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Usually MOSFETs stay in switched-off state by default and get switched on by a signal. But I want to use MOSFET the opposite way:

1) it should pull down a line to GND all the time by default, after power supply is ON

2) if needed, should be turned-off by signal from MCU, so that the line gets automatically floating

Is this possible?

In the schematics below R24 should either be connected to GND to enable charging or let float to disable it (according to datasheet MCP73833, chapters 3.4 and 4.2). As per chapter 1.0 the resistor R24 could be at least 200K to disable charging.

If the PROG pin is open or floating, the MCP73833/4 is disabled and the battery reverse discharge current is less than 2 µA. In this manner, the PROG pin acts as a charge enable and can be used as a manual shutdown.

enter image description here

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  • \$\begingroup\$ It depends if a voltage supply is available or not. \$\endgroup\$ – Transistor Sep 21 '19 at 22:06
  • \$\begingroup\$ if there is no power supply then the whole circuit is powered off. I know that MOSFET can't conduct if there is no power supply available \$\endgroup\$ – hanni76 Sep 21 '19 at 22:10
  • \$\begingroup\$ Sure, but do you require the switch to be low-impedance in that case. It's not clear in your question so I suggest you edit ... \$\endgroup\$ – Transistor Sep 21 '19 at 22:12
  • \$\begingroup\$ I want MOSFET to pull down the line as soon as the global power is applied \$\endgroup\$ – hanni76 Sep 21 '19 at 22:15
  • \$\begingroup\$ R24 is doing what you require on power-up. Now all you need to do is pull it high using another GPIO configured as an output (and connected to PROG) when required. \$\endgroup\$ – Transistor Sep 21 '19 at 22:22
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I am going to use CPC1117N solid state relay to simply disconnect R24 from GND. This completely resolves my problem with minimal PCB space.

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  • \$\begingroup\$ I am fine with my answer as it completely resolves my problem with minimal PCB space. I will close the question tomorrow. \$\endgroup\$ – hanni76 Sep 22 '19 at 15:12

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