L7981 datasheet(19/44 Pages) STMICROELECTRONICS

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L7981

Application informations

Doc ID 15182 Rev 3

19/44

6.4

Compensation network

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

of the L7981 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 (see Figure

10.). The transfer function of the PWM modulator, from the error amplifier output (COMP

pin) to the OUT pin, results:

Equation 13

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

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

Equation 14

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

Equation 15

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

modifies the PWM modulator gain (see Chapter 5.1 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 modulation and the LC output filter

The transfer function on the LC filter is given by:

PW0

---------

⋅

PW0

---------

----

COMP

REF

E/A

PWM

OUT

ESR

PW0

COMP

REF

E/A

PWM

OUT

ESR

PW0

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