10

www.irf.com

Figure 7: Typical lamp voltage at startup;

Figure 8: Improved lamp voltage at startup;

The duration of the Preheat mode as well as which mode of operation the ballast is operating in is

determined by the voltage on the CPH pin of IC2. At the completion of the UVLO mode, Preheat

mode is entered and an internal current source is activated at the CPH pin of IC2 which begins to

charge up capacitor CPH. The ballast remains in the Preheat mode until the voltage on the CPH

pin of IC2 exceeds the Ignition Ramp mode threshold (4V).

Ignition Ramp Mode

At the completion of the Preheat mode (4V < CPH

pin < 5.1V) the ballast switches to the Ignition Ramp

mode and the frequency ramps down to the ignition

frequency. The frequency ramping is accomplished

by turning off the internal open drain MOSFET on

the RPH pin of IC2 (see Figure 1, IR21571 block

diagram). Resistor RPH is no longer connected di-

rectly in parallel with resistor RT. The shift in frequency

does not occur in a step function but rather with an

exponential decay because of capacitor CRAMP in

series with resistor RPH to ground. The duration of

this frequency ramp is determined by the time con-

stant of the RC combination of capacitor CRAMP

and resistor RPH. The minimum frequency of oscil-

lation occurs at the end of this ramp and is deter-

mined by resistor RT and capacitor CT. 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

Figure 9: Upper trace: voltage on capacitor

CRAMP during Ignition Ramp mode

Lower trace: Lamp voltage during Ignition

Ramp mode.