LM1946N National Semiconductor, LM1946N Datasheet - Page 10

• 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
MORE NOISE FILTERING
The current flowing through the sense resistor and certain
loads can sometimes be very noisy particularly when the
load is a DC motor or switching supply Large amounts of
noise on the supply line can also cause problems when
threshold voltages are set to very small values In these
cases while the average current level may remain well be-
low the threshold trip point noise peaks may exceed it A
LED display could then flicker or appear dimly lit or exces-
sive software routines and processor time may be required
for a P to disregard such noise Often such noise must be
filtered directly at the inputs using the input resistors R1
and R2 and a capacitor Care must be taken however that
such a filter will not cause an erroneous output state upon
power-up or whenever switch S1 is closed The most effec-
tive general methodology to achieve this is to split the resis-
tor in the positive input lead into two resistor values and
connect a capacitor from here to the negative input For
example the 1 2k resistor R2 of Figure 10 could be re-
placed with 3 9k and 1 2k resistors as shown in Figure 16a
(R1 increasing from 6 2k to 10k to compensate) The value
of capacitor C2 depends upon the degree of filtering re-
quired the amount of noise present and the response times
desired The choice of values for the new resistors is almost
arbitrary Generally the larger value is attached to the sense
resistor for better decoupling The smaller value must be
large enough so that the DC voltage across it upon power-
up exceeds the maximum offset voltage expected of the
comparator (i e Iset R2b
that guarantees that the output will not be in an erroneous
high state upon power-up or whenever S1 is closed (Should
this feature be unnecessary to a particular application cir-
cuit the methodology described can be replaced with a sim-
ple capacitor across the comparator input pins)
l
5 0mV) It is this requirement
(Continued)
10
For extremely severe cases additional filter stages can be
cascaded at the inputs (see Figure 17 ) Since the input bias
currents of the comparator are equal at the input threshold
level the voltage drops across the 1k resistors cancel and
do not affect the DC operation of the circuit (ignoring resis-
tor match tolerance and Ios) If an application circuit is noisy
enough to require such an elaborate filter then ferrite
beads shown here as L1 and L2 will also probably help
FIGURE 17 Additional Noise Filters
FIGURE 16 Input Noise Filters for
b Over-Current Limit Detector
Various Application Circuits
a Open-Circuit Detector
TL H 8707 – 16
TL H 8707 – 17
TL H 8707 – 18