ISL6721 datasheet(17/21 Pages) INTERSIL

17 / 21 page

FN9110.4

April 13, 2007

the remaining pole and zero for the compensator are located

at:

The ratio of R15 to the parallel combination of R17 and R18

determine the mid band gain of the error amplifier.

From Equation 27, it can be seen that the control to output

transfer function frequency dependence is a function of the

output load resistance, the value of output capacitance, and

the output capacitance ESR. These variations must be

considered when compensating the control loop. The worst

case small signal operating point for the converter is at

minimum VIN, maximum load, maximum COUT, and

minimum ESR.

The higher the desired bandwidth of the converter, the more

difficult it is to create a solution that is stable over the entire

operating range. A good rule of thumb is to limit the

bandwidth to about Fsw/4. For this example, the bandwidth

will be further limited due to the low GBWP of the LM431-

based Error Amplifier and the opto-coupler. A bandwidth of

approximately 5kHz was selected.

For the EA compensation, the first pole is placed at the

origin by default (C14 is an integrating capacitor). The first

zero is placed below the crossover frequency, fco, usually

around 1/3 fco. The second pole is placed at the lower of the

ESR zero or at one half of the switching frequency. The

midband gain is then adjusted to obtain the desired

crossover frequency. If the phase margin is not adequate,

the crossover frequency may have to be reduced.

Using this technique to determine the compensation, the

following values for the EA components were selected.

R17 = R18 = R15 = 1kΩ

R20 = open

C13 = 100nF

C14 = 100pF

A Bode plot of the closed loop system at low line, max load

appears below.

π R

•

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

π R

•

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

≈

(EQ. 38)

π R

•

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

(EQ. 39)

midband

()

•

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

(EQ. 40)

FIGURE 9A. GAIN

10k

100k

10M

100M

-10

-20

-30

-40

-50

FREQUENCY (Hz)

FIGURE 9B. PHASE MARGIN

10k

100k

10M

100M

-100

-50

100

150

200

FREQUENCY (Hz)

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