IR3088A(2004) View Datasheet(PDF) - International Rectifier
Part Name
Description
MFG CO.
IR3088A Datasheet PDF : 34 Pages
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IR3088A
RFB
=
RL _ MAX
∗ VO _ NLOFST − VCS _ TOFST
I FB ∗ RL _ MAX
∗ n ∗ RO
(15)
RDRP
=
RFB
∗ RL _ MAX ∗ GCS _ MIN
n ∗ RO
(16)
Body BrakingTM Related Resistors RBBFB and RBBDRP
The body brakingTM during Dynamic VID can be disabled by connecting BBFB pin to ground. If the feature is
enabled, Resistors RBBFB and RBBDRP are needed to restore the feedback voltage of the error amplifier after
Dynamic VID step down. Usually RBBFB and RBBDRP are chosen to match RFB and RDRP respectively.
IR3088A EXTERNAL COMPONENTS
PWM Ramp Resistor RPWMRMP and Capacitor CPWMRMP
PWM ramp is generated by connecting the resistor RPWMRMP between a voltage source and PWMRMP pin as well
as the capacitor CPWMRMP between PWMRMP and LGND. Choose the desired PWM ramp magnitude VRAMP and
the capacitor CPWMRMP in the range of 100pF and 470pF, and then calculate the resistor RPWMRMP from Equation
(17). To achieve feed-forward voltage mode control, the resistor RRAMP should be connected to the input of the
converter.
RPWMRMP
=
VIN
*
fSW
* CPWMRMP
*[ln(VIN
VO
− VDAC ) − ln(VIN
− VDAC
− VPWMRMP )]
(17)
Inductor Current Sensing Capacitor CCS+ and Resistors RCS+ and RCS-
The DC resistance of the inductor is utilized to sense the inductor current. Usually the resistor RCS+ and capacitor
CCS+ in parallel with the inductor are chosen to match the time constant of the inductor, and therefore the voltage
across the capacitor CCS+ represents the inductor current. If the two time constants are not the same, the AC
component of the capacitor voltage is different from that of the real inductor current. The time constant mismatch
does not affect the average current sharing among the multiple phases, but affect the current signal ISHARE as well
as the output voltage during the load current transient if adaptive voltage positioning is adopted.
Measure the inductance L and the inductor DC resistance RL. Pre-select the capacitor CCS+ and calculate RCS+ as
follows.
RCS +
=
L RL
CCS +
(18)
The bias current flowing out of the non-inverting input of the current sense amplifier creates a voltage drop across
RCS+, which is equivalent to an input offset voltage of the current sense amplifier. The offset affects the accuracy of
converter current signal ISHARE as well as the accuracy of the converter output voltage if adaptive voltage
positioning is adopted. To reduce the offset voltage, a resistor RCS- should be added between the amplifier inverting
input and the converter output.
RCS − = RCS +
(19)
If RCS- is not used, RCS+ should be chosen so that the offset voltage is small enough. Usually RCS+ should be less
than 2 kΩ and therefore a larger CCS+ value is needed.
Page 16 of 34
9/30/04
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