JP2009011112A - Overcurrent protection circuit and electronic equipment using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an overcurrent protection circuit achieving overcurrent protection without being affected by ambient temperature. <P>SOLUTION: The overcurrent protection circuit 30 is structured by integrating: a resistor R1 for detecting an output current using a parasitic resistance component of a metal wiring connected to a power transistor 10; a first current mirror circuit, which includes a pair of bipolar transistors Q1, Q2, in which a voltage across the resistor R1 is supplied between the emitters of each transistor; a second current mirror circuit (N0, N1, N2), which maintains the collector currents I1, I2 of the transistors Q1, Q2 to a predetermined mirror ratio; and a switch N3, which is turned on and off according to a collector voltage V2 of the transistor and from one end of which an overcurrent protection signal OCP is drawn. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an overcurrent protection circuit integrated in a semiconductor device and an electronic device using the same.

  FIG. 3 is a diagram showing a conventional example of an overcurrent protection circuit.

  As shown in FIG. 3, the conventional overcurrent protection circuit generally uses a monitor transistor Trb connected in parallel with the power transistor Tra to generate a monitor current Im that exhibits the same behavior as the output current Io. Is converted into the monitor voltage Vm by flowing it into the resistor Ra, and then the overcurrent protection signal OCP is generated by comparing with the predetermined reference voltage Vref by the comparator CMP.

In addition, patent documents 1-3 etc. can be mentioned as a prior art relevant to an overcurrent protection circuit.
JP-A-5-284733 JP 2000-91854 A JP-A-2005-328606

  Certainly, the above-described conventional overcurrent protection circuit prevents the power transistor Tra from being destroyed beyond its safe operation area (ASO [Area of Safe Operation]) and causing the semiconductor device to emit smoke or ignite. It is possible.

  However, the above-described conventional overcurrent protection circuit does not directly detect the output current Io flowing through the power transistor Tra, but indirectly detects the output current Io by detecting the monitor current Im that exhibits the same behavior. Therefore, the dependency on the input voltage Vcc is large, and the overcurrent protection operating point is likely to vary.

  In the above-described conventional overcurrent protection circuit, the power transistor Tra generates heat as the large output current Io flows. However, since the monitor transistor Trb flows only a minute monitor current Im, the power transistor Tra generates almost heat. do not do. Therefore, depending on the temperature characteristics of the power transistor Tra and the monitor transistor Trb, there is a possibility that a difference occurs between the on-threshold voltages of the power transistor Tra and the monitor transistor Trb, resulting in variations in the overcurrent protection operating point.

  Further, in the above-described conventional overcurrent protection circuit, if there is a manufacturing variation between the temperature characteristic of the power transistor Tra and the temperature characteristic of the monitor transistor Trb, it is assumed that both temperature conditions are the same. However, the behaviors of the output current Io and the monitor current Im are not matched, and there is a possibility that the overcurrent protection operating point may vary.

  Further, since the conventional overcurrent protection circuit is configured to generate the monitor voltage Vm by flowing the monitor current Im into the resistor Ra, the temperature characteristic of the resistor Ra directly affects the overcurrent protection operating point. It was. Therefore, when the temperature characteristic of the resistor Ra is greatly different for each product, there is a possibility that a large variation occurs in the overcurrent protection operation point.

  In view of the above problems, an object of the present invention is to provide an overcurrent protection circuit capable of realizing overcurrent protection that is not affected by ambient temperature, and an electronic device using the same.

  In order to achieve the above object, an overcurrent protection circuit according to the present invention comprises a resistor for detecting an output current using a parasitic resistance component of a metal wiring connected to a power element; and a pair of bipolar transistors between each emitter. A first current mirror circuit to which a voltage across the resistor is applied; a second current mirror circuit that maintains a collector current of the pair of bipolar transistors at a predetermined mirror ratio; and according to a collector voltage of the bipolar transistor And a switch element from which an overcurrent protection signal is drawn out from one end of the switch element (first configuration).

  The electronic device according to the present invention is driven by a primary power supply system, a secondary power supply system driven by receiving power supply from the primary power supply system, and a power supply supplied from the secondary power supply system. And an electronic device having a load system, wherein the overcurrent protection means of the secondary power supply system includes an overcurrent protection circuit having the first configuration (second configuration). Yes.

  With the overcurrent protection circuit according to the present invention, overcurrent protection that is not affected by the ambient temperature is realized.

  FIG. 1 is a diagram showing a semiconductor device provided with an overcurrent protection circuit according to the present invention.

  As shown in FIG. 1, the semiconductor device 1 is formed by integrating a power transistor 10, a control circuit 20, and an overcurrent protection circuit 30.

  The power transistor 10 is connected between an input terminal IN to which the power supply voltage Vcc is applied and an output terminal OUT to which a load (not shown in the figure) is connected, and is turned on / off according to an instruction from the control circuit 20. In this embodiment, an N-channel field effect transistor is used.

  The control circuit 20 is a main body that performs on / off control of the power transistor 10 and has a function of determining whether or not to forcibly turn off the power transistor 10 according to the logic of the overcurrent protection signal OCP. Yes.

  Specifically, referring to FIG. 1, the control circuit 20 determines that no overcurrent has occurred in the output current Io flowing through the power transistor 10 when the logic of the overcurrent protection signal OCP is at a high level. Then, on / off control of the power transistor 10 is performed as usual. On the other hand, when the logic of the overcurrent protection signal OCP is at a low level, the control circuit 20 determines that an overcurrent has occurred in the output current Io flowing through the power transistor 10, and forcibly turns off the power transistor 10. To do.

  By such an overcurrent protection operation, it is possible to prevent the power transistor 10 from being destroyed beyond its safe operation area (ASO [Area of Safe Operation]) and causing the semiconductor device 1 to emit smoke and fire. Become.

  The overcurrent protection circuit 30 includes resistors R1 and R2, pnp bipolar transistors Q1 and Q2, N channel field effect transistors N0, N1, N2, and N3, and a constant current source CS1.

  The resistor R1 is a wiring resistance component that is parasitic on a metal wiring (for example, an aluminum wiring) extending from the input terminal IN to the drain of the power transistor 10. In order to obtain a desired resistance value R for a wiring section corresponding to the resistor R1. The width of the metal wiring is narrower than the others.

  The emitter of the transistor Q1 is connected to the drain of the power transistor 10 (corresponding to one end of the resistor R1). The emitter of the transistor Q2 is input terminal IN (corresponding to the other end of the resistor R1). The bases of the transistors Q1 and Q2 are connected to each other, and the connection node is connected to the collector of the transistor Q1.

  The drain of the transistor N0 is connected to the output terminal of the constant current source CS1. The drain of the transistor N1 is connected to the collector of the transistor Q1. The drain of the transistor N2 is connected to the collector of the transistor Q2. The gates of the transistors N0, N1, and N2 are connected to each other, and the connection node is connected to the drain of the transistor N0. The drain of the transistor N3 is connected to the input terminal IN via the resistor R2, and is connected to the overcurrent protection signal input terminal of the control circuit 20 as a lead-out terminal for the overcurrent protection signal OCP. The sources of the transistors N0, N1, N2, and N3 are all grounded.

  The operation of the overcurrent protection circuit 30 having the above configuration will be described in detail.

  In the overcurrent protection circuit 30, the transistor Q1 and the transistor Q2 form a first current mirror circuit, and the transistor N0, the transistor N1, and the transistor N2 form a second current mirror circuit. Has been.

  The transistors N1 and N2 are supplied with a first current I1 and a second current I2 (collector currents of the transistors Q1 and Q2) that mirror the reference current Iref generated by the constant current source CS1. The transistors N1 and N2 are designed so that the first current I1 and the second current I2 have a predetermined mirror ratio (I1: I2 = a: b).

  In the overcurrent protection circuit 30 having the above configuration, when the output current Io is smaller than a predetermined threshold (overcurrent detection value), the voltage drop at the resistor R1 is small and the emitter voltage of the transistor Q1 is high. A current easily flows through the transistor Q1. However, a predetermined mirror ratio is set with respect to the reference current Iref for the first current I1 flowing through the transistor Q1 and the second current I2 flowing through the transistor Q2. Therefore, the first voltage V1 appearing at the drain of the transistor N1 and the second voltage V2 appearing at the drain of the transistor N2 are lowered so as to maintain the above mirror ratio. As a result, the transistor N3 is turned off until the output current Io reaches a predetermined threshold value, and the overcurrent protection signal OCP is pulled up to the resistor R2 and becomes high level.

  On the other hand, when the output current Io reaches a predetermined threshold value (overcurrent detection value), the voltage drop at the resistor R1 is large and the emitter voltage of the transistor Q1 is low, so that it is difficult for current to flow through the transistor Q1. . However, as described above, since the predetermined mirror ratio is set for the first current I1 and the second current I2 with respect to the reference current Iref, the first voltage V1 and the second voltage V2 are Rise to maintain mirror ratio. As a result, when the output current Io reaches a predetermined threshold, the transistor M3 is transitioned from the off state to the on state, and the logical value of the overcurrent protection signal OCP is transitioned from the high level to the low level.

  As described above, if the parasitic resistance component of the metal wiring connected to the input side of the power transistor 10 is the resistor R1 for detecting overcurrent, when the output current Io flows through the power transistor 10, a voltage drop corresponding to the output current Io flows. Is generated in the resistor R1, the overcurrent protection signal OCP can be generated by comparing the voltage difference between the input terminal IN and the drain of the power transistor 10 (that is, the voltage across the resistor R1) with a predetermined threshold. It becomes possible.

  Further, if the parasitic resistance component of the metal wiring connected to the input side of the power transistor 10 is the resistor R1 for detecting overcurrent, the monitor transistor is not necessary, and the output current Io flowing through the power transistor 10 is not reduced. All can be monitored.

  In addition, since the resistance component of the contact can be reduced by using the wiring resistance as compared with the case where a separate resistance element is used, it is possible to realize a highly accurate low resistance.

  Further, in the case of the overcurrent protection circuit 30 according to the present embodiment, the temperature of the output current detection unit in the overcurrent protection circuit 30 is obtained using the temperature characteristics of the resistor R1 (metal wiring) and the temperature characteristics of the bipolar transistors Q1 and Q2. The characteristic can be made flat. In the following, such actions and effects will be described in detail.

  First, the following equation (1) is established between the transistor Q1 and the transistor Q2 by a diode equation.

In the above equation (1), the thermal voltage V _T and the resistance value R of the resistor R1 are expressed as a function of the absolute temperature T as represented by the following equation (2). In equation (2), k represents the Boltzmann coefficient, q represents the amount of charge of electrons, and α represents the temperature constant of the resistor R1.

  Here, the temperature characteristic of the output current detection unit in the overcurrent protection circuit 30 is expressed by the following expression (3) by fully differentiating the expression (1) with the temperature T.

  As can be seen from the above equation (3), in the overcurrent protection circuit 30 of the present embodiment, the temperature characteristics of the resistor R1 (metal wiring) and the temperature characteristics of the bipolar transistors Q1 and Q2 cancel each other, and the output current The temperature characteristic of the detection unit is flat (dIo / dT = 0).

  At the same time, the mirror ratio of the transistors M1 and M2 is finely adjusted according to the on-threshold voltage of the transistor M3, so that the overcurrent protection circuit 30 operates without depending on the ambient temperature.

  Accordingly, even if the ambient temperature changes, the current ratio between the first current I1 and the second current I2 is less likely to be shifted, and further, the overcurrent protection operating point is less likely to vary, which affects the ambient temperature. Overcurrent protection that is not performed can be realized.

  Further, in the semiconductor device 1 equipped with the overcurrent protection circuit 30, the variation in the overcurrent protection operation point is reduced, so that it is not necessary to provide an unnecessary margin in the safe operation region (ASO) of the power transistor 10. . Therefore, the chip area (element size of the power transistor 10) can be reduced, so that the chip cost can be reduced and the profit rate can be improved.

  FIG. 2 is a diagram (a general power system diagram) illustrating an example of an application to which the overcurrent protection circuit is applied.

  The application shown in FIG. 2 includes a primary power supply system A, a secondary power supply system B driven by receiving power supply from the primary power supply system A, and a load driven by receiving power supply from the secondary power supply system B. And a system C.

  As shown in FIG. 2, as the primary power supply system A, a DC / DC converter (or AC / DC converter) A1 or the like can be considered. The secondary power supply system B includes a low voltage drop type (LDO [Low Drop-Out] regulator B1, a motor driver B2, a high side switch B3, and a DC / DC converter (or low voltage drop type regulator) B4). The load system C may be a memory card C1, a motor C2, a USB device (including a USB connector) C3, a signal processing device (signal processing IC) C4, and the like.

  Further, the relationship of power supply capacity (required power) is expressed as follows: primary power supply system A (overcurrent protection operating point)> secondary power supply system B (overcurrent protection operating point)> load system C. Therefore, when there is a large variation in the overcurrent protection operation point of the secondary power supply system B, an unnecessary margin must be provided in the power supply capability of the primary power supply system A, resulting in a large load.

  On the other hand, if the overcurrent protection circuit 30 according to the present invention is used as the overcurrent protection means provided in each component B1 to B4 of the secondary power supply system B, the variation in the overcurrent protection operating point is reduced. Therefore, the power supply capability of the primary power supply system A can be minimized. Further, since the influence of the ambient temperature does not need to be taken into consideration, the design of the set becomes easy.

  The configuration of the present invention can be variously modified within the scope of the present invention in addition to the above embodiment.

  The present invention is a technique useful for all semiconductor devices (such as a power supply IC) including a power element.

These are figures which show the semiconductor device provided with the overcurrent protection circuit based on this invention. These are figures which show the application used as the application object of an overcurrent protection circuit. These are figures which show one example of a conventional overcurrent protection circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor device 10 Power transistor (N channel type field effect transistor)
20 control circuit 30 overcurrent protection circuit IN input terminal OUT output terminal R1 resistance (wiring resistance component)
R2 resistor Q1, Q2 pnp bipolar transistor N0, N1, N2, N3 N-channel field effect transistor CS1 constant current source Io output current Iref reference current I1, I2 first and second currents V1, V2 first and second voltages OCP overcurrent protection signal A Primary power supply system A1 DC / DC converter (or AC / DC converter)
B Secondary power supply system B1 Low voltage drop regulator (LDO regulator)
B2 Motor driver B3 High side switch B4 DC / DC converter (or LDO regulator)
C load system C1 memory card C2 motor C3 USB device C4 signal processing device (signal processing IC)

Claims (2)

A resistor for detecting the output current using the parasitic resistance component of the metal wiring connected to the power element;
A first current mirror circuit comprising a pair of bipolar transistors, wherein a voltage across the resistor is applied between the emitters;
A second current mirror circuit for maintaining a collector current of the pair of bipolar transistors at a predetermined mirror ratio;
A switch element which is turned on / off according to the collector voltage of the bipolar transistor and from which an overcurrent protection signal is drawn;
An overcurrent protection circuit characterized by being integrated. An electronic device having a primary power supply system, a secondary power supply system driven by receiving power supply from the primary power supply system, and a load system driven by receiving power supply from the secondary power supply system. And
An electronic device comprising the overcurrent protection circuit according to claim 1 as overcurrent protection means of the secondary power supply system.

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