L6566B View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566B' PDF : 51 Pages View PDF
Application information
L6566B
setpoint. This is illustrated in the diagram on the left-hand side of Figure 17 on page 29: it
shows the relationship between the voltage on the pin VFF and Vcsx (with the error amplifier
saturated high in the attempt of keeping output voltage regulation):
Equation 5
Vcsx
= 1− VVFF
3
= 1− k Vin
3
Figure 16. Typical power capability change vs input voltage in QR flyback
converters
2.5
k=0
2
system not
compensated
k
1.5
Note:
1
system optimally
compensated
k = kopt
0.5
1
1.5
2
2.5
3
3.5
4
Vin
Vinmin
If the voltage on the pin exceeds 3 V switching ceases but the soft-start capacitor is not
discharged. The schematic in Figure 17 on page 29 shows also how the function is included
in the control loop.
With a proper selection of the external divider R1-R2, i.e. of the ratio k = R2 / (R1+R2), it is
possible to achieve the optimum compensation described by the lower curve in the diagram
of Figure 16.
The optimum value of k, kopt, which minimizes the power capability variation over the input
voltage range, is the one that provides equal power capability at the extremes of the range.
The exact calculation is complex, and non-idealities shift the real-world optimum value from
the theoretical one. It is therefore more practical to provide a first cut value, simple to be
calculated, and then to fine tune experimentally.
Assuming that the system operates exactly at the boundary between DCM and CCM, and
neglecting propagation delays, the following expression for kopt can be found:
Equation 6
( ) kopt = 3 ⋅ Vinmin ⋅ Vinmax +
VR
Vinmin + Vinmax
⋅ VR
28/51
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