ir3084u International Rectifier Corp., ir3084u Datasheet - Page 12

• Manufacturer Part Number: ir3084u
• Description: Xphasetm Vr10, Vr11 & Opteron/athlon64 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 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 “0% Duty Cycle 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. If the two time constants match, the
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.
BODY DIODE
EAIN
VDAC
Page 12 of 47
. The minimum time required to reduce the current in the inductor in response to a load transient
TM
STEADY-STATE
OPERATION
L
was used. The mismatch of the time constants does not affect the measurement of
T
DUTY CYCLE INCREASE
DUE TO LOAD
INCREASE
v
C
SLEW
(
Figure 5 – PWM Operating Waveforms
s
)
= [L x (I
=
T
v
SLEW
L
(
s
)
= [L x (I
MAX
1
+
- I
sR
MIN
1
DUTY CYCLE DECREASE
DUE TO VIN INCREASE
(FEED-FORWARD)
MAX
S
)] / (Vout + V
C
S
- I
=
MIN
i
L
)] / Vout
BODY BRAKING
THRESHOLD
(
s
)
1
BODY DIODE
R
+
L
sR
DUTY CYCLE DECREASE DUE TO LOAD
DECREASE (BODY BRAKING) OR FAULT
(VCC UV, VCCVID UV, OCP, VID=11111X)
+
S
sL
C
S
)
IR3084U
9/14/2005
STEADY-STATE
OPERATION