ISL6566 datasheet(23/30 Pages) RENESAS

23 / 30 page

ISL6566

FN9178 Rev 4.00

Page 23 of 30

Mar 9, 2006

Then the resistor RS must be chosen to set the current

sense network gain, obtaining the desired full load droop

voltage. Follow the steps below to choose the component

values for this R-C network.

1. Choose an arbitrary value for CCOMP. The recommended

value is 0.01

F.

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

values for CCOMP chosen in steps 1, into Equation 26 to

calculate the value for RCOMP.

3. Use the new value for RCOMP obtained from Equation

26, as well as the desired full load current, IFL, full load

droop voltage, VDROOP, and inductor DCR in Equation

27 to calculate the value for RS.

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

to adjust the value of RCOMP to match the time constants

correctly. The effects of time constant mismatch can be seen

in the form of droop overshoot or undershoot during the

initial load transient spike, as shown in Figure 19. Follow the

steps below to ensure the R-C and inductor L/DCR time

constants are matched accurately.

1. Capture a transient event with the oscilloscope set to

about L/DCR/2 (sec/div). For example, with L = 1

H and

DCR = 1m

, set the oscilloscope to 500s/div.

2. Record

V1 and V2 as shown in Figure 19.

3. Select a new value, RCOMP,2, for the time constant

resistor based on the original value, RCOMP,1, using the

following equation.

4. Replace RCOMP with the new value and check to see that

the error is corrected. Repeat the procedure if necessary.

After choosing a new value for RCOMP, it will most likely be

necessary to adjust the value of RS to obtain the desired full

load droop voltage. Use Equation 27 to obtain the new value

for RS.

Compensation

The two opposing goals of compensating the voltage

regulator are stability and speed.

The load-line regulated converter behaves in a similar

manner to a peak current mode controller because the two

poles at the output filter L-C resonant frequency split with the

introduction of current information into the control loop. The

final location of these poles is determined by the system

function, the gain of the current signal, and the value of the

compensation components, RC and CC.

Since the system poles and zero are affected by the values of

the components that are meant to compensate them, the

solution to the system equation becomes fairly complicated.

RCOMP

DCR CCOMP



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

(EQ. 26)

IFL

VDROOP

------------------------- RCOMP DCR



(EQ. 27)

FIGURE 18. DCR SENSING CONFIGURATION

ICOMP

DCR

INDUCTOR

COUT

IL1

VL(s)

DCR

INDUCTOR

DCR

INDUCTOR

PHASE1

PHASE2

PHASE3

IL2

IL3

RCOMP

CCOMP

ISUM

IREF

ISL6566

VDROOP

IOUT

(optional)

RCOMP 2



RCOMP 1





----------



(EQ. 28)

FIGURE 19. TIME CONSTANT MISMATCH BEHAVIOR

V1

VOUT

ITRAN

V2

I

FIGURE 20. COMPENSATION CONFIGURATION FOR

LOAD-LINE REGULATED ISL6566 CIRCUIT

ISL6566

COMP

RFB

VDIFF

C2 (OPTIONAL)

ISL6566 Datasheet(PDF) 23 Page - Renesas Technology Corp

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