ISL6565BCB datasheet(21/28 Pages) RENESAS

21 / 28 page

ISL6565A, ISL6565B

FN9135 Rev 4.00

Page 21 of 28

Dec 1, 2005

The time constant of this R-C network must match the time

constant of the inductor L/DCR. Follow the steps below to

choose the component values for this R-C network.

1. Choose an arbitrary value for C. The recommended value

is 0.01

F.

2. Plug the Inductor L and DCR component values, and the

values for C chosen in steps 1, into Equation 28 to

calculate your value for R1. Do not populate R2.

Due to errors in the inductance or DCR it may be necessary

to adjust the value of R1 for each phase to match the time

constants correctly.

Once the R-C network components have been chosen, use

Equation 29 to calculate the value of RISEN. In Equation 29,

DCR is the DCR of the output inductor at room temperature,

IFL is the full load operating current, and N is the number of

phases.

Adjusting Phase Currents (ISL6565B Only)

Layout issues in the core-power regulator may cause the

currents in each phase to be slightly unbalanced. This

problem can be resolved without any changes to the layout or

any significant cost increase. The solution requires populating

R2 in certain phases (as shown in Figure 16) to create a

resistor divider ratio, K, for each phase. The time constant of

each new resistor divider R-C sense network must match the

time constant of the old sense network. Follow the steps

below to choose the component values for the resistor

divider R-C network for each phase.

1. Load the regulator to full load and allow the board to heat

until the output voltage stabilizes (usually several

minutes).

2. Measure the current flowing through each phase,

labeling the highest phase current, IHIGH, and the other,

lower phase currents ILOW(1) and ILOW(2).

3. Individually, plug the values for each low phase current,

ILOW(n), the highest phase current, IHIGH, the full load

current, ILOAD, and the number of phases, N, into

Equation 30 to calculate the resistor divider ratio, KLOW,

for each low phase. (NOTE: The phase with the highest

phase current is the reference phase and it will not use a

resistor divider network, keeping its resistor divider ratio

equal to 1.)

4. For each phase, calculate the values for the new R-C

network sense resistors, R1,new and R2,new, by plugging

in each phase’s new resistor divider ratio, KLOW, and

each phase’s present sense resistor R1, into Equations

31 and 32.

After calculating the new resistor divider sense resistors, the

phases will be balanced. It may be necessary to adjust the

RISEN resistor slightly to correct for any changes in the

desired ISEN current that results from adding the resistor

dividers.

The phase currents might also have to be adjusted if the

components of one or more phases are inhibited from

effectively dissipating their heat so that the affected phases

run hotter than desired. In this case it may be necessary to

adjust the resistor divider ratio of one or more of the R-C

networks. Doing so adjusts the current through affected

phases and can balance the temperatures of each phase.

Choose R1,new and R2,new in relation to the desired change

in temperature, as described in Equations 33 and 34, in order

to cause less current to flow in the hotter phase.

In Equations 33 and 34,

T2 is the desired temperature rise

above the ambient temperature, and

T1 is the measured

temperature rise above the ambient temperature. It is

FIGURE 16. DCR SENSING CONFIGURATION

ISEN(n)

ISL6565B

VIN

ICOMMON

PWM(n)

ISL6605

RISEN

DCR

INDUCTOR

VOUT

COUT

VC(s)

VL(s)

ISEN

DCR C



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

(EQ. 28)

RISEN

DCR IFL



70 10

–





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

(EQ. 29)

KLOW n



ILOW n

 I

HIGH

–

ILOAD N



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

(EQ. 30)

R1new





R1 n



KLOW n



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

(EQ. 31)

R2new





R1 n



– LOW n



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

(EQ. 32)

R1 new

T

----------

(EQ. 33)

R2 new

R1 R2



R1 R2 1



----------

–









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

(EQ. 34)

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