I want to use a pair of N and P channel mosfets in a totem pole arragement to shift a VFD display on and off using a 74LS247 decoder/driver. I would put the P channel with the source pin on the top and connected to the 25 volts the VFD requires. The P channel drain pin would be connected N channel drain pin. The junction of both drains would be my output (25 Volts). The source pin of the N channel would be connected to ground.
The gates of both the P and N channel mosfets would be tied together and powered from the "Open Collector" output of the 74LS247 decoder/driver to turn on and off the appropriate mosfet. I plan on using a Fairchild FDS8958A mosfet complementary pair.
Questions:
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Do I need a series resistor in series with the LS247 output and the two gates?
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What value do I need?
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Do I need pullup / pulldown resistors between the source pin and gate pins?
Best Answer
Sorry, but this isn't going to work, or at least not directly. Yes, you need a pullup, but if you use one you'll destroy the 247. Reason? The 247 outputs have a max voltage rating of 15 volts, and you need to drive to 25. If you do manage to drive the P-type gate to 25 (to turn it off) you will violate the gate-source maximum voltage Vgss of 20 volts for the N-type. And if you drive the 247 output low you will exceed the gate-source voltage rating of the P-type.