I’m trying to use the LTC4020 as a 12V to single-cell LiPo charger, at present only on LTSpice. The goal is for a 4A charging current and a simultaneous capability to supply 4A to the load.
I’ve adapted the test fixture in what I think is a reasonable way, though I have not yet closely examined stability.
The first oddity I observe is with the battery disconnected and a nominal 1A load, I do not see Vout at the 4.2V I expected, I see an output hunting between 2.4V and 3.6V. (trace-no-battery.pdf)
If I connect the battery simulation, again adapted from the test fixture, I see Vbgate possibly trying to trickle charge whilst the battery is too low, though I see very little current … I rather expected 4A/15 => ~250mA. As the battery voltage rises above about 3V, I expect to see charging start at 4A, but it does not and Vout soon falls below Vbat.
Only as Vbat approaches 4.2V does Vbgate finaly turn on the powerpath FET. But Vout remains below Vbat, so the circuit is now discharging the battery.