Datasheet LM1946N, LM1946M Datasheet (NSC)

February 1993

LM1946 Over/Under Current Limit Diagnostic Circuit

LM1946 Over/Under Current Limit Diagnostic Circuit

General Description

The LM1946 provides the industrial or automotive system
designer with over or under current limit detection superior
to that of ordinary transistor or comparator-based circuits.

Each of the five independent comparators can be used to
monitor a separate load as either an over current or under
current limit detector. Two comparators monitoring a single
load can function as a current window monitor.

Current is sensed by monitoring the voltage drop across the
wiring harness, pc board trace, or external sense resistor
that feeds the load.

Provisions for compensating the user set limits for wiring
harness resistance variations over temperature and supply
voltage variations are also available.

When a limit is reached in one of the comparators, it turns
on its output which can drive an external LED or microproc­essor.

One side of the load can be grounded (not possible with

Features

Y

Five independent comparators

Y

Capable of 20 mA per output

Y

Low power drain

Y

User set input threshold voltages

Y

Reverse battery protection

Y

60V load dump protection on supply and all inputs

Y

Input common mode range exceeds V

Y

Short circuit protection

Y

Thermal overload protection

Y

Prove-out test pin

Y

Available in plastic DIP and SO packages

Applications

Y

Lamp fault detector

Y

Motor stall detector

Y

Power supply bus monitoring

ordinary comparator designs), which is important for auto­motive systems.

Typical Application CircuitÐLamp Fault Detector (I

CC

l

1A)

L

FIGURE 1

TL/H/8707– 2

C

1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A.

TL/H/8707

Absolute Maximum Ratings

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.

Supply Voltage (V

Survival Voltage (T

and Input Pins)

CC

s

100 ms)

b

50V toa60V

Operational Voltage 9V to 26V

Internal Power Dissipation (Note 1) Internally Limited

s

Electrical Characteristics 9V

s

V

16V, Isete20 mA, T

CC

Parameter Conditions Min Typ Max Units

Quiescent Current All Outputs ‘‘Off’’ 1.40 3.00 mA

Reference Voltage I

Reference Voltage 9VsV
Line Regulation

e

10 mA 5.8 6.4 7.0 V

ref

CC

s

16V, I

e

ref

Iset Voltage Isete20 mA 1.20 1.40 1.60 V

Input Offset Voltage At Output Switch Point. V

Input Offset Current I

Input Bias Current I

9VsV

IN(a)

IN(a)

b

or I

CM

s

I

IN(b)

IN(b)

16V

,9VsV

,9VsV

CM

CM

Input Common Mode
Voltage Range

Maximum Positive Either Input. Ts100 ms
Input Transient

Maximum Negative Either Input. Ts100 ms
Input Transient

Output Saturation I
Voltage

Output Short Circuit V
Current

Output Leakage Current V

e

2 mA, 9VsV

O

e

I

10 mA, 9VsV

O

e

0Vdc, Comparator ‘‘ON’’

O

e

0Vdc. Comparator ‘‘Off’’ 0.01 1.00 mA

O

CC

CC

s

s

Test Threshold At Switch Point on Any Output
Voltage V

e

O

2V (Note 2)

Test Threshold
Current

Note 1: Thermal resistance from junction to ambient is typically 53§C/W (board mounted).

Note 2: The test pin is an active high input, i.e. all five will be forced high when this pin is driven high.

Note 3: C

ESD

e

100 pF, R

ESD

e

1.5k

Output Short Circuit to Ground or V

Operating Temperature Range (TA)

CC

Continuous

b

40§Ctoa85§C

Maximum Junction Temperature

Storage Temperature Range

b

65§Ctoa150§C

Lead Temperature (Soldering, 10 sec.)

ESD Susceptibility (Note 3) 600V

e

25§C (unless otherwise specified)

j

10 mA

e

2V

O

s

16V

s

16V 18.00 20.00 22.00 mA

g

5

g

1.0

g

0.10

g

50 mV

g

5.0 mV

g

1.00 mA

4.00 26.0 V

60 70 V

b

50

b

60 V

16V 0.80 1.00 V

16V 1.00 1.20 V

20 45 120.0 mA

0.80 1.25 2.00 V

0.2 mA

a

a

150§C

260§C

dc

dc

dc

dc

dc

dc

dc

dc

dc

dc

dc

dc

dc

dc

2

Connection Diagram

Order Number LM1946N or LM1946M

See NS Package Number M20B or N20A

Typical Test Circuit

Simplified Comparator Schematic

TL/H/8707– 20

TL/H/8707– 23

TL/H/8707– 24

3

Typical Performance Characteristics

V

sat

Peak I

vs I

Quiescent Current

O

vs V

O

CC

vs V

CC

V

vs Temperature V

set

Quiescent Current
vs V

CC

vs Temperature

ref

Iset vs Temperature Iset vs Temperature Iset vs Temperature

Test Threshold Common Mode Lower Limit

4

TL/H/8707– 4

Application Hints

THEORY OF OPERATION: UNDER-CURRENT LIMIT
DETECTOR

FIGURE 3. Equivalent Automotive Lamp Circuit

Lamp Fault Detector

The diagram of

Figure 3

represents the typical lamp circuit
found in most automobiles. Switch S1 represents a dash­board switch, discrete power device, relay and/or flasher
circuit used for turn signals. Sense resistor R
actual circuit component (such as a 0.1X 1W carbon resis­tor) 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 IL. If a fault occurs due to an open bulb
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 predeter­mined level (defined as the threshold voltage).

Typical sense voltages range from tens to hundreds of milli­volts. 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 nonlin­ear 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 meth­odologies 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 volt­age in an operational circuit, and a comparator threshold
detecting voltage of approximately 10 mV is set. Even if
component tolerances, battery line variations, and tempera­ture 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 nomi­nal 100 mV drop across the sense resistor will drop to
50 mV if one of two bulbs fail. Therefore, a threshold detec­tion voltage between 50 and 100 mV is required (since a

TL/H/8707– 6

can be an

s

S

, drop to

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 fac­tors 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 es­pecially 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). Typi­cally around 20 mA, the effect of this across the sense resis­tor R

compared to a typical load measured in amps is negli-

s

gible and can be ignored. However, when resistors R1 and
R2

(Figure 4)

voltage is effected. This occurs since each input has been

s

affected by different IR drops. The LM1946 behaves like

are added to the circuit, a shift in the threshold

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:

FIGURE 4. Input Bias Current

VthrshldeIset (R1bR2)

VthrshldaIset#R2bVoffsetbIset#R1e0

Assuming VoffsetmVthrshld:

VthrshldeIset#R1bIset#R2

VthrshldeIset (R1bR2)

TL/H/8707– 9

5

Application Hints (Continued)

Typical values are:

For values of sense voltages greater than 100 mV, the com­parator 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 resis­tors 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 microsec­onds. A minimum value of approximately 1 kX is therefore
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. Dou­bling 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.
straight line over the normal battery range of 9V to 16V for
this particular example can be given by:

e

6.2kg5%

R1

R2e1.2kg5%

Isete20 mA@25§C

Vthrshlde20 mA (6.2kb1.2k)e100 mV

FIGURE 5

Figure 5

I

(Amps)e0.62a0.069#Vbattery

L

shows this effect. A best fit

TL/H/8707– 21

TL/H/8707– 10

1

a

R4

J

Iset

Iset

e

V

e

V

CC

R4

b

1.4

CC

R4

Vref

a

b

R3

FIGURE 6

Vrefb1.4

a

1.4

#

R3

1

R3

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
exactly with only two resistors. The result is shown in

6

, giving a nominal Iset of 20 mA that tracks the bulb current

as supply varies from 9V to 16V. The graph of

Figure 5

can be realized

Figure

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 compo­nents with no temperature coefficient, then the compensa­tion to be subsequently detailed here is unnecessary. How­ever, resistors of the very small values usually required in a
lamp monitoring system are sometimes difficult or expen­sive to acquire. A convenient alternative is the wiring har­ness, 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/
designed to accommodate a wide range of temperature

C. The LM1946 has been

§

compensation techniques. If the Iset current is designed to
increase or decrease with temperature, nearly any tempera­ture coefficient can be produced in the threshold voltage of
the five input pairs.

FIGURE 7

TL/H/8707– 22

6

Application Hints (Continued)

One solution is to use a low cost thermistor in conjunction
with some low-TC resistors (see

There are three fixed resistors and one thermistor. This is
an NTC thermistor, since it has a negative temperature co­efficient. This is what is required in order to have Iset in-
crease as the temperature rises. The data sheet with the
thermistor described a number of ways to establish different
final TC’s. The thermistor itself has a very large TC which is
somewhat difficult to describe mathematically. But, if it is
used with some other fixed resistors, such as Rmin and
Rmax, definite end point limits can be established and an

Figure 8

).

FIGURE 8. Thermistor/Resistor Network

approximate staight line TC generated. See

Figure 9

for a
graphic representation of the ideal calculated values of Iset
and the actual measured values generated. Notice that
there is very close agreement between the two graphs. The
circuit actually creates an S-shaped curve around the ideal.

The low-cost thermistor is available from Keystone and is
listed as follows: RL2008-52.3K-155-D1.

OVER-CURRENT LIMIT DETECTOR

Other applications include an over-current detector, as
shown in

Figure 10

. The load represented here can be ei-

ther a single component or an entire system. Resistors R3

Thermistor
Keystone:
RL2008-52.3K-155-D1

@

25§C

100k

TL/H/8707– 11

FIGURE 9. Iset vs Temperature with

7

Figure 8

Circuit

TL/H/8707– 12

Application Hints (Continued)

FIGURE 10. Using the LM1946 as an Over-Current Limit Detector

and R4 again allow the system designer to tailor the thresh­old 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 refer­ence voltage, provides a current and threshold that is inde­pendent of the supply voltage, V
directly proportional to supply. These resistors allow thresh-

. R4 provides a current

CC

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 characteris­tics 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. Like­wise, 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 nomi­nal 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 val­ue, enough margin exists that minor variations due to tem­perature 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 cur­rent 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

TL/H/8707– 7

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.

Vthrshld-loeIset#(R10bR11)

e

Iset

Vthrshld-hi

(R13bR12)

#

TL/H/8707– 8

FIGURE 11. Current Limit Window Detector

COMPARATOR INPUT STAGE

The LM1946 IC consists of five specially designed compara­tor 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 ac­commodate 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

Figure 12

). This is vital in a system which must operate in
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 compara­tor, the two emitter-base PNP junctions become reverse bi­ased 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.

8

Application Hints (Continued)

FIGURE 12. Comparator Input Stage

TL/H/8707– 13

THE OUTPUT SECTION

The output section of the LM1946 is different from most
automotive comparators as it employs high beta proprietary
PNP transistors which are very rugged and capable of high­er output currents. Each of the five comparator outputs is
capable of at least 20 mA of drive and are internally current
limited and protected against supply overvoltage. The
LM1946 is therefore capable of driving LED’s directly and
larger bulbs via an external grounded base NPN (see

ures 13

and14). The outputs can also be wired-or together

Fig-

without harm.

For use in systems with a microprocessor flag instead of a
dashboard indicator, the LM1946 can be powered by a stan­dard 5V logic supply. This prevents the LM1946 output from
swinging above the microprocessor supply which might
cause latch problems. Since the input common mode range
is independent of supply, the inputs can still operate at any
level up to 26V. Since the outputs can source current only,
pull-down resistors as in

Figure 15

are required, their value
depending on the input drive requirements of the particular
microprocessor used. When operating with a V
less than 7V, it is important to connect the V
This forces V
internal circuitry.

to a fixed voltage which is used for bias of

REF

pin to VCC.

REF

CC

supply

FIGURE 14

TL/H/8707– 14

TEST PIN

The test pin is a high impedance logic input. Forcing this pin

t

high (

2V) forces all five comparator outputs on. This is
used to test the indicator LED display (or other output load).
The usual application circuit connects this pin to the ignition
crank line. During engine crank, therefore, the LM1946 out­put display will light, similar to the usual dashboard indica­tors. The test pin was designed to operate with the usual
transient voltages found on the crank line as long as a limit­ing resistor (e.g. 30k) separates them

(Figure 1)

.

Minimum pulse width (ms)&0.01a1.5#C1 (mF)

FIGURE 13

TL/H/8707– 19

TL/H/8707– 15

FIGURE 15

9

Application Hints (Continued)

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 be­low the threshold trip point, noise peaks may exceed it. A
LED display could then flicker or appear dimly lit, or exces­sive software routines and processor time may be required
for a mP 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 effec­tive general methodology to achieve this is to split the resis­tor 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
placed with 3.9k and 1.2k resistors as shown in
(R1 increasing from 6.2k to 10k to compensate). The value
of capacitor C2 depends upon the degree of filtering re­quired, 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 power­up 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 cir­cuit, the methodology described can be replaced with a sim­ple capacitor across the comparator input pins).

Figure 10

could be re-

Figure 16a

l

5.0mV). It is this requirement

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 resis­tor 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.

a. Open-Circuit Detector

TL/H/8707– 16

b. Over-Current Limit Detector

TL/H/8707– 17

FIGURE 16. Input Noise Filters for

Various Application Circuits

FIGURE 17. Additional Noise Filters

TL/H/8707– 18

10

Circuit Schematic

TL/H/8707– 3

FIGURE 18

11

Physical Dimensions inches (millimeters)

20-Lead Molded Dip (N)
Order Number LM1946N

NS Package Number N20A

LM1946 Over/Under Current Limit Diagnostic Circuit

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DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL
SEMICONDUCTOR CORPORATION. As used herein:

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systems which, (a) are intended for surgical implant support device or system whose failure to perform can
into the body, or (b) support or sustain life, and whose be reasonably expected to cause the failure of the life
failure to perform, when properly used in accordance support device or system, or to affect its safety or
with instructions for use provided in the labeling, can effectiveness.
be reasonably expected to result in a significant injury
to the user.

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