Power-MOSFET gate directly takes cue from comparator

Using a comparator powered by a regulated voltage to drive a MOSFET with an unregulated power supply (Figure 1) is a common practice. It would be a waste of money and power to use a regulated supply to drive the FET as many loads are insensitive to driving voltage. Adding resistors R1 and R2 to the comparator to put hysteresis in the operation and make the circuit less susceptible to noise, especially with slowly changing signals, is also usually done.



This circuit’s comparator changes with changes in the unregulated power supply. You can correct this problem by adding diode D2 and resistor R5 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 D1 to the FET gate to avoid exceeding the gate-to-source voltage. The improvements in the circuit in Figure 2 become apparent when the comparator turns off. In either case, R4 pulls the comparator’s open-collector output up to the positive power supply. In Figure 2, however, the diode isolates the hysteresis circuit from the power supply so that R4 pulls up R5 to the regulated 15V, no matter how the power supply changes.



With a legacy comparator such as Texas Instruments’ LM193, the common mode of the inputs must stay well below the power-supply rail (Figure 3). The circuit requires 1.5V head room at 25°C and 2V head room over temperature. Thus, for the circuits in Figures 1 and 2, you cannot set the threshold voltage higher than 13V. If your circuit requires a threshold voltage closer to the power rail, consider using newer parts with rail-to-rail inputs. You must use an open-collector or open-drain comparator for this hysteresis-isolation circuit to work. It would be incompatible with a totem-pole-output IC.