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Enable circuit for backup battery boost converter

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kristaps_r

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Hi!

I need to realize backup battery with boost converter in my design.
To minimize battery current I need to keep boost converter disabled until main power is lost.
Switching to backup battery should be uninterrupted.

Main power: 8V
Battery: 3V
Boost coverter: 7V

Untitled.png

Could you recommend any cheap and effective way of doing this?
 

is the enable high or low?

assuming enable is high, connect two resistors in series from 8V source to ground
connect the collector of an NPN transistor to enable
connect base of said transistor to junction of two resistors
connect emitter to ground.

when 8 V is present, the transistor will be ON, pulling enable low, boost OFF
when 8 V is not present, transistor will be OFF, letting enable go high, boost ON

add a capacitor at SYS to smooth transition

resistor values not critical
from 8V to base, about 10k
from base to ground, about 1K
transistor - any small signal device useful as a switch, like 2N2222A or 2N3019
 
Hi,

Add two resistors an NPN BJT.
Two resistors as voltage divider from 8V
Center of divider to bjt_base
Bjt emitter to GND
Bjt collector to Boost_EN

Klaus
 
Thanks for replies.
Yes enable is active high.
 

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