IR3521

Page 12

V3.03

Phase IC1

PWM Latch SET

Control IC CLKOUT

(Phase IC CLKIN)

Control IC PHSOUT

(Phase IC1 PHSIN)

Phase IC 1 PHSOUT

(Phase IC2 PHSIN)

Phase IC 2 PHSOUT

(Phase IC3 PHSIN)

Phase IC 3 PHSOUT

(Phase IC4 PHSIN)

Phase IC4 PHSOUT

(Control IC PHSIN)

Figure 5 Four Phase Oscillator Waveforms

PWM Operation

The PWM comparator is located in the phase IC. Upon receiving the falling edge of a clock, the PWM latch is set

and the PWM ramp amplitude begins to increase prompting the low side driver is turned off. After the non-overlap

time (GATEL < 1.0V), the high side driver is turned on. When the PWM ramp voltage exceeds the error amplifier’s

output voltage, the PWM latch is reset. This also turns off the high side driver, turns on the low side driver after the

non-overlap time and the PWM ramp discharged current is clamped which quickly discharges the internal capacitor

to the output voltage of share adjust amplifier, in phase IC, until the next clock pulse.

The PWM latch is reset dominant allowing all phases to go to zero duty cycle within a few tens of nanoseconds in

response to a load step decrease. Phases can overlap and go up to 100% duty cycle in response to a load step

increase with turn-on gated by the clock pulses. An error amplifier output voltage greater than the common mode

input range of the PWM comparator results in 100% duty cycle regardless of the voltage of the PWM ramp. This

arrangement guarantees the error amplifier is always in control and can demand 0 to 100% duty cycle as required.

It also favors response to a load step decrease which is appropriate given the low output to input voltage ratio of

most systems. The inductor current will increase much more rapidly than decrease in response to load transients.

This control method is designed to provide “single cycle transient response” where the inductor current changes in

response to load transients within a single switching cycle maximizing the effectiveness of the power train and

minimizing the output capacitor requirements. An additional advantage of the architecture is that differences in

ground or input voltage at the phases have no effect on operation since the PWM ramps are referenced to VDAC.

Figure 6 depicts PWM operating waveforms under various conditions.