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VBAT current draw high just after VDD removed

Question asked by Rob on Oct 12, 2017
Latest reply on Feb 21, 2018 by José Pablo Crespo

I have a custom circuit board with a STM32F407 part. Everything has worked fine with this board in past revisions, except on the most recent revision we increased the size of the output capacitor on the main regulator supplying power to the microcontroller. This caused an unexpected problem where the VBAT pin draws more current from the coincell backup battery when the main power is removed.

 

To quickly summarize, we have a BR1225 coincell connected to the VBAT pin to keep the RTC running when power is removed. The LDO supplying 3.3V to the board has a beefy 330uF capacitor on the output to reduce transient current spikes when the SD card (also on the board) is inserted or removed.

 

When the main power input is removed from the board, the 330uF capacitor can take some time to fully discharge to 0V. During the time the capacitor is discharging, the current draw on the VBAT pin is quite high. It does eventually settle down to 1uA once the output cap is completely discharged, but since the board is frequently turned on and off the coincell battery is depleted much faster than it was when we used a smaller output capacitor.

 

Has anyone else experienced this issue or have suggestions on how to fix it? I’ve considered adding some kind of auto discharge circuit to the board but wanted to explore other options before going down that road. Maybe I’m missing some setting with the Power Voltage Detector or brownout options.

 

Thanks in advance,

Rob

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