AN487 APPLICATION NOTE

8/13

The compensation network introduces a low frequency pole and a zero that usually is put at the frequency

of the resonant pole of the output LC filter. The second high frequency pole is usually at a frequency of no

interest. If needed , more sophisticated compensation circuits can be used by feedback with the opamp.

An example is shown in fig.15.

Figure 15. One pole, two zero compensation network.

Such a configuration introduces a low frequency pole and two zeros Z1 = 1/2

ΠR1C1 and Z2 = 1/2ΠR2C2.

Note that due to the high output impedance it is present also a second pole p2 = gm/2PC1. Usually it is

better to use the highest possible value for R1, to have a low value for C1 in such a way to put p2 at the

highest frequency. Limitations to R1 value are put by offset voltage due to opamp. input bias currents.

If a resistive divider is used at the output of the power supply, for voltages higher than 5.1V, it is possible

to introduce a second zero with the network of fig.16.

Figure 16. Compensation network for output voltages higher than 51V.

Such a configuration introduce 2 zeros at:

;

and 2 poles at:

;

;

Z

1

1

2

ΠR

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

=

Z

2

1

2

ΠR

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

=

P

1

1

2

ΠR

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

=

P

2

1

2

ΠR

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

=

Rx

R

R

1

R

2

+

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

=