Vbat with diode to VDD.

[ingwmeier]

Posted on November 30, 2012 at 10:07

AN2586 say

s:

Battery backup

To retain the content of the Backup registers when VDD is turned off, the VBAT pin can be

connected to an optional standby voltage supplied by a battery or another source.

The VBAT pin also powers the RTC unit, allowing the RTC to operate even when the main

digital supply (VDD) is turned off. The switch to the VBAT supply is controlled by the power

down reset (PDR) circuitry embedded in the Reset block.

If no external battery is used in the application, it is highly recommended to connect VBAT

 

externally to VDD.

 

 

I made test

s with a 1N

4448

D

iode conn

ecte

d to Vbat

, so that an optional Battery ca

n be used for certain applications.

Are there an

y concerns about using a Diode instead a direct connect to VDD?

Regards

We

rner

#vbat

[jpeacock2399]

Posted on November 30, 2012 at 15:49

You shouldn't have any problems.  I use a pair of 1N4448s as steering diodes to run Vbat from Vdd when powered, or from a supercap when not powered.  The diode is actually a benefit; it keeps the Vbat volatge below Vdd due to voltage drop across the diode.