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ATMEL Corporation |
Flasher, 30-mW Shunt
U643B
Description
The bipolar integrated circuit U643B is used in relay-
controlled 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
D Temperature and supply voltage compensated
flashing frequency
D Frequency doubling indicates lamp outage
D Relay driver output with high current
carrying capacity and low saturation voltage
D Minimum lamp load for flasher operation: ≥ 1 W
D Very low susceptibility to EMI
D Protection according to ISO/TR 7637/1 level 4
Block Diagram
C1
4
6
Measuring comparator K 1
V6–81 mV
K1 20 kW
2
5 kW
7
R3
Shunt
30 mW
+ VS
+ 49
K2
R1
Pulse
generator
49 a–Comparator K2
20 kW
49 a–Comparator K3
K3
5
20 kW
95 9756
3
Relay
8 R2
23 V
5 kW
IC Ground
1
1.5...2.2 kW
R4
470 W
49 a
5 W 21 W 21 W 21 W 21 W 5 W
GND
– 31
Indicator lamps
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
U643B-FP
SO8
Rev. A3, 10-Apr-01
1 (5)
U643B
Pin Description
Pin Symbol
Function
1 GND IC ground
2 VS Supply voltage VS
3 REL Relay driver
4 OSC C1 Oscillator
5 OSC R1 Oscillator
6 VS Supply voltage VS
7 LD Lamp failure detection
8 SI Start input (49a)
GND 1
8 SI
VS 2
REL 3
U643B
7 LD
6 Vs
OSC 4
5 OSC
94 9290
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 automo-
tive relay with a minimum coil resistance of 60 W.
Pin 4 and 5 Oscillator
Flashing frequency, f1, is determined by the R1C1 compo-
nents as follows (see figure 1):
f1 [ R1
1
C1
1.5 Hz
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 mW.
Comparator K1‘s reference voltage is matched to the
characteristics of filament lamps (see “control signal
threshold” 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.
where
C1 47 mF
R1 + 6.8 kW to 510 kW
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-
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 u 5 kW 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
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