Ultra low power for STM32L433 using VBAT pin for power in shutdown

[freeflyer]

I am designing a compact, low power device using the STM32L433.

The device is powered by a 1S battery and I use a buck-boost converter (TPS63070) to generate the 3.3V supply.

However, at the moment the buck-boost converter is permantely powered (enable pin permentatly pulled high).

I use both shutdown and standby power modes, which use WKUP2 (pin PC13) and RTC to wake the STM32.  But I have just noticed the VBAT pin which could potentially reduce power consumption drastically.  In both shutdown and standby power modes, I use RTC and LSE.

However, there are two issues I am not sure how to resolve.

1. The WKUP2 pin currently uses the 3.3V from the buck-boost converter (I copied the same circuit used on the Nucleo develpment board).  But if I disable the buck-boost converter then there will be no 3.3V supply to the switch for the WKUP pin, so the STM32 would never wake.  The circuit is shown below..

2. The battery is 1S so could be up to 4.2V, but the VBAT pin can only handle voltatges of 1.55 to 3.6 V.  So how do I power the VBAT pin using the 1S battery ?

At the moment the current draw In shutdown is around 0.5mA (probably due to the buck-boost converter always running), but I believe if I can disable the buck-boost converter and use the VBAT pin then this could potentially reduce to tens of nA which would make a significant power saving, so I am very keen to implement this if I can resolve the above issues.

[TDK]

Use an LDO with a very low quiescent current. There are thousands of these devices. Here is one basically at random:

AP7354

It would be better (more efficient) to use a battery that lies within the allowable voltage range, but this will get you to the sub-uA range easily.

[freeflyer]

Thanks TDK, I need a 1S battery 500mAh+ as it’s a Bluetooth audio device driving a headphone speaker when running. 

Are you suggesting to use a coin cell in addition to the 1S battery?