L5986 datasheet(18/39 Pages) STMICROELECTRONICS

18 / 39 page

Application information

L5986 - L5986A

18/39

Figure 8.

The error amplifier, the PWM modulation and the LC output filter

The transfer function on the LC filter is given by:

Equation 13

where:

Equation 14

Equation 15

As seen in Chapter 5.3 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.

6.4.1

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

COMP

REF

E/A

PWM

OUT

ESR

PW0

COMP

REF

E/A

PWM

OUT

ESR

PW0

LC s

()

π f

zESR

⋅

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

π Qf

⋅

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

π f

⋅

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

⎝⎠

⎛⎞ 2

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

OUT

⋅

ESR

OUT

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

⋅⋅

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

zESR

π ESR C

OUT

⋅⋅

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

OUT

ESR

()

⋅⋅

⋅

OUT

ESR

⋅⋅

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

OUT

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

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