I have a circuit where upon startup, it selects a voltage source with a programmed current limit via a resistor. Though I want a uC down the line to change this when it first powers up to protect other circuitry. Therefore I want to use a digital potentiometer. The datasheet tells me the power on value but what is the value between wiper and terminal A when the power is off? I could place a "startup" via a transistor and then have the uC disconnect the resistor by influencing the transistor but I wanna save on these 2 parts if possible.

I am using this digital potentiometer: https://ww1.microchip.com/downloads/en/DeviceDoc/20001978D.pdf

This is my circuit I am about the finish up:

enter image description here

Edit to solve chicken and egg problems. enter image description here


1 Answer 1


I think the circuit has a chicken-and-egg kind of issue. The pot needs supplies to operate, but supply also needs pot to operate.

The digital resistor state is random when supply is less than 1.8V and it won't be within specs of the datasheet.

However if supply is below that, like 0V, the analog muxes that connect the wiper cannot turn on at all, so resistance would be quite high, as in disconnected.

And on a side note, all the pot pins must be within 0.3V of supply voltages, so with 0V supply, the protection diodes start to conduct at about 0.3V on the wiper pin and backfeed current to pot supply.

And the powerswitch would see the high current limit resistance and would have quite a low current limit, so it might turn off when starting up.

  • \$\begingroup\$ That was my fear. Thanks for informing about the high resistance when the pot is underpowered. I will revise the circuit and post. \$\endgroup\$
    – joe
    Jan 6, 2021 at 23:39
  • \$\begingroup\$ Edited the circuit. I think this will work. It will boot up +V to have I_limit of 100mA. Then when the pot turns on, it will be ~150mA. Then I can set is from 100mA to full current. It won't be linear but I found the values to be fine when I threw it into a spreadsheet. \$\endgroup\$
    – joe
    Jan 7, 2021 at 0:02

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