IR3508

Page 9 of 19

October 27, 2008

PHASE IC

CLOCK

PULSE

EAIN

VDAC

PWMRMP

GATEH

GATEL

STEADY-STATE

OPERATION

DUTY CYCLE DECREASE

DUE TO VIN INCREASE

(FEED-FORWARD)

DUTY CYCLE INCREASE

DUE TO LOAD

INCREASE

STEADY-STATE

OPERATION

DUTY CYCLE DECREASE DUE TO LOAD

DECREASE (BODY BRAKING) OR FAULT

(VCCLUV, OCP, VID=11111X)

Figure 3: PWM Operating Waveforms

In a conventional synchronous buck converter, the minimum time required to reduce the current in the inductor in

response to a load step decrease is;

O

MIN

MAX

SLEW

V

I

I

L

T

)

(

*



The slew rate of the inductor current can be significantly increased by turning off the synchronous rectifier in response

to a load step decrease. The switch node voltage is then forced to decrease until conduction of the synchronous

minimum time required to reduce the current in the inductor in response to a load transient decrease is now;

BODYDIODE

O

MIN

MAX

SLEW

V

V

I

I

L

T





)

(

*

Since the voltage drop in the body diode is often comparable to the output voltage, the inductor current slew rate can be

increased significantly. This patented technique is referred to as “body braking” and is accomplished through the “body

braking comparator” located in the phase IC. If the error amplifier’s output voltage drops below the output voltage of the

share adjust amplifier in the phase IC, this comparator turns off the low side gate driver.

Lossless Average Inductor Current Sensing

Inductor current can be sensed by connecting a series resistor and a capacitor network in parallel with the inductor and

measuring the voltage across the capacitor, as shown in Figure 4. The equation of the sensing network is,

CS

CS

L

L

CS

CS

L

C

C

sR

sL

R

s

i

C

sR

s

v

s

v







1

)

(

1

1

)

(

)

(