ISL6566 datasheet(15/30 Pages) RENESAS

15 / 30 page

ISL6566

FN9178 Rev 4.00

Page 15 of 30

Mar 9, 2006

As shown in Figure 6, a voltage, VDROOP, proportional to the

total current in all active channels, IOUT, feeds into the

differential remote-sense amplifier. The resulting voltage at

the output of the remote-sense amplifier is the sum of the

output voltage and the droop voltage. As Equation 4 shows,

feeding this voltage into the compensation network causes

the regulator to adjust the output voltage so that it’s equal to

the reference voltage minus the droop voltage.

The droop voltage, VDROOP, is created by sensing the

current through the output inductors. This is accomplished

by using a continuous DCR current sensing method.

Inductor windings have a characteristic distributed

resistance or DCR (Direct Current Resistance). For

simplicity, the inductor DCR is considered as a separate

lumped quantity, as shown in Figure 7. The channel current,

IL, flowing through the inductor, passes through the DCR.

Equation 5 shows the s-domain equivalent voltage, VL,

across the inductor.

The inductor DCR is important because the voltage dropped

across it is proportional to the channel current. By using a

simple R-C network and a current sense amplifier, as shown

in Figure 7, the voltage drop across all of the inductors DCRs

can be extracted. The output of the current sense amplifier,

VDROOP, can be shown to be proportional to the channel

currents IL1, IL2, and IL3, shown in Equation 6.

If the R-C network components are selected such that the

R-C time constant matches the inductor L/DCR time

constant, then VDROOP is equal to the sum of the voltage

drops across the individual DCRs, multiplied by a gain. As

Equation 7 shows, VDROOP is therefore proportional to the

total output current, IOUT.

By simply adjusting the value of RS, the load line can be set

to any level, giving the converter the right amount of droop at

all load currents. It may also be necessary to compensate for

any changes in DCR due to temperature. These changes

cause the load line to be skewed, and cause the R-C time

constant to not match the L/DCR time constant. If this

becomes a problem a simple negative temperature

coefficient resistor network can be used in the place of

RCOMP to compensate for the rise in DCR due to

temperature.

Note: An optional 10nF ceramic capacitor from the ISUM

pin to the IREF pin is recommended to help reduce any

noise affects on the current sense amplifier due to layout.

Output-Voltage Offset Programming

The ISL6566 allows the designer to accurately adjust the

offset voltage by connecting a resistor, ROFS, from the OFS

pin to VCC or GND. When ROFS is connected between OFS

and VCC, the voltage across it is regulated to 1.5V. This

causes a proportional current (IOFS) to flow into the OFS pin

and out of the FB pin. If ROFS is connected to ground, the

voltage across it is regulated to 0.5V, and IOFS flows into the

FB pin and out of the OFS pin. The offset current flowing

through the resistor between VDIFF and FB will generate the

desired offset voltage which is equal to the product (IOFS x

RFB). These functions are shown in Figures 8 and 9.

VL s



IL sL DCR







(EQ. 5)

VDROOP s





DCR

------------- 1





sRCOMP CCOMP





--------------------------------------------------------------------------

RCOMP

-----------------------

IL1 IL2 IL3

 DCR





(EQ. 6)

VDROOP

RCOMP

--------------------- IOUT DCR



(EQ. 7)

FIGURE 7. DCR SENSING CONFIGURATION

ICOMP

DCR

INDUCTOR

VOUT

COUT

IL1

VL(s)

DCR

INDUCTOR

DCR

INDUCTOR

PHASE1

PHASE2

PHASE3

IL2

IL3

RCOMP

CCOMP

ISUM

IREF

ISL6566

VDROOP

IOUT

(optional)

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