# Backfeeding boost converter

I'm designing a circuit that can be powered by either USB or a single NiMH cell. The NiMH will be boosted to 3.3V, and then an LDO will reduce both that and the USB 5V to 3.1V. I've attached a quick sketch.

As shown, the boost converter would need be able to resist backfeeding from the USB power. This is an example of a converter that I'm considering:

This converter has output overvoltage protection, short circuit protection, and output disconnect at shutdown. I've read the datasheet but I'm not entirely clear what the first two mean. Do they mean that I would be fine applying 5V to the output while the device is in operation? If so, would any current pass back through to the battery? Or, would I need to use the EN pin to shutdown the converter and disconnect the output when USB power is supplied?

Also, is the diode shown necessary?

Thanks!

For most regular boost converters, the series diode in the output will resist back-feeding USB voltages onto the boost converter output (reference the ideal diode below): -

The above is a simple representation of a boost converter but, importantly, it shows the diode I mentioned. However, you should use a Schottky diode because the maximum voltage than can be safely applied at the output of your chosen device is 5.0 volts and, it's likely that the raw USB output will naturally be around this level.

These sections should explain your other questions: -

Do they mean that I would be fine applying 5V to the output while the device is in operation?

No, 5 volts is right on the cusp of the absolute maximum ratings irrespective of using the EN pin to shut down the device.

• Thanks for your help. To make sure I understand, with this particular converter the only safe option is to add a Schottky diode on the output, correct? I see that there are others that can tolerate 5.5V or more, so I assume those would be okay? Would I need to look for one that has load disconnect? I gather from your reply that this may not be necessary as long as the output pin is tolerant of at least 5.5V? Nov 11, 2022 at 19:37
• It would be more convenient to find one that can deal with 5 volts with ease. Nov 11, 2022 at 21:52
• To follow up on this: I've had trouble finding another boost converter that will work well for this application. If I use a silicon diode in series with the USB power to drop it to around 4.5V, would I then be okay backfeeding the converter? It's not entirely clear to me what the best practice is for this sort of application. Nov 16, 2022 at 19:58
• Some people use power path controllers that have an internal SPST switch made from MOSFETs that drop very little voltage and, importantly, provide isolation between supply A and supply B. Nov 16, 2022 at 20:03
• Thanks-- I asked TI about this and they verified that it is fine to backfeed 4.5V, so I think I'll use a silicon diode and drop the voltage a bit. Nov 17, 2022 at 22:03