 to which the feedback voltage Vfb is input to the gate and a PMOS transistor M6 to which the overcurrent detection voltage Vsub is input to the gate, and the feedback voltage Vfb and the overcurrent detection voltage Vsub are Amplify the difference. The second differential pair 12 includes a PMOS transistor M7 to which the second reference voltage Vref2 is input to the gate and a PMOS transistor M8 to which the third reference voltage Vref3 is input to the gate, and the second reference voltage Vref2 The difference from the third reference voltage Vref3 is amplified. The current mirror 13 includes an NMOS transistor M9 and an NMOS transistor M10. The drain of the transistor M9 is connected to the drain of the transistor M5, the drain of the transistor M7, and the gate of the overcurrent control transistor M3. The drain of the transistor M10 is connected to the drain of the transistor M6 and the drain of the transistor M8, and the gate is connected to the gate of the transistor M9 and its own drain. The first current source I12 supplies a constant current to the first differential pair 11. The second current source I13 supplies a constant current to the second differential pair 12.

  The overcurrent control transistor M3 constituting the overcurrent control circuit 7 is an NMOS transistor, the source is connected to the ground G2, and the drain is connected to the source of the first output transistor M1. The overcurrent control transistor M3 has a voltage indicating that the overcurrent control voltage Vcont input to the gate is in a state of Vsub + Vref3 (second added voltage) <Vfb + Vref2 (first added voltage) (normal time) Is turned on, and when it is a voltage indicating that Vsub + Vref3 (second added voltage) ≧ Vfb + Vref2 (first added voltage) (when overcurrent occurs), it is turned off.

  Hereinafter, the operation of the constant voltage power supply circuit 1 will be described. First, the regulation operation will be described. When power is supplied to Vdd of the regulator circuit 4, the first reference voltage Vref1 is generated, and the output voltage Vout is divided by the resistors R1 and R2 to generate the feedback voltage Vfb.

  The amplifier A1 compares the first reference voltage Vref1 and the feedback voltage Vfb, and supplies a differential amplification signal to the gate of the first output transistor M1. The differential amplification signal is converted into a drain current by the first output transistor M1, and controls the base current of the second output transistor Q1. As a result, a constant output voltage Vout is generated.

  Here, in the normal state (when no overcurrent occurs), the overcurrent control transistor M3 is on, so the drain current of the first output transistor M1 is not controlled by the overcurrent control transistor M3.

Therefore, the feedback voltage Vfb becomes equal to the first reference voltage Vref1 by the feedback loop of the amplifier A1, and therefore the output voltage Vout is
Vout = ((R1 + R2) / (R2)) × Vref1
It becomes. Here, the resistance value of the resistor R1 is R1, and the resistance value of the resistor R2 is R2. (Hereinafter, the resistance value of the resistor Rxx will be referred to as Rxx, and the voltage value at the point indicated as Vxxx in the names of the node and terminal will be referred to as Vxxx.)

  Next, the overcurrent protection operation will be described. The overcurrent is generated by detecting the voltage (Vin, Vshort) across the current detection resistor R3 by the overcurrent detection circuit 5 which is a subtracting amplifier circuit and comparing it by the overcurrent control signal generation circuit 6, and the difference between the two voltages is the reference. This is done by determining whether or not the threshold is exceeded.

The amplifier A2 is a feedback circuit including resistors R10 and R11, and outputs an overcurrent detection voltage Vsub so that the inverting input terminal (−) becomes equal to the potential of the non-inverting input terminal (+). Here, when the amplification factor of the amplifier A3 when the resistance values of the resistors R10 and R12 and the resistance values of the resistors R11 and R13 are the same is A, the overcurrent detection voltage Vsub is expressed by the following equation (1). The
R10 = R12, R11 = R13
Vsub = A × (Vin−Vshort)
A = (R11 / R10)
Vin−Vshort = Iout × R3
Therefore,
Vsub = (R11 / R10) × Iout × R3 (1)

  Therefore, the voltage across the current detection resistor R3, that is, a voltage obtained by multiplying the difference voltage between the input voltage Vin and the short-circuit voltage Vshort by A is output as the current detection signal Vsub.

  Next, the generation operation of the overcurrent control voltage Vcont will be described. In principle, the overcurrent control signal generation circuit 6 performs normal regulation operation and overcurrent based on the result of comparing the feedback signal Vfb and the overcurrent detection voltage Vsub by differential amplification of the PMOS transistors M5 and M6. An overcurrent control voltage Vcont that switches between protection operations is generated. However, in this embodiment, the result of comparing the second reference voltage Vref2 and the third reference voltage Vref3 by differential amplification of the NMOS transistors M7 and M8 is the result of the feedback signal Vfb and the overcurrent detection voltage Vsub. Is added to the comparison result.

  In the state of Vsub + Vref3 <Vfb + Vref2 (normal time), the NMOS transistor M9 is turned off and the overcurrent control transistor M3 is turned on. Thereby, the on / off control of the second output transistor is performed only by the first output transistor M1, and the normal regulation operation is performed.

  On the other hand, in the state of Vsub + Vref3 ≧ Vfb + Vref2 (when an overcurrent occurs), the first output transistor M1 is turned on due to the decrease in the output voltage Vout, but the NMOS transistor M9 is turned on and the overcurrent control transistor M3 is turned off. . As a result, the base current of the second output transistor Q1 is reduced, the second output transistor Q1 is turned off, and an overcurrent protection operation is performed.

When the threshold voltage of the overcurrent detection voltage Vsub at the time of switching between the normal regulation operation and the overcurrent protection operation is Vth, Vth is expressed by the following equation (2).
Vth = Vfb + Vref2-Vref3 (2)
Here, Vfb is the voltage of the feedback voltage Vfb, Vref2 is the voltage of the second reference voltage Vref2, and Vref3 is the voltage of the third reference voltage Vref3. This is a fixed value determined by setting the resistance values of R1 and R2.

Further, since the load current Iout where Vth = Vsub is the overcurrent detection current Ipeak, when this is added to the equation (1), Ipeak is expressed by the following equation (3).
(R11 / R10) × Ipeak × R3 = Vfb + Vref2−Vref3
Ipeak = (R10 / (R11 × R3)) × (Vfb + Vref2−Vref3) (3)

  Further, since Vfb is 0 V, the short-circuit current Ishort is expressed by the following formula (4).

  Ishort = (R10 / (R11 × R3)) × (Vref2−Vref3) (4)

  FIG. 2 shows current interruption characteristics (DC characteristics) in the overcurrent protection operation. In the figure, the horizontal axis represents the load current Iout, and the vertical axis represents the output voltage Vout. This figure shows a current interruption characteristic called “f-shaped characteristic”.

  The U-shaped characteristic is so called because the relation between the load current Iout and the output voltage Vout becomes a “F” shape by combining the characteristics of the normal regulation operation and the characteristics of the overcurrent protection operation. In the figure, point A indicates a point where the load current Iout is 0, point B indicates a point where the presence or absence of overcurrent is detected and the operating characteristics are switched, and point C indicates a point where the load is completely short-circuited.

  The operating point of normal regulation operation moves from point A to point B as the load current Iout increases. Thereafter, when the load current Iout increases and exceeds a preset current value, the occurrence of overcurrent is detected, and the operation is switched to the overcurrent protection operation. Thereafter, when the load current Iout is limited in the overcurrent protection operation, the output voltage Vout decreases with a decrease in the impedance of the load, so that the current interruption characteristic moves from the point B to the point C.

  That is, when the load current Iout increases from 0 in the normal regulation operation, the operating point moves from A to B point. Thereafter, when the load current Iout exceeds the overcurrent detection current and the overcurrent protection operation is started, the operating point moves from the B point toward the C point in accordance with a decrease in the impedance of the load. The load current Iout at point C when the load is completely short-circuited and the output Vout becomes 0 becomes the short-circuit current Ishort.

  Here, the points A, B, and C can be set by a calculation formula at the circuit design stage, but change due to fluctuations in the input voltage Vin, variations in circuit constants, temperature changes, and the like. However, according to the present embodiment, even when the input voltage Vin fluctuates greatly, fluctuations at the point B of the overcurrent detection current Ipeak and the point C of the short circuit current Ishort in the characteristics of the overcurrent protection operation are suppressed. The overcurrent detection circuit 5 operates according to the equation (1) as described above, and detects the overcurrent based on the difference between the input voltage Vin and the short-circuit voltage Vshort, so that the input voltage Vin is large (for example, 1 to 10 times). This is because the overcurrent can be accurately detected even when the change occurs. Further, the overcurrent detection voltage Vsub is converted into a low potential signal with the ground G1 as a reference, and also functions as a level shifter.

  Further, the point B of the overcurrent detection current Ipeak is, as shown in the equation (3), a current detection resistor R3, resistors R1, R2, R10, R11, R12, R13, R20, R21, R22, and the first reference. It is determined by the voltage Vref1. That is, since the input voltage Vin is not included in the expression (3), the point B of the overcurrent detection current Ipeak is not affected even when the input voltage Vin changes greatly.

  Further, the point C of the short-circuit current Ishort is that the accuracy of the short-circuit current Ishort is represented by the equation (4) as follows: current detection element R3, resistors R11, R12, R13, R20, R21, R22, and the first reference voltage. It is determined by Vref1. That is, since the input voltage Vin is not included in the expression (4), even if the input voltage Vin changes greatly, the point C of the short-circuit current Ishort is not affected.

  In addition, since the first output transistor M1 and the overcurrent control transistor M3 are vertically stacked (in series connection), fluctuations in drain currents of both transistors M1 and M3 due to channel length modulation are suppressed. Since this effect is the same for the fluctuation of the input voltage Vin, the fluctuations at the point B of the overcurrent detection current Ipeak and the point C of the short-circuit current Ishort are suppressed.

  As described above, according to the present embodiment, even when the input voltage Vin varies, switching between the normal regulation operation and the overcurrent protection operation is performed appropriately, and high-precision operation characteristics are realized. It becomes possible. Therefore, the constant voltage power supply circuit 1 can be particularly suitably used for an in-vehicle electronic device or the like that receives the input voltage Vin from a battery whose output voltage varies greatly.

Embodiment 2
FIG. 3 shows a configuration of the constant voltage power supply circuit 21 according to the second embodiment. The difference between the constant voltage power supply circuit 21 and the constant voltage power supply circuit 1 according to the first embodiment is the overcurrent control signal generation circuit 26.

  The overcurrent control signal generation circuit 26 according to the present embodiment is a folded cascode type amplifier circuit that performs the same operation as the overcurrent control signal generation circuit 6 according to the first embodiment. The overcurrent control signal generation circuit 26 includes PMOS transistors M21 and M22, NMOS transistors M23 to M26, and a third constant current source I21 in addition to the constituent elements of the overcurrent control signal generation circuit 6. The configuration shown in FIG. 3 is an application of the configuration of a well-known folded cascode type amplifier circuit, and therefore description of the configuration and operation is omitted.

  Since the folded cascode type amplifier circuit has a wider input voltage range than a normal differential amplifier circuit, it can operate even when the input level of the overcurrent detection voltage Vsub is low. That is, overcurrent can be detected even if the difference between the input voltage Vin and the short-circuit voltage Vshort is small. As a result, it is possible to improve the characteristics in the vicinity of the point B of the overcurrent detection current Ipeak of the current interruption characteristic as compared with the first embodiment. Alternatively, the current drop around the current detection resistor R3 is reduced by reducing the current detection resistor R3 while maintaining the accuracy near the point B, and the overcurrent detection operation is performed even if the difference between the input voltage Vin and the short-circuit voltage Vshort is small. It is also possible to be able to perform. This embodiment is particularly suitable for an in-vehicle electronic device that receives power supply from a battery having a large voltage fluctuation because of such advantages.

Embodiment 3
FIG. 4 shows the configuration of the constant voltage power supply circuit 31 according to the third embodiment. The constant voltage power supply circuit 31 according to the present embodiment realizes the same operation as the overcurrent protection operation in the first and second embodiments by digital processing by program control.

  Unlike the first and second embodiments, the regulator circuit 34 according to the present embodiment does not have a circuit that generates the second and third reference voltages Vref2 and Vref3.

  The overcurrent detection circuit 35 according to the present embodiment includes a subtractor 41 and an amplifier A3. The input voltage Vin and the short-circuit voltage Vshort are input to the subtractor 41, and when the difference between the voltages Vin and Vshort exceeds a predetermined value, the amplifier A3 outputs an overcurrent detection voltage Vsub that is proportional to the short-circuit voltage Vshort.

  The overcurrent control signal generation circuit 36 according to the present embodiment includes a logic circuit 45, a first AD converter 46, a second AD converter 47, and a DA converter 48. The logic circuit 45 is a microcomputer that performs arithmetic processing according to a program, a PLD (Programmable Logic Device), or the like. The first and second AD converters 46 and 47 AD convert the feedback voltage Vfb, the first reference voltage Vref1 and the overcurrent detection voltage Vsub, and output each digital data to the logic circuit 45. The DA converter 48 DA converts the digital data generated by the logic circuit 45 and outputs an overcurrent control voltage Vcont.

  The logic circuit 45 uses the digital data corresponding to the second reference voltage Vref2 and the third reference voltage Vref3 based on the digital data of the first reference voltage Vref1 supplied from the first and second AD converters 46. The digital data corresponding to is generated. After that, the logic circuit 45 generates the feedback voltage Vfb, the second reference voltage Vref2, and the second reference voltage Vref2 based on the digital data of the feedback voltage Vfb, the overcurrent detection voltage Vsub, the second reference voltage Vref2, and the third reference voltage Vref3. The digital data corresponding to the overcurrent control voltage Vcont, which is a comparison result of the first addition voltage obtained by adding the above, the overcurrent detection voltage Vsub, and the second addition voltage obtained by adding the third reference voltage Vref3, is output. The DA converter 48 DA-converts the digital data and outputs an overcurrent control voltage Vcont.

  Thereby, the overcurrent control signal generation circuit 36 can generate the overcurrent control voltage Vcont, similarly to the overcurrent control signal generation circuits 6 and 26 according to the first and second embodiments. The operation of the overcurrent control transistor M3 is the same as in the first and second embodiments.

  As described above, the above-described overcurrent protection operation can be performed by using software means. This makes it possible to share hardware with various electronic devices. In addition, it is possible to improve maintenance by updating the program. Further, according to the present embodiment, a circuit such as a voltage dividing resistor for generating the second and third reference voltages Vref2 and Vref3 can be eliminated.

  As mentioned above, the invention made by the present inventor has been specifically described based on the embodiment. However, the present invention is not limited to the embodiment, and various modifications can be made without departing from the scope of the invention. Needless to say. For example, in the above-described embodiment, the configuration in which the current detection resistor R3 is included in the regulator circuits 4 and 34 is shown. However, this is for convenience in the drawings, and defines the technical scope of the present invention. It is not a thing. In addition, it goes without saying that the constant voltage power supply circuit according to the above embodiment can be suitably used not only in an in-vehicle electronic device but also in various electronic devices such as home appliances.

1, 2, 36 Constant voltage power supply circuit 2 Input terminal 3 Output terminal 4, 34 Regulator circuit 5, 35 Overcurrent detection circuit 6, 26, 36 Overcurrent control signal generation circuit 7 Overcurrent control circuit 11 First differential pair 12 second differential pair 13 current mirror 41 subtractor 45 logic circuit 46 first AD converter 47 second AD converter 48 DA converter A1, A2, A3 amplifier M1, M2, M3, M9, M10 NMOS Transistors (MA: first output transistor, M3: overcurrent control transistor, M9: fifth transistor, M10: sixth transistor)
M5, M6, M7, M8 PMOS transistors (M5: first transistor, M6: second transistor, M7: third transistor, M8: fourth transistor)
I12 1st constant current source I13 2nd constant current source I21 3rd constant current source R1, R2, R3, R10, R11, R12, R13, R20, R21, R22 resistance (R3: current detection resistance)
Q1 PNP transistor (second output transistor)
V11 Reference power supply Vin Input voltage Vout Output voltage Vfb Feedback voltage Vref1 First reference voltage Vref2 Second reference voltage Vref3 Third reference voltage Vshort Short-circuit voltage Vsub Overcurrent detection voltage Vcont Overcurrent control voltage (overcurrent control signal)

Claims (10)

A circuit for adjusting and outputting an input voltage to a constant voltage, the first output transistor being controlled by a comparison result between a feedback voltage proportional to the output voltage and a first reference voltage, and the first output A regulator circuit comprising a second output transistor that is controlled by a drain current of the transistor and that switches connection between an input terminal that inputs the input voltage and an output terminal that outputs the output voltage;
The input voltage, which is a voltage across a current detection resistor disposed between the input terminal and the second output transistor, is compared with a short-circuit voltage, and a difference between the input voltage and the short-circuit voltage is a predetermined value. An overcurrent detection circuit that outputs an overcurrent detection voltage proportional to the short-circuit voltage when exceeded,
A first added voltage obtained by adding the feedback voltage and a second reference voltage generated by dividing the first reference voltage, and the overcurrent detection voltage and the first reference voltage are divided and generated. An overcurrent control signal generation circuit that outputs an overcurrent control signal that is a comparison result with a second added voltage obtained by adding the third reference voltage to be added;
Provided between the source of the first output transistor and the ground, and turned off when the overcurrent control signal indicating that the second added voltage is equal to or higher than the first added voltage is output. An overcurrent control circuit comprising an overcurrent control transistor;
A constant voltage power supply circuit comprising: The overcurrent detection circuit includes:
An amplifier that amplifies the difference between the input voltage and the short-circuit voltage;
A transistor having a source connected to ground, a gate connected to the output terminal of the amplifier, and a drain connected to the terminal for inputting the short-circuit voltage of the amplifier and the overcurrent control signal generation circuit;
Comprising
The constant voltage power supply circuit according to claim 1. The overcurrent control signal generation circuit
A first differential pair including a first transistor that inputs the feedback voltage to the gate, and a second transistor that inputs the overcurrent detection voltage to the gate;
A second differential pair consisting of a third transistor having the second reference voltage input to the gate and a fourth transistor having the third reference voltage input to the gate;
A drain connected to the drain of the first transistor, a drain of the third transistor, and a gate of the overcurrent control transistor; and a drain connected to the drain of the second transistor and the fourth transistor. A current mirror comprising a sixth transistor having a gate connected to the drain of the fifth transistor and a gate connected to the gate of the fifth transistor;
A first current source for supplying a constant current to the first differential pair;
A second current source for supplying a constant current to the second differential pair;
Comprising
The constant voltage power supply circuit according to claim 1. The overcurrent control signal generation circuit comprises a folded cascode type circuit,
The constant voltage power supply circuit according to claim 1. The overcurrent control signal generation circuit converts analog data related to the first addition voltage and the second addition voltage into digital data, and based on the digital data, the first addition voltage and the second addition Comparing the voltages, and generating the overcurrent control signal based on the comparison result;
The constant voltage power supply circuit according to claim 1. The current control transistor is a discrete element;
The constant voltage power supply circuit according to claim 1. The current detection resistor is a discrete element.
The constant voltage power supply circuit according to claim 1. It has a U-shaped current interruption characteristic,
The constant voltage power supply circuit according to claim 1. Receiving the input voltage from the vehicle battery,
The constant voltage power supply circuit according to claim 1. A circuit for adjusting and outputting an input voltage to a constant voltage, the first output transistor being controlled by a comparison result between a feedback voltage proportional to the output voltage and a first reference voltage, and the first output An overcurrent protection method for a constant voltage power supply circuit, comprising: a second output transistor that switches connection between an input terminal that is controlled by a drain current of a transistor and inputs the input voltage; and an output terminal that outputs the output voltage,
Comparing the input voltage, which is a voltage across a current detection resistor disposed between the input terminal and the second output transistor, with a short-circuit voltage;
When the difference between the input voltage and the short circuit voltage exceeds a predetermined value, outputting an overcurrent detection voltage proportional to the short circuit voltage;
A first added voltage obtained by adding the feedback voltage and a second reference voltage generated by dividing the first reference voltage, and the overcurrent detection voltage and the first reference voltage are divided and generated. Outputting an overcurrent control signal which is a comparison result with a second added voltage obtained by adding the third reference voltage to be added;
An overcurrent provided between the source of the first output transistor and the ground when the overcurrent control signal indicating that the second added voltage is equal to or higher than the first added voltage is output. Turning off the control transistor;
An overcurrent protection method for a constant voltage power supply circuit comprising:

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