LM1946N National Semiconductor, LM1946N Datasheet - Page 6

• Manufacturer Part Number: LM1946N
• Description: IC DIAGNOSTIC CIRCUIT 20 DIP
• Manufacturer: National Semiconductor

Specifications of LM1946N

• Function: Current Sense
• Voltage - Input: 9 ~ 26 V
• Current - Output: 20mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 20-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Accuracy: -
• Sensing Method: -
• Other names: *LM1946N; LM1946

Application Hints
Typical values are
For values of sense voltages greater than 100 mV the com-
parator output is off (low) Sense voltages less than 100 mV
turn the output on (high)
It’s also important that the output of the comparator be in
the ‘‘off’’ state when the inputs are taken to ground i e S1
is opened and the lamp is turned ‘‘off’’ The input section of
LM1946 has been designed to turn ‘‘off’’ when the inputs
are grounded and therefore not deliver an erroneous bulb
out indication The comparator is only activated when the
inputs are above ground by at least 3V
R1 and R2 are necessary for another reason These resis-
tors protect the input terminals of the IC from the many
transients in an automobile found on the battery line some
of which can exceed a thousand volts for a few microsec-
onds A minimum value of approximately 1 k
recommended
COMPENSATING FOR BATTERY VOLTAGE
The current through a typical automotive lamp whether a
headlight or dashboard illumination lamp will vary as battery
voltage changes The change however is nonlinear Dou-
bling the battery voltage does not double the lamp current
This occurs since a higher voltage will heat the filament
more increasing its resistance and allowing less current to
flow than expected Figure 5 shows this effect A best fit
straight line over the normal battery range of 9V to 16V for
this particular example can be given by
R1
R2
Iset
Vthrshld
e
e
I
e
L
(Amps)
6 2k
1 2k
20 A
e
g
g
20 A (6 2k
5%
5%
e
25 C
0 62
FIGURE 5
a
(Continued)
b
0 069 Vbattery
1 2k)
e
100 mV
is therefore
TL H 8707–21
6
6 giving a nominal Iset of 20 A that tracks the bulb current
Thus in actual use the LM1946 threshold voltage should
track the variations in bulb current with respect to battery
voltage To accomplish this Iset should have a component
that varies with the battery As shown in the LM1946 circuit
schematic of Figure 18 the Iset pin is two diode drops
above ground or approximately 1 4V A resistor from this
pin to the 6 4V reference sets the fixed component of Iset a
resistor to the battery line sets the variable component
Thus the best fit straight line in Figure 5 can be realized
exactly with only two resistors The result is shown in Figure
as supply varies from 9V to 16V The graph of Figure 7
shows the final result comparing a typical sense voltage
across Rs with the comparator threshold voltage as the
supply varies
COMPENSATING FOR AMBIENT
TEMPERATURE VARIATION
If the sense resistors used in a system are perfect compo-
nents with no temperature coefficient then the compensa-
tion to be subsequently detailed here is unnecessary How-
ever resistors of the very small values usually required in a
lamp monitoring system are sometimes difficult or expen-
sive to acquire A convenient alternative is the wiring har-
ness a length of wire or even a trace on a printed circuit
board All of these are of copper material and therefore can
vary by as much as 3900 ppm C The LM1946 has been
designed to accommodate a wide range of temperature
compensation techniques If the Iset current is designed to
increase or decrease with temperature nearly any tempera-
ture coefficient can be produced in the threshold voltage of
the five input pairs
Iset
Iset
e
V
R4
e
CC
V
a
CC
FIGURE 6
FIGURE 7
R4
Vref
R3
b
1 4
b
a
1 4
Vref
R3
R3
1
b
a
1 4
R4
1
TL H 8707 – 10
TL H 8707 – 22