Hi guys, Recently I had a trouble with a Li-ion battery and the way the battery charger IC was charging the battery. I wanted to charge the battery while power on the circuit. The circuit I was using was:
1- Charging circuit never finish charging the battery. If the load is constantly drawing current out of the Li-Ion battery, the charge management system will never be terminated properly by minimum current.
2- “Power ON” issues. If your load is a MCU with a step-up converter you can experiment some RESETS due to high current peaks.
3- Battery life cycle. The system load continuously discharges the Li-Ion battery and costs a battery’s life cycle.
The keyword to solve this trouble is “Load Sharing”, microchip describes it well in Microchip AN1149 and Microchip AN1260. Adding load sharing to your battery charger IC only requires an additional 3 components (Rpull, Q1, D1).
The circuit disconnects the battery from the load when the USB power is present. The battery and the Load are powered from the USB with different current paths.
Q1 – is a P channel MOSFET. When 5V from USB is applied, Q1 will turn off and disconnect the battery to the load (Vgs > 0). The load will then use power from USB through D1. ( Q1 must have a low Ron to minimize power loss )
D1 – must be a schottky diode to prevent internal diode of Q1 pulling current from the battery to the load.
Rpull – is to make sure Q1 turns ON after removing USB from the circuit, 0V is applied to the gate connecting the battery to the load. You can easy prove the circuit doing a simulation with LTSpice and test if the circuit will work or not.
V1 – USB power. V2 – Li-ion battery Power. R2 – Resistance to simulate Load.
Q1 – P Mosfet. Rpull – Pull down resistor. D1 – Schottky diode.
Here are the results:
The circuit with 5V USB connected.
As you can see the battery is disconnected from the load and the load is sinking current from the USB ( 47 mA out of the USB, 47 mA in R2).
The circuit with 5V USB removed.
As soon as you remove the USB, the battery start sourcing current to the load ( 42 mA out of Li-ion battery, 42 mA in R2). There are some discrepancies with the current output of the Li-ion battery and the Load due to the leakage current of the schottky.