Why use two fets for reverse voltage protection?

[trevwhite]

I was looking at a TI reference design and they use 2 P channel fets in series on the input from a coin cell battery.

I attached the schematic section. Why have they used two, I thought one would be good enough?

Trev

[Leon23]

Putting the mosfets/diodes in series seems to make the reverse voltage protection to withstand twice the reverse voltage compared to just having one mosfet/diode in series.

[bktemp]

The limiting factor is often the gate voltage (I have never seen a mosfet with Vdsmax lower than Vgdsmax), so putting them in series does not increase the reverse maximum voltage.

I would assume it is for safety in case one mosfet fails, or maybe to reduce the leakage current.

[AndyC_772]

In this design, though, they're in series, and pointing the same way.

I've done many designs using a series pair of FETs facing opposite ways, so they can block current flow in either direction. That's not what they're doing here, though.

I'd guess at minimising reverse leakage through the body diode. The thing being powered must be very high impedance if it's being powered from a coin cell, so perhaps a single reverse-biased diode isn't enough to prevent the supply voltage going negative?

[tszaboo]

Probably leakage. At a given temperature, the leakage is the (somewhat linear) function of drain source voltage, if Vds is smaller, the leakage is smaller. With two in series, Vds will be half-ish. I'm not sure, if this is an economic way to protect the device though.

[Paul Price]

Somebody has a interest in selling a big overstock of  P-Chan MOSFETS, no other reason.

[Niklas]

The additional MOSFET is related to the reverse current rating from the coin cell manufacturer. Check the note above Figure 3 in the user manual for the demo board.