L6718 datasheet(63/71 Pages) STMICROELECTRONICS

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L6718

System control loop compensation

poles; both poles are fixed once the output filter is designed (LC filter resonance

ω LC) and

the zero (

ω ESR) is fixed by ESR and the droop resistance.

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

ω F=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 a

frequency lower than the above reported zero.

The compensation network can be designed as follows:

Equation 15

Equation 16

12.1

Compensation network guidelines

The compensation network design assures a system that responds 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 follows (see Figure 15):

–

Increase RF to increase the system bandwidth accordingly.

–

Decrease RF to decrease the system bandwidth accordingly.

–

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

system phase margin.

Even with fast compensation network design the load requirement can be limited by the

inductor value because it 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

ZF(s)

GLOOP(s)

wLC = wF

wESR

RF[dB]

ZF(s)

GLOOP(s)

wLC = wF

wESR

RF[dB]

AM12890v1

ΔV

OSC

⋅

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

------

⋅

ESR

()

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

⋅⋅

⋅

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

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