IR3521

Page 13

V3.03

PHASE IC

CLOCK

PULSE

VDAC

EAIN

PWMRMP

GATEL

GATEH

DUTY CYCLE DECREASE

DUE TO VIN INCREASE

(FEED-FORWARD)

DUTY CYCLE INCREASE

DUE TO LOAD

INCREASE

STEADY-STATE

OPERATION

STEADY-STATE

OPERATION

DUTY CYCLE DECREASE DUE TO LOAD

DECREASE (BODY BRAKING) OR FAULT

(VCC UV, OCP, VID FAULT)

Figure 6 PWM Operating Waveforms

Body Braking

TM

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 rectifier’s body diode occurs. This increases the voltage across the inductor from Vout to Vout +

decrease is now;

BODYDIODE

O

MIN

MAX

SLEW

V

V

I

I

L

T







)

(

*

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

increased by 2X or more. This patent pending 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 VDAC voltage or a programmable voltage, 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 7. 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

)

(

)

(

Usually the resistor Rcs and capacitor Ccs are chosen so that the time constant of Rcs and Ccs equals the time

voltage across Ccs is proportional to the current through L, and the sense circuit can be treated as if only a sense

inductor DC current, but affects the AC component of the inductor current.