ir3500v International Rectifier Corp., ir3500v Datasheet - Page 11

• Manufacturer Part Number: ir3500v
• Description: Xphase3 Vr11.1 Cpu Vtt 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
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 method 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 7. 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
Page 11 of 34
. The minimum time required to reduce the current in the inductor in response to a load transient
TM
Figure 7 - Inductor Current Sensing and Current Sense Amplifier
L
was used. The mismatch of the time constants does not affect the measurement of
v
C
CSOUT
(
s
)
=
v
Current
Sense Amp
L
T
T
(
SLEW
SLEW
s
)
1
i
+
L
=
=
sR
L
L
V
1
CS
O
( *
( *
L
R
C
+
CS
I
I
CS
V
MAX
MAX
v
BODYDIODE
V
L
=
O
R
C
v
−
−
i
L
CS
c
L
CS
I
I
(
s
MIN
MIN
)
1
)
)
+
R
sR
V
L
O
+
CS
C
sL
C
O
CS
L
). If the two time constants match, the
July 03, 2008
IR3500V