L7987 datasheet(28/37 Pages) STMICROELECTRONICS

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Application information

L7987

28/37

DocID025589 Rev 3

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

assuming that CP << CF and RS << RU.



Starting from Equation 19 on page 24, in case of type III compensation network and

MLCC ceramic output capacitors the control loop gain module at s = 2



• FBW allows

to fix the RF/RU ratio:

Equation 26

After choosing the regulator bandwidth (typically FBW < 0.2 • FSW) and a value for RU,

usually between 1 k

 and 50 k, in order to achieve CF and CP not comparable with

parasitic capacitance of the board, the RF required value is computed by Equation 26.



Select CF in order to place FZ1 below FLC (typically 0.1 • FLC)



Select CP in order to place FP1 at 0.5 • FSW

Equation 27



Select CS in order to place FZ2 at FLC



Select RS in order to place FP2 at 0.5 • FSW

Equation 28

)

(











III

LOOP



 R

0.1 fLC



------------------------------------------------;

 R

0.5 fSW



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

 R

fLC



----------------------------------;

 C

0.5 fSW



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

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