Digital Versus Analog Power Control—A Fight To The... Draw?

For instance, the DPWM must have finer resolution than the ADC. Otherwise, a 1-LSB change in the ADC output could cause the DPWM to shift the output voltage to a level greater than what the ADC sees as 1 LSB. As a result, the output voltage would toggle between two values in the steady state, a situation called "limit cycling."

Avoiding that cycling isn't necessarily easy, though. That's because giving the DPWM finer resolution means stepping up its pulse rate. (Pulse rate determines how many bits can be generated in any given time period.) However, the DPWM pulse rate limits the time it has in which to compress all those bits from the controller. The example that's in the Artesyn white paper addresses a hypothetical DPWM with a 1-MHz switching rate and a 10-bit ADC. Calculations show that the modulator would require a pulse rate greater than 1 GHz.

Such a high rate is impractical, of course, so digital controller designers must come up with an alternative solution. One is to introduce some DPWM clock dither. The regulator output filter averages any pulse train that's fed into it. That makes it possible to adjust the width of every mth output pulse by the equivalent of 1 LSB.

This increases or decreases the average value of the resulting pulse train by (1/m) times the resolution of 1 LSB. If 1 LSB at the controller input moves the output pulse train average by 10 mV, which shortens every fourth pulse by an amount of time corresponding to 10 mV, the average output voltage through the filter then will reduce by 10 mV/4 or 2.5 mV.

ALTERNATIVE APPROACHES
While almost all digital controllers use ADCs and stored-program controllers, this isn't the only possible approach. Last year, Zilker Labs noted that achieving the kind of step responses (hundreds of amperes per nanosecond) imposed by the latest Pentium-class processors required a pretty fast and, therefore, power-hungry DSP or micro in the controller.

As a lower-power alternative, the company introduced a controller based on comparators (rather than an ADC) and a state machine (rather than a stored-program approach). (See "PMBus Controller Takes A State Machine Approach" at www. electronicdesign.com, ED Online 11614.)

Furthermore, the simple buck and boost topologies described earlier aren't the only ways to accomplish digital regulation. Vicor presents a vastly different approach based on a much more sophisticated regulator topology than the simple buck or boost described above and a repartitioning of the elements in the power-distribution paradigm. (See ED Online 3185, 5948, 9092, and 11119 at www.electronicdesign.com.)

In the long run, digital control was a breakthrough technology. But many of digital control's benefits have now migrated back into analog-control regulators.