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Can I leave VBAT unconnected?

re.wolff9
Senior
Posted on April 11, 2018 at 14:03

Hi, 

I Have a project that can be populated with or without RTC and a battery. Can I leave the VBAT pin unconnected when building PCBs without the battery?

I'm using an STM32F072. 

#stm32f0 #stm32 #vbat
11 REPLIES 11
Posted on April 11, 2018 at 17:53

If no battery, both RM0091 and AN4080 recommend to connect it to VDD.

JW

Posted on April 11, 2018 at 20:08

Well, yes. You're right. But I was hoping I would not need extra components when I don't mount the battery. Or what if I do mount the battery-holder, but the user forgets to replace the battery when it's empty? That creates a very similar situation as what I have now: Simply a floating VBAT. 

I just checked the 'decoupling your STM32F0xx' section in the datasheet. No capacitor on VBAT. And then RM0091 says to install a capacitor when connecting to VDD...  

Posted on April 11, 2018 at 21:28

I guess you can leave it unconnected as long as you don't use the RTC and other VBAT-related features.

Note that I am not ST so do this on your own risk.

JW

T J
Lead
Posted on April 12, 2018 at 00:12

use a jumper or solder blob under the battery itself.

if the battery is not installed, you must solder the blob , shorting out the Vbat to VDD

Posted on April 12, 2018 at 00:30

Hello,

Capacitor between VBAT pin and ground is not mandatory. If you don't plan to use backup domain, please simply connect VBAT to VDD.

Regards

Szymon

Posted on April 12, 2018 at 01:03

A capacitor to ground on the VBAT pin is usually detrimental to the battery life

Not recommended unless you use a high quality poly cap.

Posted on April 19, 2018 at 07:32

Why does a capacitor drain the battery? Why would a better cap not do this? You're thinking about leakage current?  I don't think of ceramic caps as 'leaky'. I'll see if I can measure the leakage of the 100nF 50V ceramic 0805 capacitors that I have. 

Posted on April 20, 2018 at 01:15

If you are using a normal Coin Type cell,

I found the basic 0.1uF bypass cap,

reduced the voltage on the battery below 3V within a month.

Posted on April 21, 2018 at 16:22

I measured the leakage of my standard 100nF 50V capacitor. It is about 0.1 nA at 10V and about 1nA at 50V. 

A 200mAh coin cell would be depleted by such a current after about 63 years.....