I’m trying to understand how exactly the reverse voltage protection circuit found on the DRV3383 breakout board works:
I’m super confused about the role of components R1, D1, and R2:
As I understand it, Q1 is a PMOS FET providing reverse voltage protection, but why is R1 in between the gate and the ground? Can’t it be tied directly to ground?
Is D1 an overvoltage clamp? Wouldn’t the 0-11v range then get clamped to 0-6.8v?
I’m assuming that C1 is the bulk capacitor for the motor supply voltage VM, but why is there a 1ohm resistor in series with it?
In your post you referred to the DRV3383, but I think you probably meant our DRV8838 carrier. The MOSFET used in the reverse protection circuit can only handle a maximum of 8V between the gate and source, and the Zener diode in the circuit prevents that voltage from ever going above 6.8V. The resistor is used to pull down the gate in normal operation and drop the extra voltage if VM is greater than 6.8V. The capacitors and R2 are not really part of the ‘reverse voltage circuit’. The capacitors are used to smooth out the input voltage. The one ohm resistor adds resistance in series with the capacitor, which can help protect the board from LC voltage spikes(which are discussed in this application note).
Ah thanks for that explanation, that makes a lot more sense!
Just to solidify my understanding: the reason low voltages don’t work with the carrier board is because a low VIN keep the gate-source voltage above the treshold voltage; however once VIN exceeds 1.5V or so, Vgs is negative enough to turn on the PMOS, but it will never go more negative than -6.8V due to the protecting Zener diode?
Yes, your understanding is right, though the threshold voltage for the MOSFET in the DRV8838’s reverse protection circuit is actually around 0.9V, so it can start turning on at voltages a little lower than 1.5V.
It sounds like you are trying to make your own reverse voltage protection circuit. If so, the MOSFET value would depend on the circuit you want to protect. You might find the following video helpful in understanding reverse protection circuits like this. It also has some suggestions for specific parts.
