L6710

20/34

The worst condition depends on the input voltage available and the output voltage selected. Anyway the

worst case is the response time after removal of the load with the minimum output voltage programmed

and the maximum input voltage available.

MAIN CONTROL LOOP

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 Cur-

rent Mode control loop fixes the output voltage equal to the reference programmed by VID. Figure 13 re-

ports the block diagram of the main control loop.

Figure 13. Main Control Loop Diagram

Current Sharing (CS) Control Loop

Active current sharing is implemented using the information from Tran conductance differential amplifier

in an average current mode control scheme.

read current and this reference is converted to a voltage with a proper gain and it is used to adjust the duty

cycle whose dominant value is set by the error amplifier at COMP pin (See fig. 14).

The current sharing control is a high bandwidth control loop allowing current sharing even during load tran-

sients.

The current sharing error is affected by the choice of external components; choose precise Rg resistor

(±1% is necessary) to sense the current.

The current sharing error is internally dominated by the voltage offset of Tran conductance differential am-

plifier; considering a voltage offset equal to 2mV across the sense resistor, the current reading error is

given by the following equation

t

app lic atio n

L

∆I

⋅

V

IN

V

OUT

–

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

t

re m ov al

L

∆I

⋅

V

OUT

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

=

=

L1

L2

+

+

PWM1

1/5

+

-

1/5

4/5

PWM2

FB

COMP

ERROR

AMPLIFIER

REFERENCE

PROGRAMMED

BY VID

CURRENT

SHARING

DUTY CYCLE

CORRECTION

D02IN1392

∆I

REA D

I

MAX

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

2mV

R

SENSE

I

MAX

⋅

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

=