Your circuit has problems above +5VDC as drawn. The PNP 3906 turns your n-channel MOSFET on. Everything works OK until you go above +5.6VDC, then it all falls apart. Here's why:
The emitter of the PNP goes to +supply. The output of your 74LS00 cannot be pulled above +5VDC. The base of the PNP cannot be pulled lower than one diode drop below your supply voltage. At 10VDC, the base is at 9.4V, the 74LS00 output is at 5V, current flows through your 100K resistor (44uA) and you cannot turn off the n-channel MOSFET. The p-channel MOSFET turns on and crowbars your supply. This is not a good thing.
Please see the attached gif. I redrew your circuit so that it will work from 5V to 24V. Above 24V you may rupture the p-channel MOSFET gate to source oxide. The PNP now turns the p-channel MOSFET off instead of on. The PNP base is driven by the NPN collector which acts a voltage level shifter. The MOSFET action will be 'fast off, slow on' which should prevent shoot-thru drain currents. This is not a high-speed circuit (about 1kHz max). High-speed circuits become more complex.
Mariss |