UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
Since a fluorescent lamp is a pressurized gas filled tube less pronounced as the lamp is over-driven as shown in
(usually Argon and Mercury vapor), it shouldn’t be sur- Fig. 15. The expected life of the lamp will also degrade,
prising that temperature plays a major role in the lamp as illustrated in Fig. 16, when the lamp is operated above
characteristics. Fig. 13 depicts the variations in striking rated current.
and operating voltage for a 150 x 3mm lamp over tem-
perature, illustrating the importance of taking tempera-
ture effects into account when designing the converter.
Cold Cathode Fluorescent Lamp Efficiency
Trade-Offs
Although CCFLs offer high output light efficiency com-
pared to other lamp types such as incandescent, only a
percentage of the input energy is converted to light. As il-
lustrated in Fig. 17, 35% of the energy is lost in the elec-
trodes, 26% as conducted heat along the tube. A portion
of the Ultra Violet energy gets converted into visible light
by the lamp phosphor, where the remainder is converted
into radiated heat. Finally, Mercury atoms convert 3% of
the initial energy into visible light. The result is typically
15% overall electrical to optical energy conversion in the
lamp.
The lumen output of the backlight system is temperature
dependent as well, and may need to be accounted for in
applications requiring tight lumens regulation over a wide
temperature range. Fig. 14 shows the temperature ef-
fects on lumens for the lamp operated at 5mA.
Since lamp current is roughly proportional to luminosity, it
may be tempting to operate the lamp at a RMS current
higher than specified in the manufacturer’s data sheet.
While the lamp will continue to operate tens of percent
above the rated current, the luminosity gain becomes
Striking Voltage
5mA
140
120
100
80
1200
1000
800
600
400
200
0
60
40
20
0
0
20
40
60
0
20
40
60
AMBIENT TEMPERATURE (°C)
80
AMBIENT TEMPERATURE (°C)
Figure 13. Temperature effects on voltage.
Figure 14. Temperature effects on lumens.
120
100
80
60
40
20
0
100
5mA
RATED
LAMP
10
1
50
75
100
125
150
175
200
0
2
4
6
LAMP CURRENT (mA)
8
10
% RATED LAMP CURRENT
Figure 15. Lumens output versus current.
Figure 16. Lamp life versus current.
16
TI [ TEXAS INSTRUMENTS ]
