IR3505M International Rectifier Corp., IR3505M Datasheet - Page 8

• Manufacturer Part Number: IR3505M
• Description: Xphase3 Phase Ic
• Manufacturer: International Rectifier Corp.

PWM Operation
The PWM comparator is located in the phase IC. Upon receiving the falling edge of a clock pulse, the PWM latch is
set; the PWM ramp voltage begins to increase; the low side driver is turned off, and the high side driver is turned on
after the non-overlap time. When the PWM ramp voltage exceeds the error amplifier’s output voltage the PWM latch
is reset. This turns off the high side driver, turns on the low side driver after the non-overlap time, and activates the
ramp discharge clamp. The clamp drives the PWM ramp voltage to the level set by the share adjust amplifier 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.
An additional advantage of this PWM modulator 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 3 depicts PWM operating waveforms under various conditions.
Control IC CLKOUT
(Phase IC CLKIN)
Control IC PHSOUT
(Phase IC1 PHSIN)
Phase IC1
PWM Latch SET
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)
Page 8 of 19
Figure 2 Four Phase Oscillator Waveforms
Sept 26
th
IR3505
, 2007