L7985 datasheet(19/42 Pages) STMICROELECTRONICS

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DocID022446 Rev 7

19/42

L7985

Application information

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

Equation 18

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 9 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 19

where:

Equation 20

Equation 21

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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