IR3088 データシートの表示（PDF） - International Rectifier
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IR3088 Datasheet PDF : 34 Pages
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IR3088
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)
STEADY-STATE
OPERATION
Figure 4. PWM Operating Waveforms
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 * (IMAX − IMIN )
VO
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 +
VBODYDIODE. The minimum time required to reduce the current in the inductor in response to a load transient
decrease is now;
TSLEW
=
L * (IMAX − IMIN )
VO + VBODYDIODE
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,
vC
( s)
=
vL
(
s)
1
+
1
sRCS
CCS
=
iL
(
s)
1
RL + sL
+ sRCSCCS
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 (RL). 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 RL was used. The mismatch of the time constants does not affect the measurement of
inductor DC current, but affects the AC component of the inductor current.
Page 9 of 34
9/30/04
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