People, I have very basic question on 'How to make a switch using a PMOS' transistor.

Following are the specifications: Drain - Output Pin Source - 4.2V (From Lithium-ion Battery) Gate - 3.3V / OV (Control Pin)

I am using NDS8434 PMOS. Gate threshold for this is less than -1V.

Question: Ideally, if I am connecting Gate to 3.3V then PMOS should be OFF and when I am connecting 0V then PMOS should be ON, But this is not the case?

  • \$\begingroup\$ Following the product's datasheet the minimum gate threshold voltage is reported on -0.4V at room temperature. The max is -1V. So, no with 0V on the gate you can't turn it on. You need at least -0.4V on the gate-source. \$\endgroup\$ – dirac16 Jan 31 '17 at 11:35
  • \$\begingroup\$ @dirac16 What about turning it OFF? I think, 3.3V at Gate should turn it off, right? \$\endgroup\$ – Rudraksh007 Jan 31 '17 at 11:45
  • \$\begingroup\$ You don't need only a 3.3V on the gate to turn it off. Any positive voltage such as 0V or greater should do it. \$\endgroup\$ – dirac16 Jan 31 '17 at 11:54
  • \$\begingroup\$ If your source is at 4.2V no it will not turn off. You still have -0.9V as your g-s voltage. \$\endgroup\$ – RoyC Jan 31 '17 at 11:54
  • \$\begingroup\$ Sorry, I answered in general. In that case, as @RoyC explained it's the voltage on the gate-source that determines if it's on or off. \$\endgroup\$ – dirac16 Jan 31 '17 at 11:57

You need to level shift your 3.3V to 4.2V to use a pmos device use a circuit like this.


simulate this circuit – Schematic created using CircuitLab

Note The polarity of the input is changed the pmos transistor will be on when the input is high.

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