STM32f091 VBat pin sinking a lot of mA's

[marcel23]

Posted on June 09, 2017 at 22:53

I'm trying to set-up a STM32F091 with a battery connected straight into the VBAT pin. For the first couple times I turned on this vbat at 3v, It worked ok(no weird behavior). However, after trying to program the MCU via SWD, this VBat pin became a low impedance path to ground(around 150 Ohms) sinking 20mA. At first I thought this low impedance path was in my board, but then I removed the mcu and measured the resistance on the chip between pins VBAT and Ground, getting 159 ohms as a result. This problem manifested itself in two different chips and similar resistances were measured.(150 and 250 ohms)

I'm inclined to believe this is a problem, since the VBat domain is supposed to power the RTC and back-up register, definitely not sinking 20mA @ 3v. I have not found any information on the device errata sheet and the data sheet says this pin draws uA's when powering the RTC domain.

The circuit is a straight connection between the battery and this pin. All others power pins are connected according to the datasheet. An interesting fact is that the ADC channel 18(ADC_IN18) measures the voltage on VBat pin correctly.

Have anyone experienced this before? Am I missing something when using Vbat or is this a configuration issue?

#vbat-battery-backup #stm32f0 #stm32-vbat-rtc

[Zt Liu · Accepted solution]

Posted on June 10, 2017 at 17:39

This is impossible for a healthy STM32F0, I'm afraid ur mcus are damaged somehow. 

I am just using F091 and making use of its low power mode. 

Even in the beginning I connect VBAT directly to VCC, (of course VCC is regulated.)

In stop/standby with RTC on mode, the whole mcc sinks only about few uA from 3.3V VCC.

By the way, check the following values from datasheet

1 To exclude custom board hardware issue, I would first use a nucelo board, disconnect VBat and VCC, and perform the current measurement. 

2 If Nucleo board works fine, I would check on the custom board:

    VBAT and VCC(and some other power pin) isolation,  

    VBAT to Gnd impedance, and

    LSE circuitry. 

VBT draws current only when VCC is not present, and it powers RTC, oscillator and backup register only. 

Good Luck!

[raptorhal2]

Posted on June 12, 2017 at 15:58

The circuit is a straight connection between the battery and this pin.

Read the warning in the STM32L09 Reference Manual section on Battery Backup Domain (probably Section 6.1.2). Is your battery at or more than 0.6V below VCC?

Cheers, Hal

[Zt Liu]

Posted on June 14, 2017 at 04:25

Using two AA batteries Vbat around 2.6V.

Turn VCC off, here.

with VCC On.

VBat to Gnd impedance, there are few Mega Ohms.

As for your horrible mA current consumption....

Suppose you mcu is healthy and your board is correctly lay out.

Is it possible that your current measuring tool is problematic?

Can you test it with a reference current, like a current source?

[marcel23]

Posted on July 18, 2017 at 17:46

Thank you all. Actually the issue was a problematic PSU damaging the MCU -> 

https://electronics.stackexchange.com/questions/310059/stm32f091-vbat-pin-sinking-a-lot-of-mas