I'm designing a prototype FPGA board and I have an unused IO bank that I'd like to expose to pin headers. The IO bank has a separate power supply, which I'd like to power from the internal 3.3V supply.

At the same time, I'd like to allow the user to provide their own supply, between 1.8V and 3.3V.

Now, I could obviously use a switch, like this.


simulate this circuit – Schematic created using CircuitLab

However, I was wondering if there is an easy way to detect the external voltage and switch automatically. I came up with the following, but I'm pretty sure I'm going to have a hard time finding a FET with threshold voltage range that would work here. I'm also a bit annoyed by the drop on the diode (but the current draw will not exceed 100mA).


simulate this circuit

What's the best way to implement the automatic switch?

  • \$\begingroup\$ If the "user" is expected to interface some connector, then you might specify that they need to add some "bridge" between the pins on their side, such that it will act as a switch on yours. \$\endgroup\$
    – Eugene Sh.
    Mar 27 at 20:54
  • \$\begingroup\$ What is the maximum voltage drop you can tolerate through a switch? At 100 mA perhaps an analog switch such as a MAX20337 could be used. At 100 mA and VCCEN = 1.8V the maximum on resistance is 0.40 Ω, so would drop 40 mV. \$\endgroup\$ Mar 27 at 21:09
  • \$\begingroup\$ @ChesterGillon That's actually perfect, thanks! \$\endgroup\$
    – avakar
    Mar 27 at 21:26

1 Answer 1


but the current draw will not exceed 100mA

At 100 mA an analog switch could be suitable. E.g. a MAX20337 which is described as:

Ultra-Small, Low-RON, Beyond-the-Rails DPST/SPDT Analog Switches

Industry's Smallest DPST/SPDT Can Switch ±5.5V Signals with a Single VCC as Low as 1.6V

Connected so that the EXT supply is connected to the NO side of the switch and the CB control input:


simulate this circuit – Schematic created using CircuitLab

For the CB input the VIH is 1.4 V so can be driven from an external supply of 1.8 V to 3.3 V.

RON is a max of 0.33 Ω at VCCEN = 2.5 V, so at 100 mA the switch will drop a maximum of 33 mV.

If a lower voltage drop through the switch is required, could look for an alternative with a lower RON. The MAX20337 was just the first found on a search.


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