The PMM originates from a 4-wire fan header on a personal computer motherboard, an open drain output with a maximum pull-up of 3.3 volts that can sink up to 5mA. I am using an after market heat sink on the CPU so the boxed INTEL fan is useless. Also because it is the CPU, I need to insure that the fan will not stall, hence the minimum 5V output requirement.
I want to be able to use the FG signal (tachometer) of the controlled 3-wire brushless fan which is 2 pulses per revolution, pull-down to ground. At low duty cycles the the PWM on time is less than the FG signal on time, so i would need to stretch or average the FG signal to make it usable.
It has also been suggested that I use a PIC. The circuit would be easy to build, but the last time I did any programing it was in FORTRAN, so I would need copious help to write the necessary code.
What I'm looking for is something that has a small physical footprint and can be knocked out on perf-board with out too much fuss. I would be just as happy with a discrete component circuit, but have been unable to find any design information to pursue that type of build.