IR3081 View Datasheet(PDF) - International Rectifier
Part Name
Description
Manufacturer
IR3081 Datasheet PDF : 20 Pages
| |||
IR3081
This control method is designed to provide “single cycle transient response” where the inductor current changes in
response to load transients within a single switching cycle maximizing the effectiveness of the power train and
minimizing the output capacitor requirements. An additional advantage is that differences in ground or input voltage
at the phases have no effect on operation since the PWM ramps are referenced to VDAC.
Body BrakingTM
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;
TSLEW = [L x (IMAX - IMIN)] / Vout
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 + VBODY DIODE. The
minimum time required to reduce the current in the inductor in response to a load transient decrease is now;
TSLEW = [L x (IMAX - IMIN)] / (Vout + VBODY DIODE)
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 4 depicts PWM operating waveforms under various conditions
PHASE IC
CLOCK
PULSE
EAIN
PWMRMP
VDAC
GATEH
91% VDAC
GATEL
STEADY-STATE
OPERATION
DUTY CYCLE INCREASE
DUE TO LOAD
INCREASE
DUTY CYCLE DECREASE
DUE TO VIN INCREASE
(FEED-FORWARD)
DUTY CYCLE DECREASE DUE TO LOAD
DECREASE (BODY BRAKING) OR FAULT
(VCC UV, VCCVID UV, OCP, VID=11111X)
Figure 4 – PWM Operating Waveforms
STEADY-STATE
OPERATION
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,
vC
( s)
=
vL
(s)
1
+
1
sRS
CS
=
i
L
(s
)
1
RL + sL
+ sRS CS
Page 9 of 20
9/1/03
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