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need help with low part count circuit to interface 3-wire DC fan to INTEL 4-wire pwm fan output on motherboard. Direct pwm drive will spoil the tachometer output of the 3-wire fan. The pwm control signal is 25kHz, 3.3Vpp, 5mA maximum sink. Want to use high side drive to preserve fan tachometer output and have a 6V to ~11V @ 500mA output to the 3-wire fan. I am thinking a 20Hz first order low pass filter + complementary pair amp + P channel enhancement MOSFET will work, but need help selecting a MOSFET for linear mode.
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