battery charging - Solar powered single cell LiFePo4 charger circuit

As a follow-up to this question I've come up with this schematic for a solar powered battery charger:

Subcircuits, from left to right:

• Solar cell (just a voltage source), 12V, 10W.


• Buck regulator with current limit set to about 300 mA,


• output voltage selection: 3.5V (float) or 4V (fast charging)


• LiFePo4 battery with charge enable transistors (M2 and M3)


• 3V LDO for the application circuit which includes an MCU

M2 and M3 are used to enable charging. If M2 is off, the battery can supply the application through M2's internal body diode. The gate voltage of M2 can rise above 3V, which is the reason why M3 exists.

M1 is used to alter the FB voltage divider. When M1 is off, the buck regulator's output voltage is 3.5V. When it is on, the output voltage is 4V.

I have selected the regulators as shown above mainly because they were available in LTspice, but also because U1 (the buck regulator) has current limiting and because U2 (the LDO) has a low dropout voltage. I don't need a shutdown pin on that one.

The design suggested in the linked question also includes a signal to detect if solar power is available, but I have omitted that in this schematic.

I also omitted the battery protection IC (BQ29706) because I could not simulate it. It will disconnect the battery from the charger when the battery voltage reached 3.85V (in fast charging mode), so that the buck regulator's voltage rises to 4V. With a simple voltage divider this can be detected by the MCU (not shown), which will then let the battery relax to 3.5V.

I chose a LiFePo4 battery because I want the circuit to operate at both low (say -20°C) and high temperatures (60°C). On average it will draw very little current and I want it to work over a full year. Peak current is 300 mA for a few milliseconds every now and then, and less than 1 mA while sleeping.

Specific questions:

• Are there alternative parts that you would recommend? Is anything wrong with the parts I chose?


• If I wanted to add MPPT to this, how could I get finer control over the solar cell current? The selectable voltage divider I'm using now is a bit coarse.


• Apart from the battery protection, what other means of protection should I consider?



• is it safe to add another transistor next to M1, below R2, in order to shave away those 20µA through the fixed part (R1 + R2) of the voltage divider?