L6701 datasheet(37/44 Pages) STMICROELECTRONICS

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L6701

15 System Control Loop Compensation

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15.1

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 19):

●

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 19-left, further tuning the Compensation network cannot give any

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

capacitance improves the system response.

Figure 19. Best Load Transient achievable (d=0) and RF-CF impact on Bandwidth.

ZF(s)

GLOOP(s)

LC = ωF

ESR

RF[dB]

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