L6717ATR datasheet(51/57 Pages) STMICROELECTRONICS

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L6717A

System control loop compensation

Figure 18. Control loop bode diagram and fine tuning (not in scale)

To obtain the desired shape an R

-C

series network is considered for the Z

(s)

implementation. A zero at

=1/R

is then introduced together with an integrator. This

integrator minimizes the static error while placing the zero

in correspondence with the L-

C resonance assures a simple -20dB/Dec. shape of the gain.

In fact, considering the usual value for the output filter, the LC resonance results to be at

frequency lower than the above reported zero.

Compensation network can be simply designed placing

and imposing the cross-

over frequency

as desired obtaining (always considering that

might be not higher than

1/10th of the switching frequency F

11.1

Compensation network guidelines

The compensation network design assures to having system response according to the

cross-over frequency selected and to the output filter considered: it is anyway possible to

further fine-tune the compensation network modifying the bandwidth in order to get the best

response of the system as follow (See

Figure 18):

–

Increase R

to increase the system bandwidth accordingly;

–

Decrease R

to decrease the system bandwidth accordingly;

–

Increase C

to move

to low frequencies increasing as a consequence the

system phase margin.

Having the fastest compensation network gives not the confidence to satisfy the

requirements of the load: the inductor still limits the maximum dI/dt that the system can

afford. In fact, when a load transient is applied, the best that the controller can do is to

“saturate” the duty cycle to its maximum (d

MAX

) or minimum (0) value. The output voltage

dV/dt is then limited by the inductor charge / discharge time and by the output capacitance.

In particular, the most limiting transition corresponds to the load removal since the inductor

results being discharged only by V

OUT

(while it is charged by d

MAX

-V

OUT

during a load

appliance).

(s)

LOOP

(s)

ESR

[dB]

(s)

LOOP

(s)

ESR

[dB]

ΔV

OS C

⋅

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

---

ES R

()

⋅

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

⋅⋅

⋅

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

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