Assembling and testing the MP3432+MP2145 NiMH-li-ion boost+buck driver :
The MP3432 was used specifically (instead of MP3431) because it has a passthrough mode, meaning it stops boosting when the input voltage is higher that the output voltage and behave like a piece of wire, with some resistance (high side FET+ boost inductor = 10mΩ+8.2mΩ), this output voltage being the boosted voltage, not the LED voltage, presently the boosted voltage is set at 3.37V, then this serves as the input voltage for the constant current buck driver.
If you look at Vout at 3Vin and 4Aout, it’s lower than at 4Vin, that’s because the 3.37V boosted voltage is not high enough to allow 4Aout at ~3.28Vout, which is the LED Vf + drop across the longish wires and current shunt, used for accurate current measurement. It could only do ~1.7A.
So I used a lower resistance shunt there, lowering the output votage needed for 4A. A 519A at 4A has a Vf around 3.2V, so it should be OK, if a higher Vf LED is used then the boosted voltage should be set a bit higher to allow full output at any (li-ion) Vin.
I was a bit worried about the passthrough mode because it only works in PSM, meaning the switching frequency decreases in the audible range at very low output and could generate noise, like my first driver with the TPS61288 did, but I hear nothing so that’s great. Plus now the power consumption will be lower at very low output vs USM (ultrasonic mode, freq>30kHz).
Considering I used 4020 inductors (4x4x2mm) the efficiency is really good. Though the fact that it peaks at relatively high output current means that I might have used a too small inductance (680nH, 8.2mΩ) for the boost converter. It would be good to test with XGL4025 (4x4x2.5mm), like 900nH (7.4mΩ), or 1uH (8.5mΩ), it could maybe break the 90% barrier with NimH voltages.