5

4727B–AUTO–09/05

U643B

4.6

Pin 7, Lamp Outage Detection

The lamp current is monitored via an external shunt resistor R

with its reference voltage of typically 81 mV (V

according to the following calculation:

Nominal current of 1 lamp: 21W / (V

Nominal current of 2 lamps: 2

× 21W / (V

The detection threshold should be set in the middle of the current range:

I

2.7A

Thus, the shunt resistor is calculated as:

R

R

Comparator K1's reference voltage is matched to the characteristics of filament lamps (see Con-

trol Signal Threshold in “Electrical Characteristics” on page 6).

The combination of shunt resistor and resistance of wire harness prevents pin 7 from a too high

voltage in the case of shorted lamps.

4.7

Pin 8, Start Input

Start condition for flashing: the voltage at pin 8 has to be below K3 threshold (flasher switch

closed).

Humidity and dirt may decrease the resistance between 49a and GND. If this leakage resistance

is > 5 k

Ω, the IC still remains in the OFF condition. In this case the voltage at pin 8 is between

the thresholds of comparators K2 and K3.

During the bright phase the voltage at pin 8 is above the K2 threshold, during the dark phase it is

below the K3 threshold. For proper start conditions a minimum lamp wattage of 1W is required.