L6711 datasheet(41/50 Pages) STMICROELECTRONICS

41 / 50 page

L6711

System control loop compensation

41/50

System control loop compensation

The control loop is composed by the Current Sharing control loop and the Average Current

Mode control loop. Each loop gives, with a proper gain, the correction to the PWM in order

to minimize the error in its regulation: the Current Sharing control loop equalize the currents

in the inductors while the Average Current Mode control loop fixes the output voltage equal

to the reference programmed by VID. Figure 21 shows the block diagram of the system

control loop.

Figure 21.

Main control loop diagram

The average current mode control loop is reported in Figure 22. The current information IFB

sourced by the FB pin flows into RFB implementing the dependence of the output voltage

from the read current.

The system can be modeled with an equivalent single phase converter which only

difference is the equivalent inductor L/3 (where each phase has an L inductor).The ACM

control loop gain results (obtained opening the loop after the COMP pin):

Where:

●

RSENSE is the mosfet RdsON or the Inductor DCR depending on the sensing element

selected;

●

is the equivalent output resistance determined by droop;

●

ZP(s) is the impedance resulting by the parallel of the output capacitor (and its ESR)

and the applied load Ro;

●

ZF(s) is the compensation network impedance;

●

ZL(s) is the parallel of the three inductor impedance;

●

A(s) is the error amplifier gain;

4/5

ZF(s)

PLIFIER

TIO

1/5

IINFO1

IINFO2

IINFO3

IFB

L OOP s

()

PWM Z

F s

() R

D ROOP

P s

()

⋅

P s

() Z

L s

()

F s

()

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

()

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

⎝⎠

⎛⎞ R

⋅

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

–

D ROOP

SEN SE

--------------------- RFB

⋅

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