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RD0802

Figure 4.5: Lamp Voltage and Arc Current during preheat and ignition

It should be noted in this example that the VCO voltage and consequently the ballast

frequency varies only by quite a small amount between the four different dimming levels,

although the lamp arc current varies considerably. This is because the system is operating

above the point in the dimming response curve known as the "knee", this is illustrated

below.

Lamp

Current

Ballast Running

Frequency

Ballast / Lamp Operating Characteristic

Figure 4.6: Lamp current against ballast frequency

R6 is also required in order to ensure that the DIM pin of IC1 will be biased above 0V

before the voltage at VCC exceeds the UV+ threshold at startup. This prevents the

IRS2530D from shutting down during the first few cycles of hard switching that occur

when the half bridge starts to oscillate at maximum frequency.