L5983 datasheet(19/37 Pages) STMICROELECTRONICS

19 / 37 page

Application informations

L5983

19/37

The transfer function on the LC filter is given by:

Equation 13

where:

Equation 14

Equation 15

As seen in Chapter 4.3 two different kind of network can compensate the loop. In the two

following paragraph the guidelines to select the Type II and Type III compensation network

are illustrated.

Figure 9.

Error amplifier, PWM modulator and LC output filter

5.4.1

Type III compensation network

The methodology to stabilize the loop consists of placing two zeros to compensate the effect

of the LC double pole, so increasing phase margin; then to place one pole in the origin to

minimize the dc error on regulated output voltage; finally to place other poles far away the

zero dB frequency.

LC s

()

π f

zESR

⋅

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

π Qf

⋅

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

π f

⋅

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

⎝⎠

⎛⎞ 2

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

OUT

⋅

ESR

OUT

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

⋅⋅

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

zESR

π ESR C

OUT

⋅⋅

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

OUT

ESR

()

⋅⋅

⋅

OUT

ESR

⋅⋅

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

OUT

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

COMP

REF

E/A

PWM

OUT

ESR

PW0

COMP

REF

E/A

PWM

OUT

ESR

PW0

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