L6701

15 System Control Loop Compensation

35/44

15

System Control Loop Compensation

The control loop is composed by the Current Sharing control loop (See Figure 17) 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 17 shows the block diagram of the

system control loop.

read current.

Figure 17. 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

(

for fully differential current sense);

●

●

●

●

A(s) is the error amplifier gain;

L3

L2

L1

PWM3

PWM2

PWM1

4 / 5

Reference

ERROR AMPLIFIER

COMP

FB

1 / 5

1 / 5

1 / 5

CURRENT SHARING

DUTY CYCLE

CORRECTION

G

()

PWM Z

() R

DROOP

Z

()

+

()

⋅⋅

Z

() Z

()

+

[]

Z

()

As

()

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

1

1

As

()

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

+

⎝⎠

⎛⎞ R

FB

⋅

+

⋅

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

–

=

R

D ROOP

DCR

R

D

⋅

=

R

DR OO P

3

DCR

R

D

⋅⋅

=