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Startup Mode
When power is initially applied to the ballast, the voltage on the VCC pin of IC2 (IR21571) begins to
charge up. The voltage for IC2 is derived from the current supplied from the rectified AC line through
startup resistor R14. During this initial startup when the VCC voltage of IC2 is below its rising under-
voltage lock-out threshold (11.4V), IC2 is in its UVLO and also its micro-power mode. The micro-
power mode of the IC2 allows the use of a large value,
low wattage startup resistor (R14). When the voltage
on IC2 reaches the rising under-voltage lockout thresh-
old, the oscillator is enabled (this assumes that there
are no fault conditions) and drives the half-bridge out-
put MOSFETs (M2 and M3). When the half-bridge is
oscillating, capacitor C16, diodes D5 and D6 form a
snubber /charge pump circuit which limits the rise and
fall time at the half-bridge output and also supplies the
current to charge capacitor C12 to the VCC clamp volt-
age (approx. 15.6V) of IC2. The voltage for IC1 is de-
rived from the current supplied from another snubber/
charge pump circuit formed by capacitor C14 and di-
odes D1 and D3. When the rising under-voltage lock-
out threshold of IC1 is reached, it starts to oscillate
and drive MOSFET M1 to boost and regulate the bus
voltage to 400 VDC. An oscillograph of the startup of
the VCC of IC1, VCC of IC2 and half-bridge output
voltage are shown in Figure 3. (For a complete de-
scription of the operation of IC1, refer to the Motorola Semiconductor MC34262 data sheet.)
Preheat Mode
When the ballast reaches the end of the UVLO mode, the Preheat mode is entered. At this point the
oscillator of IC2 has begun to operate and the half-bridge output is driving the resonant load (lamp)
circuit. The oscillator section of IC2 is similar to oscillators found in many popular PWM voltage
regulator ICs and consists of a timing capacitor and resistor connected to ground. Resistors RT and
RPH program a current which determines the ramp up time of capacitor CT and resistor RDT deter-
mines the ramp down time. The downward ramping time of CT is the deadtime between the switch-
ing off of the LO (HO) and the switching on of the HO (LO) pins on IC2. The Preheat mode frequency
of oscillation is selected such that the voltage appearing across the lamp is below the minimum lamp
ignition voltage while supplying enough current to preheat the lamp filaments to the correct emission
temperature within the Preheat mode period. The preheating of the lamp filaments is performed with
a constant current during the Preheat mode. The waveform in Figure 4 shows the lamp filament
current while Figure 5 shows lamp filament voltage during the normal Startup, Preheat, Ignition
Ramp and Run modes of the ballast.
Figure 3: Top trace: Half-bridge output voltage
Middle trace: VCC of IC2
Bottom trace: VCC of IC1
