Is it possible to use LT1129CT as constant current source?

[isaac12345]

Hi,

Is it possible to use a LT1129CT as a constant current source for charging a NiMh battery, like an LM317T? If not, are there any other LDOs that can be used for this purpose?

 

[keith1200rs]

I cannot see why it shouldn't work. The devices are similar in the way they operate.

Keith.

 

[isaac12345]

I tried to, but it didnt work. My input voltage was 13.6 and my battery pack's voltage was approx 12. So the differential voltage was well above 400mv. Does the adjust pin have to be connected to ground as well? I simply connected it to the the output pin via a 16 ohm resistor.

 

[keith1200rs]

I didn't realise the LT1129 has a 3.75V reference compared to the more normal 1.25V used in the LM317. That means that you have to drop 3.75V across the series resistor. So, the benefit of it being LDO is rather wasted by the extra voltage you need to drop across the current sense resistor.

If you are wanting to charge a battery with a voltage close to its fully charged voltage you might be better off with a discrete design. A switcher or a battery charging IC are other options.

Keith.