Datasheet

Ballast section L6585DE
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6.4 Old lamp management during run mode
During run mode, an old lamp can exhibit three different abnormal behaviors:
● Rectifying effect
● Over-current
● Hard switching event
6.5 Rectifying effect
The rectifying effect is related to a differential increase of the ohmic resistance of the two
cathodes. The lamp equivalent resistance is therefore higher when the lamp current flows in
one direction than in the other. The current waveform is distorted and the mean value of the
lamp current is no longer zero. The EOL pin is the input of an internal window comparator
that can be triggered by a voltage variation due to rectifying effect.
The reference of this comparator and the amplitude of the window can be set by connecting
a suitable resistor to EOLP pin as indicated in following table:
The reference of this comparator can be set at a fixed voltage or at the same voltage as the
CTR pin.
The fixed reference configuration (see
Figure 12) can be used when the lamp is connected
to ground, and requires two Zener diodes in order to shift the mean value of the lamp
voltage to 2.5 V. The values of the two Zeners affect the symmetry of the intervention of the
protection: the best symmetry is obtained choosing two values whose difference is equal to
twice the reference voltage:
● V
UP
= V
REF
+ V
Z2
+ V
F1
+ W/2
● V
DOWN
= V
REF
– (V
Z1
+ V
F2
) – W/2
● V
UP
= - V
DOWN
●
2 V
REF
= V
Z1
− V
Z2
Where V
UP
and V
DOWN
are the maximum allowed values of V
K
The tracking configuration (see Figure 13) is useful when the lamp is connected between
choke and blocking capacitor in the block capacitor-to-ground configuration. In this
configuration the voltage across the blocking capacitor is affected by the voltage ripple
superimposed on the PFC output. Using a reference affected by the same ripple helps to
reject it and avoid premature triggering of the comparator.
As soon as the comparator is triggered, a Tch cycle starts in order to improve the noise
immunity.
Table 5. EOL window comparator configuration table
EOLP resistor range Reference Window amplitude (Wv)
22 k ÷ 27 k V
CTR
+160 mV / -150 mV
75 k ÷ 91 k 2.5 V 240 mV
220 k ÷ 270 k V
CTR
+ 250 mV / -240 mV
> 680 k 2.5 V 720 mV