21/35

L6918 L6918A

MAIN CONTROL LOOP

The four phases control loop is composed by two dual phases devices that are independent each other. So, the

compensation network and the control loop stability of each device don't depend on the other except for the fact

that the other converter represents a load for this one.

The L6918/A control loop is composed by the Current Sharing control loop and the Average Current Mode con-

trol 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 12 reports the

block diagram of the main control loop

Figure 12. Main Control Loop Diagram

CURRENT SHARING (CS) CONTROL LOOP

The devices are configured to work in a four synchronized phase application. Since the application is composed

by two-phase devices that share reference and synchronization signals, the current sharing between the phases

is realized in two different steps:

1. Sharing between the phases of the same device;

2. Sharing between devices.

The Current Sharing between phases of the same device uses the internal current information to correct the

PWM signal in order to equalize the current. Active current sharing is implemented using the information from

Tran conductance differential amplifier in an average current mode control scheme. A current reference equal

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. 13).

The current sharing control is a high bandwidth control allowing current sharing even during load transients.

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 mismatch of

Tran conductance differential amplifier between phases; considering a voltage mismatch equal to 2mV across

the sense resistor, the current reading error is given by the following equation:

Where

For Rsense=4m

Ω and Imax=40A the current sharing error is equal to 2.5%, neglecting errors due to Rg and

Rsense mismatches.

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

SEN SE

I

MAX

⋅

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

=