AN3040

Experimental results

Doc ID 16181 Rev 2

17/27

4.2

Protections

With old lamps, abnormal behavior may occur during run mode as a result of the rectifying

effect.

This effect relates 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. Figure 8 shows the behavior of a dual lamp ballast during a rectifying

effect. In the EOL pin, as soon as the internal window comparator is triggered by a voltage

again triggered, the L6585DE stops.

Figure 8.

Run mode, rectifying effect

When an old lamp is connected to the ballast, the strike voltage is higher than the nominal

voltage and may also be higher than the safety threshold. In this case, the lamp can take

longer than usual to ignite or may not ignite at all. In both cases, because of the frequency

drop, the voltage at the output of the ballast can easily reach dangerous values during this

ignition time.

The same problem occurs if any one of the lamp’s four tubes is broken: the lamp cannot

ignite and the lamp voltage must be limited. Figure 9 and Figure 10 show how the four-lamp

ballast ignites when two lamps are broken.

if one of the four lamps is not ignited, the IC is latched.