IR3081 International Rectifier Corp., IR3081 Datasheet - Page 8

• Manufacturer Part Number: IR3081
• Description: Xphasetm Vr 10 Control Ic
• Manufacturer: International Rectifier Corp.

PWM Operation
The PWM comparator is located in the Phase IC. Upon receiving a clock pulse, the PWM latch is set; the PWMRMP
voltage begins to increase; the low side driver is turned off, and the high side driver is then turned on after the non-
overlap time. When the PWMRMP voltage exceeds the Error Amplifier’s output voltage, the PWM latch is reset.
This turns off the high side driver and then turns on the low side driver after the non-overlap time; it activates the
Ramp Discharge Clamp, which quickly discharges the PWMRMP capacitor to the VDAC voltage of the Control 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 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 4 depicts PWM operating waveforms under various conditions.
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VPHASE4&5 (4.5V)
VPHASE3&6 (3.5V)
VPHASE2&7 (2.5V)
VPHASE1&8 (1.5V)
VVALLEY (1.00V)
VPEAK (5.0V)
CLK1
CLK2
CLK3
CLK4
CLK5
CLK6
CLK7
CLK8
Figure 3. 8 Phase Oscillator Waveforms
50% RAMP
DUTY CYCLE
SLOPE = 80mV / % DC
SLOPE = 1.6mV / ns @ 200kHz
SLOPE = 8.0mV / ns @ 1MHz
10/01
IR3081
/04