L6751C datasheet(46/57 Pages) STMICROELECTRONICS

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System control loop compensation

L6751C

46/57

DocID025037 Rev 1

Figure 16. Control loop bode diagram and fine tuning

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

implementation. A zero at

F = 1/RFCF is then introduced together with an integrator. This

integrator minimizes the static error while placing the zero

F in correspondence with the L-

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

In fact, considering the usual value for the output filter, the LC resonance results as being at

a frequency lower than the above reported zero.

The compensation network can be designed as follows:

Equation 22

Equation 23

11.1

Compensation network guidelines

The compensation network design assures a system response according to the crossover

frequency selected and to the output filter considered: it is however possible to further fine

tune the compensation network by modifying the bandwidth in order to get the best

response of the system as follows (see Figure 16):

–

Increase RF to increase the system bandwidth accordingly.

–

Decrease RF to decrease the system bandwidth accordingly.

–

Increase CF to move F to low frequencies, increasing as a consequence the

system phase margin.

Having the fastest compensation network does not guarantee that the load requirements

are satisfied: 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 as being

discharged only by VOUT (while it is charged by VIN - VOUT during a load appliance).

ZF(s)

GLOOP(s)

LC = ω F

ESR

RF[dB]

ZF(s)

GLOOP(s)

LC = ω F

ESR

RF[dB]

AM14822v1

RFB VOSC



VIN

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

------

FSW L



RLL ESR



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



CO L



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

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