L7987 datasheet(25/37 Pages) STMICROELECTRONICS

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L7987

Application information

5.4.1

Type II compensation network

If the equivalent series resistance (RES) of the output capacitor introduces a zero with

a frequency lower than the desired bandwidth (that is: 2



• RES • CO > 1/BW), this zero

helps stabilize the loop. Electrolytic capacitors show non-negligible ESR (> 30 m

 typically),

so with this kind of output capacitor the type II network combined with the zero of the ESR

allows to stabilize the loop.

Figure 11. Type II compensation network

The type II compensation network transfer function, from VOUT to COMP, is computed in

Equation 20.

Equation 20

Equation 21

The following suggestions can be followed for a quite common compensation strategy,

assuming that CP << CF.



Starting from Equation 19, in case of type II compensation network and electrolytic

output capacitors the control loop gain module at s = 2



• FBW allows to fix the RF/RU

ratio:

Equation 22

















































)

(

)

(

)

(

COMPII



























;



)

(











zESR

LOOP



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