3.6
Lamp Outage Detection (Pin 7)
The lamp current is monitored via an external shunt resistor R
3
and an internal comparator K1
with its reference voltage of typically 81 mV (V
S
= 12V). The outage of one lamp out of two
lamps is detected according to the following calculation:
Nominal current of 1 lamp:
Nominal current of 2 lamps:
21W/(VS = 12V):
2
×
21W/(V
S
= 12V):
I
lamp
= 1.75A
I
lamp
= 3.5A
We recommend setting the detection threshold in the middle of the current range:
I
outage
≈
2.7A
Thus, the shunt resistor is calculated as:
R
3
= V
T
(K1)/I
outage
R
3
= 81 mV/2.7 A = 30 mΩ
Comparator K1’s reference voltage is matched to the characteristics of filament lamps (see sec-
tion “Control Signal Threshold”).
The combination of the shunt resistor and the resistance of the wire harness prevents pin 7 from
a too high voltage in the case of shorted lamps.
3.7
Start Input (Pin 8)
Start condition for flashing:
The voltage at pin 8 has to be less than V
S
– 5V (flasher switch closed).
Humidity and dirt may decrease the resistance between 49a and GND. If this leakage resistance
is > 5 kΩ the IC is still kept in its off-condition. In this case the voltage at pin 8 is greater than
V
S
– 5V.
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 10W is required.
4
U2043B
4720B–AUTO–09/05
ATMEL [ ATMEL CORPORATION ]
