JP2012211805A - Current detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an integrated circuit capable of setting an overcurrent detection value by a resistance value of the external resistance.SOLUTION: A current detection circuit includes: a power MOSFET Q1; a series body of a parallel-connected sense MOSFET Q2 and a sense resistor Rs; a current amplifier OTA 1 which detects and amplifies a current of the sense resistor Rs; a resistor Rsns which converts an output current signal of the current amplifier OTA 1 into a voltage signal. The current flowing through an external resistance Rset connected to an external terminal is converted into a voltage to be used as a reference voltage, and overcurrent detection can be performed by comparing voltage drop of the resistor Rsns with the reference voltage set by the external resistance Rset.

Description

  The present invention relates to an integrated circuit including a MOSFET current detection circuit, and more particularly to a circuit for setting an overcurrent protection start point at an external terminal in an overcurrent detection circuit including a sense MOSFET.

  Conventionally, as a current detection means of a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a current flowing in a power MOSFET is detected by using a sense MOSFET and a sense resistor as in, for example, US Pat. There is a current detection circuit. In the prior art, a plurality of small capacity MOSFETs called cells are connected in parallel to form a power MOSFET, and a sense MOSFET composed of a single cell provided on the same semiconductor chip is connected in parallel. Since these many cells are manufactured by the same manufacturing process and have the same characteristics, when the same gate-source voltage is applied to each cell, the current of each of the power MOSFET and the sense MOSFET is changed to that cell. Flows according to the number ratio. For example, when the ratio of the number of cells of the power MOSFET and the sense MOSFET is 3000: 1, the current flows divided into the respective MOSFETs at a ratio of 3000: 1. Since the current flowing through the sense MOSFET flows in proportion to the power MOSFET, if this sense MOSFET current is detected using a sense resistor, this detection signal can be used as a limiter detection signal for limiting the load current. .

FIG. 3 shows a configuration diagram of a conventional step-down DC-DC converter using a sense MOSFET. In FIG. 3, the reference numeral Q1 indicates a power MOSFET as a high-side switching element, and the reference numeral Q2 indicates a current detection sense MOSFET of the power MOSFET Q1. The source terminals of the power MOSFET Q1 and the sense MOSFET Q2 are connected to each other, and the drain terminal of the power MOSFET Q1 is connected to the DC power source VIN. The drain terminal of the sense MOSFET Q2 is connected to the DC power source VIN via the sense resistor Rs. A connection point where the source terminals of the power MOSFET Q1 and the sense MOSFET Q2 are connected is connected to the output terminal via the reactor L. A flywheel diode SBD is connected between the connection point where the source terminals of the power MOSFET Q1 and the sense MOSFET Q2 are connected to the ground terminal, and a smoothing capacitor C and a load are connected between the output terminal and the ground terminal. Reactor L and smoothing capacitor C constitute a DC smoothing circuit at the output of the step-down DC-DC converter.
Both ends of the sense resistor Rs are connected to an inversion terminal (−) and a non-inversion terminal (+) of a current detection circuit OTA1 (Operational Transconductance Amplifier: a transconductance amplifier). The current detection circuit OTA detects a voltage drop of the sense resistor Rs and outputs it as an N times current signal. This current signal is passed through the resistor Rc to be converted into a current and voltage, and is input to the non-inverting terminal of the voltage comparator COMP1. The reference voltage Vc is connected to the inverting terminal of the voltage comparator COMP1, and when the current signal voltage exceeds the reference voltage Vc, an H-level overcurrent detection signal (OCP signal) is output from the output of the voltage comparator COMP1, not shown. The control circuit of the DC-DC converter is fed back so as to narrow the pulse width of the drive signal Sg1.
In addition, the drive signal Sg1 is input to the gates of the power MOSFET Q1 and the sense MOSFET Q2 via the buffer circuit Bf, and the power MOSFET Q1 and the sense MOSFET Q2 are on / off controlled.

U.S. Pat. No. 4,553,084

  The current detection circuit provided in the prior art integrated circuit has an overvoltage based on a signal obtained by voltage-converting the current divided by the ratio of the number of cells of the power MOSFET and the sense MOSFET and a reference voltage to be compared with the voltage-converted signal. Current protection start point is fixed.

For example, when the current detection circuit of the prior art is applied to the detection of the power MOSFET current of the step-down DC-DC converter, the switching frequency considering the efficiency, the inductance value of the reactor, the capacity of the output smoothing capacitor, or the rating When factors such as output current are optimized, there is a case where appropriate protection cannot be performed at a fixed overcurrent protection start point.
In other words, depending on the rated power of the connected load, the fixed overcurrent value increases the overcurrent value more than necessary, and measures such as reinforcing the printed circuit board pattern width and raising the component rating by one rank are required. Therefore, it has been a hindrance to downsizing at low cost.

  In view of the above problems, an object of the present invention is to solve the problems of the prior art and provide a current detection circuit capable of setting an overcurrent protection start point in an integrated circuit by an external terminal.

A current detection circuit of the present invention is an overcurrent detection circuit incorporated in an integrated circuit having an external terminal to which an external resistor is connected, and is connected in parallel to a power MOSFET that supplies power to a load. A sense amplifier and a series body of a sense resistor, a current amplifier that detects and amplifies the current flowing through the sense resistor, a sense current voltage conversion circuit that converts an output current signal of the current amplifier into a voltage signal, and is connected to an external terminal A reference current generation circuit that generates a reference current according to the resistance value of the external resistor, a current-voltage conversion circuit that converts the reference current generated by the reference current generation circuit into a reference voltage, and a sense current-voltage conversion circuit The overcurrent detection signal is generated by comparing the voltage signal with the reference voltage converted by the current-voltage conversion circuit.
An overcurrent detection circuit incorporated in an integrated circuit having an external terminal to which an external resistor is connected, the power MOSFET supplying power to a load, a sense MOSFET connected in parallel to the power MOSFET, and a sense resistor A current amplifier for detecting and amplifying the current flowing through the sense resistor, a sense current voltage conversion circuit for converting the output current signal of the current amplifier into a voltage signal, and an external resistor connected to the external terminal. A reference current generation circuit that generates a reference current according to the resistance value and a constant current source are provided. The reference current according to the resistance value of the external resistor is subtracted from the constant current value of the constant current source, and the subtracted current is obtained. By comparing the voltage signal of the current-voltage conversion circuit that converts to the reference voltage and the voltage signal of the sense current-voltage conversion circuit with the reference voltage converted by the current-voltage conversion circuit, an overcurrent detection signal Generated and characterized in that.

According to the present invention, it is possible to provide a current detection circuit that can freely set an overcurrent protection start point by using one pin of an external terminal of an integrated circuit.
Even if the external resistance is released, the load and the DC-DC converter can be protected by setting the overcurrent protection start point to the minimum set value.

It is a circuit block diagram which shows the circuit structure of the current detection circuit of Embodiment 1 by this invention. It is a circuit block diagram which shows the circuit structure of the current detection circuit of Embodiment 2 by this invention. It is a circuit block diagram which shows the circuit structure of the current detection circuit of a prior art.

  Hereinafter, a step-down DC-DC converter using an overcurrent detection circuit according to an embodiment of the present invention will be described in detail with reference to the drawings. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

(Embodiment 1)
FIG. 1 shows a first embodiment in which a current detection circuit according to the present invention is applied to a current detection circuit of a power MOSFET of a step-down DC-DC converter.

In FIG. 1, the circuit configuration corresponding to the reference voltage Vc connected to the inverting input terminal of the voltage comparator COMP1 is different from the conventional overcurrent detection circuit.
The voltage corresponding to the reference voltage Vc is a voltage across the resistor Rocp by a series circuit of a constant current source CC1 connected to an internal power supply REG (not shown) and the resistor Rocp.

  Here, in parallel with the resistor Rocp, the buffer amplifier B-AMP including the transistor Qb and the operational amplifier OP1 and the external resistor Rset are connected, and the current I2 flowing through the buffer amplifier B-AMP through the current I1 of the constant current source CC1 is the resistor Rocp. Is shunted with the current I3 flowing through the.

  The reference voltage VREF is connected to the non-inverting terminal of the operational amplifier OP1 of the buffer amplifier B-AMP, the emitter terminal of the transistor Qb is connected to the inverting terminal, and the external terminal T2 is connected. Since the output terminal of the operational amplifier OP1 and the base terminal of the transistor Qb are connected, the operational amplifier OP1 and the transistor Qb constitute a voltage follower circuit that outputs the external terminal T2. The collector of the transistor Qb of the buffer amplifier B-AMP is connected to a connection point between the constant current source CC1 and the resistor Rocp.

A constant current source CC1 connected to the internal power supply REG supplies a constant current I1, and a current I3 flows through the resistor Rocp and a current I2 flows through the buffer amplifier B-AMP. Accordingly, the voltage across the resistor Rocp corresponding to the reference voltage Vc of the overcurrent detection circuit is the product of the resistance value Rocp of the resistor Rocp and the current I3. The current I3 is a value obtained by subtracting the current I2 from the constant current I1 to the buffer amplifier B-AMP, and the current I2 is a value obtained by dividing the reference voltage VREF by the external resistance Rset.
Accordingly, the voltage Vocp_ref across the resistor Rocp is expressed by the following equation.
Vocp_ref = (I1-I2) × Rocp
= (I1− (VREF / Rset)) × Rocp Formula 1
However, VREF / Rset does not exceed the current I1 from the constant current source CC1.
The current Is flowing through the sense MOSFET Q2 for detecting the current Ip of the power MOSFET Q1 is output as the current Isns multiplied by N by the current detection circuit OTA1, and flows through the resistor Rsns. Since the resistor Rsns is connected to the non-inverting terminal of the comparator COMP1, the voltage Vsns of the resistor Rsns is compared with the reference voltage Vocp_ref of the overcurrent detection circuit.
Therefore, when the voltage Vsns exceeds the reference voltage Vocp_ref, it is determined as an overcurrent, and an overcurrent protection signal OCP (H level signal) is output from the output terminal of the comparator COMP1 to a control circuit of a DC-DC converter (not shown) and driven. The pulse width of the signal Sg1 is reduced, and the overcurrent protection operation is performed.

Here, as shown in Expression 1, the reference voltage Vocp_ref of the overcurrent detection circuit has the resistance value of the external resistor Rset as a parameter. Accordingly, it is possible to set the overcurrent detection value by changing the resistance value of the external resistor Rset.
From Equation 1, as the value of the external resistance Rset becomes higher, it approaches the maximum overcurrent detection point where Vocp_ref = I1 × Rocp, and conversely, the minimum overcurrent point becomes lower as the value of the external resistance Rset becomes lower. Overcurrent detection approaching can be set.

(Embodiment 2)
FIG. 2 is a diagram showing a configuration example in which the setting of overcurrent detection based on the value of the external resistance Rset in FIG. 1 is reversed.
Specifically, a circuit corresponding to a current inverter of the current I2 flowing through the external resistor Rset in FIG. 1 of the first embodiment is added.

The current inverter circuit C-INV includes a current mirror circuit including MOSFETs Q3 and Q4 and a constant current source CC2, and is connected between a connection point between the constant current source CC1 and the buffer amplifier B-AMP and a resistor Rocp.
As shown in FIG. 2, the constant current source CC2 of the current inverter circuit C-INV is connected to the internal power supply REG, and the output of the constant current source CC2 is connected to the collector of the buffer amplifier B-AMP transistor and the drain of the MOSFET Q3. Yes. The drain of the MOSFET Q3 is connected to the gate of the MOSFET Q3 and the gate of the MOSFET Q4. The drain of the MOSFET Q4 is connected to a connection point between the constant current source CC1 and the resistor Rocp. Each source of MOSFETs Q3 and Q4 is connected to GND. Note that the current ratio of the current mirror circuit composed of the MOSFETs Q3 and Q4 is set to 1: 1.

The current I4 of the constant current source CC2 is divided into a current I2 flowing through the buffer amplifier B-AMP and a current I5 flowing through the MOSFET Q3 of the current mirror circuit, and a current obtained by subtracting the current I2 from the current I1 flows.
I5 = I4-I2 Formula 2
The current flowing through MOSFET Q4 of the current mirror circuit has the same current value I5 as the current flowing through MOSFET Q3, and shunts current I1 of constant current source CC1. Since the current I1 of the constant current source CC1 is divided into the current I5 flowing through the MOSFET Q4 and the current I3 flowing through the resistor Rocp, the current I3 flowing through the resistor Rocp is expressed by the following equation.
I3 = I1-I5 Formula 3
However, the current I5 does not exceed the current I1 from the constant current source CC1.
When the current I1 of the constant current source CC1 and the current I4 of the constant current source CC2 are set to the same current value, and formulas 2 and 3 are arranged,
I3 = I1- (I4-I2) = I1- (I1-I2) = I2 Formula 5
Accordingly, the voltage Vocp_ref across the resistor Rocp is expressed by the following equation.
Vocp_ref = I2 × Rocp
= (VREF / Rset) × Rocp Equation 6

As is clear from Equation 6, the minimum overcurrent detection point approaches as the resistance value of the external resistor Rset increases, and the maximum overcurrent detection point approaches as the resistance value of the external resistor Rset decreases. It becomes possible.
In other words, even if the external resistance is released due to some factor, the overcurrent detection point is set to the minimum value, so that damage to the load or DC-DC converter can be prevented.

  Although the present invention has been described above by way of specific examples, it is needless to say that this is an exemplification, and modifications can be made without departing from the spirit of the present invention. In the above embodiment, an example in which the present invention is applied to the current detection circuit of the power MOSFET of the step-down DC-DC converter has been described. However, the present invention may be applied to a current detection circuit of the power MOSFET of the step-up DC-DC converter. The present invention is not limited to the above-described embodiments, such as being applied to a DC / AC conversion inverter circuit.

VIN DC power supply Q1 Power MOSFET
Q2 sense MOSFET
Q3, Q4 MOSFET
Qb transistor Rs sense resistor SBD flywheel diode C smoothing capacitor L reactor OTA1 current detection circuit OP1 operational amplifier Bf buffer circuit CC1, CC2 constant current source VREF, Vc reference voltage REG internal power supply Rsns, Rocp, Rset resistance

Claims (2)

An overcurrent detection circuit incorporated in an integrated circuit having an external terminal to which an external resistor is connected,
A power MOSFET for supplying power to the load;
A sense MOSFET connected in parallel to the power MOSFET and a series body of a sense resistor;
A current amplifier for detecting and amplifying the current flowing through the sense resistor;
A sense current voltage conversion circuit for converting an output current signal of the current amplifier into a voltage signal;
A reference current generating circuit that generates a reference current according to a resistance value of the external resistor connected to the external terminal;
A current-voltage conversion circuit that converts the reference current generated by the reference current generation circuit into a reference voltage;
An overcurrent detection circuit that generates an overcurrent detection signal by comparing a voltage signal of the sense current / voltage conversion circuit with the reference voltage converted by the current / voltage conversion circuit. An overcurrent detection circuit incorporated in an integrated circuit having an external terminal to which an external resistor is connected,
A power MOSFET for supplying power to the load;
A sense MOSFET connected in parallel to the power MOSFET and a series body of a sense resistor;
A current amplifier for detecting and amplifying the current flowing through the sense resistor;
A sense current voltage conversion circuit for converting an output current signal of the current amplifier into a voltage signal;
A reference current generating circuit that generates a reference current according to a resistance value of the external resistor connected to the external terminal;
With a constant current source,
A current-voltage conversion circuit that subtracts a reference current corresponding to a resistance value of the external resistor from a constant current value of the constant current source, and converts the subtracted current into a reference voltage;
An overcurrent detection circuit that generates an overcurrent detection signal by comparing a voltage signal of the sense current / voltage conversion circuit with the reference voltage converted by the current / voltage conversion circuit.

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