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ST7LITE0xxx MICROCONTROLLED BALLAST

3 ABNORMAL CONDITION HANDLING

3.1 LAMP REMOVAL

This is the most usual error condition during the ballast life. It is relatively common that a lamp

is changed without turning off the mains switch. So the detection of a lamp removal and con-

sequent insertion must be implemented. In Figure 11 a detection circuit is shown. When a

lamp is present in the application the voltage on the processor input pin is zero (logical 0),

while if the lamp is removed the voltage on the input pin rise up to 5V (logical 1). A simple bit-

check is enough to detect the lamp presence and then switch the micro to Stop mode when-

ever it detects the lamp removal.

Figure 11. Detection Circuit for Lamp Removal/Insertion

3.2 LAMP FAILS TO START

This condition can occur only during the ignition state, when the voltage rises without any sign

of lamp ignition. That can be caused for example by filament damage or an old lamp. There

are three security protections implemented. None of them, however, adds a single component

into the application because all of them use the existing possibilities and opportunities given

by ST7LITE0xxx. During the ignition phase, the software checks whether the current or the

voltage exceeds the preset value. If so, it will immediately stop the driver and switch into the

stop state. In addition to these, two protections rather than one were implemented to prevent

the sense circuit failure. This protection allows only a limited number of frequency steps in the

ignition state. In the event that both current and voltage sensors fail and this protection is not

implemented, the frequency will decrease along the resonant curve and the ballast could be

damaged.

R26

10k

C11

10nF

Digital In

+5V

R25

1M

Lamp Filament