VBAT current draw high just after VDD removed

[coppercrimp]

Posted on October 12, 2017 at 03:13

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

#vbat #stm32f4 #rtc-backup #high-currrent-draw-on-vbat Note: this post was migrated and contained many threaded conversations, some content may be missing.

[coppercrimp]

Posted on October 16, 2017 at 07:10

Anyone have any ideas on this?

[coppercrimp]

Posted on January 17, 2018 at 02:37

I figured I'd try revisiting this problem again because it's still something I'm struggling to find a solution to. To recap, I have a custom circuit board with an STM32F407. The circuit has a CR1225 coin-cell connected to VBAT which I use to keep the RTC going when main battery power is removed.

A linear regulator providing the 3.3V rail to the circuit has a 330uF capacitor at it's output. This large output capacitor was added to reduce transient voltage drops when an SD card was inserted or removed. An unexpected result occurred when the original smaller output capacitor was replaced by the 330uF one: The coin-cell backup battery was suddenly dying very quickly.

After much time spent troubleshooting, I discovered that the current draw from the coin-cell was very high during the time the 330uF output capacitor was discharging. I.e. the 3.3V rail does not drop to zero immediately upon removing the main battery input from the circuit. Instead there's a period of about 15-20 seconds where the voltage slowly ramps down. During this time the microcontroller circuitry must enter some strange quasi-state that draws tons of current from the coin-cell battery.

I've looked into creating a circuit that automatically shorts the output capacitor to ground when input power is removed, however a workable solution is eluding me. Anyone out there care to take a stab at how I can fix this issue?

Thanks in advance,

Rob

[AVI-crak]

Posted on January 17, 2018 at 03:58

The current consumption from the battery CR1225 is available for the remaining permitted write to the spare area of the memory (PWR_CR1_DBP). Reset the resolution can (need) on the sensor PVD_IRQHandler.

[T J]

Posted on January 17, 2018 at 04:57

I think you will find that the VBat pin only needs a few uA but must be above 2.6ish volts to maintain ram/rtc

You could use a resistor, 10k, 100k should work.maybe 1M would work.

then the VBat pin is supplied by the 330uF whilst its above 2.6V, and by the Battery after that.

do not put a bypass cap across the battery, it will be the problem.

[Jan Waclawek]

Posted on January 17, 2018 at 09:17

Isn't VDDA behind a filter with long time constant? What are the brownout fuse settings?

JW

[coppercrimp]

Posted on January 18, 2018 at 19:11

Hi JW,

Thanks for your reply. The brownout fuse setting is currently set to brownout level 3 threshold, which according to the datasheet should trigger a brownout reset on a falling edge of 2.83V. I've tried other brownout settings but none seem to have an effect on the problem

The 3.3 volt power rail drops to under a volt in less then a second when input power is removed. But the 330uF output capacitor keeps the power rail around 500mV for over a minute. During this time the STM32 RTC circuitry continues to draw quite a bit of current from the coin cell

[coppercrimp]

Posted on January 18, 2018 at 19:12

Hi TJ,

Thanks for your suggestion. Where are you suggesting I put the resistor? Do you mean to put the resistor in series with the coin-cell battery and VBAT pin?

Thanks again,

Rob

[waclawek.jan]

Posted on January 18, 2018 at 20:49

Isn't VDDA behind a filter with long time constant? Is VSSA properly connected?

JW

[coppercrimp]

Posted on January 18, 2018 at 21:12

JW, I'm not using the ADC or other analog features on my board. I have VDDA connected to 3.3V power rail and bypassed to ground with a 0.1uF ceramic capacitor. VSSA is connected directly to ground