Can I leave VBAT unconnected?

[re.wolff9]

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

[waclawek.jan]

Posted on April 11, 2018 at 17:53

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

JW

[re.wolff9]

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...  

[waclawek.jan]

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]

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

[Szymon PANECKI]

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

[T J]

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.

[re.wolff9]

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. 

[T J]

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.

[re.wolff9]

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.....