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Fig 19. Brownout shutdown and restart

Trace

Color

Signal

2

Red

VDC Pin Voltage

3

Blue

Lamp Arc Current

4

Green

Lamp Voltage

The IRPLDIM3 ballast also includes end of life protection, which is enabled only after successful

ignition when the ballast is in run mode. The lamp voltage is divided down through REOL1, REOL2,

REOL3 and REOL4. This divided voltage is fed to two back to back zener diodes, DEOL1 (7.5 V) and

DEOL2 (5.1 V). The SD pin is internally biased at 2 V with a window comparator that has 1 V and 3 V

thresholds. If the positive peak of the lamp voltage becomes large enough to allow DEOL1 to reverse

breakdown then the SD pin voltage will rise above 3 V. A delay is incorporated by clocking the oscillator

pulses with the fault counter so that after a number of cycles the IRS2158D will shut down. The delay is

normally around 1ms and was added to provide some immunity to transients and false shutting down of the

end of life circuit. In the same way if the negative peak of the lamp voltage drops low enough for DEOL2

to reverse breakdown then the SD pin will fall below 1 V. After the same delay period the IRS2158D will

also shut down. This end of life circuit will therefore cause the ballast to shut down if an aging lamp is

connected if it produces an asymmetric voltage in either direction, due to the rectifying effect.

End of lamp life shutdown is often a mandatory requirement for electronic ballasts.

In the event of a ballast output becoming open circuit during running, for example if the lamp is

broken or cracked but the filament is still intact, the SD pin will not detect a fault. However the ballast will

still shut down because the CS pin will detect the transients caused by hard switching of the half bridge

MOSFETs. In this case the fault counter will be active so that approximately 60 consecutive transients need

to be detected by the CS pin before shut down. The broken lamp test is easily simulated by disconnecting

the high voltage end of the lamp during running, which is the end that is not connected to the SD pin lamp

out detection circuit.