ir3502b International Rectifier Corp., ir3502b Datasheet - Page 11

• Manufacturer Part Number: ir3502b
• Description: Xphase3 Control Ic
• Manufacturer: International Rectifier Corp.

Body Braking
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;
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 + V
minimum time required to reduce the current in the inductor in response to a load transient decrease is now;
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, enabling the bottom
FET body diode to take over. There is 100mV upslope and 200mV down slope hysteresis for the body braking
comparator.
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 6. The equation of the sensing network is,
Usually the resistor Rcs and capacitor Ccs are chosen, such that, the time constant of Rcs and Ccs equals the time
constant of the inductor, which is the inductance L over the inductor DCR R
voltage across Ccs is proportional to the current through L, and the sense circuit can be treated as if only a sense
PHASE IC
CLOCK
PULSE
PWMRMP
GATEH
GATEL
Page 11 of 38
EAIN
VDAC
TM
STEADY-STATE
OPERATION
DUTY CYCLE INCREASE
DUE TO LOAD
INCREASE
v
C
Figure 5 PWM Operating Waveforms
(
s
)

v
L
T
T
(
SLEW
SLEW
s
)
1



sR
L
L
V
1
CS
DUTY CYCLE DECREASE
DUE TO VIN INCREASE
(FEED-FORWARD)
O
( *
( *
C

I
I
CS
V
MAX
MAX
BODYDIODE
V

O


i
L
I
I
(
MIN
MIN
s
)
1
)
)

R
sR
L
DUTY CYCLE DECREASE DUE TO LOAD
DECREASE (BODY BRAKING) OR FAULT
(VCCLUV, OCP, VID=11111X)

CS
sL
C
CS
L
. If the two time constants match, the
IR3502B
STEADY-STATE
OPERATION
V3.2
BODYDIODE
. The