ISL6569ACR datasheet(18/22 Pages) RENESAS

18 / 22 page

ISL6569A

FN9092 Rev 2.00

Page 18 of 22

Dec 29, 2004

COMPENSATING LOAD-LINE REGULATED

CONVERTER

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 effected by the values

of the components that are meant to compensate them, the

solution to the system equation becomes fairly complicated.

Fortunately there is a simple approximation that comes very

close to an optimal solution. Treating the system as though it

were a voltage-mode regulator by compensating the L-C

poles and the ESR zero of the voltage-mode approximation

yields a solution that is always stable with very close to ideal

transient performance.

The feedback resistor, RFB, has already been chosen as

outlined in Load-Line Regulation Resistor. Select a target

bandwidth for the compensated system, f0. The target

bandwidth must be large enough to assure adequate

transient performance, but smaller than 1/3 of the per-

channel switching frequency. The values of the

compensation components depend on the relationships of f0

to the L-C pole frequency and the ESR zero frequency. For

each of the three cases which follow, there is a separate set

of equations for the compensation components.

In Equations 27, L is the per-channel filter inductance

divided by 2 (the number of active channels); C is the sum

total of all output capacitors; ESR is the equivalent-series

resistance of the bulk output-filter capacitance; and VPP is

the peak-to-peak sawtooth signal amplitude as described in

Figure 6 and Electrical Specifications.

Once selected, the compensation values in Equations 27

assure a stable converter with reasonable transient perfor-

mance. In most cases, transient performance can be

improved by making adjustments to RC. Slowly increase the

value of RC while observing the transient performance on an

oscilloscope until no further improvement is noted. Normally,

CC will not need adjustment. Keep the value of CC from

Equations 27 unless some performance issue is noted.

The optional capacitor C2, is sometimes needed to bypass

noise away from the PWM comparator (see Figure 13). Keep

a position available for C2, and be prepared to install a high-

frequency capacitor of between 22pF and 150pF in case any

trailing edge jitter problem is noted.

Compensation without load-line regulation

The non load-line regulated converter is accurately modeled

as a voltage-mode regulator with two poles at the L-C

resonant frequency and a zero at the ESR frequency. A

type-III controller, as shown in Figure 14, provides the

necessary compensation.

The first step is to choose the desired bandwidth, f0, of the

compensated system. Choose a frequency high enough to

assure adequate transient performance but not higher than

1/3 of the switching frequency. The type-III compensator has

an extra high-frequency pole, fHF. This pole can be used for

added noise rejection or to assure adequate attenuation at

the error-amplifier high-order pole and zero frequencies. A

good general rule is to chose fHF =10 f0, but it can be higher

if desired. Choosing fHF to be lower than 10 f0 can cause

problems with too much phase shift below the system

bandwidth.

FIGURE 13. COMPENSATION CONFIGURATION FOR

LOAD-LINE REGULATED ISL6569A CIRCUIT

COMP

RFB

IOUT

VDIFF

VDROOP

C2 (OPTIONAL)

 LC

------------------- f0

RFB

f

0Vpp LC

0.75VIN

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

0.75VIN

V

PPRFBf0

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

Case 1:

 LC

------------------- f0

C ESR



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



RFB

VPP 2

2 f

2 LC

0.75 VIN

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

0.75VIN



2 f

2 V

PPRFB LC

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

Case 2:

(EQ. 27)

C ESR



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

RFB

 f

0VppL

0.75 VIN ESR



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

0.75VIN ESR

 C

V

PPRFBf0 L

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

Case 3:

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