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I am currently designing a circuit which utilises a 18650 battery which passes through a TPS63000 buck-boost converter, converting the voltage to a steady 3.3V. I require the circuit to interface with an FTDI board occasionally, where the FTDI board supplies 3.3V to the circuit, allowing the board to function with the battery disconnected. I would like to know how I would design the circuit so that plugging in the FTDI's power pins would not damage any components. I have seen suggestions for Schottky diodes being placed in series with both the buck-boost converter and the FTDI's 3.3V supply, however the forward voltage drop would likely be too great for the microcontroller to be powered (STM32).

I would appreciate any help on the matter, even if it was to tell me I was missing something blatantly obvious 😅.

Thanks again!

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  • \$\begingroup\$ After reading the data sheet I'm tempted to say that it will work without a Schottky diode in series with Vout. \$\endgroup\$
    – Andy aka
    Dec 21 '20 at 11:55
  • \$\begingroup\$ however the forward voltage drop would likely be too great for the microcontroller Don't assume, go look it up in the datasheet. Many modern uCs work down to a supply voltage of 1.8 V so maybe you're in luck. It can be that a supply drop resets the uC (brown out detection), re-programming this detection can fix that though. Don't investigate too much into "non standard" solutions until you have proven that it cannot be done in the "standard" way. \$\endgroup\$ Dec 21 '20 at 12:36
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You can try the same scheme as used on and Arduino Uno. Sense the power supply voltage using a voltage divider and an op-amp. If it lower than your threshold, connect the board power to the FTDI's power via a p-channel MOSFET.

See the circuit in the top left side in this schematic, consisting of RN1A, RN1B, U1A, U1B and T1.

I hope you can get an idea of how to build a circuit for your application. If you can't come up with an exact circuit, I can edit this post to include a custom schematic for your application.

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