IR3505M International Rectifier Corp., IR3505M Datasheet - Page 9

• Manufacturer Part Number: IR3505M
• Description: Xphase3 Phase 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
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.
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,
Usually the resistor Rcs and capacitor Ccs are chosen so 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
resistor with the value of R
inductor DC current, but affects the AC component of the inductor current.
BODYDIODE
PHASE IC
CLOCK
PULSE
PWMRMP
GATEH
GATEL
Page 9 of 19
. The minimum time required to reduce the current in the inductor in response to a load transient
EAIN
VDAC
TM
STEADY-STATE
OPERATION
L
was used. The mismatch of the time constants does not affect the measurement of
v
C
Figure 3 PWM Operating Waveforms
DUTY CYCLE INCREASE
DUE TO LOAD
INCREASE
(
s
)
=
v
L
T
T
(
SLEW
SLEW
s
)
1
+
=
=
sR
L
L
V
1
CS
O
( *
( *
C
+
DUTY CYCLE DECREASE
DUE TO VIN INCREASE
(FEED-FORWARD)
I
I
CS
V
MAX
MAX
BODYDIODE
V
=
O
i
−
−
L
I
I
(
s
MIN
MIN
)
1
)
)
+
R
sR
L
+
CS
sL
DUTY CYCLE DECREASE DUE TO LOAD
DECREASE (BODY BRAKING) OR FAULT
(VCCLUV, OCP, VID=11111X)
C
CS
L
). If the two time constants match, the
Sept 26
th
IR3505
, 2007
STEADY-STATE
OPERATION