
FUNCTIONAL DESCRIPTION
The circuit is designed to drive the direction indi-
cator flasher relay. The application circuit shows
the typical system configuration with the external
components. Its consists of a network (R
1
C
1
) to
determine the oscillator frequency, shunt resistor
(R
S
) to detectdefectivebulbs and two current lim-
iting resistors (R
2
/R
3
) to protect the IC against
load dump transients.
The lightbulbsL
2
, L
3
, L
4
, L
5
, are the turn signal in-
dicators with the dashboard-light L
6
. The S
1
switch
position is sensed
and R
lamp
by input 8. The flashing cycle is started
across
resistor R
2
by closing S
1
: then, after a delay time t
d
typically
equal to 1.5 ms, the relay is actuated and the pin
3 goes high switching on the corresponding
lamps L
2
, L
3
, (or L
4
, L
5
). These lamps will flash at
the oscillator frequencynot depending on the bat-
tery voltage value (8 - 18 V). The flashing cycle
stops and the circuit is reset to the initial position
when the switch S
1
is open.
The lamp failure detection function senses the
current through the shunt resistor R
S
. When one
of the lightbulbs is defective the voltage drop
across R
S
is reduced to a half and the failure is
indicatedby doublingthe flashing frequency.
ELECTRICAL CHARACTERISTICS
(– 20
°
C < T
amb
<, 100
°
C, 8V < V
S
< 18V unless otherwise speci-
fied.)
Symbol
V
S
V2 – V1
V2 – V3
R2
K
N
Parameter
Test Condition
Min.
8
27
Typ.
Max.
18
34
1.7
3.6
1.74
Unit
V
V
V
K
Operating Voltage
Clamping Voltage
Output Saturation Voltage
Starter Resistance
Oscillator Constant K
N
(normal
Operation)
Temperature Coefficient of Kn
see note 1
I
RL
= 250mA
see note 2
Fn = 1/KnRoCo
Osc. Frequency
See Note 3
1.27
C
T
–1.5
10-3
50
1/
°
C
D.C.
K
C
Duty Cycle (normal operation)
Oscillator Constant K
C
(lamp failure detection)
Duty Cicle (lamp failure
detection)
Current Consumption Relay off
I
pin 1
45
0.53
55
0.74
%
F
C
= 1KcRoCo
Osc. Frequency
DC
LF
35
40
45
%
I
Q
V
S
= 8V
V
S
= 13.5V
V
S
= 18V
R
3
= 220
V
S
= 13.5V
–20 < T
amb
< 100
°
C
2.2
2.7
3.3
3.9
4.3
4.7
mA
mA
mA
V
th
Lamp FailureThreshold
(see note 4)
65
85
95
mV
Notes :1.
This voltage is the threshold used to protect the circuit against overvoltage : if V
is > than this threshold, the relay will be on
and the voltage across the circuit will maintain constant increasing the current inthe protective resistor R
3
.
This is the maximum value for operation. This value must be higher than 1 K Ohms in orderto limit the current in pin 8 during
dumps. A recommended value for application should be 1,5 K Ohms.
The external leakage from the blinker unit to ground must be with an equivalent resistor higher than 5,6 K Ohms to avoid para-
sitic operationwhen the switch S
1
is off.
This temperature coefficient is usefullto compensate the drift of the external timing network(R
1
, C
1
).
This threshold is calculated for a 20 m Ohm shunt. The threshold is dependant of V
bat
as the bulb current.
2.
3.
4.
5.
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