Selecting the right FETs as the power switch is important, when switching 5A at 20V.
My initial pick was the Alpha & Omega AON7407. It has very good Rds_on but was lacking at V_drain-source with only -20V max and V_gate-source of only 8V. Better candidates turned out to be:
Diodes DMP3013 with V_ds = -30V and V_gs = 25V; Rds_on 9.5mOhm (V_gs=10V)
Rohm RQ3E120 with V_ds = -30V and V_gs = 20V; Rds_on 8.8mOhm (V_gs=10V)
The following scope plots all show:
Ch1: signal GATES
Ch2: signal VBUSIN
Ch3: signal SOURCE
Ch4: signal VBUSOUT
Math: Ch3 - Ch4 = SOURCE - VBUSOUT to measure Vds
The 1st step to 5V is the initial USB power up, the trigger point is the successful power contract negotiation (blue,light blue and purple trace rise to 20V) and~140msec later, the FETs finally turn on(green trace starts raising) and the input voltage drops slightly because of the 4.85A current draw.
Q1, Q2 = AON7407: