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Figure 7: Typical lamp voltage at startup;
f Startup = f P reheat
Figure 8: Improved lamp voltage at startup;
f Startup > f P reheat
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
10
Figure 9: Upper trace: voltage on capacitor
CRAMP during Ignition Ramp mode
Lower trace: Lamp voltage during Ignition
Ramp mode.
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