LM1946N National Semiconductor, LM1946N Datasheet - Page 8

• 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
and R4 again allow the system designer to tailor the thresh-
old limit to the V I characteristics of each particular system
The input threshold voltage is determined by and directly
proportional to Iset into pin 20 R3 from the on-chip refer-
ence voltage provides a current and threshold that is inde-
pendent of the supply voltage V
directly proportional to supply These resistors allow thresh-
olds to be either independent of or directly proportional to
supply voltage or anything in between For example the
values in Figure 10 are tailored to match the V I characteris-
tics of the bulb filament used in earlier examples However
if the load had purely resistive characteristics Iset and the
threshold would be set with R4 only eliminating R3 Like-
wise if the load current was independent of supply such as
in many systems powered by a voltage regulator Iset would
be better set by R3 only eliminating R4 Further details on
this and how to handle variations with ambient temperature
with resistor and thermistor combinations are discussed in
detail in previous sections Compensation for temperature
variations however is rarely necessary since short circuit or
over-current values are usually much greater than the nomi-
nal value For example if the load in Figure 10 represented
a DC motor the circuit could be used to detect the motor
stall condition Stall current through the sense resistor Rs
would typically be five times the nominal running current By
setting the threshold at three times the nominal current val-
ue enough margin exists that minor variations due to tem-
perature can be ignored The variation in stall current due to
battery or supply voltage can be significant however Being
approximately proportional Iset would best be set in this
case by R4 only
WINDOW DETECTOR
The availability of more than one comparator per IC allows
many other applications One is the current sense window
detector Many times it is useful to know that a certain cur-
rent is within both an upper and lower limit Using two of the
LM1946 comparators and the circuit of Figure 11 will ac-
complish this In this particular case high and low limits
FIGURE 10 Using the LM1946 as an Over-Current Limit Detector
(Continued)
CC
R4 provides a current
8
Figure 12 ) This is vital in a system which must operate in
are approximately 3A and 1A respectively The outputs can
be kept separate or wired-or as shown to a single output
load as a simple out-of-bounds detector
COMPARATOR INPUT STAGE
The LM1946 IC consists of five specially designed compara-
tor input circuits to monitor the IR drop across the wiring
harness or the sense resistor between the battery and the
light bulb These comparators have been designed to ac-
commodate a wide range of input signals without damage to
the IC or the load circuitry The inputs can easily withstand a
common mode voltage above the positive supply since the
inputs are the emitters of two matched PNP devices (see
the conditions present under the hood of an automobile
The inputs can also survive when taken well below ground
If a negative voltage is present at the inputs of the compara-
tor the two emitter-base PNP junctions become reverse bi-
ased and block any current flow in or out of the device To
disable an unused comparator it is recommended that the
inputs be connected to ground
FIGURE 11 Current Limit Window Detector
Vthrshld-lo
Vthrshld-hi
TL H 8707– 7
e
e
Iset (R10
Iset (R13
TL H 8707 – 8
b
b
R11)
R12)