LM1946N National Semiconductor, LM1946N Datasheet - Page 5

• 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
THEORY OF OPERATION UNDER-CURRENT LIMIT
DETECTOR
The diagram of Figure 3 represents the typical lamp circuit
found in most automobiles Switch S1 represents a dash-
board switch discrete power device relay and or flasher
circuit used for turn signals Sense resistor R
actual circuit component (such as a 0 1
tor) or it can represent the resistance of some or all of the
wiring harness The load represented here as a single bulb
can just as easily be two or more bulbs in parallel such as
front and rear parking lights or left and right highbeams etc
One of the easiest methods to electronically monitor proper
bulb operation is to sense the voltage developed across R
by the bulb current I
filament the load current and sense voltage V
zero (or to half their former values in the case of two bulbs
wired in parallel) A comparator circuit can then monitor this
sense voltage and alert the system or system user (e g
power an LED) if this sense voltage drops below a predeter-
mined level (defined as the threshold voltage)
Typical sense voltages range from tens to hundreds of milli-
volts Not only does this sense voltage vary nonlinearly with
the battery voltage it may vary significantly with ambient
temperature depending on the temperature coefficient (TC)
of the sense resistor or wiring harness Since these nonlin-
ear characteristics can vary from system to system and
sometimes even within a single system provisions must be
made to accommodate them There are two general meth-
odologies to accomplish this
The first method uses only one bulb per monitoring circuit A
sense resistor is selected to give 50–100 mV of sense volt-
age in an operational circuit and a comparator threshold
detecting voltage of approximately 10 mV is set Even if
component tolerances battery line variations and tempera-
ture coefficients cause the sense voltage to vary 3 1 or
more circuit operation will not be affected
The second method must be used if two or more bulbs are
wired in parallel and it is necessary to detect if any single
lamp fails This is often desirable as it reduces the number
of comparators and displays and system cost by at least a
factor of two In this case the sense voltage will drop by
only half (or less) of it’s original value For example a nomi-
nal 100 mV drop across the sense resistor will drop to
50 mV if one of two bulbs fail Therefore a threshold detec-
tion voltage between 50 and 100 mV is required (since a
FIGURE 3 Equivalent Automotive Lamp Circuit
L
Lamp Fault Detector
If a fault occurs due to an open bulb
1W carbon resis-
TL H 8707 – 6
s
can be an
S
drop to
s
5
10 mV threshold would alert the system only if both bulbs
failed) Yet a fixed threshold of 75 mV may not work if the
nominal 100 mV sense voltage can vary 3 1 due to the fac-
tors mentioned earlier What is required is a comparator with
a threshold-detecting voltage that tracks the nominal sense
voltage as battery line and ambient temperature change
Thus while the sense voltage may nominally be anywhere
from 50 to 150 mV the threshold voltage will always be
roughly 75% of it or 37 mV to 112 mV and will detect the
failure of either of two bulbs
The LM1946 integrated circuit contains five comparators es-
pecially designed for lamp monitoring requirements Since
all lamps in a system share the same battery voltage and
ambient temperature accommodations for these variations
need to be made only once at the IC and each threshold of
the five comparators then tracks these variations
SETTING THE COMPARATOR THRESHOLD VOLTAGE
The threshold voltage at which the comparator output
changes state is user-set in order to accommodate the
many possible system designs The input bias currents are
purposely high to accomplish this and are each equal to the
user-set current into the Iset pin (more on this later) Typi-
cally around 20 A the effect of this across the sense resis-
tor R
gible and can be ignored However when resistors R1 and
R2 (Figure 4) are added to the circuit a shift in the threshold
voltage is effected This occurs since each input has been
affected by different IR drops The LM1946 behaves like
any other comparator in that the output switches when the
input voltage at the IC pins is zero millivolts (ignoring offset
voltage for the moment) If the output therefore has just
switched states due to just the right threshold voltage
across the sense resistor then the sum of voltages around
the resistor loop should equal zero
Assuming Voffset m Vthrshld
s
Vthrshld
Vthrshld
Vthrshld
compared to a typical load measured in amps is negli-
FIGURE 4 Input Bias Current
a
e
e
Iset R2
Iset R1
Iset (R1
b
b
b
R2)
Voffset
Iset R2
b
Vthrshld
Iset R1
e
Iset (R1
TL H 8707– 9
e
0
b
R2)