L6918 datasheet(23/35 Pages) STMICROELECTRONICS

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L6918 L6918A

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 2V

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

gain results:

With further simplifications, it results:

Considering now that in the application of interest it can be assumed that Ro>>RL; ESR<<Ro and

RDROOP<<Ro, it results:

The ACM control loop gain is designed to obtain a high DC gain to minimize static error and cross the 0dB axes

with a constant -20dB/dec slope with the desired crossover frequency

ωT. Neglecting the effect of ZF(s), the

transfer function has one zero and two poles. Both the poles are fixed once the output filter is designed and the

zero is fixed by ESR and the Droop resistance.

To obtain the desired shape an RF-CF series network is considered for the ZF(s) implementation.

A zero at

ωf=1/RFCF is then introduced together with an integrator. This integrator minimizes the static error

LOO P s

()

PWM Z

F s

() R

DRO OP

P s

()

⋅

P s

() Z

L s

()

F s

()

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

()

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





⋅

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

–

DR O OP

Rsense

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

⋅

PWM

---

∆V

OS C

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

⋅

LOO P s

()

---

∆V

OS C

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

F s

()

P s

() Z

L s

()

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

--------

P s

()

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





⋅⋅

⋅

–

LOO P s

()

---

∆V

OSC

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

F s

()

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

DRO OP

-------

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

1s C o

DRO OP//Ro

E SR

()

⋅⋅

---

2Ro

⋅

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

Co ESR

-------

⋅

⋅⋅

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

⋅⋅

⋅

–

LOO P s

()

---

∆V

OSC

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

F s

()

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

1s Co

DRO OP

ESR

()

⋅⋅

---

2Ro

⋅

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

C o ESR

-------

⋅

⋅⋅

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

⋅⋅

⋅

–

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