I built a power switch circuit based on a LTC4412 that will disconnect my battery when an auxilary power source is available. Schematic is built exactly as presented in the data sheet:
The wall adapter input, when available, is a regulated 6V DC supply. The battery is a single-cell lipo that should be between 4.2-3.7V. The load goes toward a 3.3V low-dropout voltage regulator.
Testing the operation of this circuit, I found that with just the battery hooked up, the LTC4412 pulls its gate low and Vbat is fed into the regulator. With the auxilary source enabled I verified that the gate pin measures a high. So far so good. Now for the problem: the MOSFET does not seem to open completely. I'm measuring 6V at both the drain and gate pins, but the PSU also hits the current limiter and I read about 5.5V at the source pin (thus going into the battery!). I've replaced the MOSFET but the problem remains.
So, my best guess is that I did not choose a correct MOSFET for the job. The part I'm using is the
NDS332p, with important parameters
V
DS(MAX): -20V
V
GS(VT): -0.6V
RDS
on: 0.4Ohms typical
The V
GS(VT) seems most important and should work with the 6V-4.2V difference between aux and battery supply.
How can I figure out what's happening here?