TPS54A20: The issue regarding the on-resistance of two different low-side MOSFETs

Hi, 

Phase A and Phase B have different internal circuits as shown on section 8.1.2: https://www.ti.com/lit/ds/symlink/tps54a20.pdf#page=22. Difference in circuit results in different resistances used.

Essentially Phase A has a few extra functionalities that Phase B doesn't like:

- An LDO to manage HS boostrap driver by producing an output at the VGA pin. The LDO isn't needed for PhaseB as bootstrap cap should be already charged from VG input rail. 

- Extra SCAP pin connection which connects to the internal series capacitor (between the source of the high-side MOSFET and the drain of the low-side MOSFET in Phase A) used to determine SS sequence start as described on section 7.3.12 Series Capacitor Monitoring https://www.ti.com/lit/ds/symlink/tps54a20.pdf#page=19 and therefore turns on the HS FET of phase B (Q1b).

Regards,

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