# get 100mA pulses from 20mA source to charge li-ion

I have only 20mA in my circuit and after trying to charge a battery with TP4056 it doesn't work. TP4056 is a linear charger so I need to try with LTC4002 or even better to improve efficiency. Anyway, my first attempt was to implement a charge pump like this:

My goal is to obtain 100mA or more (8v) pulses from 20mA @ 8V input. To do this, I used a supercapacitor connected to a hysteresis comparator so when Vcap is >5V output is enabled, so pulse can be done and feed the li-ion charger, and when Vcap is bellow 1V this output is disabled so supercapacitor can charge again.

This is the schematic: (Some resistor values was changed over the tests...)

and this is the expected output:

I tested it with a load and circuit seems to be successfully enabled, but if load is present seems like is not able to disable comparator output and some weird waveforms happens.

Maybe this schematic is not the best approach to solve the problem: any suggestions?

------- EDIT -------------------------------------------------------------------

This power supply is for a GSM device with peaks of 2A but average of 60mA. The device will sleep some days and some hours a day, so can be "recharged" in this moments. Supercapacitor can do the job of charge/discharge, but since consumption is higher that the one who a supercapatitor can give, I considered a battery.

## Solar chargers have similar pulsed charge currents, but higher than mine.

• If your source can only supply 20 mA, you can't get an average charging current of more than 20 mA, and you'll probably waste a significant amount of power in your pulse charge circuit, leaving even less current for charging the battery.. Commented Nov 8, 2019 at 17:05
• I dont want to get more than 20mA average, but peaks of 100mA. Maybe pulse charging circuit waste a significant amount of power.. any suggestions? Commented Nov 8, 2019 at 17:09
• Please don’t! Why not boosting the voltage and limit the charge current, and probably use some other charge IC? Charge termination will be a challenge. Commented Nov 8, 2019 at 17:26
• boosting the voltage gives me less current... ¿? LTC4002 is a switch mode charger instead linear, so it can be better, but I don't know if support low current charging. Charge termination needs constant current, right? Commented Nov 8, 2019 at 17:45
• Sorry, buck. I mean buck. Sort of. Read the datasheet for the battery. It’s usually detecting when the current falls below 100 mA in CV mode. Commented Nov 8, 2019 at 18:07