L5988D datasheet(28/52 Pages) STMICROELECTRONICS

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Application information

L5988D

28/52

DocID15324 Rev 4

have to be chosen in order to sustain load transient and to have a fast response to the

transient.

In Table 13 some capacitor series are listed.

6.4

Compensation network

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

of the L5988D 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. The transfer

function PWM modulator, from the error amplifier output (COMP pin) to the OUT pin, results:

Equation 20

where VS is the sawtooth amplitude and H represent its reliance on the switching frequency.

As seen in Section 5.2 on page 15, the voltage feed forward generates a sawtooth

amplitude directly proportional to the input voltage, that is:

Equation 21

The internal saw tooth is designed in order to have the maximum amplitude at the natural

switching frequency of the device.

At fSW0 = 400 kHz the PWM modulator can be written as:

Equation 22

The synchronization of the device with an external clock fed into the SYNCH pin and the

adjustment of the switching frequency through the FSW pin modify the gain of the internal

saw tooth (see Section 5.2 on page 15).

Table 13. Output capacitors

Manufacturer

Series

Cap value (

F)

Rated voltage (V)

ESR (m

)

MURATA

GRM32

22 to 100

6.3 to 25

< 5

GRM31

10 to 47

6.3 to 25

< 5

PANASONIC

ECJ

10 to 22

6.3

< 5

EEFCD

10 to 68

6.3

15 to 55

SANYO

TPA/B/C

100 to 470

4 to 16

40 to 80

TDK

C3225

22 to 100

6.3

< 5

GPW0 fSW0



VIN

--------- HfSW0





VS fSW0



KVIN HfSW0





GPW0 400 kHz



----

VIN

VS 400 kHz



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

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