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.
– 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)
IN
1K2
2
27
IN
1K1
2
3
4
5
6
7
8
9
10
11
IN
1K2
IN
1K5
REF
K5
IN
2K5
IN
1K3
REF
K3
IN
2K3
IN
3K3
IN
1K7
REF
K7
In
1K5
3
26
IN
FK3
REF
K5
4
25
IN
1K4
IN
2K5
5
24
REF
K4
IN
1K3
6
23
IN
F
12
IN
2K7
IN
3K7
OUT
OSC
IN
FK5
IN
FK6
GND
V
S
V
B
IN
1K6
REF
K6
IN
F
REF
K4
IN
1K4
IN
FK3
IN
1K1
REF
K1
REF
K3
7
22
REF
K6
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
IN
2K3
8
21
IN
1K6
IN
3K3
9
20
V
B
IN
1K7
10
19
V
S
REF
K7
11
18
GND
IN
2K7
12
17
IN
FK6
IN
3K7
13
16
IN
FK5
OUT
14
94 8831
15
OSC
Input 1; single comparator K2
Input 1; double comparator K5
Reference for threshold voltage;
double comparator K5
Input 2; double comparator K5
Input 1; triple comparator K3
Reference for threshold voltage;
triple comparator K3
Input 2; triple comparator K3
Input 3; triple comparator K3
Input 1; triple comparator K7
(U2481B with latch)
Reference for threshold voltage;
triple comparator K7
(U2481B with latch)
Input 2; triple comparator K7
(U2481B with latch)
Input 3; triple comparator K7
(U2481B with latch)
Output for pilot lamp
R/C combination for oscillator
Fuse monitor input ; double
comparator K5
Fuse monitor input; single
comparator K6
Ground
Supply voltage
Reference voltage of compara-
tors
Input1; single comparator K6
Reference for threshold voltage;
single comparator K6
Input; brake fuse comparator
Reference for threshold voltage;
single comparator K4
Input 1; single comparator K4
Fuse monitor input; triple
comparator K3
Input 1; single comparator K1
Reference for threshold voltage;
single comparator K1
Figure 2. Pin configuration
Rev. A1: 21.08.1995
Preliminary Information
3 (11)
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:
Pin 20:
R = 150
W;
K = 0.1
mF
R = 1000
W;
K = 0.1
mF
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 V
ref
< V
Min
with
V
Min
= 0.33 V
B
.
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
REF
K5
and R = 2 kW each for IN
1K5
and IN
2K5
.
Triple comparators:
The reference inputs (K3 and K7)
sink currents of typically 30
mA.
All inputs (IN
1K3
, IN
2K3
IN
3K3,
and IN
1K7
, IN
2K7
, IN
3K7
) sink the identical typical
current of 10
mA
during switch-over. Therefore the
external protection resistor must provide R = 1 kW for
REF
K3
and REF
K7
and R = 3 kW at each input.
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 f
osc
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:
f
osc
+
1
+
c1
osc
f
osc
(0.74 R
osc
)
2260
W)
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 IN
FK3
, IN
FK5
and IN
FK6
with their EXOR gates
can be used for additional fuse monitoring.
Detection threshold for a blown fuse is V
ref
< V
Min
with
V
Min
= 0.33 V
B
(V
B
= reference voltage of comparators).
Pin IN
F
is used for brake fuse monitoring. With a voltage
drop V
F
> 4.8 V across the fuse, the comparator detects
a blown fuse. A voltage drop V
F
< 2.8 V represents a
“good” fuse. U2481B’s comparator K7 is connected to a
latch, thus a brake lamp outage is memorized.
With the recommended values
R
osc
= 82 kW and C
osc
= 10 nF
f
osc
≈
1.58 kHz
Comparators
Comparators K1 and K2:
threshold voltage of typically
V
th
= 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
V
th
= 3.5 mV; designed for all other monitor purposes.
Due to the integrated compensation circuitry, these
comparators are suitable for pc layer copper shunts.
Rev. A1: 21.08.1995
Preliminary Information
5 (11)
Microcontroller MCU
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