ISL6566ACR datasheet(14/29 Pages) RENESAS

14 / 29 page

ISL6566A

FN9200 Rev 2.00

Page 14 of 29

Jul 27, 2005

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 ISL6566A 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.

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

IL3

VL(s)

DCR

INDUCTOR

DCR

INDUCTOR

PHASE3

PHASE2

PHASE1

IL2

IL1

RCOMP

CCOMP

ISUM

IREF

ISL6566A

VDROOP

IOUT

(optional)

To External Driver

PHASE Pin

E/A

OFS

VCC

GND

0.5V

1.5V

GND

ROFS

RFB

VDIFF

ISL6566A

FIGURE 8. POSITIVE OFFSET OUTPUT VOLTAGE

PROGRAMMING

VREF

VOFS

IOFS

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