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Very quick question:

Assuming I am using a pmos as a high side switch and my supply voltage and the voltage at the gate I want to switch the transistor with (V_logic) are the same. I usually find something like this: pmos pullup

Now I actually don't quite understand why I need the pullup here. Wouldn't the circuit work just fine without it as well?

Axodarap
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3 Answers3

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You need the pullup because otherwise you risk to have it open accidentally by having it floating. You normally can put even higher R value, like 100kR for keeping the FET blocked. Idea is to have a resistor with a big value not to lose current when commanding it. When you don't command it, you have it closed (blocked) by default because of that resistor.

Florinel Cocos
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It depends what is driving the gate.

If it is a logic chip, it does not need a pull-up, because it has a push-pull output.

If it is a MCU, it needs a pull-up to keep the FET off, until someone has programmed the MCU with a program that configures the pin as push-pull output. It also may take milliseconds before MCU powers up and starts executing any code to configure the pin as push-pull output.

Justme
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The main function of that resistor is to make sure the PMOS is not in conduction when the V_logic is for example high impedance.

If V_logic is the an output pin of a microcontroller, it is probable that the after reset the pin is defualt configured as an input so the gate is floating. Imagine you are debugging and there is a breakpoint before setting the input as output or there is a firmware issue that keeps restarting the microcontroller. In this case, without the RGS (R_pu) the PMOS may be in condition and this can be a problem.

MF3
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  • (in condition if not in blessed circumstances?) – greybeard Apr 16 '23 at 13:06
  • I see. But if I can guarantee that V_logic is always stable logic high or low I technically wouldn't need it, would I? – Axodarap Apr 18 '23 at 07:07
  • Ask your self: What am I controlling? Do I care if the output is always ON if something happens? Because the controlling part can always fail (bad soldering, component burned, ...). – MF3 Apr 18 '23 at 08:19