Integrated circuit and power supply circuit

1. An integrated circuit for a power supply circuit configured to generate an output voltage at a target level from an alternating current (AC) voltage input thereto, the power supply circuit including

an inductor to which a rectified voltage corresponding to the AC voltage is applied,

a power transistor configured to control an inductor current flowing through the inductor,

the integrated circuit comprising:

a first terminal that receives a power supply voltage for operating the integrated circuit, the power supply voltage being generated according to a variation in the inductor current;

a second terminal to which a control electrode of the power transistor is coupled;

a first drive circuit configured to drive the power transistor via the second terminal during a first time period to turn on the power transistor; and

a second drive circuit configured to drive the power transistor via the second terminal during a second time period to turn on the power transistor, the second time period including at least a part of the first time period, the second drive circuit having driving capability lower than that of the first drive circuit.

2. The integrated circuit according to claim 1 , further comprising a control circuit configured to control the first drive circuit such that a voltage at the second terminal is within a predetermined range, based on the voltage at the second terminal upon turning on the power transistor.

3. The integrated circuit according to claim 2 , wherein the control circuit includes:

a first determination circuit configured to determine whether the voltage at the second terminal is higher or lower than the predetermined range; and

an adjustment circuit configured to

increase the first time period, when the voltage at the second terminal is lower than the predetermined range, and

reduce the first time period, when the voltage at the second terminal is higher than the predetermined range,

based on a result of determination of the first determination circuit.

4. The integrated circuit according to claim 3 , wherein

the first determination circuit further determines whether the voltage at the second terminal is within the predetermined range, and

the adjustment circuit maintains the first time period, when the voltage at the second terminal is within the predetermined range.

5. The integrated circuit according to claim 1 , wherein the second drive circuit drives the power transistor via the second terminal during the second time period, the second time period including the first time period and being longer than the first time period.

6. The integrated circuit according to claim 1 , wherein the first time period includes

a first sub-period, and

a second sub-period subsequent to the first sub-period, the driving capability being higher in the first sub-period than in the second sub-period.

7. The integrated circuit according to claim 6 , wherein the first drive circuit includes

a first transistor coupled between the first terminal and the second terminal, and

a voltage output circuit configured to receive a control signal, and to apply, to a control electrode of the first transistor, a control voltage for increasing an on-resistance of the first transistor in the first time period, in response to the control signal upon turning on the power transistor.

8. The integrated circuit according to claim 7 , wherein

the first drive circuit further includes a clamp element coupled between the first terminal and the control electrode of the first transistor, and

the voltage output circuit is a voltage divider circuit that includes a resistor coupled to the clamp element in parallel, the voltage divider circuit having a voltage divider resistance value that changes in response to the control signal.

9. The integrated circuit according to claim 8 , wherein

the resistor coupled to the clamp element in parallel is a first variable resistor, and

the voltage divider circuit further includes

a second variable resistor, and

a second transistor coupled between the first variable resistor and the second variable resistor, the second transistor being configured to be on during the first time period, and

a resistance value of the first variable resistor and a resistance value of the second variable resistor change in the first time period in response to the control signal.

10. The integrated circuit according to claim 7 , wherein the voltage output circuit is a voltage divider circuit configured to divide the power supply voltage, to thereby apply a divided voltage to the control electrode of the first transistor in response to the control signal.

11. The integrated circuit according to claim 1 , wherein

the second drive circuit includes a source circuit configured to output, from the second terminal, a source current corresponding to a difference between a first level and a voltage at the second terminal, and

the first level is higher than a threshold of the power transistor and lower than a withstand voltage of the power transistor.

12. The integrated circuit according to claim 11 , wherein

the second drive circuit receives a sink current from the second terminal, and

the second drive circuit further includes a sink circuit configured to increase the sink current when the voltage at the second terminal exceeds a second level, the second level being higher than the first level and lower than the withstand voltage of the power transistor.

13. The integrated circuit according to claim 1 , further comprising a second determination circuit configured to determine whether the power supply voltage is higher than a predetermined level, wherein

the first drive circuit

drives the power transistor during the first time period, when the power supply voltage is higher than the predetermined level, and

drives the power transistor for a third time period corresponding to the target level, when the power supply voltage is lower than the predetermined level; and

the second drive circuit

drives the power transistor during the second time period, when the power supply voltage is higher than the predetermined level, and

stops operating, when the power supply voltage is lower than the predetermined level.

14. A power supply circuit configured to generate a direct-current (DC) voltage from an alternating current (AC) voltage inputted thereto, the power supply circuit comprising:

an inductor to which a rectified voltage corresponding to the AC voltage is applied;

a power transistor configured to control an inductor current flowing through the inductor; and

an integrated circuit configured to drive the power transistor, the integrated circuit including

a first terminal that receives a power supply voltage for operating the integrated circuit, the power supply voltage being generated according to a variation in the inductor current,

a second terminal to which a control electrode of the power transistor is coupled,

a first drive circuit configured to drive the power transistor via the second terminal during a first time period to turn on the power transistor, and

a second drive circuit configured to drive the power transistor via the second terminal during a second time period to turn on the power transistor, the second time period including at least apart of the first time period, the second drive circuit having driving capability lower than that of the first drive circuit.