| 摘要 |
PROBLEM TO BE SOLVED: To vary the frequency characteristic of a low pass filter without using an analog switch. SOLUTION: In a high switching frequency mode, a drive circuit 21 performs switching of an FET 11 and an FET 12, and an FET 21 and an FET 22 are turned off, based on a digital signal D0(H) obtained by a comparison circuit 1 by performing PWM (Pulse Width Modulation) with a triangular wave of 250 kHz generated by a reference oscillator 5. From an output from a common point O1, a signal component is extracted and noise by the switching is filtered out by the use of a low pass filter 31. In a low switching frequency mode, the drive circuit 21 performs switching of the FET 21 and FET 22, and the FET 11 and FET 12 are turned off, based on a digital signal D0(L) obtained by the comparison circuit 1 by performing PWM with a triangular wave of 50kH generated by the reference oscillator 5. From an output from a common point O2, a signal component is extracted and noise by the switching is filtered out according to a filter characteristic by a combination of the filter 31 and an inductor L2. COPYRIGHT: (C)2006,JPO&NCIPI
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