R5975DTR datasheet(21/47 Pages) STMICROELECTRONICS

21 / 47 page

DocID026878 Rev 1

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R5975D

Closing the loop

7.2

LC filter

The transfer function of the LC filter is given by:

Equation 6

where RLOAD is defined as the ratio between VOUT and IOUT.

If RLOAD >> ESR, the previous expression of ALC can be simplified and becomes:

Equation 7

The zero of this transfer function is given by:

Equation 8

F0 is the zero introduced by the ESR of the output capacitor and it is very important to

increase the phase margin of the loop.

The poles of the transfer function can be calculated through the following expression:

Equation 9

In the denominator of ALC the typical second order system equation can be recognized:

Equation 10

If the damping coefficient

 is very close to zero, the roots of the equation become a double

root whose value is

n.

Similarly for ALC, the poles can usually be defined as a double pole whose value is:

Equation 11

ALC s



RLOAD 1 ESR COUT s







2 LC

OUT

ESR RLOAD

 s ESR C

OUT RLOAD



 R

LOAD







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

ALC s



1ESR COUT s



LCOUT s

ESR COUT s1



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

 ESR C

OUT





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

FPLC1 2



ESR COUT





4LCOUT





–





–

2LCOUT



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



n s







FPLC





LCOUT





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

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