L6751B datasheet(47/58 Pages) STMICROELECTRONICS

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L6751B

System control loop compensation

Doc ID 024028 Rev 1

47/58

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

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

integrator minimizes the static error while placing the zero

ωF in correspondence with the L-

C resonance 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 21

Equation 22

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 ωF to low frequencies, increasing as a consequence the

system phase margin.

Having the fastest compensation network does not guarantee 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 being

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

ZF(s)

GLOOP(s)

wLC = wF

wESR

RF[dB]

ZF(s)

GLOOP(s)

wLC = wF

wESR

RF[dB]

AM14847v1

ΔV

OSC

⋅

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

------

⋅

ESR

()

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

⋅⋅

⋅

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

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