IR3084MPBF datasheet(29/45 Pages) IRF

29 / 45 page

IR3084

Page 29 of 45

07/20/2005

IR3086 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 (18). To achieve feed−forward voltage mode control, the resistor RRAMP should be connected to

the input of the converter.

)]

PWMRMP

DAC

ln(V

)

DAC

*[ln(V

PWMRMP

−

(18)

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.

+ =

(19)

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.

−

∗

CSIN

(20)

If RCS− is not used, RCS+ should be chosen so that the offset voltage is small enough. Usually RCS+ should be

less than 2 k

and therefore a larger CCS+ value is needed.

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