L5987ATR View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

L5987ATR

3 A step-down switching regulator

STMicroelectronics 

Application information

L5987

6.4

Compensation network

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

loop of the L5987 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 the 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 14

GPW0 = V--V---I-sN--

where VS is the sawtooth amplitude. As seen in Section 5.1 on page 9, the voltage

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

is:

Equation 15

VS = K  VIN

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

Equation 16

GPW0 = V--V---I-sN-- = K-1-- = 9

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 10. The error amplifier, the PWM modulation and the LC output filter

VCC

VREF

FB

VS

E/A

COMP

PWM

GPW0

OUT

L

ESR

GLC

COUT

20/40

DocID14972 Rev 4

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