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Figure 6, Normal Run mode, Upper trace: voltage

across RCS, Middle trace: IC2 LO pin voltage,

Lower trace: IC2 HO-VS pin voltage

Figure 7, 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 over-current protection is disabled. However, at the end of Preheat

mode (the beginning of the Ignition mode) the hard-switching and over-current detection are en-

abled. If at any time thereafter the voltage magnitude across resistor RCS rises above the over-

current threshold (1.6V) 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 8 shows the voltage across resistor RCS and the voltage appear-

ing across the lamp when the ballast detects a failure to ignite the lamp and goes into Fault mode.

Figure 9 shows the voltage appearing across the lamp during the tail end of the Preheat mode and

the Ignition 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 10

shows this hard-switching condition. Figure 11 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 re-

placement is performed.