IR3087

PWM Control Method

The PWM block diagram of the XPhase

trailing edge modulation is used. A high-gain wide-bandwidth voltage type error amplifier in the Control IC is used

for the voltage control loop. An external RC circuit connected to the input voltage and ground is used to program the

slope of the PWM ramp and to provide the feed-forward control at each phase. The PWM ramp slope will change

with the input voltage and automatically compensate for changes in the input voltage. The input voltage can change

due to variations in the silver box output voltage or due to the wire and PCB-trace voltage drop related to changes

in load current.

+

-

10K

SHARE

ADJUST

ERROR

AMPLIFIER

CURRENT

SENSE

AMPLIFIER

X34

X

0.91

20mV

+

-

10K

SHARE

ADJUST

ERROR

AMPLIFIER

CURRENT

SENSE

AMPLIFIER

X34

20mV

X

0.91

+

-

DACIN

PWMRMP

BIASIN

RAMPIN-

RAMPIN+

GATEH

SCOMP

ISHARE

CSIN+

GATEL

EAIN

CSIN-

SYSTEM

REFERENCE

VOLTAGE

ENABLE

CLOCK

PULSE

GENERATOR

BODY

BRAKING

COMPARATOR

RAMP

DISCHARGE

CLAMP

PWM

LATCH

S

RESET

DOMINANT

PHASE IC

R

PWM

COMPARATOR

RDRP

RPWMRMP

RPHS2

CSCOMP

RPHS1

RCS

RVFB

CPWMRMP

CSCOMP

CCS

RPWMRMP

RCS

+

-

CPWMRMP

CCS

RPHS2

RPHS1

+

-

GND

VOUT

BIASIN

VDAC

VBIAS

DACIN

PWMRMP

RAMPIN+

VOSNS-

VOSNS+

VOSNS-

ISHARE

RAMPIN-

IIN

VDRP

EAIN

GATEH

SCOMP

CSIN-

CSIN+

GATEL

EAOUT

RMPOUT

VIN

FB

IROSC

SYSTEM

REFERENCE

VOLTAGE

VDAC

BODY

BRAKING

COMPARATOR

RAMP

DISCHARGE

CLAMP

ENABLE

CLOCK

PULSE

GENERATOR

VBIAS

REGULATOR

IFB

VDRP

AMP

50%

DUTY

CYCLE

RAMP GENERATOR

VVALLEY

VPEAK

ERROR

AMP

R

S

RESET

DOMINANT

PWM

LATCH

PHASE IC

COUT

CONTROL IC

PWM

COMPARATOR

Figure 2. PWM Block Diagram

Frequency and Phase Timing Control

An oscillator with programmable frequency is located in the Control IC. The output of the oscillator is a 50% duty

cycle triangle waveform with peak and valley voltages of approximately 4.6V and 0.9V respectively. This signal is

used to program both the switching frequency and phase timing of the Phase ICs. The Phase IC is programmed by

comparator in the Phase ICs detects the crossing of the oscillator waveform over the voltage generated by the

resistor divider and triggers a clock pulse that starts the PWM cycle. The peak and valley voltages track the VBIAS

voltage reducing potential Phase IC timing errors. Figure 3 shows the Phase timing for an 8 phase converter. Note

that both slopes of the triangle waveform can be used for phase timing by swapping the RMPIN+ and RMPIN– pins,

as shown in Figure 2.

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