Schematic Is there anything wrong with this circuit?

The goal is to have a battery powered device that is occasionally plugged into computer via USB. When plugged into the computer, I want to switch power from the battery to solely USB power. I don't want any battery charging.

This is how I expect the circuit to work:

Q1 is a P Channel MOSFET. It is the switch that connects or disconnects the cathode of the battery from the circuit. With only battery power connected, Q1 gate is 0v and source is 3.6v (from the battery), which drives the MOSFET on and sends power to the load. The diode is to keep from back feeding regulator.

When USB is plugged in, "EN" is pulled to 3.3V which will turn off MOSFET and disconnect battery from the circuit. The regulator steps 5V down to 3.3V and powers the load. There any problems with switching power this way? This is the best circuit I could think of that will have the least parasitic losses and no voltages drops when load is using just battery. It is important the entire system uses no more than 10-15uA. Parasitic losses from this circuit I calculated at about 4.3uA

  • \$\begingroup\$ No, you should not use a PMOS as a low-side switch...you won't be able to pull the source lower than the transistor's threshold voltage. Use an NMOS instead. \$\endgroup\$ Dec 14, 2019 at 14:17
  • 2
    \$\begingroup\$ Maybe I'm missing something, but wouldn't the battery just conduct through the body diode when the FET was off? \$\endgroup\$
    – Stiddily
    Dec 14, 2019 at 14:40
  • \$\begingroup\$ @Stiddily you didn't miss anything, on the contrary: Well noticed \$\endgroup\$
    – Huisman
    Dec 14, 2019 at 14:55
  • \$\begingroup\$ NMOS would also still conduct (when source is grounded), so better replace the FET with a NPN (and add a resistor preceeding its base). \$\endgroup\$
    – Huisman
    Dec 14, 2019 at 14:57
  • \$\begingroup\$ Im confused, both a NMOS and PMOS would conduct through the body diode when off? How is it possible they both would conduct? \$\endgroup\$
    – James Pie
    Dec 14, 2019 at 15:10


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