A7985ATR datasheet(21/44 Pages) STMICROELECTRONICS

21 / 44 page

DocID023128 Rev 5

21/44

A7985A

Application information

In this way the PWM modulator gain results constant and equal to:

Equation 19

The synchronization of the device with an external clock provided through the SYNCH pin

can modify the PWM modulator gain (see Section 5.1 on page 11 to understand how this

gain changes and how to keep it constant in spite of the external synchronization).

Figure 9. The error amplifier, the PWM modulator and the LC output filter

The transfer function on the LC filter is given by:

Equation 20

where:

Equation 21

Equation 22

As seen in Section 5.3 on page 14, two different kinds of network can compensate the loop.

In the two following paragraphs the guidelines to select the Type II and Type III

compensation network are illustrated.

GPW0

VIN

---------

----

COMP

REF

E/A

PWM

OUT

ESR

PW0

COMP

REF

E/A

PWM

OUT

ESR

PW0

GLC s



 f

zESR



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

 Qf



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

 f



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



 2

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

fLC



LCOUT



ESR

ROUT

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



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

fzESR

 ESR C

OUT



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



ROUT LCOUT ROUT ESR







LCOUT ROUT ESR



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

ROUT

VOUT

IOUT

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

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