Today’s GHz-level processors frequently operate at only a fraction of their maximum performance levels, but the power consumption is only slightly lower. When the processor is idle, leakage current losses are the main culprit. Reducing the core voltage to zero could eliminate the losses, but was not an option due to the time required (historically, 100 ěsec or more) for the power supply to shut down and restore the voltage. A technique based on Switched-Current Power Converter (SCPC) technology has a very fast dynamic response (less than 2 ěsec) and provides engineers with a new option.
SCPC technology uses a power supply topology, a totem-pole power converter, based on a current source instead of the traditional voltage source. Building on this approach, a recently issued patent (U.S. Patent No. 7,098,638) shows the use of passive ballasting to divide a constant current into a large number of smaller constant currents. The ballasting technique uses very small parallel inductors, and the self-inductance of parallel circuit board traces provides sufficient inductance for most applications. The converter produces a voltage that can be ramped at up to 1,500 mV/ěsec and precisely responds to any processor voltage identification (VID) commands.
With the new circuitry for dynamic voltage control, turn on from 0A and 0V to any VID level, and any load occurs in less than 2 ěsec. For load dump from any VID and any load to 0V and 0A — and essentially any voltage transition at any load — switching occurs within 1 ěsec. As a result, the processor’s leakage current can be cut to zero in the off-period. Turning on the processor’s core only when an operation occurs and turning it off immediately afterward can reduce power consumption as much as 60 percent.