VBAT pin of STM32F334C8?

[cupoftea]

Hi,
We have a 600W Mains PSU with big_battery backup.........there is a micro controlling the switch in/out of the big_battery when the mains fails.

When only the big_battery is connected, the micro must detect this, and then switch the big_battery on to the mains power module......and indeed allow the big_battery to power the micro instead of the coin cell.

The micro needs to be able to detect if the big_battery is there or if the mains is there...........so the micro has a coin cell connected to its VBAT pin (pin1).
Thing is, when we manually switch in the big_battery, the micro does not switch on the PFET which switches the big_battery into the cct.
We just get zero volts out of the micro pin thats supposed to switch on the n channel signal fet. (which would then switch on the series PFETs)
So i am guessing that the VBAT pin is ESD damaged, and the micro is not "seeing" the coin cell?

Is it true that the VBAT pin has no ESD diodes?...and likely the VBAT pin was ESD zapped when the guy manually inserted the coin cell?

When we manually switch in the 3V3 supply (which ultimately comes from the big_battery), the micro works fine....its just not working when only supplied by the coin cell...do you agree the VBAT pin is likely ESD damaged?

STM32F334C8

https://www.st.com/en/microcontrollers-microprocessors/stm32f334c8.html

 

[KlausST]

Hi,

What about a schematic?

You know that VBAT is an input and not meant as output to switch a PFET?

Klaus

 

[FvM]

Not sure what you're trying to achieve. VBAT is supplying RTC and RAM, not processor core.

 

[cupoftea]

Thanks, yes the VBAT pin is not the pin thats being used to switch on the signal NFET. But the signal NFET gets switched on whilst the micro is being supplied by the coin cell connected to the VBAT pin.

The schem is in a load of separate pages and very difficult to navigate.

 

[KlausST]

Hi,

whilst the micro is being supplied by the coin cell connected to the VBAT pin.

As FvM correctly states, and the datasheet, too: VBatt does not supply the microcontroller core.
When VBatt is valid while VCC is down, the microcontroller will not work.

Klaus

 

[stenzer]

Hi,

Figure 12 on page 48 of the datasheet clearly indicates what's supplied by VBAT, which has been and already pointed out by @KlausST and @FvM .

BR