PDA

View Full Version : LDO voltage regulator lipoly question


devmonkey
Jun 09, 2004, 06:41 PM
Hi,

I have been looking for an ldo regulator to build an ultra simple lipoly charger from. It needs to be ldo since I need to charge 3S (12.6v) from around 14v. Micrel make some excellent regulators.

http://www.micrel.com/_PDF/mic29150.pdf

My question is this: The datasheet says that the devices include current limiting and that in overload conditions the output current will be constant. Does this imply that If i was to pick a regulator that was rated at the max charge current (say 1.5A) I required and simply set it to regulate to 12.6v open circuit that in the initial part of the charge it would drop the output voltage to lower the current to 1.5A?

If this is correct then the charger could be built with only 4 components. They also have an error flag which indicates the device is in an over current condition. This could be used to light an LED during the constant current part of the charge.

What do you think?

Joe

devmonkey
Jun 10, 2004, 07:51 AM
Well I have ordered enough parts to make 6 chargers so we'll see. Any thoughts on the automatic currnet limiting?

Cheers

Joe

Bruce Abbott
Jun 10, 2004, 08:01 AM
The regulator is guaranteed to pass 1.5A, but current limit may occur anywhere up to 3.5A (typically 2.1A). Due to this ambiguity I would not recommend using the internal current limit.

devmonkey
Jun 10, 2004, 06:42 PM
Hi Bruce,

I am charging 4.7AH cells so 3.5A would be fine. That given do you think that this device would on its own create a reliable charger?

Cheers

Joe

Bruce Abbott
Jun 10, 2004, 11:04 PM
It should be OK so long as the heatsink can keep it cool. Maximum power loss when charging 3 cells from a 14V supply is about 17 Watts.