How do I get a high side switch to turn on when some voltage goes high?

Paul Andrews wrote 04/10/2018 at 03:43 0 points

I have a board powered by USB with battery backup. It has several chips on it running at 3V3. When USB is removed, the battery takes over, but I want one of the 3V3 chips to shut down for as long as USB power is off. When USB power is off, The voltage on the +5V rail drops to around 1.5V, not zero, because of leakage through a couple of diodes.

So, I thought, time for a transistor to switch the power to the chip. However, it needs to be a high-side switch, because I want to switch the power going into the chip. High side switches (PNP or p-channel MOSFET) are turned ON when their base/gate voltage goes low. I want it to turn ON when the base/gate voltage goes high. Also, I think MOSFETs are out, because of the forward voltage drop - remember I want to switch the 3V3 supply going in to a 3V3 chip, otherwise maybe I could have looked at depletion mode MOSFETS(?).

I briefly wondered if I could use a NPN, as I can guarantee that the base voltage (5V) is higher than the emitter voltage (3V3) when I want it to turn on. However, it looks like the emitter voltage is roughly equal to the base voltage when the base voltage is low (1.5V), even though I thought it should be cleanly off at that point.

So, what do I do?