JP2012064009A - Voltage output circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a voltage output circuit including an overcurrent protection circuit with improved detection accuracy of output current.SOLUTION: A voltage output circuit 10 comprises: an output transistor 11 that has a first voltage-current property between drain-source voltages Vds1 and Vds2 and to which conduction is controlled according to a first node voltage Vn1; a voltage-dividing circuit 12 that outputs feedback voltages Vr1 and Vr2 obtained by dividing an output voltage Vout; an output current detection transistor 51 that has a second voltage-current property approximately similar to the first voltage-current property between drain-source voltages Vds3 and Vds4, and to which conduction is controlled according to Vn1 since Vds3 is provided when the drain-source voltage of the output transistor 11 falls under Vds1 and Vds4 is provided when it falls under Vds2, based on Vr2; and an output current limiting circuit 16 to which conduction is achieved when a drain current of the output current detection transistor 51 exceeds an allowable value, and that raises Vn1 to limit a current applied to the output transistor 11.

Description

  Embodiments described herein relate generally to a voltage output circuit.

  Conventionally, in a voltage output circuit that outputs a constant voltage to the output terminal regardless of voltage fluctuations at the input terminal, an overcurrent protection circuit that protects the output transistor by limiting the output current when an overcurrent flows to the output terminal is provided. Have.

  In this overcurrent protection circuit, a detection current corresponding to the output current is supplied to the output current detection transistor, and when the detection current reaches an allowable value, the detection current is maintained at the allowable value and the output voltage is reduced to 0V. Some are configured to exhibit drooping overcurrent protection characteristics.

  At this time, in the output transistor, the output current reaches the overcurrent protection operation current, the output voltage drops to 0 V, and an overcurrent protection short-circuit current substantially equal to the overcurrent protection operation current flows.

  However, due to the difference between the voltage current characteristics of the output transistor and the output current detection transistor, there is a problem that the output current and the detection current are shifted, and the overcurrent protection short-circuit current becomes larger than the overcurrent protection operating current. .

  For this reason, the output transistor may be overheated while the output voltage drops to 0 V, causing thermal destruction. On the other hand, in order to avoid overheating of the output transistor, if the overcurrent protection operating current is set to a low value in advance and the current output capability of the output transistor is limited, the performance of the voltage output circuit is degraded.

JP 2009-193414 A

  The present invention provides a voltage output circuit having an overcurrent protection circuit with improved output current detection accuracy.

  According to one embodiment, in the voltage output circuit, the output transistor has a first voltage-current characteristic between the first and second drain-source voltages, and the first terminal to which the input voltage is applied and the output voltage Is connected between the second terminals to which is applied, the gate electrode is connected to the first node, and conduction is controlled according to the voltage of the first node. The voltage dividing circuit outputs a first feedback voltage and a second feedback voltage obtained by dividing the output voltage. The error amplifier compares the first feedback voltage with a reference voltage, and feeds back the voltage to the first node so that the first feedback voltage and the reference voltage are equal. The output current detection circuit has a second voltage-current characteristic substantially similar to the first voltage-current characteristic between the third and fourth drain-source voltages, and the output transistor is based on the second feedback voltage. When the drain-source voltage of the output transistor is the first drain-source voltage, the third drain-source voltage is applied, and when the output transistor drain-source voltage is the second drain-source voltage The fourth drain-source voltage is applied to the gate electrode, the voltage of the first node is applied to the gate electrode, and an output current detection transistor whose conduction is controlled according to the voltage of the first node is included. The output current limiting circuit includes an output current limiting transistor connected between the first terminal and the first node, and is turned on when a drain current of the output current detection transistor exceeds an allowable value, and the voltage of the first node To limit the current flowing through the output transistor.

The block diagram for demonstrating the structure of the voltage output circuit which concerns on an Example. The circuit diagram which shows the voltage output circuit which concerns on an Example. The figure which shows the voltage-current characteristic of the output transistor which concerns on an Example, and an output current detection transistor. The figure which shows the operating characteristic of the output transistor and output current detection transistor which concern on an Example. The figure which shows the overcurrent protection characteristic of the voltage output circuit which concerns on an Example. The circuit diagram which shows the voltage output circuit of the 1st comparative example which concerns on an Example. The figure which shows the operating characteristic of the output transistor and output current detection transistor of the 1st comparative example which concerns on an Example. The figure which shows the overcurrent protection characteristic of the voltage output circuit of the 1st comparative example which concerns on an Example. The circuit diagram which shows the voltage output circuit of the 2nd comparative example which concerns on an Example. The figure which shows the operating characteristic of the output transistor and output current detection transistor of the 2nd comparative example which concerns on an Example.

  Embodiments of the present invention will be described below with reference to the drawings.

  The voltage output circuit according to this embodiment will be described with reference to FIGS. FIG. 1 is a block diagram for explaining the configuration of the voltage output circuit of this embodiment, and FIG. 2 is a circuit diagram showing the voltage output circuit. The present embodiment is an example of a voltage output circuit that indirectly detects an output current and has a drooping type overcurrent protection circuit. This voltage output circuit is also called a series regulator or a voltage regulator.

  As shown in FIG. 1, the voltage output circuit 10 of this embodiment includes a P-channel output transistor 11, a voltage divider circuit 12, an error amplifier 13, a driver circuit 14, an output current detection circuit 15, and an output current. The limiting circuit 16 is configured. The output current detection circuit 15 and the output current limit circuit 16 are the overcurrent protection circuit 17.

  The output transistor 11 has a source electrode connected to the first terminal 18, a drain electrode connected to the second terminal 19, a gate electrode connected to the first node N 1, and a voltage Vn 1 (hereinafter referred to as the first node N 1) at the first node N 1. The conduction is controlled in accordance with the node voltage Vn1).

  The output transistor 11 is a P-channel MOS transistor designed to allow large current to flow efficiently with a large mutual conductance Gm, for example, a high-voltage double diffusion insulated gate field effect transistor (DMOS transistor: Double Diffusion MOS FET). However, a normal PMOS transistor having a short gate length and a large gate width may be used.

  An input voltage Vin is applied to the first terminal 18, and a load 20 is connected to the second terminal 19. An output voltage Vout is applied to the load 20, and an output current Iout is supplied to the load 20.

  The voltage dividing circuit 12 has a series circuit of first to third resistors R1, R2, and R3 connected between the second terminal 19 and the reference potential GND. The voltage dividing circuit 12 divides the output voltage Vout, outputs a first feedback voltage Vr1 from a connection node between the first resistor R1 and the second resistor, and outputs a first feedback voltage from a connection node between the second resistor R2 and the third resistor R3. A second feedback voltage Vr2 greater than Vr1 is output.

The first feedback voltage Vr1 and the second feedback voltage Vr2 are expressed by the following equations.
Vr1 = Vout · R1 / (R1 + R2 + R3) (1)
Vr2 = Vout · (R1 + R2) / (R1 + R2 + R3) (2)
The error amplifier 13 compares the first feedback voltage Vr1 with the reference voltage Vref generated by the reference voltage source 21, and feeds back to the first node voltage Vn1 so that the first feedback voltage Vr1 and the reference voltage Vref are equal. The conduction of the transistor 11 is controlled.

  The driver circuit 14 outputs a first node voltage Vn1 as a buffer amplifier of the error amplifier 13.

  The output current detection circuit 15 is provided with a drain-source voltage corresponding to the drain-source voltage of the output transistor 11 based on the second feedback voltage Vr2, and an output whose conduction is controlled according to the first node voltage Vn1. The output current Iout is indirectly detected with the drain current (detection current Isense) of the current detection transistor.

  When the detection current Isense reaches an allowable value (corresponding to the overcurrent protection operation current of the output current Iout), the output current limiting circuit 16 increases the first node voltage Vn1 and limits the current flowing through the output transistor 11.

  Next, the configurations of the error amplifier 13, the driver circuit 14, the output current detection circuit 15, and the output current limiting circuit 16 will be described.

As shown in FIG. 2, in the error amplifier 13, NMOS transistors 31 and 32 are differentially connected. The PMOS transistors 33 and 34 constitute a current mirror circuit, and drive the NMOS transistors 31 and 32 with an equal current. The NMOS transistor 35 is a current source that causes the error amplifier 13 to operate by passing a constant current according to the bias voltage Vb.
The operation of the error amplifier 13 is well known and will not be described.

  The driver circuit 14 has a series circuit of a PMOS transistor 41 and an NMOS transistor 42. The PMOS transistor 41 has a gate electrode connected to the output terminal of the differential amplifier 13 and a drain electrode connected to the first node N1. The NMOS transistor 42 is a current source that supplies a constant current to the driver circuit 14 in accordance with the bias voltage Vb.

  In the output current detection circuit 15, an output current detection transistor (PMOS transistor) 51 is connected between the first terminal 18 and the second node N2. A drain-source voltage control circuit 52 is connected between the second node N2 and the output current detection resistor R6 connected to the reference potential GND. The gate electrode of the output current detection transistor 51 is connected to the first node N1.

  The drain-source voltage control circuit 52 includes a buffer 53 having a second feedback point pressure Vr2 input to the input terminal, and fourth and fifth resistors R4 connected between the first terminal 18 and the output terminal of the buffer 53. , R5, and a PMOS transistor 54 connected between the second node N2 and the output current detection resistor R6 and having a gate electrode connected to a connection node of the fourth and fifth resistors R4, R5. .

  The buffer 53 is configured by, for example, a differential amplifier in which a negative potential terminal and an output terminal are connected, has a high input impedance and a low output impedance, and has a gain of 1. The buffer 53 electrically separates the voltage dividing circuit 12 and the output current detection circuit 15 and transmits only the voltage information of the second feedback voltage Vr2.

  The voltage Vn2 at the second node N2 (hereinafter referred to as the second node voltage Vn2) is expressed by the following equation.

Vn2 = Vr2 + R4 · (Vin−Vr2) / (R4 + R5) + Vgs54 (3)
Here, Vgs 54 is a gate-source voltage of the PMOS transistor 54.

  In the output current limiting circuit 16, an output current limiting transistor (PMOS transistor) 61 is connected between the first terminal 18 and the first node N1. A series circuit of an overcurrent limiting resistor R7 and an NMOS transistor 62 is connected between the first terminal 18 and the reference potential GND.

  The gate electrode of the output current limiting transistor 61 is connected to a connection node between the overcurrent limiting resistor R 7 and the NMOS transistor 62. The gate electrode of the NMOS transistor 62 is connected to the connection node between the output current detection resistor R 6 and the PMOS transistor 62.

  Thereby, in the overcurrent protection circuit 17, the detection current Isense flowing through the output current detection transistor 51 is converted into a current detection voltage Vsense that flows through the output current detection resistor R6 and is proportional to the output current Iout. When the current detection voltage Vsense exceeds the threshold value of the NMOS transistor 62, a drain current flows through the NMOS transistor 62, and a voltage drop occurs in the overcurrent limiting resistor R7.

  Further, the output current limiting transistor 61 whose gate electrode is connected to the overcurrent limiting resistor R7 operates to cause a drain current to flow. The drain electrode of the output current limiting transistor 61 is connected to the first node N1, and acts to raise the first node voltage Vn1 when the output current limiting transistor 61 allows the drain current to flow.

  The relationship between the operation in which the output voltage Vout is held constant by the first feedback voltage Vr1 and the overcurrent protection circuit 17 is that the output current Iout starts flowing when the first node voltage Vn1 first decreases, When the current exceeds a certain constant current, the output current limiting transistor 61 operates to increase the first node voltage Vn1, so that the first node voltage Vn1 cannot be further decreased.

  Due to this action, the drain current (output current Iout) of the output transistor 11 cannot be increased any more. As a result, when the load 20 further increases, the output voltage Vout starts to drop while maintaining the output current Iout.

  The output current Iout no longer increases and the output current at which the output voltage Vout starts to decrease is called an overcurrent protection operation current ICL. The current that flows when the load 20 increases and the output voltage Vout finally decreases to 0 V is called an overcurrent protection short-circuit current ISC.

  Next, voltage-current characteristics of the output transistor 11 and the output current detection transistor 51 will be described. In general, the drain current Ids of a MOS transistor is expressed by the following equation.

Ids = (1/2) · (W / L) · K · (Vgs−Vth) ² (4)
Here, W is the gate width, L is the gate length, K is a constant determined by the thickness of the gate oxide film, the dielectric constant, Vgs is the gate-source voltage, and Vth is the threshold value. From equation (4), when the gate-source voltage Vgs is the same, the drain current Ids is proportional to the gate width W and inversely proportional to the gate length L.

  In the output transistor 11, the gate length L is set as short as possible. This is to increase the current output capability per unit area as much as possible to suppress the increase in chip size.

  On the other hand, in the output current detection transistor 51, the gate length L is set longer than the gate length of the output transistor 11. Since the detection current Isense is a reactive current that flows to the reference potential GND, the detection current Isense needs to be as small as possible, for example, one of tens of thousands, for example. Originally, it is desirable that the output current detection transistor 51 and the output transistor 11 have the same gate length L, but the gate width W of the output current detection transistor 51 is less than the minimum dimension of the layout, which makes it difficult to actually manufacture. .

  3A and 3B are diagrams showing voltage-current characteristics of the output transistor 11 and the output current detection transistor 51. FIG. 3A shows the voltage-current characteristics of the output transistor 11, and FIG. It is a figure which shows the voltage-current characteristic.

  As shown in FIG. 3A, in the output transistor 11, when the gate length L is shortened, the mutual conductance Gm increases and a larger drain current Ids flows. Since the distance between the drain and source (channel length) becomes narrow, it is easily affected by the drain-source voltage. Even in the pentode region (saturation region), the change in the drain current Ids increases with respect to the change in the drain-source voltage.

  As shown in FIG. 3B, in the output current detection transistor 51, since the gate length L is longer than that of the output transistor 11, the mutual conductance Gm becomes small and the drain current Ids becomes small. Since the drain-source distance (channel length) is wide, it is difficult to be affected by the drain-source voltage. In the pentode region, the drain current Ids is substantially constant with respect to changes in the drain-source voltage. In other words, the output resistance of the drain increases.

  That is, the output transistor 11 and the output current detection transistor 51 are fundamentally different in voltage / current characteristics. When the output transistor 11 and the output current detection transistor 51 are operated with the same drain-source voltage, the drain current (detection current Isense) of the output current detection transistor 51 is not proportional to the drain current (output current Iout) of the output transistor 11. Deviation occurs.

  In the voltage output circuit 10 described above, based on the second feedback voltage Vr2, output is performed so that the second voltage-current characteristic of the output current detection transistor 51 is substantially similar to the voltage-current characteristic in the first operating range of the output transistor 11. The second operating range of the current detection transistor 51 is selected.

  The selection of the second operating range of the output current detection transistor 51 is performed by controlling the drain-source voltage of the output current detection transistor 51 by the drain-source voltage control circuit 52 based on the second feedback voltage Vr2. It is.

The drain-source voltage Vds51 of the output current detection transistor 51 is expressed by the following equation.
Vds51 = Vin−Vn2 (5)
By operating the output transistor 11 and the output current detection transistor 51 with different drain-source voltages so that the first and second voltage-current characteristics of the output transistor 11 and the output current detection transistor 51 are substantially similar, sufficient accuracy can be obtained. Thus, it is possible to obtain a detection current Isense proportional to the output current Iout.

  Next, the operation range of the output transistor 11 and the output current detection transistor 51 will be described. FIG. 4 is a diagram showing the operating range of the output transistor 11 and the output current detection transistor 51. In FIG.

  As shown in FIG. 4, the first operating range ΔV1 of the output transistor 11 is between the first drain-source voltage Vds1 and the second drain-source voltage Vds2 (ΔV1 = Vds2−Vds1). In the first operating range ΔV1, the output transistor 11 has a first voltage-current characteristic 71.

  The second operating range ΔV2 of the output current detection transistor 51 is between the third drain-source voltage Vds3 and the fourth drain-source voltage Vds4 (ΔV2 = Vds4-Vds3). In the second operating range ΔV2, the output current detection transistor 51 has a second voltage-current characteristic 72 that is substantially similar to the first voltage-current characteristic 71.

  The first drain-source voltage Vds1 is a drain-source voltage when the voltage output circuit 10 outputs the rated output voltage Vout. The second drain-source voltage Vds2 is a drain-source voltage when the voltage output circuit 10 passes the overcurrent protection short-circuit current ISC (Vout = 0).

The first and second drain-source voltages Vds1, Vds2 are expressed by the following equations.
Vds1 = Vin−Vout (6)
Vds2 = Vin (7)
The first operating range ΔV1 of the output transistor 11 is in the pentode region, and the first voltage-current characteristic 71 indicates that the drain current is changed from Id1 to Id2 when the drain-source voltage changes from Vds1 to Vds2, as described with reference to FIG. (ΔI1 = I2−I1).

  On the other hand, the second operating range of the output current detection transistor 51 is between the triode region and the pentode region, and the second voltage-current characteristic 72 shows that when the drain-source voltage changes from Vds3 to Vds4, the drain current changes from Id3. It changes to Id4 (ΔI2 = I4-I3).

  Assuming that the drain resistance is the reciprocal of the ratio of the drain current change amount to the drain-source voltage change amount, the first drain resistance Rd1 = ΔV1 / ΔI1 of the output transistor 11 and the second drain resistance Rd2 of the output current detection transistor 51 = ΔV2 / ΔI2 is substantially equal (Rd1≈Rd2).

  Therefore, for the output transistor that operates according to the first voltage-current characteristic 71 in the first operating range ΔV1, the operating range of the output current detection transistor 51 is set to the second operating range ΔV2, and the operation is performed according to the second voltage-current characteristic 72. By doing so, it is possible to obtain the detection current Isense proportional to the output current Iout with sufficient accuracy.

Here, the third and fourth drain-source voltages Vds3 and Vds4 are expressed by the following equations.
Vds3 = Vin−Vr2−R4 · (Vin−Vr2) / (R4 + R5)
-Vgs54 (8)
Vds4 = Vin−R4 · Vin / (R4 + R5) −Vgs54 (9)
A specific example will be described assuming a certain operating condition. It is assumed that the voltage output circuit 10 performs a rated operation with an input voltage Vin of 6V and an output voltage Vout of 5V. At this time, the first drain-source voltage Vds1 is 1V, and the second drain-source voltage Vds2 is 6V.

  Comparing the static characteristics of the output transistor 11 and the output current detection transistor 51, the third drain-source voltage Vds 3 that gives a second voltage-current characteristic 72 substantially similar to the first voltage-current characteristic 71 is 0.6 V, 4 Assume that the drain-source voltage Vds4 is 1.1V. The third and fourth drain-source voltages Vds3 and Vds4 can be arbitrarily set depending on the transistors used.

  Further, it is assumed that some element constants are set, for example, the fifth resistor R5 is 10 kΩ, and the gate-source voltage Vgs54 of the PMOS transistor 54 is 0.8V.

  Accordingly, the first to third resistors R1, R2, and R3 are set so that the fourth resistor R4 is 21.5 kΩ and the second feedback voltage Vr2 is 1.579 V when the output voltage Vout is 5 V.

  As another example, it is assumed that the voltage output circuit 10 is rated at an input voltage Vin of 2.8V and an output voltage Vout of 1.8V. At this time, the first drain-source voltage Vds1 is 1V, and the second drain-source voltage Vds2 is 2.8V.

  When the fifth resistor R5 is 10 kΩ, the gate-source voltage Vgs54 of the PMOS transistor 54 is 0.8 V, the fourth resistor R4 is 1.2 kΩ, and the second feedback voltage Vr2 is 1 when the output voltage Vout is 1.8 V. The first to third resistors R1, R2, and R3 are set to be .12V.

  The above-described element constants are realistic values. Thereby, the voltage output circuit 10 is a realizable circuit.

  FIG. 5 is a diagram showing overcurrent protection characteristics of the voltage output circuit 10. As shown in FIG. 5, the voltage output circuit 10 operates at a normal output voltage of 5 V so that a normal operating current Iconst flows through the load 20 as an output current.

  When the load 20 increases and the output current Iout reaches the overcurrent protection operation current ICL from the normal operation current Iconst, the overcurrent protection circuit 17 of the voltage output circuit 10 maintains the overcurrent protection operation current ICL. In this state, the output voltage Vout is lowered to 0V, and a drooping type overcurrent protection characteristic in which the overcurrent protection operation current ICL and the overcurrent protection short circuit current ISC1 are equal is shown.

  Therefore, there is no possibility that the output transistor 11 of the voltage output circuit 10 is overheated and thermally destroyed while the output voltage Vout drops to 0V.

  FIG. 6 is a diagram showing a voltage output circuit of the first comparative example, and FIG. 7 is a diagram showing voltage-current characteristics in the operating range of the output transistor and output current detection transistor of the first comparative example. Here, the first comparative example is a voltage output circuit that does not have the drain-source voltage control circuit 52.

  As shown in FIG. 6, in the voltage output circuit 80 of the first comparative example, since the output current detection circuit 81 does not have the drain-source voltage control circuit 52, the second node N2 and the output current detection resistor R6 are directly connected. Has been. Therefore, the third resistor R3 is not necessary for the voltage dividing circuit 82.

  As shown in FIG. 7, the first operating range ΔV1 of the output transistor 11 is the same as the first operating range ΔV1 shown in FIG. On the other hand, the second operation range ΔV2 of the output current detection transistor 51 is different from the second operation range ΔV2 shown in FIG.

In the second operating range ΔV2 of the first comparative example, the third and fourth drain-source voltages Vds3 and Vds4 are expressed by the following equations.
Vds3 = Vin−Isense · R6 (10)
Vds4 = Vds2 (11)
As a result, since the second voltage-current characteristic 73 is not substantially similar to the first voltage-current characteristic 71, the first drain resistance Rd1 is significantly smaller than the second drain resistance Rd2 (Rd1 << Rd2), and the detection current Isense is It is not proportional to the output current Iout.

  As shown in FIG. 8, the overcurrent protection characteristic of the voltage output circuit 80 is that when the output current Iout reaches the overcurrent protection operation current ICL, the output current Iout cannot maintain the overcurrent protection operation current ICL, The current overruns by ΔI1 from the protection operating current ICL, and the output voltage Vout drops to 0V. Therefore, since the overcurrent protection short-circuit current ISC2 larger than the overcurrent protection short-circuit current ISC1 flows, the output transistor 11 of the voltage output circuit 80 may be overheated and may be thermally destroyed.

  FIG. 9 is a diagram showing a voltage output circuit of the second comparative example, and FIG. 10 is a diagram showing voltage-current characteristics in the operating range of the output transistor and output current detection transistor of the second comparative example. Here, the second comparative example is a voltage output circuit configured such that the drain-source voltage of the output current detection transistor 51 matches the drain-source voltage of the output transistor 11.

  As shown in FIG. 9, in the output current detection circuit 91 of the voltage output circuit 90 of the second comparative example, the positive input terminal of the differential amplifier serving as the buffer 53 is connected to the second terminal 19 and the negative input terminal is the PMOS transistor. The output terminal is connected to the gate electrode of the PMOS transistor 54.

  As a result, the second node voltage Vn2 becomes equal to the output voltage Vout, so that the drain-source voltage of the output current detection transistor 51 matches the drain-source voltage of the output transistor 11.

As shown in FIG. 10, in the second operating range ΔV2 of the second comparative example, the third and fourth drain-source voltages Vds3 and Vds4 are expressed by the following equations.
Vds3 = Vds1 (12)
Vds4 = Vds2 (13)
As a result, since the second voltage-current characteristic 74 is not substantially similar to the first voltage-current characteristic 71, the first drain resistance Rd1 is smaller than the second drain resistance Rd2 (Rd1 <Rd2), and the detection current Isense is the output current. It is not proportional to Iout.

  The overcurrent protection characteristic of the voltage output circuit 90 is substantially the same as the overcurrent protection characteristic of the voltage output circuit 80 shown in FIG. 8, and the description thereof is omitted.

  On the other hand, in the voltage output circuit 10 of the present embodiment, as described above, since the second voltage-current characteristic 72 is substantially similar to the first voltage-current characteristic 71, the first drain resistance Rd1 is the same as the second drain resistance Rd2. The detection current Isense is approximately proportional to the output current Iout with sufficient accuracy (Rd1≈Rd2).

  When the output current Iout reaches the overcurrent protection operating current ICL, the output voltage Iout drops to 0V while maintaining the overcurrent protection operating current ICL. Therefore, there is no possibility that the output transistor 11 of the voltage output circuit 10 is overheated and thermally destroyed.

  As described above, in the voltage output circuit 10 of this embodiment, the drain-source voltage control circuit 52 performs the second operation corresponding to the first operation range ΔV1 of the output transistor 11 based on the second feedback point pressure Vr2. The output current detection transistor 51 is operated in the range ΔV2.

  As a result, the first voltage-current characteristic 71 of the output transistor 11 and the second voltage-current characteristic 72 of the output current detection transistor 51 are substantially similar, and a current proportional to the output current Iout can flow through the output current detection transistor 51. . Therefore, the voltage output circuit 10 having the overcurrent protection circuit 17 with improved output current detection accuracy can be obtained.

  Here, the case where the second feedback voltage Vr2 is larger than the first feedback voltage Vr1 (Vr2> Vr1) has been described. However, even if the second feedback voltage Vr2 is smaller than the first feedback voltage Vr1 (Vr1> Vr2), Is possible.

  Although the case where the output circuit 10 has the drooping type overcurrent protection characteristic has been described, the present invention can also be applied to an output circuit having a U-shaped overcurrent protection characteristic.

  However, since the overcurrent protection short circuit current ISC is configured to be smaller than the overcurrent protection operating current ICL, the F-shaped overcurrent protection characteristic is less likely to cause the output transistor 11 to overheat and to be thermally destroyed. However, since the output current detection accuracy is improved, there is an advantage that a stable operation can be obtained.

  Further, the output current detection circuit 15 is not limited to the voltage output circuit, and may be similarly applied as long as it is a circuit intended to detect the current flowing through the output element using another element that operates in proportion to the operation of the output element. Is possible. For example, in an OP amplifier, the present invention can be applied to the case where it is desired to improve the output current detection accuracy for output protection.

  The above-described embodiments are merely exemplary and are not intended to limit the scope of the invention. Indeed, the novel circuits described herein may be embodied in a variety of other forms, and various omissions may be made in the form of circuits described herein without departing from the spirit or spirit of the invention. Replacements and changes may be made. The appended claims and their equivalents or equivalent methods are intended to include such forms or modifications as would fall within the scope and spirit or spirit of the present invention.

10, 80, 90 Voltage output circuit 11 Output transistor 12, 82 Voltage dividing circuit 13 Error amplifier 14 Driver circuit 15, 81, 91 Output current detection circuit 16 Output current limiting circuit 17 Overcurrent protection circuit 18 First terminal 19 Second terminal 20 Load 21 Reference voltage source 31, 32, 35, 42, 62 NMOS transistor 33, 34, 41, 54 PMOS transistor 51 Output current detection transistor 52, 92 Drain / source voltage control circuit 53 Buffer 61 Output current limiting transistor 71 1st Voltage-current characteristics 72, 73, 74 Second voltage-current characteristics N1 First node N2 Second node Vref Reference voltage Vr1 First feedback voltage Vr2 Second feedback voltage Vb Bias voltage ΔV1 First operation range ΔV2 Second operation range R1 First Resistor R2 Second resistor R3 Third resistor R4 Fourth resistor R5 First 5 resistor R6 output current detection resistor R7 overcurrent limiting resistor

Claims (5)

A first voltage-current characteristic between the first and second drain-source voltages, connected between a first terminal to which an input voltage is applied and a second terminal to which an output voltage is applied; An output transistor connected to a first node and controlled in conduction according to a voltage of the first node;
A voltage dividing circuit for outputting a first feedback voltage and a second feedback voltage obtained by dividing the output voltage;
An error amplifier that compares the first feedback voltage with a reference voltage and feeds back to the voltage of the first node so that the first feedback voltage and the reference voltage are equal;
Between the third and fourth drain-source voltages, the second voltage-current characteristic is substantially similar to the first voltage-current characteristic, and the drain-source voltage of the output transistor is based on the second feedback voltage. Is the first drain-source voltage, the third drain-source voltage is applied, and when the output transistor drain-source voltage is the second drain-source voltage, the fourth drain-source voltage is applied. An output current detection circuit including an output current detection transistor to which a source-to-source voltage is applied, a voltage of the first node is applied to a gate electrode, and conduction is controlled according to the voltage of the first node;
An output current limiting transistor connected between the first terminal and the first node, and is turned on when a drain current of the output current detection transistor exceeds an allowable value; An output current limiting circuit for limiting the current flowing through the transistor;
A voltage output circuit comprising:   The voltage output circuit according to claim 1, wherein a gate length of the output transistor is shorter than a gate length of the output current detection transistor. The output current detection circuit includes:
The output current detection transistor is connected between the first terminal and a second node;
A buffer in which the second feedback voltage is input to an input terminal; a series circuit of a fourth resistor and a fifth resistor connected between the first terminal and the output terminal of the buffer; the second node; and the reference The voltage output according to claim 1, further comprising: a transistor connected between a sixth resistor connected to a potential and a gate electrode connected to a connection node of the fourth and fifth resistors. circuit. When the input voltage is Vin, the second feedback voltage is Vr2, the fourth and fifth resistors are R4 and R5, the gate-source voltage of the transistor is Vgs, and the voltage of the second node is Vn2. The drain-source voltage Vds of the output current detection transistor is
Vds = Vin−Vn2
= Vin− (Vr2 + R4 (Vin−Vr2) / (R4 + R5) + Vgs)
The voltage output circuit according to claim 3, wherein the voltage output circuit is given by:   A ratio between the first and second drain-source voltages and the first and second drain currents flowing through the output transistor when the first and second drain-source voltages are given is defined as a first drain resistance. , A ratio of the third and fourth drain-source voltages to the third and fourth drain currents flowing through the output current detection transistor when the third and fourth drain-source voltages are applied is given by 2. The voltage output circuit according to claim 1, wherein when the drain resistance is applied, the first drain resistance and the second drain resistance are substantially equal.

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