ir3500m International Rectifier Corp., ir3500m Datasheet - Page 10

• Manufacturer Part Number: ir3500m
• Description: Xphase3 Vr11.0 & Amd Pvid Control Ic
• Manufacturer: International Rectifier Corp.

Frequency and Phase Timing Control
The oscillator and system clock frequency is programmable from 250kHz to 9MHZ by an external resistor (ROSC).
The control IC system clock signal (CLKOUT) is connected to CLKIN of all the phase ICs. The phase timing of the
phase ICs is controlled by the daisy chain loop, where control IC phase clock output (PHSOUT) is connected to the
phase clock input (PHSIN) of the first phase IC, and PHSOUT of the first phase IC is connected to PHSIN of the
second phase IC, etc. and PHSOUT of the last phase IC is connected back to PHSIN of the control IC. During
power up, the control IC sends out clock signals from both CLKOUT and PHSOUT pins and detects the feedback at
PHSIN pin to determine the phase number and monitor any fault in the daisy chain loop. Figure 5 shows the phase
timing for a four phase converter. The switching frequency is set by the resistor ROSC as shown in Figure 23. The
clock frequency equals the number of phase times the switching frequency.
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 then
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, then turns on the low side driver after the non-overlap time,
and activates the ramp discharge clamp. The ramp discharge clamp quickly discharges the PWM ramp capacitor to
the output voltage of the share adjust amplifier in the 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
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 10 of 47
Figure 5 - Four Phase Oscillator Waveforms
June 12, 2007
IR3500