L6714 datasheet(59/70 Pages) STMICROELECTRONICS

59 / 70 page

L6714

System control loop compensation

59/70

Compensation network can be simply designed placing

ωF=ωLC and imposing the cross-over

frequency

ωT as desired obtaining (always considering that ωT might be not higher than

1/10th of the switching frequency FSW):

21.1

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 24):

●

Increase RF to increase the system bandwidth accordingly;

●

Decrease RF to decrease the system bandwidth accordingly;

●

Increase CF to move ωF 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 (dMAX) 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 VOUT (while it is charged by dMAXVIN-VOUT during a load

appliance).

Referring to Figure 24-left, further tuning the Compensation network cannot give any

improvements unless the output filter changes: only modifying the main inductors ot the

output capacitance improves the system response.

Figure 24. RF-CF impact on bandwidth.

∆V

OSC

⋅

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

---

DROOP

ESR

()

⋅

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

⋅⋅

⋅

----

⋅

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

ZF(s)

GLOOP(s)

LC = ωF

ESR

RF[dB]

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