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ST7LITE0xxx MICROCONTROLLED BALLAST

is a simple software trick using a variable which counts the number of these interrupts. If you

want an event to occur after a certain time (e.g. switch from preheating to ignition mode after

one second), you should watch this global variable and when it reaches the desired value a

proper procedure is run. The time from 1ms up to the dozen of minutes can be measured this

way (with the step of 1ms).

To connect the analog world to the digital core there is an analog to digital converter (ADC) im-

plemented in the ST7LITE0xxx. This ADC has two input ranges, the first measures the analog

voltage from 0 to 5V in order to obtain a digital value ranging from 0 to 255 (8-bit resolution).

The second turns on an integrated amplifier with a gain of eight, which means that it can

measure the voltage in the range from 0 to 250mV. This integrated amplifier is very useful, es-

pecially when measuring the small voltage drop on the current sense resistors.

Lamp current and voltage must be measured to have complete information about the ballast

circuitry. In Figure 4 and Figure 5 are the circuits used to filter the voltage on the current sense

resistor. The first filter is used to obtain the peak current value and the second one to get an

average current.

Two circuits are used for voltage measurement, similar to the current measurement. The first,

used to measure a peak value, is depicted in Figure 6. It is a simple voltage divider with output

in the range from 0 to 5V. To avoid an error caused by the voltage drop on the diode D3, the

divider has been split into two parts. The second measurement circuit, used to obtain the

voltage DC offset on the lamp, is shown in Figure 7. Because the offset can be either positive

or negative the circuit has been adapted to raise the zero point to 2.5V. This means that a

measured voltage smaller than 2.5V results in a negative offset and voltage higher than 2.5V

gives a positive offset.

Figure 4. Peak Current Sensor

Figure 5. DC Current Sensor

Sense Resistor

Analog Input

D4

BAT46

D4

BAT46J

R38

12k

R38

12k

R37

8k2

R37

8k2

C12

150n

C12

150n

Analog Input

Sense Resistor

R33

10k

R33

10k

C19

C19

470nF