Dual-Edge PWM Improves Multiphase Regulators

Jul 1, 2007 12:00 PM
By Weihong Qiu, Principal Applications Engineer of Computing Products, and Greg Miller, Vice Preside

Advanced Pulse Positioning

Conversely, the leading-edge modulation scheme has an excellent turn-on response but a poor turn-off response. Conventional dual-edge modulation, based on a triangular-ramp waveform, provides significant improvements over either of the single-edge schemes, but can still have half-switching-cycle delays for the turn-on and turn-off edges. Based on this observation, the APP PWM scheme is introduced to minimize both turn-on and turn-off delays by combining the benefits of the trailing-edge and leading-edge modulators. The block diagram and operational waveforms of this scheme are shown in Fig. 3.

The APP scheme has a down-ramp signal (V_RAMPDOWN), which is the same as the ramp in the conventional leading-edge modulator. When V_COMP becomes higher than V_RAMPDOWN, a single pulse is generated at the one-shot's output (labeled SET) to trigger the R-S flip-flop and start a PWM pulse at T1, as shown in Fig. 3. Once the PWM signal goes high, another ramp signal, V_RAMPUP, starts to increase with a fixed slew rate. When the voltage of V_RAMPUP equals the voltage of the V_COMP signal, it will terminate the PWM pulse at T2. Therefore, the PWM duty cycle depends on V_RAMPUP and V_COMP, just as it does in the conventional trailing-edge modulator, while the PWM pulse position is determined by V_RAMPDOWN and V_COMP, which is similar to the conventional leading-edge modulator. The APP PWM modulator has the advantages of both the leading-edge and trailing-edge modulators. The PWM pulse can be turned on and off at any time with minimal delay.

Free from the limits of the clock-signal triggering, the APP modulation scheme can respond to the change of the V_COMP signal instantaneously. If the transient event happens, and the error amplifier output V_COMP increases and intersects V_RAMPDOWN at T3, the PWM pulse will start to increase the inductor current instantaneously. At T4, the voltage on V_RAMPUP equals the voltage on V_COMP, and the PWM pulse stops without any delay. Therefore, the system can respond to the transient event and settle very rapidly.

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© 2007 Penton Media, Inc.