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6.2 Under-Voltage Fault Counter

The IC also includes an under-voltage fault counter at the VSENSE pin. Once the lamp

has ignited, the lamp voltage will decrease sharply to a very low voltage (20V typical).

As the lamp warms up, the lamp voltage will slowly increase until the nominal running

voltage is reached (100V typical). If the lamp voltage remains too low for too long, then

this is a lamp fault condition and the ballast must shutdown. To detect this, the VSENSE

pin includes an under-voltage threshold of VOV(1/7.5). If the voltage at the VSENSE pin

remains below VOV(1/7.5) and the under-voltage fault counter times out (295sec typical,

with CTCLK=0.18uF), then the lamp is not warming up properly due to a lamp fault

condition (end of life, etc.) and the IC will enter fault mode and shutdown. If the voltage

at the VSENSE pin increases above VOV(1/7.5) before the under-voltage counter times

out, then the lamp has successfully warmed up and the IC will remain in general mode.

Figure 6.2 shows some waveforms when the ballast goes into Fault Mode due to under-

voltage fault.

Figure 6.2: Under-voltage fault: CH1 is TCLK pin voltage, CH2 is VSENSE voltage, CH3 is LO voltage

and CH4 is VCC voltage

6.3 Fast Transient Under-Voltage Fault Counter

During normal running conditions, fast transient under-voltage spikes can occur on the

lamp voltage due to instabilities in the lamp arc.

The resulting transients on the

VSENSE pin will cycle below and above the VOV(1/7.5) threshold quickly (<50us). If the

number of events of these transients exceeds the maximum number of events of the

fault counter (16,384 events typical), then the IC will enter fault mode and shutdown.