STM32f4 + standbymode : need idea

[ASSAAD.ASSAAD]

Posted on August 20, 2016 at 11:00

Hello ; 

I am using STM32F405 in my device ; I have some problem to configure the device in very low power mode ; The device should work from 18V/5000mah battery . The device should do selftest everyday : I use RTC alarm to wakeup from standby mode. The device should give an alarm if the battery is not connected ; what I do now is I charge a supercap and put the device in standbymode ;   the supercab and teh battery are connected to LDO  that power the STM32F4 ; STM32F4 is in standby mode either wakeuo by RTC alarm  very one hour to check if the battery removed or not  either wake up by a wakeup button for normal operation of teh device . I need the total current being consumed  from teh battery to be less than 100uA while I know the standby mode of  stm32f4 is about 20uA  ( max ) .  but I am getting 800uA ! ; I think it is due to the supercap or to the LDO ; But I have to put LDO ; DO you sugegst any other idea for this issue ? Down is the  shematic of the power 

[Tesla DeLorean]

Posted on August 20, 2016 at 14:47

How is the VBAT connected on the STM32F4 side?

What is the part# for the super-cap? Could they be on the low voltage side? You have an enable pin on the LDO, so you can easily turn that off. Is the 3V3 powering other things?

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Posted on August 20, 2016 at 16:26

Hi Clive ; Tobe honest I was waiting your reply ; thank you first . 

The LDO have to be on all the time because the MCU in standby mode and waiting for wakeup PA0 or RTC alarm ; the supercaaps are 2 capacitors in series C19 and C34 . the 3.3 is only powering the MCU and connected to Vbat too . 

[Tesla DeLorean]

Posted on August 20, 2016 at 17:44

Yes I can see how you have them in series, I'm interested in the specific part number.

You can power the F4 while it is in standby via the the VBAT pin, I think a diode and super-cap arrangement there to power VBAT, and VDD absent the primary supply, and a way of enabling the LDO as it exits STANDBY. ie a GPIO with a weak pull-down, that is driven high by some very compact code in Reset_Handler A super-cap on the low voltage side would be more efficient, and still permit you to swap out the 18V battery.

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Posted on August 21, 2016 at 12:36

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Posted on August 21, 2016 at 12:41

Hi Clive ; 

I attached the shematic in the above post ; I hope I could undertood your idea ; Q23 + Q24 will get a ground from button , then Q24 will wakeup the MCU  form standby and Q23 will turn LDO  on ; then the MCU will turn the LDO  Via R31 . I used Q23 to  turn LDO incase of the supercap are disharged . Vbat here 18volt ; I connected Vcell of the MCU to 3V batetry . I wish you give me your comments and suggestions . Thank you in advanced