Datasheet
Application information L6563S
26/43 Doc ID 16116 Rev 4
The twice-mains-frequency (2•f
L
) ripple appearing across C
FF
is triangular with a peak-to-
peak amplitude that, with good approximation, is given by:
where f
L
is the line frequency. The amount of 3rd harmonic distortion introduced by this
ripple, related to the amplitude of its 2
•f
L
component, will be:
Figure 37 shows a diagram that helps choose the time constant R
FF
·C
FF
based on the
amount of maximum desired 3rd harmonic distortion. Note that there is a minimum value for
the time constant R
FF
• C
FF
below which improper activation of the VFF fast discharge may
occur. In fact, the twice-mains-frequency ripple across CFF under steady state conditions
must be lower than the minimum line drop detection threshold (V
VFF_min
= 40 mV).
Therefore:
Always connect R
FF
and C
FF
to the pin, the IC will not work properly if the pin is either left
floating or connected directly to ground.
Figure 37. R
FF
·C
FF
as a function of 3rd harmonic distortion introduced in the input
current
FFFFL
MULTpk
FF
CRf41
V2
V
+
=Δ
FFFFL
3
CRf2
100
%D
π
=
min_L
min_VFF
max_MULTpk
FFFF
f4
1
V
V
2
CR
−
Δ
>⋅
D %
3
0.1110
0.01
0.1
1
10
f = 50 Hz
L
f = 60 Hz
L
R · C [s]
FFFF