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Fig 17. Lamp Voltage and Current – Correct Ignition

Protection Circuits

The SD pin of the IRS2158D is used for lamp removal protection. If there is no lamp present the

voltage at SD pin will be pulled above the 5 V threshold via RPU1 and RPU2 and RSD charging CSD1 and

CSD2 through DSD1 and DSD2. When a lamp is in circuit the voltage at the junction of RPU2 and RSD

will be held low via DS1 and RLS. In this way when a lamp is removed the ballast shuts down and the

lamp is replaced with a good one the ballast starts up again. In the IRPLDIM3 design the pull up of the SD

pin for lamp removal detection is connected to the DC bus instead of VCC as in some other designs. This is

because an additional 12 V zener diode DSD2 has been added, so that any voltage at the lower cathode

produced during preheat, which can contain some DC offset will never be sufficient to falsely trip the lamp

removal shut down circuit. In addition to this pulling up through resistors to the DC bus provides a more

rapid response without draining current from VCC and maintains the SD pin voltage firmly above the 5 V

threshold when no lamp is present under all conditions of line input voltage.

During preheat and ignition modes the end of life protection function, which monitors the lamp

voltage through the SD pin is not enabled, therefore the ballast will operate as long as the SD pin remains

below 5 V. The SD pin is normally biased at 2 V from an internal voltage source within the IRS2158D.