L7987L datasheet(23/41 Pages) STMICROELECTRONICS

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L7987L

Application information

5.4

Compensation network

The compensation network must assure stability and good dynamic performance. The loop

of the L7987L device is based on the voltage mode control. The error amplifier is an

operational amplifier with high bandwidth. So, by selecting the compensation network the

E/A is considered as ideal, that is, its bandwidth is much larger than the system one.

Figure 10. Switching regulator control loop simplified model

The transfer function of the PWM modulator, from the error amplifier output (COMP pin) to

the LX pin results in an almost constant gain, due to the voltage feed-forward which

generates a sawtooth with amplitude VS directly proportional to the input voltage:

Equation 14

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

can modify the PWM modulator gain (see Section 4.1 on page 11 to understand how this

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

GPWO

VIN

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kFF

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