L6710 datasheet(21/34 Pages) STMICROELECTRONICS

21 / 34 page

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L6710

Figure 14. Current Sharing Control Loop.

Where

∆IREAD is the difference between one phase current and the ideal current (IMAX/2).

For Rsense=4m

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

and Rsense mismatches.

Average Current Mode (ACM) Control Loop

The average current mode control loop is reported in figure 15. 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;

■ 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 two inductor impedance;

■ A(s) is the error amplifier gain;

■

is the ACM PWM transfer function where Vosc is the oscillator ramp amplitude and

has a typical value of 3V

Removing the dependence from the Error Amplifier gain, so assuming this gain high enough, the control

loop gain results:

With further simplifications, it results:

PWM1

1/5

IINFO2

IINFO1

PWM2

COMP

VOUT

CURRENT

SHARING

DUTY CYCLE

CORRECTION

D02IN1393

LOOP s )

()

PWM Z

F s

() R

DROOP

P s

()

⋅

P s

() Z

L s

()

F s

()

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

()

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





⋅

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

–

DROOP

Rsense

---------------------- R

⋅

PWM

---

∆V

OSC

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

⋅

LOOP s

()

---

∆V

OSC

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

F s

()

P s

() Z

L s

()

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

--------

P s

()

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





⋅⋅

⋅

–

LOOP s

()

---

∆V

OSC

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

F s

()

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

DROOP

-------

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

1s Co

DROOP //Ro

ESR

()

⋅

---

2Ro

⋅

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

Co E SR

-------

⋅

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

⋅⋅

⋅

–

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