L7986TA datasheet(19/43 Pages) STMICROELECTRONICS

19 / 43 page

L7986TA

Application information

Doc ID 022098 Rev 3

19/43

Equation 16

where VS is the sawtooth amplitude. As seen in Section 5.1, the voltage feed-forward

generates a sawtooth amplitude directly proportional to the input voltage, that is:

Equation 17

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

Equation 18

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

can modify the PWM modulator gain (see Section 5.1 to understand how this gain changes

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

Figure 8.

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

The transfer function on the LC filter is given by:

Equation 19

where:

PW0

---------

⋅

PW0

---------

----

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

⋅

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

⎝⎠

⎛⎞

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

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