LM317: Power management forum

Hi Pan,

I would recommend using circuit 2 for this application, due to it being a lower component count and eliminating the series resistance between the battery and ground.

For circuit 2, I'd recommend adding a Schottky diode between the top of R121 (where R121 and R5 connect) and the battery to prevent the battery being loaded by the feedback network or the LM317L when the charging input is not connected.

Additionally, for circuit 2, the resistor values do not appear to be correct for the ML2032 - from Maxell's information here they recommend a 3.0 to 3.3V charging voltage, and no more than 2mA of charge current. From section 9.3.6 of the LM317 datasheet and choosing a 1mA current limit and 3.2V charging voltage, I calculate that R5 should be 1.24kΩ, R120 should be 15.6kΩ, and R121 should be 10kΩ. If the diode is added, the divider may need to be adjusted to account for the diode voltage drop. 

In circuit 1 the charging current must flow through R154 and causing an increase in voltage at 3V0_RTC of up to 0.68V with the resistor values shown. If this is acceptable, I would double check the other resistor values - I calculate the output voltage of this configuration with the resistor values given to be 7.88V. Replacing R142 and R146 with a 2.74kΩ resistor should limit the maximum output voltage to approximately 3.2V. 

Best Regards,

Alex Davis