I have a customer who as floated a proposed circuit that includes an external bootstrap Schottky from Vin-to-BOOT and an external Schottky catch diode from PH-to-PGND for the basic TPS50601-SP application circuit shown in Figure 29 on Page 28 of the TPS50601-SP data sheet (January 2015):
I am of the opinion that neither of these components is necessary, as they will only add losses in the circuit: from Cj losses for the devices required to provide the appropriate Vf's at each location. This is particulatly so for the catch diode since a device rated for 12A (6A, derated) over temperature will have a not-insignificant Cj that will be presented at the switching node, and thus incur AC charging losses by the 50601 IC. Also, since the 50601 senses output (inductor) current in both directions, will the presence of the external catch diode with a lower clamping voltage interfere with the proper operation of the IC? {I don't think either diode is necessary, as they ONLY incur losses to the circuit without much benefit, but I'd like TI Application's opinion on this!]
Finally, is the back-side of the die metalized, and what is the metalization?? Also, should the back-side be connected to ground. In the DLA SMD specification on Page 19, the Table in Figure A-1 "Die bonding pad locations and electrical functions" states that "Substrate is not connected". However in the TI data sheet, the thermal pad on the packaged IC's require that this pad be connected to power ground externally. Should the die-back-side be connected to ground or left floating?? [I think it should be grounded, but I just want to make sure!]
Any insight into these questions is GREATLY appreciated.
