L5988D datasheet(31/52 Pages) STMICROELECTRONICS

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L5988D

Application information

The guidelines for positioning the poles and the zeroes and for calculating the component

values can be summarized as follow:

Choose a value for R1, usually between 1 k and 5 k.

Choose a gain (R5/R8) in order to have the required bandwidth (BW), that means:

Equation 29

where K is the feed forward constant and 1/K is equals to 18.

Calculate C5 by placing the zero at 50% of the output filter double pole frequency (fLC):

Equation 30

Calculate C6 by placing the second pole at four times the system bandwidth (BW):

Equation 31

Set also the fist pole at four times the system bandwidth and also the second zero at

the output filter double pole:

Equation 32

The suggested maximum system bandwidth is equals to the switching frequency divided by

3.5 (FSW/3.5), anyway lower than 120 kHz if the FSW is set higher than 500kHz.

For example with VOUT = 1.2 V, VIN = 12 V, IO = 4 A, L = 4.7 H, COUT = 47 F, the type III

compensation network is:

Equation 33

fLC

----------

---- R8



 R

5 fLC



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

 R

5 C5 4BW



–



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

4BW



fLC

----------------- 1

–

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

 R

7 4BW

 

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



R8 4.7k

R6 4.7k

R7 56

R5 1.2K

C7 10nF

C5 22nF

C6 1nF





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