L6566ATR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566ATR' PDF : 51 Pages View PDF
L6566A
Application information
be 50% higher than at minimum line, as shown by the upper curve in the diagram of
Figure 16. The L6566A has the line feedforward function available to solve this issue.
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
It acts on the overcurrent setpoint Vcsx, so that it is a function of the converter’s input voltage
Vin (output of the PFC pre-regulator) sensed through a dedicated pin (15, VFF): the higher
the input voltage, the lower the setpoint. This is illustrated in the diagram on the left-hand
side of Figure 17: it shows the relationship between the voltage on the VFF and Vcsx pin
(with the error amplifier saturated high in an attempt to maintain the output voltage
regulation):
Equation 5
Vcsx
= 1− VVFF
3
= 1− k Vin
3
If the voltage on the pin exceeds 3 V, switching ceases but the soft-start capacitor is not
discharged. The schematic in Figure 17 also shows 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, easily
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:
Doc ID 13794 Rev 4
27/52
Share Link: 
