L6713ATR datasheet(52/64 Pages) STMICROELECTRONICS

52 / 64 page

System control loop compensation

L6713A

52/64

System control loop compensation

The control loop is composed by the current sharing control loop (See Figure 9) 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 25 shows the block

diagram of the system control loop.

The system control loop is reported in Figure 26. The current information IDROOP sourced by

the DROOP pin flows into RFB implementing the dependence of the output voltage from the

read current.

Figure 25.

Main control loop

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

is the equivalent inductor L/N (where each phase has an L inductor). The control loop gain

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

Where:

●

DCR is the Inductor parasitic resistance;

●

is the equivalent output resistance determined by the droop

function;

●

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 N inductor impedance;

●

A(s) is the error amplifier gain;

●

is the PWM transfer function where

ΔV

OSC is the oscillator ramp

amplitude and has a typical value of 3 V.

PWM3

PWM2

PWM1

4 / 5

VREF

ERROR AMPLIFIER

COMP

ZF(s)

ZFB(s)

DROOP

IDROOP

COUT

ROUT

1 / 5

CURRENT SHARING

DUTY CYCLE

CORRECTION

IINFO1

IINFO3

IINFO2

(PHASE2 Only applies when using 3-PHASE Operation)

LOOP s

()

PWM Z

F s

() R

DROOP

P s

()

⋅⋅

P s

() Z

L s

()

[]

F s

()

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

()

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

⎝⎠

⎛⎞

⋅

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

–

DROOP

DCR

------------- R

⋅

PWM

---

ΔV

OSC

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

⋅

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