Overcurrent protection circuit, power supply control device, inverting type switching power supply

1. An overcurrent protection circuit comprising:

a low-pass filter configured to generate a smoothing voltage corresponding to a voltage across a lower transistor forming a half-bridge output stage of an inverting type switching power supply;

a sense transistor; and

a sense resistor,

wherein an overcurrent protection signal is generated based on the smoothing voltage,

wherein a first end of the sense transistor is connected to a first end of the lower transistor, and the sense transistor is turned on and off in synchronization with the lower transistor,

wherein the sense resistor is connected between a second end of the sense transistor and a second end of the lower transistor, and

wherein the low-pass filter generates the smoothing voltage by smoothing a node voltage appearing at a connection node between the sense transistor and the sense resistor.

2. The overcurrent protection circuit of claim 1 , further comprising:

a first node;

a second node;

a bias current generation circuit configured to supply a bias current to each of the first node and the second node;

a first transistor connected between the first node and the second end of the lower transistor and having a gate to which the smoothing voltage is input;

a second transistor connected between the second node and the second end of the lower transistor and having a gate connected to the second end of the lower transistor;

a resistor connected in series with the first transistor between the first node and the second end of the lower transistor; and

a comparator configured to compare a first node voltage appearing at the first node and a second node voltage appearing at the second node to generate the overcurrent protection signal.

3. The overcurrent protection circuit of claim 2 , wherein the first transistor and the second transistor have the same on-threshold voltage.

4. The overcurrent protection circuit of claim 2 , wherein the bias current generation circuit includes:

a reference voltage generator configured to generate a reference voltage by flowing a reference current through a reference resistor;

a voltage/current converter configured to convert the reference voltage into the bias current; and

a current mirror configured to supply the bias current to each of the first node and the second node.

5. The overcurrent protection circuit of claim 4 , wherein the reference resistor is an on-resistor of a transistor configured to have the same temperature characteristics as the lower transistor.

6. The overcurrent protection circuit of claim 1 , wherein a current capacity of the sense transistor is smaller than a current capacity of the lower transistor.

7. The overcurrent protection circuit of claim 1 , wherein the low-pass filter is an RC filter including a resistor and a capacitor.

8. A power supply control device comprising the overcurrent protection circuit of claim 1 .

9. An inverting type switching power supply comprising the power supply control device of claim 8 .