ir3082pbf International Rectifier Corp., ir3082pbf Datasheet - Page 9

• Manufacturer Part Number: ir3082pbf
• Description: Xphase Amd Opterontm/athlon 64tm 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
DIODE
now;
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 Amp’s output voltage drops below 91%
of the VDAC voltage this comparator turns off the low side gate driver.
Figure 5 depicts PWM operating waveforms under various conditions.
PHASE IC
CLOCK
PULSE
PWMRMP
GATEH
GATEL
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. 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.
. The minimum time required to reduce the current in the inductor in response to a load transient decrease is
EAIN
VDAC
Page 9 of 33
TM
STEADY-STATE
OPERATION
L
was used. The mismatch of the time constants does not affect the measurement of
DUTY CYCLE INCREASE
DUE TO LOAD
INCREASE
v
Figure 5 – PWM Operating Waveforms
C
(
s
)
=
v
L
T
T
(
SLEW
SLEW
s
)
1
+
=
=
sR
L
L
V
1
DUTY CYCLE DECREASE
DUE TO VIN INCREASE
(FEED-FORWARD)
CS
O
⋅
⋅
(
(
C
+
I
I
CS
MAX
MAX
V
V
BODYDIODE
=
O
−
−
i
L
I
I
(
MIN
MIN
s
)
91% VDAC
1
)
)
+
R
sR
L
+
CS
sL
DUTY CYCLE DECREASE DUE TO LOAD
DECREASE (BODY BRAKING) OR FAULT
(VCC UV, OCP, VID=11111)
C
CS
L
). If the two time constants match, the
IR3082PbF
01/31/05
STEADY-STATE
OPERATION
BODY