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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