U643B
Flasher, 30-m Shunt
Description
The bipolar integrated circuit U643B is used in relaycontrolled automotive flashers where a high-level EMC is required.
Lamp outage is indicated by frequency doubling during hazard warning as well as direction mode.
Features
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Temperature and supply voltage compensated |
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Minimum lamp load for flasher operation: ≥ 1 W |
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flashing frequency |
Very low susceptibility to EMI |
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Frequency doubling indicates lamp outage |
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Protection according to ISO/TR 7637/1 level 4 |
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Relay driver output with high current carrying capacity and low saturation voltage
Block Diagram
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+ VS |
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+ 49 |
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6 |
2 |
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C1 |
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Measuring comparator K 1 |
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R3 |
30 m |
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Shunt |
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4 |
V6±81 mV |
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20 k |
5 k |
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K1 |
7 |
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K2 |
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R1 |
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Pulse |
3 |
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generator |
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49 a±Comparator K2 |
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Relay |
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20 k |
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R2 |
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49 a±Comparator K |
8 |
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3 |
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49 a |
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5 |
K3 |
23 V |
1.5...2.2 k |
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20 k |
5 k |
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IC Ground |
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1 |
R4 |
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95 9756 |
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470 |
5 W |
21 W |
21 W |
21 W |
21 W 5 W |
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GND |
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Indicator lamps |
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± 31 |
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Figure 1. Application circuit as a car flasher, Resistor R1 and R2: 1/4 Watt
R4 for protection against continuous reversed polarity: 0.5 Watt
Ordering Information
Extended Type Number |
Package |
Remarks |
U643B |
DIP8 |
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U643B-FP |
SO8 |
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Rev. A3, 10-Apr-01 |
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1 (5) |
U643B
Pin Description
Pin |
Symbol |
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Function |
1 |
GND |
IC ground |
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2 |
VS |
Supply voltage VS |
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3 |
REL |
Relay driver |
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4 |
OSC |
C1 |
Oscillator |
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OSC |
R1 |
Oscillator |
5 |
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6 |
VS |
Supply voltage VS |
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7 |
LD |
Lamp failure detection |
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8 |
SI |
Start input (49a) |
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Functional Description
Pin 1, GND
The integrated circuit is protected against damage via resistor R4 to ground (±31) in the case of battery reversal. An integrated protection circuit together with external resistances R2 and R4 limits the current pulses in the IC.
Pin 2, Supply voltage, VS - Power
The arrangement of the supply connections to Pin 2 must be so as to ensure that, on the connection printed circuit board (PCB), the resistance of VS to Pin 6 is lower than that to Pin 2.
Pin 3, Relay control output (driver)
The relay control output is a high-side driver with a low saturation voltage and capable to drive a typical automotive relay with a minimum coil resistance of 60 .
Pin 4 and 5 Oscillator
Flashing frequency, f1, is determined by the R1C1 components as follows (see figure 1):
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1 |
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f1 |
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Hz |
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R1 C1 1.5 |
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C1 |
47 F |
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R1 |
6.8 k to 510 k |
In the case of a lamp outage (see Pin 7) the oscillator frequency is switched to the lamp outage frequency f2 with f2 2.2 f1.
Duty cycle in normal flashing mode: 50%
Duty cycle in lamp outage mode: 40% (bright phase)
Pin 6, Supply voltage, Sense
For accurate monitoring via the shunt resistor, a mini-
GND |
1 |
8 |
SI |
VS |
2 |
7 |
LD |
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U643B |
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REL |
3 |
6 |
Vs |
OSC |
4 |
5 |
OSC |
94 9290
Figure 2. Pinning
mized layer resistance from point VS / shunt to Pin 6 is recommended.
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 typ. 81 mV (VS = 12 V). The outage of one lamp out of two lamps is detected according to the following calculation:
Nominal current of 1 lamp: 21 W / (VS = 12 V):
Ilamp = 1.75 A
Nominal current of 2 lamps: 2 x 21 W / (VS = 12 V):
Ilamp = 3.5 A.
The detection threshold is recommended to be set in the
middle of the current range: Ioutage 2.7 A Thus the shunt resistor is calculated as:
R3 = VT (K1) / Ioutage
R3 = 81 mV/2.7 A = 30 m .
Comparator K1`s reference voltage is matched to the characteristics of filament lamps (see ªcontrol signal thresholdº in the data part).
The combination of shunt resistor and resistance of wire harness prevents Pin 7 from a too high voltage in the case of shorted lamps.
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 49 a and GND. If this leakage resistance is 5 k the IC is still kept in its 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 1 W is required.
2 (5) |
Rev. A3, 10-Apr-01 |