The VTM's fast transient response makes it a candidate to replace a conventional VRM, as shown in Figure g of "Mixing And Matching FPA Building Blocks." The resulting FPA implementation for an 80-A VRM will be considerably smaller than the standard SIP-style VRM. Like the VRM, the required PRM and VTM chips may be mounted vertically.
A standard VRM may consist of a four-phase synchronous buck VRM that delivers 80 A at 1.5 V. Assume 20 A and 300 kHz per phase. For such a VRM, the efficiency could be in the 75% to 79% range, with current density of approximately 10 A/in.2 and price of about 65 to 95 cents/A.
In contrast, an FPA equivalent solution would offer 90% efficiency, 40-A/in.2 current density, and 65-cent/A cost. Moreover, the FPA solution operates off a high-voltage bus, while the VRM requires another dc-dc stage to generate its lower input voltage. As noted previously, the FPA approach takes one more IC inserted into the voltage feedback loop to perform VID control.