Low power low voltage cap discharge issue preventing MCU boot.

[saber63]

A simplified schematic is attached. The issue is that when the battery is removed the circuit drains down to ~0.7v and then just hangs out... For ever.... Like 15+min...  (There may be additional leakage from another dc-dc's enable pin back to the MCU, but thats not the main point of the discussion)

This 3.3v rail not getting close enough to 0volts prevents the 32.768khz TCXO from turning on when power is returned!

We dont want to just slap on 1meg resistance as our entire system power aims for ~170uA average.

Trying to think of a low voltage cutoff circuit or chip that would either cleanly remove the 3v3 power below say 2.5v. or some how only draw current below 2.5v to bleed off the caps quicker...

Other ideas would be to power the txco from the mcu... But im not sure the gpio wont float to ~0.7v

Thanks!

[SteveThackery]

I can't see the battery in the diagram. But a 1M resistor will only add 3.3uA to the 170uA already drawn. Seems too small to worry about.

[thm_w]

Does toggling the EN pin on the TCXO work to wake it up? Or is it still stuck in that frozen state.

[saber63]

The 7.2v on the left is the battery/incoming power.
The concern is that even with a bleed resistance of manageable size, the sub 1 volt error window would be long enough to get caught when plugging in power source die contact or switch boucing. Yes the occurrence rate would drop from every time to.... Still possible...
Id like it to not be possible.

The enable pin is an excellent question, will need todo some rework to test. But will do!

Thanks!

[Psi]

You can get chips designed to cleanly startup/shutdown ICs via enable/reset pins based on voltage and some delay.

TC51 comes to mind, but there are lots of them.  TC51 draws 1uA I think.

They're often called voltage supervisor ICs or Reset ICs

https://www.microchip.com/en-us/product/TC51

Actually you probably want one that cuts off a bit lower than that one.

[saber63]

Ah! Yes! Thats the family of chips I was thinking of!

The TC51 looks pretty spot on actually,  ill shop around a bit as well.

Thanks!