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IRPLLNR3

I

Preheat Mode

When the ballast reaches the end of the UVLO mode, the Preheat mode is entered. At this point the

ballast control oscillator of the IR2167 has begun to operate and the half-bridge output is driving the

resonant load (lamp) circuit. The ballast control section oscillator of the IR2167 is similar to oscillators

found in many popular PWM voltage regulator ICs and consists of a timing capacitor and resistor

connected to ground. Resistors RT and RPH program a current which determines the ramp up time

of capacitor CT and resistor RDT determines the ramp down time. The downward ramping time of CT

is the dead time between the switching off of the LO (HO) and the switching on of the HO (LO) pins on

the IR2167. The Preheat mode frequency of oscillation is selected such that the voltage appearing

across the lamp is below the minimum lamp ignition voltage while supplying enough current to preheat

the lamp filaments to the correct emission temperature within the Preheat mode period. The preheat-

ing of the lamp filaments is performed with a constant voltage during the Preheat mode. The wave-

form in Figure 2 shows the lamp filament current while Figure 3 shows lamp filament voltage during

the normal Startup, Preheat and Ignition Ramp modes of the ballast.

Figure 1:

Top trace: Half-bridge output voltage

Middle trace: Bus Voltage

Bottom trace: VCC of the IR2167