Comparator directly controls power-MOSFET gate

摘要

It is common practice to power a MOSFET with a comparator and with an unregulated voltage and to power the comparator driving it from a regulated one (Figure 1). Many loads are insensitive to driving voltage, so it would be a waste of money and power to use a regulated supply to drive the FET. It is also common practice to add resistors R_1 and R_2 to the comparator to put hysteresis in the operation, making the circuit less susceptible to noise, especially with slowly changing signals. This circuit's comparator changes with changes in the unregulated power supply. You can correct this problem by adding diode D_2 and resistor R_5 to the circuit (Figure 2). This approach isolates the hysteresis circuit from the unregulated output and instead drives it from the same regulated supply that drives the comparator. When the comparator is on, it drives the FET just as the original circuit does, pulling the P-channel FET gate toward ground. In both cases, you connect zener diode D_1 to the FET gate to avoid exceeding the gate-to-source voltage.