How to OR together 3 li-poly batteries and protect batteries fromundervoltage

[Michael]

Hi there - I need to combine together 3 lithium polymer battery packs.
Each battery pack is a 4 cell (in series) pack, so the voltage of each
pack could be as high as say around 18V, and I can't let it fall below
12V otherwise damage to the pack could occur. Once the battery pack is
shut off I need drain on the pack to be brought to a minimum - I'd
like it to be well under a milli-amp.

Further, Each battery will be connecting to a separate (but identical)
PCB. the PCBs will have 3 connections going between all of them - a
power ground (the battery ground), ORed power high, and a low current
switch connection (to turn off all batteries, regardless of voltage
level).

I need to do the switching on the high side of the batteries,
unfortunately. I need to protect the batteries from charging each
other as that would (clearly) be less than good. So my first
inclination is to have the positive side of my batter packs connected
to the anode of a schottky, with the schottky's cathode connected to
the source of a N-FET, and the drain connected to the ORed power
supply. An unvervoltage protection circuit would be ORed with the
switch line, and with some sort of charge pump that would drive the
FET.

This would work. This is also a lot of parts - and I'm very space
constrained.

I stated looking around and have found such a thing as an "N+1 and
ORing Power Rail Controller". For example, the TI TPS2413:
http://focus.ti.com/lit/ds/symlink/tps2413.pdf

This part won't work due to the 16.5V max bus voltage limitation, but
otherwise, it looks like it handles... everything.

Does this sort of part sound like the right solution for me? Does
anybody know of a similar part that can handle my bus voltage?

Thanks!

-Michael

 

[Martin Griffith]

Hi there - I need to combine together 3 lithium polymer battery packs.
Each battery pack is a 4 cell (in series) pack, so the voltage of each
pack could be as high as say around 18V, and I can't let it fall below
12V otherwise damage to the pack could occur. Once the battery pack is
shut off I need drain on the pack to be brought to a minimum - I'd
like it to be well under a milli-amp.

Further, Each battery will be connecting to a separate (but identical)
PCB. the PCBs will have 3 connections going between all of them - a
power ground (the battery ground), ORed power high, and a low current
switch connection (to turn off all batteries, regardless of voltage
level).

I need to do the switching on the high side of the batteries,
unfortunately. I need to protect the batteries from charging each
other as that would (clearly) be less than good. So my first
inclination is to have the positive side of my batter packs connected
to the anode of a schottky, with the schottky's cathode connected to
the source of a N-FET, and the drain connected to the ORed power
supply. An unvervoltage protection circuit would be ORed with the
switch line, and with some sort of charge pump that would drive the
FET.

This would work. This is also a lot of parts - and I'm very space
constrained.

I stated looking around and have found such a thing as an "N+1 and
ORing Power Rail Controller". For example, the TI TPS2413:
http://focus.ti.com/lit/ds/symlink/tps2413.pdf

This part won't work due to the 16.5V max bus voltage limitation, but
otherwise, it looks like it handles... everything.

Does this sort of part sound like the right solution for me? Does
anybody know of a similar part that can handle my bus voltage?

Thanks!

-Michael

The Linear LTC1473 works up to 30V, but is a dual, the ltc1479 is
triplle, but is a larger package

HTH


martin

 

[Michael]

The Linear LTC1473 works up to 30V, but is a dual, the ltc1479 is
triplle, but is a larger package

HTH

martin

Hi Martin - I'm looking for a single controller - since I need one
controller per board. Looking at some parts in the same family as
those two Linear parts I stumbled across the LT4351 (http://
www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1142,C1079,P2173,D2942).
It looks pretty good - though it doesn't have an enable. It looks like
I could use the overvoltage pin as an enable however, as I'm not
worried about an overvoltage condition occuring.

-Michael