Skip to main content
coppercrimp
coppercrimp
Associate III
October 12, 2017
Question

VBAT current draw high just after VDD removed

  • October 12, 2017
  • 5 replies
  • 4845 views
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.
This topic has been closed for replies.

5 replies

coppercrimp
coppercrimpAuthor
Associate III
October 16, 2017
Posted on October 16, 2017 at 07:10

Anyone have any ideas on this?

coppercrimp
coppercrimpAuthor
Associate III
January 17, 2018
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

Jan Waclawek
Jan Waclawek
Visitor II
January 17, 2018
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
coppercrimpAuthor
Associate III
January 18, 2018
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

AVI-crak
AVI-crak
Senior
January 17, 2018
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
T J
Senior III
January 17, 2018
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.

coppercrimp
coppercrimpAuthor
Associate III
January 18, 2018
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

T J
T J
Senior III
January 18, 2018
Posted on January 18, 2018 at 23:51

Yes, A series resistor between the Coin Cell and the Processor pin.

please make sure you DO NOT have a bypass cap across the battery. It would be your problem if you had one.

a bypass cap would leak your battery flat in a week

José Pablo Crespo
José Pablo Crespo
Visitor II
February 20, 2018
Posted on February 20, 2018 at 14:34

Hi Rob, did you leave unconnected PDR_ON, or tied to GND?

coppercrimp
coppercrimpAuthor
Associate III
February 20, 2018
Posted on February 20, 2018 at 18:47

Hi Jose, I'm using the STM32F407VGT chip. It's the LQFP100 pin package which does not have a PDR_ON pin. Thanks for your response though!

-Rob

José Pablo Crespo
José Pablo Crespo
Visitor II
February 21, 2018
Posted on February 21, 2018 at 09:57

Hello Rob, I think you might be able to fix it using some kind of power

supply supervisor circiut, enabling reset when Vdd drops, because when

resets are active, conmmunication between Vbat side and power side

stops. May be configuring the BOR in a different way. But I didn't use

407...

Good luck

Pablo Crespo

RE: VBAT CURRENT DRAW HIGH JUST AFTER VDD REMOVED

reply from Rob in STM32 MCUs Forum - View the full discussion

Terms & ConditionsAccessibility statement