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RD-0613

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large value, low-wattage startup resistor (RSUPPLY). When the voltage on the IRS2168D reaches the

rising under-voltage lockout threshold (12.5 V), the gate driver oscillator is enabled (this assumes that

there are no fault conditions) and drives the half-bridge output MOSFETs (MHS and MLS). When the

half-bridge is oscillating, capacitor CSNUB, diodes DCP1 and DCP2 form a snubber/charge pump

circuit which limits the rise and fall time at the half-bridge output and also supplies the current to charge

capacitor CVCC2 to the VCC clamp voltage (approx. 15.6 V). When the rising under-voltage lockout

threshold of the IRS2168D is reached, the power factor control output also starts to oscillate and drives

MOSFET MPFC to boost and regulate the bus voltage to 500 V DC.

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 IRS2168D has begun to operate and the half-bridge output is driving

the resonant load (lamp) circuit.

There is an initial startup frequency that is higher than the preheat frequency that lasts for only a short

duration. This is done to ensure that the initial voltage appearing across the lamp at the startup of

oscillation does not exceed the minimum lamp ignition voltage. If, at the initiation of oscillation of the

half-bridge, the voltage across the lamp is large enough, a visible undesired flash of the lamp can occur.

This in effect is a cold strike of the lamp and can shorten the life of the lamp.

The ballast control section oscillator of the IRS2168D consists of an internal timing capacitor and an

external timing resistor (RFMIN). Resistors RFMIN and RPH program a current that determines the

ramp up time of the internal timing capacitor. The preheat frequency is determined by the equivalent

resistance formed by the parallel combination of RFMIN and RPH. The preheat frequency 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 their correct emission temperature within the

Preheat Mode time period. The preheating of the lamp filaments is performed using voltage mode

heating that consists of a constant voltage across the lamp filaments. The waveform in Figure 7 shows

the CPH voltage and lamp voltage during normal Preheat, Ignition, and Run Modes. Figure 8 shows the

half-bridge voltage (VS pin) during Preheat Mode.