7/16

ST7LITE0xxx MICROCONTROLLED BALLAST

2.2 PREHEAT STATE

Software has full control during the whole procedure in the preheat state. This means that you

select the desired preheating current and then a control algorithm regulates the current. The

algorithm used is the simplest one: the step-regulation. The software raises the frequency by

one step (125ns) every time the measured current is bigger than the desired value, and vice

versa.

In addition, it is possible, by changing only one constant, to set the preheating time from zero

(cold start) up to a few seconds.

2.3 IGNITION STATE

The purpose of this state is obvious, the software decrease the frequency (increases the

voltage) to ignite the lamp.

As an extra feature, you can adjust the ignition speed (the time between each frequency step)

in the software. To detect the moment when the lamp ignites, a simple assumption is used that

the voltage across the lamp will significantly decrease after ignition. After detecting the ignition

the software moves to the next phase.

2.4 LAMP STABILIZATION STATE

After the ignition of the lamp, a constant frequency is set. You should preselect this frequency

in the software as an expected frequency value for the lamp used. This state serves for stabi-

lization of all the lamp characteristics as well as the sensor circuits.

2.5 RUN STATE

At this final phase, the software measures the current to get a given power in the lamp. The

calculation of the active power flowing through the lamp is very simple, because the DC cur-

rent flowing through the sense resistor is the current from the supply. The supply is presumed

to be a constant 400V and so the power is linearly proportional to the DC current. Then the

measured value is compared with a preset value and consequently the software tries to cor-

rect it. Again, the same step regulation control algorithm as in the preheat phase is used.

There is no need to have high speed of this control. Since the gas inside the tube has a long

response time, it needs a few milliseconds to stabilize after each change; the speed can be

relatively slow (less than a few hertz).