Answer:
Start-up / Ramp-up:
1. Once the LDO (MAX17651) is enabled, the output would rise up. MAX16945 would be kept off via the PGOOD pin.
Note: it ramps up within hundreds of us (per datasheet figures).
2. Once the LDO has reached 3V (92% typ), PGOOD goes high and releases the MAX16945-EN enabling the charge-pump.
3. LDO output stabilizes at 3.3V. During that time, the inverting charge-pump gradually “increases” to -3.3V.
1. Once the LDO (MAX17651) is enabled, the output would rise up. MAX16945 would be kept off via the PGOOD pin.
Note: it ramps up within hundreds of us (per datasheet figures).2. Once the LDO has reached 3V (92% typ), PGOOD goes high and releases the MAX16945-EN enabling the charge-pump.
3. LDO output stabilizes at 3.3V. During that time, the inverting charge-pump gradually “increases” to -3.3V.
Ramp-down:
1. Whether the 12V input goes down or the LDO gets disabled, PGOOD would end up going low.
If for whatever reason PGOOD doesn’t go low, it wouldn’t matter much since the inverting charge-pump would follow the 3.3V rail anyway (VOUT = -1 * VIN).
2. At that point, both outputs would discharge at a rate based on the load and output capacitance. Assuming both VDD and VSS having the same decoupling capacitors Vss is expected to discharge first based on larger negative current demand on Vss, see below.
1. Whether the 12V input goes down or the LDO gets disabled, PGOOD would end up going low.
If for whatever reason PGOOD doesn’t go low, it wouldn’t matter much since the inverting charge-pump would follow the 3.3V rail anyway (VOUT = -1 * VIN).2. At that point, both outputs would discharge at a rate based on the load and output capacitance. Assuming both VDD and VSS having the same decoupling capacitors Vss is expected to discharge first based on larger negative current demand on Vss, see below.