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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 13 shows the voltage which appears across resis-

tor 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 13 are the gate drive signals for the low side MOSFET (LO pin) and the high side

MOSFET (HO-VS pin).

Figure 13: Normal Run mode, Upper trace: voltage

across RCS, Middle trace: IC2 LO pin voltage,

Lower trace: IC2 HO-VS pin voltage

Figure 14: Normal lamp ignition: Lamp voltage during

the end of Preheat mode, Ignition Ramp mode

and the beginning of the Run mode

During the Preheat mode the voltage across resistor RCS is not measured. However, at the end of

Preheat mode (the beginning of the Ignition Ramp mode) the hard-switching and over-current detec-

tion are enabled. If at any time thereafter the voltage magnitude across resistor RCS rises above the

over-current (CS+) threshold of the CS pin of IC2, a lamp fault condition is signaled and the half-

bridge output MOSFETs’, (M2 and M3) are turned off and the ballast goes into Fault mode. This can

happen if the lamp fails to ignite or if the upper filament is open. For failure to ignite the lamp, the

current in the half-bridge increases and thus the voltage across resistor RCS increases above the

over-current threshold signaling a fault. Figure 15 shows the voltage across resistor RCS and the

voltage appearing across the lamp when the ballast detects a failure to ignite the lamp and goes into

Fault mode. The CS+ threshold is determined by resistor ROC. An internal current source of 50uA is

connected to the OC pin of IC2 which when applied to resistor ROC sets a voltage at the OC pin.

This voltage is the CS+ threshold of IC2. Figure 16 shows the voltage appearing across the lamp

during the tail end of the Preheat mode and the Ignition Ramp mode for a failure of the lamp to ignite

condition. If the upper filament is open, the half-bridge output hard-switches and each time the low

side MOSFET (M3) is turned on a large current pulse occurs and thus a large voltage pulse occurs

across resistor RCS signaling a fault, Figure 17 shows this hard-switching condition. Figure 18 shows

the lamp voltage during the Preheat mode and beginning of Ignition Ramp mode for this hard-

switching condition when the lamp fault condition is detected. The ballast will remain in Fault mode

until either the line voltage is cycled or a lamp replacement is performed.