14

RD-0613

www.irf.com

f

osc

t

Preheat

Ignition

Run

f

Preheat

f

Run

Start

f

Fig. 9: Oscillator Frequency versus Time During Normal Operating Conditions

Figure 9 shows a plot of the half-bridge oscillation frequency as a function of time for normal Preheat,

Ignition and Run ballast operating modes. The duration of the Preheat Mode, as well as all ballast

operating modes, are determined by the voltage on the CPH pin of the IRS2168D. At the end of UVLO

Mode, Preheat Mode is entered and the external capacitor at the CPH pin of the IRS2168D begins to

charge through the external resistor (RCPH) from CPH to VCC. The ballast remains in Preheat Mode

time the ballast then enters Ignition Mode.

Ignition Mode

and the frequency begins to ramp down to the run frequency at a rate determined by the time constant

RPH(CVCO). During this ramping downward of the frequency, the voltage across the lamp increases in

magnitude as the frequency approaches the resonant frequency of the LC load circuit until the lamp

ignition voltage is exceeded and the lamp ignites. The maximum ignition voltage that can be generated

is determined from the value of RCS, and the ignition frequency must be higher than the run frequency

so that the frequency sweeps through the resonance frequency to ensure lamp ignition. If the lamp does

not ignite, then the ignition regulation feature of the IRS2168D will regulate the ballast output voltage

to a constant level (programmed by RCS) for the duration of Ignition Mode. Figure 10 shows the lamp

voltage during ignition ramp and ignition regulation during a lamp non-strike condition.