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Fig 8. Ideal Ballast Operating Points

The operating points here display an ideal condition where the ignition frequency is above the run

frequency and the preheat frequency is somewhat higher than the ignition frequency. The run frequency

also meets the target of 40 kHz. In general it is preferable to keep the run frequency (or minimum

frequency) at 40 kHz or above to avoid possible interference with infra red remote controls, some of which

operate at around 35 kHz.

In order to use a more practical value, it was decided to fix LRES at 4 mH for this design, which is

more easily available. In the BDA “Tank Components” section the “Fix L” box should be checked and the

value of 4 mH entered in place of 5.9 mH. After the “Calculate” button has been clicked a message will

appear stating that the ballast run frequency is now higher than the ignition frequency. Although many

other ballast control IC’s are not capable of operating under this set of conditions, the IRS2158D may be

used without problems.

The graph for the operating points now looks like this: