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Automotive Lamp Outage Monitor
U2481B/ U2482B
Description
The U2481B and the U2482B multicomparator circuits
are designed to monitor automotive illumination by
sensing the voltage drop across shunt resistors. Provided
with extremely low comparator thresholds, these ICs can
be used together as a chip set to build an entire lamp
monitor system. Single operation of both circuits is also
possible.
As a special feature for brake lamp monitoring, the
U2481B contains a latch stage connected to one of its
triple comparators.
With
– 4 single comparators
– 1 double comparator and
– 2 triple comparators
each of the integrated circuits monitors up to 12 lamps
and up to 6 fuses.
All comparator stages are combined together to control
the common output stage.
Features
D Extremely low comparator thresholds of typically
3.5 mV
D Internal compensation for copper shunts
D Internal compensation for voltage-dependent
bulb characteristic
D Comparator input voltage may exceed supply voltage
D Internal protection measures for pulses
according to ISO TR 7637/1
D ESD according to MIL-SID-883 C test method 3015.7
– Human body model: 4 kV
– Machine model:
200 V
EMI protection (TEM cell up to 100 V/m)
Benefits
D Extremely low threshold voltage gives low voltage
drop via shunt resistor. Power dissipation of the lamp
monitor module is minimized
D Perfect EMC in conjunction with an appropriate
pc board layout
Applications
Both ICs can be used in any kind of vehicle with a 12 V
supply. It is sufficient to insert small resistors into the
lamp wiring and to provide the dashboard instrument
cluster with a pilot lamp. These ICs increase the safety
and add comfort features.
Rev. A1: 21.08.1995
Preliminary Information
1 (11)

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Block Diagram
28 Ref
REFK1
27
IN1K1
E1
K1 13 mV
+
IT
IT
REFK2
IN1K2
1
2
Ref K2 13 mV
E1
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VB
20
VB
Voltage drift
of comparator
threshold
IT = f (VB)
Power
on
Reset
VS Stabilized
voltage
Vstab = 5.2 V
POR
IT
Divider
Oscillator
Clock 1 Clock 2
2-TOsc 20-TOsc
Cl1 Cl2
19
VS
15 OSC
REFK3
IN1K3
IN2K3
IN3K3
7
6
8
9
26
INFK3
REFK4
IN1K4
24
25
INFK5 16
REFK5
IN1K5
IN2K5
INF
4
3
5
VB
23
Ref K3 3.5 mV
E1
E2
E3
Ref K4 3.5 mV
E1
Ref K5 3.5 mV
E1
E2
+
REFK7
IN1K7
IN2K7
IN3K7
11
10
12
13
Ref K7 3.5 mV
E1
E2
E3
Up down counter
Cl2 T = 1.2 s
Start High at
Set
VS
forward counter end
Out
Start High at
return counter zero
Reset
Cl1
14
OUT
Out Reset
Latch for
stoplamps
Set
POR
K6 Ref
3.5 mV E1
17
INFK6
22
REFK6
21
IN1K6
Up down counter
Cl2 T = 1.2 s
Start High at
forward counter end
Start High at
return counter zero
Cl1
VS
* Connection to
GND only for
U2482B (latch
disabled
18
*
GND
94 8833
Figure 1.
2 (11)
Preliminary Information
Rev. A1: 21.08.1995

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Pin Description
REFK2 1
28 REFK1
IN1K2 2
27 IN1K1
In1K5 3
REFK5 4
26 INFK3
25 IN1K4
IN2K5 5
24 REFK4
IN1K3 6
REFK3 7
23 INF
22 REFK6
IN2K3 8
IN3K3 9
21 IN1K6
20 VB
IN1K7 10
19 VS
REFK7 11
18 GND
IN2K7 12
17 INFK6
IN3K7 13
16 INFK5
OUT 14
15 OSC
94 8831
Figure 2. Pin configuration
U2481B/ U2482B
Pin Symbol
Function
1 REFK2 Reference for threshold voltage
single comparator K2
2 IN1K2 Input 1; single comparator K2
3 IN1K5 Input 1; double comparator K5
4 REFK5 Reference for threshold voltage;
double comparator K5
5 IN2K5 Input 2; double comparator K5
6 IN1K3 Input 1; triple comparator K3
7 REFK3 Reference for threshold voltage;
triple comparator K3
8 IN2K3 Input 2; triple comparator K3
9 IN3K3 Input 3; triple comparator K3
10 IN1K7 Input 1; triple comparator K7
(U2481B with latch)
11 REFK7 Reference for threshold voltage;
triple comparator K7
(U2481B with latch)
12 IN2K7 Input 2; triple comparator K7
(U2481B with latch)
13 IN3K7 Input 3; triple comparator K7
(U2481B with latch)
14 OUT Output for pilot lamp
15 OSC R/C combination for oscillator
16 INFK5 Fuse monitor input ; double
comparator K5
17 INFK6 Fuse monitor input; single
comparator K6
18 GND Ground
19 VS Supply voltage
20 VB Reference voltage of compara-
tors
21 IN1K6 Input1; single comparator K6
22 REFK6 Reference for threshold voltage;
single comparator K6
23 INF Input; brake fuse comparator
24 REFK4 Reference for threshold voltage;
single comparator K4
25 IN1K4 Input 1; single comparator K4
26 INFK3 Fuse monitor input; triple
comparator K3
27 IN1K1 Input 1; single comparator K1
28 REFK1 Reference for threshold voltage;
single comparator K1
Rev. A1: 21.08.1995
Preliminary Information
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U2481B/ U2482B
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TELEFUNKEN Semiconductors
VBatt
A
B
150 W
R1
1000 W
R2 C2
0.1 mF
Fuse
Shunt
Shunt
C1
15 mF
VB
1 kW
20
28
VS
19
Osc
15
27
2 kW
U2481B
U2482B
2 kW
16
1 kW
4
Out
14
Rosc
82 kW
Cosc
10 nF
2 kW
3
2 kW
5
95 10689
Out
Pilot
lamp
1.2 W
Kl 61
A: Example of a “single comparator” using the fuse as a shunt resistor
B: Example of a “double comparator” using copper shunts and additional fuse monitoring
Figure 3. Basic application diagram
4 (11)
Preliminary Information
Rev. A1: 21.08.1995

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Functional Description
Power Supply, Pins 19 and 20
The Ics have two supply pins. The IC itself is supplied via
Pin 19 (stabilized voltage, POR-circuitry). Pin 20
supplies only the current sources of the comparators. Both
pins have an an internal 21 V Zener Diode for protection.
For reasons of interference protection and surge
immunity the supply voltage pins have to be provided
with RC-circuitries as shown in figure... The resistors
limit the current in the case of overvoltage, whereas the
capacitors smooth the supply voltage.
Recommended values:
Pin 19:
R = 150 W; K = 0.1 mF
Pin 20:
R = 1000 W; K = 0.1 mF
An external diode protects the IC against battery reversal.
Power-On-Reset
When the supply voltage is switched on, a power-on-reset
pulse is generated internally which resets the brake lamp
monitor latch and the counter stages.
Oscillator, Pin 15
The RC-oscillator is the time base for clock 1 and clock 2
(see block diagram) which are desired from a divider
stage and fed to the two upward and downward counters.
The oscillator frequency fosc is mainly determined by the
external R/C components and an integrated resistor. The
capacitor K is charged by the external resistor and
discharged by the integrated one. Because of the temper-
ature characteristic and the tolerances of the integrated
2 kW-resistor, the external one has to have considerable
higher value in order to achieve a stable frequency.
Calculation of the frequency:
fosc
+
1
fosc
+
1
c osc
(0.74 Rosc ) 2260 W)
With the recommended values
Rosc = 82 kW and Cosc = 10 nF
fosc 1.58 kHz
Comparators
Comparators K1 and K2: threshold voltage of typically
Vth = 13 mV; designed to monitor high- and low-beam
bulbs, where the fuses can be used as shunt resistors.
Comparators K3 to K7: threshold voltage of typically
Vth = 3.5 mV; designed for all other monitor purposes.
Due to the integrated compensation circuitry, these
comparators are suitable for pc layer copper shunts.
U2481B/ U2482B
A bulb is detected as “good” as long as the voltage drop
via the shunt resistor exceeds the threshold voltage. The
pilot lamp is off. A failed bulb is detected if the voltage
drop via the shunt resistor is smaller than the threshold
voltage. The pilot lamp is on. An external transistor is
necessary to drive the pilot lamp.
In order to disable monitoring when the lamps are
switched off, therefore the comparator outputs are
disabled if the reference voltage Vref < VMin with
VMin = 0.33 VB.
As protection against transients on the supply, all
comparator inputs require external resistors. Their
tolerances directly influence the comparator threshold
accuracy. Resistors with a " 1% tolerance are
recommended.
Single comparators: The inputs (K1, K2, K4, K6) sink
currents of typically 10 mA each. During switch-over, the
currents of reference and input pins are identical.
Therefore identical external protection resistors
(R = 1 kW) are required.
Double comparator: The reference input (K5) sinks a
current of typically 20 mA; its inputs sink currents of
typically 10 mA each during switch-over. Therefore the
external protection resistors must provide R = 1 kW for
REFK5 and R = 2 kW each for IN1K5 and IN2K5.
Triple comparators: The reference inputs (K3 and K7)
sink currents of typically 30 mA. All inputs (IN1K3, IN2K3
IN3K3, and IN1K7, IN2K7, IN3K7) sink the identical typical
current of 10 mA during switch-over. Therefore the
external protection resistor must provide R = 1 kW for
REFK3 and REFK7 and R = 3 kW at each input.
Fuse Monitoring
The internal EXOR conjunction of comparators K1 and
K2 allows fuse monitoring for both lamps. Even a
simultaneous blowout of both fuses is detected.
The pins INFK3, INFK5 and INFK6 with their EXOR gates
can be used for additional fuse monitoring.
Detection threshold for a blown fuse is Vref < VMin with
VMin = 0.33 VB (VB = reference voltage of comparators).
Pin INF is used for brake fuse monitoring. With a voltage
drop VF > 4.8 V across the fuse, the comparator detects
a blown fuse. A voltage drop VF < 2.8 V represents a
“good” fuse. U2481B’s comparator K7 is connected to a
latch, thus a brake lamp outage is memorized.
Rev. A1: 21.08.1995
Preliminary Information
5 (11)