I'm working on a battery-powered device using an ESP32. It needs 3.3v, and my battery is Li-Ion, so I've got 4.2-3.5ish (of useful voltage, anyway) - so max dropout has to be around 200mV. Most of the time, my device sleeps and consumes <10uA, so I need an LDO that's got a really low quiescent.
There are two other design considerations: First, sometimes I plug it in to USB to charge, so my available voltage goes up to ~5.1V with a safety margin. Second, my device sometimes spikes up to 600mA. This means I might need to dissipate up to ~1.1W.
I haven't been able to find an LDO that can meet all those requirements. Looking at block diagrams, however, the LDO itself doesn't seem terribly complex. I'm debating building one out of discrete components using a small LDO as a voltage reference so it's temperature compensated, and an op-amp driving a PMOS FET (or a few in series to spread out the heat).
Is there any reason I shouldn't do this? Or is there any other way to get the current I need? I looked into putting LDOs in parallel, but a balancing resistor always seems to add too much voltage drop, and not keep the current evenly balanced enough. I don't THINK a switching regulator is an option because of the necessary super-low quiescent current.
I would appreciate some input! Thanks.