change AA to 1s Li-ion

[reaper38]

Hello, I want to change 2 AA batteries in series to one li-ion cell. Problem is that I want to use power indicator from device, so voltage must drop as cell discharing from 2,4 to 1,8 volts. Any suggestion how to do it?

[mariush]

estimate the average power consumption of that device and the peak current (both at 1.8v, the minimum voltagE0, then maybe pick a buck switching regulator with small enough quiescent current and enough current output for your needs, and configure it to track the discharge rate of the battery ( 4.2v full to 3.6v discharged ... so 2.4v to 1.8v ... basically half the input voltage)
High frequency dc-dc regulators would use very little space, it would be just an issue of balancing the maximum frequency (for tiny inductor and capacitors) vs efficiency at some output current range.

For example see TPS63030 which uses only a 1.5uH and a bunch of tiny ceramic capacitors and can output up to 800mA of current.
Or see NCP1406 / NCV1406 for lower frequency.
Or see TPS62240 for 300mA maximum current.

Since you want exactly half the voltage (1.8v .. 2.4v for a battery that would have 3.6v to 4.2v) ... for low currents (like 50-100mA), you could look into voltage doubler chips which can also be configured to output half the input voltage.
For example,
For maximum 40mA of current , see LM2665M6
For maximum 60mA of current, you could try TPS60403
For max. 100mA, you have LM2660MX
for max 200mA you have LM2662MX
See the datasheets included on the pages linked above.

[reaper38]

Thanks for your reply.  How to make it track voltage? Not connect feedback pin?

[ebclr]

Li-ion have a very different discharge curve than AAA cell

If you let the cell discharge more than 3.1 V , cell live will be reduced or even kill the cell



This paper have some useful information

http://www.ti.com/lit/an/snva533/snva533.pdf