U643B-MY View Datasheet(PDF) - Atmel Corporation

Part Name

Description

Manufacturer

U643B-MY

Flasher/ 30-mohm Shunt

Atmel Corporation 

U643B

4.6 Pin 7, Lamp Outage Detection

The lamp current is monitored via an external shunt resistor R3 and an internal comparator K1

with its reference voltage of typically 81 mV (VS = 12V). The outage of one lamp is detected

according to the following calculation:

Nominal current of 1 lamp: 21W / (VS = 12V): Ilamp = 1.75A

Nominal current of 2 lamps: 2 × 21W / (VS = 12V): Ilamp = 3.5A.

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

Ioutage ≈ 2.7A

Thus, the shunt resistor is calculated as:

R3 = VT (K1) / Ioutage

R3 = 81 mV/2.7A = 30 mΩ

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 kΩ, 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.

5

4727B–AUTO–09/05

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