| 摘要 |
PROBLEM TO BE SOLVED: To provide a power supply circuit wherein an excessive input current passed through at start-up is limited, and even if voltage step-up operation is repeatedly started and stopped according to PWM signals, the output voltage can be immediately raised to a desired voltage. SOLUTION: The power supply circuit generates output voltage Vout obtained by a drive circuit 13 switching a FET 12 to step up the input voltage from a direct-current power supply 1 and causes white light emitting diodes LED1 to LED6 to emit light. The power supply circuit is provided with: an ON/OFF circuit 21 that starts and stops the driving operation of the FET 12 of the drive circuit 13 according to brightness control signals; an output voltage detection circuit 24 that detects that the output voltage Vout is higher than a predetermined voltage and outputs a detection signal; and a soft start circuit 20 that controls the drive circuit 13 as follows: if a detection signal from the output voltage detection circuit 24 has been supplied when the brightness control signal is entered, the drive circuit is not caused to operate. If the detection signal has not been supplied, the drive circuit is caused to gently step up the output voltage Vout. COPYRIGHT: (C)2005,JPO&NCIPI
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