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RD-0613

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If at any time during Preheat Mode or Run Mode the voltage across resistor RCS rises above the over-

current threshold (1.25 V) for 65 events, the PFC and half-bridge MOSFETs, (MPFC, MHS and MLS)

are turned off and the ballast goes into Fault Mode. During the Ignition Mode, the over-current is

disabled and the ignition regulation feature limits the maximum current flowing in the resonant tank

and half-bridge. An over-current condition can occur if the lamp fails to ignite or the lamp is broken

(an open circuit cathode or broken lamp). If a cathode is broken (open circuit), the half-bridge output

hard-switches. Each time the low-side MOSFET (MLS) is turned on, a large current pulse occurs and

thus a large voltage pulse occurs across resistor RCS signaling a fault. The ballast will remain in Fault

Mode until the AC line voltage is reset or a lamp replacement is performed.

During an end-of-life lamp fault condition, the lamp voltage can increase or decrease asymmetrically.

The resulting excessive voltage across the lamp filaments can cause the lamp ends to reach temperatures

high enough to melt the tube glass. The lamp can then fall out of the fixture and cause harm or damage.

To protect against this condition, resistors REOL1, REOL2, REOL3, REOL4, and zener diodes DEOL1

and DEOL2, are used for end-of-life protection. The end-of-life window comparator at the SD/EOL pin

is enabled in Run Mode. If the voltage on SD/EOL pin falls outside the range of the internal 1 V – 3 V

window comparator, the IC will enter Fault Mode. The SD/EOL pin is internally biased at 2 V with an

internal +/-10 µA OTA. The value of REOL4, DEOL1 and DEOL2 are selected such that the SD/EOL

pin remains at 2 V during normal operation, but increases above 3 V or decreases below 1 V during an

end-of-life fault condition. The lamp voltage end-of-life threshold can be adjusted by changing the value

of resistor REOL4 and/or zener diodes DEOL1 and DEOL2. A threshold of 30% higher than the

nominal running lamp voltage is typical.

PFC Control Section

The IRS2168D contains control circuitry for driving a boost-type power factor correction (PFC) circuit.

This is necessary for producing sinusoidal input current at the mains input that is “in phase” with the

mains voltage and contains minimal total harmonic distortion (THD). It is also convenient to use the

boost converter to regulate the DC bus voltage to a constant DC level. The PFC control is achieved

using five control pins. The DC bus voltage is sensed with a resistor divider at the VBUS pin. The loop

compensation speed is programmed with an external capacitor at the COMP pin. The cycle-by-cycle

zero-crossing of the boost inductor current is detected at the ZX pin. The gate drive for the boost

MOSFET is provided by the PFC pin. The cycle-by-cycle over-current protection is performed by the

OC pin. The following waveforms show the operation of the PFC at 120 VAC (Figure 13) and 290 V

AC (Figure 14) input line conditions.