L5983_08 datasheet(17/37 Pages) STMICROELECTRONICS

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Application information

L5983

17/37

5.4

Compensation network

The compensation network has to assure stability and good dynamic performance. The loop

of the L5983 is based on the voltage mode control. The error amplifier is a voltage

operational amplifier with high bandwidth. So selecting the compensation network the E/A

will be considered as ideal, that is, its bandwidth is much larger than the system one.

The transfer functions of PWM modulator and the output LC filter are studied (Figure 9 on

page 18). The transfer function of the PWM modulator, from the Error Amplifier output

(COMP pin) to the OUT pin, results:

Equation 10

where VS is the sawtooth amplitude. As seen in Chapter 4.1, the voltage feed forward

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

Equation 11

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

Equation 12

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

modifies the PWM modulator gain (see Chapter 4.1 to understand how this gain changes

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

The transfer function on the LC filter is given by:

Equation 13

where:

Equation 14

PW0

---------

⋅

PW0

---------

----

LC s

()

π f

zESR

⋅

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

π Qf

⋅

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

π f

⋅

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

⎝⎠

⎛⎞ 2

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

OUT

⋅

ESR

OUT

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

⋅⋅

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

zESR

π ESR C

OUT

⋅⋅

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

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