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L6917B

s

Current Sharing (CS) Control Loop

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

internally built; the error between the 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.

11).

The current sharing control is a high bandwidth control loop 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 offset of Tran

conductance differential amplifier; considering a voltage offset equal to 2mV across the sense resistor, the cur-

rent 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.

Figure 11. Current Sharing Control Loop

s

Average Current Mode (ACM) Control Loop

The average current mode control loop is reported in figure 12. The current information IFB sourced by the FB

pin flows into RFB implementing the dependence of the output voltage from the read current.

The ACM control loop gain results (obtained opening the loop after the COMP pin):

Where:

–

is the equivalent output resistance determined by the droop function;

load Ro;

I

RE AD

∆

I

MAX

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

2m V

R

SENSE

I

MAX

⋅

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

=

L1

L2

+

+

PWM1

1/5

1/5

PWM2

COMP

CURRENT

SHARING

DUTY CYCLE

CORRECTION

D02IN1393

G

()

PWM Z

() R

DROOP

Z

()

+

()

⋅

⋅

Z

() Z

()

+

()

Z

()

As

()

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

1

1

As

()

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

+





R

FB

⋅

+

⋅

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

=

R

DROOP

R

sense

R

g

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

R

FB

⋅

=