The current op-amp buffers on the voltage measuring circuit are the MAX9944 which I had originally thought were rail-to-rail input op-amps. In actuality the input voltage is limited to Vcc - 1.8 according to the datasheet. We have observed a range of up to 2V while connected to a 3V3 rail. To get the full range of our power supply we would need to replace the op-amp with a rail-to-rail input that has similar input characteristics to the MAX9944.
Another solution is powering the op-amp with the Vcc rail (either 5V for USB or 3.7V nominal for li-ion battery) since the voltage drop under battery is changing at a low frequency and typically PSRR are very high. The headroom on the op-amp will have to be enough to handle the battery drop down to approximately (3.4V). Possibly have to test at what voltage the power supply stopped outp[utting.
The current op-amp buffers on the voltage measuring circuit are the MAX9944 which I had originally thought were rail-to-rail input op-amps. In actuality the input voltage is limited to
Vcc - 1.8according to the datasheet. We have observed a range of up to2Vwhile connected to a3V3rail. To get the full range of our power supply we would need to replace the op-amp with a rail-to-rail input that has similar input characteristics to the MAX9944.Another solution is powering the op-amp with the Vcc rail (either 5V for USB or 3.7V nominal for li-ion battery) since the voltage drop under battery is changing at a low frequency and typically PSRR are very high. The headroom on the op-amp will have to be enough to handle the battery drop down to approximately (3.4V). Possibly have to test at what voltage the power supply stopped outp[utting.