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Waveforms

Figure 4 shows the voltage appearing across the lamp while Figure 5 shows the current flowing

through the lamp during Startup, Preheat, Ignition and Dim modes.

Figure 4, Lamp voltage during Startup, Preheat,

Ignition and Dim (100%)

Figure 5, Lamp current during Startup, Preheat,

Ignition and Dim (100%) (100mA/div.)

Normal Powerdown

A Normal Powerdown occurs when the AC line voltage is disconnected from the ballast. When this

occurs the voltage on the VDC pin of IC2 drops below the line fault threshold (3V) and IC2 shuts

down in a controlled fashion. The oscillator is stopped, the half-bridge driver outputs (LO and HO)

are turned off and capacitor CPH is discharged. IC2 also goes into its UVLO/micro-power mode and

the bus voltage begins to collapse.

Fault Mode

Fault mode is when the ballast driver is shutdown due to the detection of a lamp fault. Note that when

the ballast is in this Fault mode the power factor correction section of the ballast is also shutdown

and the bus voltage will drop to the non-boosted/unregulated level. There are several lamp fault

conditions which can put the ballast into the Fault mode. The lamp fault conditions detected include:

near/below resonance (under-current) detection, hard-switching detection and over-current detec-

tion. Resistor RCS in the source lead of the low side MOSFET (M3) serves as the current sensing

point for the half-bridge which is used to detect these lamp fault conditions. In operation when the

half-bridge is oscillating, a voltage appears across RCS whenever the low side MOSFET, M3, is

turned on or the high side MOSFET, M2, is turned off. The magnitude of this voltage directly relates

to the current in the lamp resonant circuit. Figure 6 shows the voltage which appears across resistor

RCS during normal Run mode conditions while Figure 14 shows the voltage appearing across the

lamp during the end of Preheat mode, Ignition Ramp mode and the beginning of Run mode. Also

shown in Figure 7 are the gate drive signals for the low side MOSFET (LO pin) and the high side

MOSFET (HO-VS pin).