ir3500a International Rectifier Corp., ir3500a Datasheet - Page 11

• Manufacturer Part Number: ir3500a
• Description: Xphase3 Vr11.0 & Amd Pvid Control Ic
• Manufacturer: International Rectifier Corp.

.
The error amplifier is a high speed amplifier with 110 dB of open loop gain. It is not unity gain stable.
PHASE IC
CLOCK
PULSE
PWMRMP
GATEH
GATEL
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,
BODYDIODE
EAIN
VDAC
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. The minimum time required to reduce the current in the inductor in response to a load transient
TM
STEADY-STATE
OPERATION
DUTY CYCLE INCREASE
DUE TO LOAD
INCREASE
Figure 6 - PWM Operating Waveforms
T
T
SLEW
SLEW
=
=
L
L
V
DUTY CYCLE DECREASE
DUE TO VIN INCREASE
(FEED-FORWARD)
O
( *
( *
+
I
I
V
MAX
MAX
BODYDIODE
V
O
−
−
I
I
MIN
MIN
)
)
DUTY CYCLE DECREASE DUE TO LOAD
DECREASE (BODY BRAKING) OR FAULT
(VCC UV, OCP, VID FAULT)
July 3, 2008
IR3500A
STEADY-STATE
OPERATION