IR3082P

bF

Page 21 of 33

01/31/05

No Load Output Voltage Setting Resistor RFB and Adaptive Voltage Positioning Resistor RDRP

A resistor between FB pin and the converter output is used to create output voltage offset VO_NLOFST, which is the

difference between VDAC voltage and output voltage at no load condition. Adaptive voltage positioning lowers the

converter voltage by Ro times Io, where Ro is the required output impedance of the converter.

RFB is not only determined by IFB, the current flowing out of the FB pin as shown in Figure 14, but also affected by

the total input offset voltage of current sense amplifiers. RFB and RDRP are determined by (15) and (16)

respectively.

MAX

L

FB

O

TOFST

CS

NLOFST

O

MAX

L

FB

R

I

R

n

V

V

R

R

_

_

_

_

⋅

⋅

⋅

−

⋅

=

(15)

O

MIN

CS

MAX

L

FB

DRP

R

n

G

R

R

R

⋅

⋅

⋅

=

_

_

(16)

IR3086A 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.

)]

ln(

)

[ln(

PWMRMP

DAC

IN

DAC

IN

PWMRMP

SW

IN

O

PWMRMP

V

V

V

V

V

C

f

V

V

R

−

−

−

−

⋅

⋅

⋅

=

(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.

+

+ =

CS

L

CS

C

R

L

R

(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. The resistor RCS- is determined by the ratio of the bias current from the non-inverting

input and the bias current from the inverting input.

+

−

+

−

⋅

=

CS

CSIN

CSIN

CS

R

I

I

R

(19)