ESP32 Forum

On ESP-WROOM-32 Vcc and Vcc_RTC are tied together. Is there any to actually use the RTC with a backup battery? Normally I would expect to put in a common anode pair of Schottky diodes between Vcc, Vbat and Vcc_RTC...

I meant common cathode, obviously... *plonk*

In deep sleep mode analog and IO supply rails contribute less than a microamp of leakage, so you may possibly add these diodes to module's VCC.

So you'd be suggesting something like a 3.6 V output from the regulator, to compensate for the diode drop? I'm assuming the main Vcc isn't as happy about ~3.0 V as Vcc_RTC would be.

It might make more sense to just use an external RTC, which is a darn shame...

There are many LDOs with ultra low quiescent current ( < 2uA ) and also step-down regulators with even lower quiescent current which can be used, so there's actually no need for separate RTC battery.
TPS62743(1) for example has less than 1 uA quiescent current with 90% efficiency at <10uA load so higher voltage batteries can be used.
Usually you won't find such regulators on common "development boards", but if you are designing you own board and need very low power consumption, such regulators shuould be used.

Well, it's not that I need low power consumption in normal operation (the project will be powered from an external bus, which in turn is AC powered), it is that I need an RTC. There is going to be other logic on the board that would kill the battery quite quickly, so there needs to be isolation even if ESP32 is in hibernation mode. Since ESP-WROOM-32 doesn't have the 32 kHz crystal anyway, it seems that the gain from using the builtin RTC over an external i2c is lost by the extra complexity in the power circuit, which is a real shame. That being said, it doesn't even lose any pins.