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AN487 APPLICATION NOTE

Neglecting the high frequency behavior (in the hy-potesis that in the overall frequency compensation of

the loop the second pole of the operational amplifier is far below the 0 dB axis), we can make a first order

approx. by which the error amplifier can be schematized by the equivalent circuit of fig.12.

Figure 12. Error amplifier equivalent circuit.

by which

The error amplifier is inserted in the regulation loop and can be easily compensated, thanks to its high

output impedance, with a network between its output and ground. The typical compensated network is

shown in fig.13.

Figure 13. Compensation network of the error amplifier.

The transfer function is:

In the hypotesis that Rc<Ro and Cc>Co, the Bode diagram of the compensated amplifier is reported (see fig.14).

Figure 14. Bode plot showing gain and phase of compensated error amplifier.

A

()

G

M

R

o

1SR

+

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

⋅

=

A

()

G

v

R

+

()

S

2

R

SR

R

R

++

() 1

++

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⋅

=