ir3088a International Rectifier Corp., ir3088a Datasheet - Page 9

• Manufacturer Part Number: ir3088a
• Description: Xphase Ic With Fault And Overtemp Detect
• 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
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 Amplifier’s output voltage drops below
91% of the VDAC voltage this comparator turns off the low side gate driver.
Lossless Average Inductor Current Sensing
Inductor current can be sensed by connecting a resistor and a capacitor in parallel with the inductor and measuring
the voltage across the capacitor, as shown in Figure 5. 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.
PHASE IC
CLOCK
PULSE
PWMRMP
GATEH
GATEL
BODYDIODE
Page 9 of 33
EAIN
VDAC
. The minimum time required to reduce the current in the inductor in response to a load transient decrease
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
C
Figure 4. PWM Operating Waveforms
(
s
)
=
v
L
T
T
(
SLEW
SLEW
s
)
1
+
=
=
sR
L
L
V
DUTY CYCLE DECREASE
DUE TO VIN INCREASE
(FEED-FORWARD)
1
CS
O
( *
( *
C
+
I
I
CS
V
MAX
MAX
BODYDIODE
V
=
O
−
−
i
L
I
I
(
s
MIN
MIN
91% VDAC
)
1
)
)
+
R
sR
L
+
CS
DUTY CYCLE DECREASE DUE TO LOAD
DECREASE (BODY BRAKING) OR FAULT
(VCC UV, VCCVID UV, OCP, VID=11111X)
sL
C
CS
L
). If the two time constants match, the
1/15
IR3088A
/05
STEADY-STATE
OPERATION