L6722 datasheet(27/34 Pages) STMICROELECTRONICS

27 / 34 page

L6722

11 System control loop compensation

27/34

System control loop compensation

The control loop is composed by the Current Sharing control loop (See Figure 16) and the

Voltage 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 Voltage control loop fixes the output voltage equal to the reference.

Figure 16 shows the block diagram of the system control loop.

The system Control Loop is reported in Figure 17. The current information IDROOP sourced by

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

read current (when DROOP is enabled).

Figure 16. Main control loop

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

Where:

●

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 three inductor impedance;

●

A(s) is the error amplifier gain;

PWM3

PWM2

PWM1

4 / 5

Reference

ERROR AMPLIFIER

COMP

ZF(s)

IDROOP

COUT

ROUT

1 / 5

CURRENT SHARING

DUTY CYCLE

CORRECTION

IINFO1

IINFO3

IINFO2

LOOP s

()

PWM

F s

()

DROOP

P s

()

⋅⋅

P s

() Z

L s

()

[]

F s

()

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

()

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

⎝⎠

⎛⎞

⋅

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

–

DROOP

DCR

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

⋅

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