Datasheet LM1819N Datasheet (NSC)

LM1819 Air-Core Meter Driver

LM1819 Air-Core Meter Driver

February 1995

General Description

The LM1819 is a function generator/driver for air-core
(moving-magnet) meter movements. A Norton amplifier and
an NPN transistor are included on chip for signal condition­ing as required. Driver outputs are self-centering and devel-

g

op

4.5V swing at 20 mA. Better than 2% linearity is guar-

anteed over a full 305-degree operating range.

Typical Application

Features

Y

Self-centering 20 mA outputs

Y

12V operation

Y

Norton amplifier

Y

Function generator

Applications

Y

Air-core meter driver

Y

Tachometers

Y

Ruggedized instruments

TL/H/5263– 1

FIGURE 1. Automotive Tachometer Application. Circuit shown operates

with 4 cylinder engine and deflects meter pointer (270

) at 6000 RPM.

§

Order Number LM1819M or LM1819N

See NS Package Number M14A or N14A

*TRW Type X463UW Polycarbonate Capacitor

**RN60D Low TC Resistor (

²

Components Required for Automotive Load Dump Protection

²²

Available from FARIA Co.

C

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

g

100 ppm)

P O Box 983, Uncasville, CT 06382
Tel. [phone]

TL/H/5263

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

Power Dissipation (note 1) 1300 mW

a

(pin 13) 20V

Operating Temperature

Storage Temperature

b

40§Ctoa85§C

b

65§Ctob150§C

Lead Temp. (Soldering, 10 seconds) 260§C

BV

CEO

20V

MIN

Electrical Characteristics V

S

e

13.1V T

e

25§C unless otherwise specified

A

Symbol Parameter Pin(s) Conditions Min Typ Max Units

I

S

V

REG

V

REF

h

FE

k Function Generator Gain Meter Deflection/DV

Note 1: For operation above 25§C, the LM1819 must be derated based upon a 125§C maximum junction temperature and a thermal resistance of 76§C/W which
applies for the device soldered in a printed circuit board and operating in a still-air ambient.

Supply Current 13 Zero Input Frequency

(See

Figure 1

)

Regulator Voltage 11 I

Regulator Output Resistance 11 I

Reference Voltage 4 I

Reference Output Resistance 4 I

Norton Amplifier Mirror Gain 5, 6 I

e

0 mA 8.1 8.5 8.9 V

REG

e

0 mA to 3 mA 13.5 X

REG

e

0 mA 1.9 2.1 2.3 V

REF

e

0 mAto50mA 5.3 kX

REF

j

20 mA 0.9 1.0 1.1

BIAS

NPN Transistor DC Gain 9, 10 125

Function Generator Feedback 1 V
Bias Current

Drive Voltage Extremes, 2, 12 I
Sine and Cosine

Sine Output Voltage 2 V
with Zero Input

Function Generator Linearity FSDe305

e

1

LOAD

e

8

5.1V

e

V

REF

20 mA

§

g

b

50.75 53.75 56.75

8

g

4

350 0

1.0 mA

65 mA

4.5 V

a

350 mV

g

1.7 %FSD

/V

§

Application Hints

AIR-CORE METER MOVEMENTS

Air-core meters are often favored over other movements as
a result of their mechanical ruggedness and their indepen­dence of calibration with age. A simplified diagram of an air­core meter is shown in

Figure 2

. There are three basic
pieces: a magnet and pointer attached to a freely rotating
axle, and two coils, each oriented at a right angle with re­spect to the other. The only moving part in this meter is the
axle assembly. The magnet will tend to align itself with the
vector sum of H fields of each coil, where H is the magnetic
field strength vector. If, for instance, a current passes
through the cosine coil (the reason for this nomenclature
will become apparent later) as shown in

Figure 3(a)

, the
magnet will align its magnetic axis with the coil’s H field.
Similarly, a current in the sine coil (

Figure 3(b)

) causes the
magnet to align itself with the sine H field. If currents are
applied simultaneously to both sine and cosine coils, the
magnet will turn to the direction of the vector sum of the two

H fields

(Figure 3(c)).

H is proportional to the voltage applied
to a coil. Therefore, by varying both the polarity and magni­tude of the coil voltages the axle assembly can be made to
rotate a full 360
ter through a minimum of 305

. The LM1819 is designed to drive the me-

§

.

§

FIGURE 2. Simplified Diagram of an Air Core Meter.

2

TL/H/5263– 2

Application Hints (Continued)

FIGURE 3. Magnet and pointer position are controlled by the H field generated by the two drive coils.

In an air-core meter the axle assembly is supported by two
nylon bushings. The torque exerted on the pointer is much
greater than that found in a typical d’Arsonval movement. In
contrast to a d’Arsonval movement, where calibration is a
function of spring and magnet characteristics, air-core me­ter calibration is only affected by the mechanical alignment
of the drive coils. Mechanical calibration, once set at manu­facture, can not change.

Making pointer position a linear function of some input is a
matter of properly ratioing the drive to each coil. The H field
contributed by each coil is a function of the applied current,
and the current is a function of the coil voltage. Our desired
result is to have i (pointer deflection, measured in degrees)
proportional to an input voltage:

e

i

kV

IN

[1]

where k is a constant of proportionality, with units of de­grees/volt. The vector sum of each coils’ H field must follow
the deflection angle i. We know that the axle assembly
always points in the direction of the vector sum of H
and H
formula:

. This direction (see

COSINE

e

(i)

arctan

Figure 4

) is found from the

À

H

l

SINE

l/l

H

COSINE

Ó

l

SINE

[2]

Recalling some basic trigonometry,

e

(i)

arctan(sin (i) / cos(i ))

FIGURE 4. The vector sum of H
in a direction i measured in a clockwise direction from
H

.

COSINE

COSINE

and H

[3]

TL/H/5263– 4

points

SINE

(c)(b)(a)

TL/H/5263– 3

Comparing[3]to[2]we see that if H
of i, and H
ate a net H field whose direction is the same as i. And since

varies as the cosine of i, we will gener-

COSINE

varies as the sine

SINE

the axle assembly aligns itself with the net H field, the point­er will always point in the direction of i.

THE LM1819

Included in the LM1819 is a function generator whose two
outputs are designed to vary approximately as the sine and
cosine of an input. A minimum drive of

g

20 mA atg4V is
available at pins 2 (sine) and 12 (cosine). The common side
of each coil is returned to a 5.1V zener diode reference and
fed back to pin 1.

For the function generator, kj54
input (pin 8) is internally connected to the Norton amplifier’s
output. V
ference of the voltages at pins 8 (Norton output/function

as considered in equation[1]is actually the dif-

IN

/V (in equation 1). The

§

generator input) and 4. Typically the reference voltage at pin
4 is 2.1V. Therefore,

e

b

i

k(V

V

)e54 (V

8

REF

b

2.1)

8

As V8varies from 2.1V to 7.75V, the function generator will
drive the meter through the chip’s rated 305

range.

§

Air-core meters are mechanically zeroed during manufac­ture such that when only the cosine coil is driven, the point­er indicates zero degrees deflection. However, in some ap­plications a slight trim or offset may be required. This is
accomplished by sourcing or sinking a DC current of a few
microamperes at pin 4.

A Norton amplifier is available for conditioning various input
signals and driving the function generator. A Norton amplifi­er was chosen since it makes a simple frequency to voltage
converter. While the non-inverting input (pin 6) bias is at one
diode drop above ground, the inverting input (5) is at 2.1V,
equal to the pin 4 reference. Mirror gain remains essentially
flat to I
designed to source current into its load. To bypass the Nor-

MIRROR

e

5 mA. The Norton amplifier’s output (8) is

ton amplifier simply ground the non-inverting input, tie the
inverting input to the reference, and drive pin 8 (Norton out­put/function generator input) directly.

An NPN transistor is included on chip for buffering and
squaring input signals. Its usefulness is exemplified in

ures 1

&6where an ignition pulse is converted to a rectan-

Fig-

gular waveform by an RC network and the transistor. The
emitter is internally connected to ground. It is important not
to allow the base to drop below

b

5Vdc, as damage may
occur. The 2.1V reference previously described is derived
from an 8.5V regulator at pin 11. Pin 11 is used as a stable
supply for collector loads, and currents of up to 5 mA are
easily accommodated.

[4]

3

Application Hints (Continued)

TACHOMETER APPLICATION

A measure of the operating level of any motor or engine is
the rotational velocity of its output shaft. In the case of an
automotive engine the crankshaft speed is measured using
the units ‘‘revolutions per minute’’ (RPM). It is possible to
indirectly measure the speed of the crankshaft by using the
signal present on the engine’s ignition coil. The fundamental
frequency of this signal is a function of engine speed and
the number of cylinders and is calculated (for a four-stroke
engine) from the formula:

e

f

n0/120 (Hz) (5)

e

where n
the crankshaft in RPM. From this formula the maximum fre­quency normally expected (for an 8 cylinder engine turning
4500RPM) is 300 Hz. In certain specialized ignition systems
(motorcycles and some automobiles) where the coil wave­form is operated at twice this frequency (
systems are identified by the fact that multiple coils are used
in lieu of a single coil and distributor. Also, the coils have
two outputs instead of one.

A typical automotive tachometer application is shown in

ure 1

the RC network and NPN transistor. The frequency of the
pulse train at pin 9 is converted to a proportional voltage by
the Norton amplifier’s charge pump configuration. The igni­tion circuit shown in
tems. The switching element ‘‘S’’ is opened and closed in
synchronism with engine rotation. When ‘‘S’’ is closed, en­ergy is stored in Lp. When opened, the current in Lp diverts
from ‘‘S’’ into C. The high voltage produced in Ls when ‘‘S’’
is opened is responsible for the arcing at the spark plug.
The coil voltage (see
the LM1819 tachometer circuit. This waveform is essentially
constant
venting negative voltages from reaching the chip. C4 and
R5 form a low pass filter which attenuates the high frequen­cy ringing, and R7 limits the input current to about 2.5mA.
R6 acts as a base bleed to shut the transistor OFF when
‘‘S’’ is closed. The collector is pulled up to the internal regu­lator by R
pulse.

Many ignition systems use magnetic, hall effect or optical
sensors to trigger a solid state switching element at ‘‘S.’’
These systems (see the LM1815) typically generate pulses
of constant
charge pump directly.

number of cylinders, and 0erotational velocity of

e

f

0/60). These

Fig-

. The coil waveform is filtered, squared and limited by

Figure 5

is typical of automotive sys-

Figure 6

) can be used as an input to

duty cycle

. D4 rectifies this waveform thereby pre-

. The output at pin 9 is a clean rectangular

REG

width

and amplitude suitable for driving the

The charge pump circuit in

Figure 7

can be operated in two
modes: constant input pulse width (C1 acts as a coupling
capacitor) and constant input duty cycle (C1 acts as a differ­entiating capacitor). The transfer functions for these two
modes are quite diverse. However, deflection is always di­rectly proportional to R2 and ripple is proportional to C2.

The following variables are used in the calculation of meter
deflection:

symbol description

n number of cylinders

0, 0

engine speed at redline and idle, RPM

IDLE

i pointer deflection at redline, degrees

e charge pump input pulse width, seconds

V

peak to peak input voltages, volts

IN

Di maximum desired ripple, degrees

k function generator gain, degrees/volt

f,f

Where the NPN transistor and regulator are used to create a
pulse V
grees (a typical pointer is about 3 degrees wide) depending

input frequency at redline and idle, Hz

IDLE

e

8.5V. Acceptable ripple ranges from 3 to 10 de-

IN

on meter damping and the input frequency.

The constant pulse width circuit is designed using the fol­lowing equations:

(1) 100 mA

(2) C

(3) R

(4) C

V

IN

k

k

3mA

R1

10e

t

1

R

1

R1i

1

f

e

f

IDLE

120R1i

VINn0ek

e

R2Din0

1

IDLE

e

2

VINek

e

2

R2Di

The constant duty cycle equations are as follows:

t

R

3kX

REG

R

1

C

1

R

Z

C

2

The values in

e

0

V

REG

mode. For distributorless ignitions these same equations will
apply if

4

s

s

e

e

6000RPM, ie270 degrees, ee1 ms, VINis

b

VINx10

e/10(R

i/3.54n0C

REG

R

REG

a

e

1

R1)

i/425fC

1

425C1/Di

Figure 1

b

0.7V, and Die3 degrees in the constant duty cycle

0/60 is substituted for

were calculated with ne4,

f

.

4

Equivalent Schematic

TL/H/5263– 12

5

Typical Applications

FIGURE 5. Typical Pulse-Squaring Circuit for

Automotive Tachometers.

TL/H/5263– 10

FIGURE 6. Waveforms Encountered in Automotive

Tachometer Circuit.

Voltage Driven Meter with Norton Amplifier Buffer

TL/H/5263– 9

TL/H/5263– 11

FIGURE 7. Tachometer Charge Pump.

Deflectione54 (V

0to305

*Full scale deflection is adjusted by trimming R

b

.7)R2/R1(degrees)

IN

deflection is obtained with .7 to 5V input.

§

TL/H/5263– 5

.

2

6

Typical Applications (Continued)

Unbuffered Voltage Driven Meter

Deflectione54(V

0to305

Full scale deflection is adjusted by trimming the input voltage.

Deflectione54R2IIN(degrees)

Inputs of 0 to 100 mA deflect the meter 0 to 270

*Full scale deflection is adjusted by trimming R

b

2.1) (degrees)

IN

deflection is obtained for inputs of 2.1 to 7.75V.

§

.

§

.

2

TL/H/5263– 6

Current Driven Meter

TL/H/5263– 7

7

Typical Applications (Continued)

Level Shifted Voltage Driven Meter

Deflectione54VIN(degrees)

Inputs of 0 to 5.65V deflect the meter through a range of 0 to 305

Full scale deflection is adjusted by trimming the input voltage.

TL/H/5263– 8

.

§

8

Physical Dimensions inches (millimeters)

14-Lead (0.150×Wide) Molded Small Outline Package, JEDEC

Order Number LM1819M

NS Package Number M14A

9

Physical Dimensions inches (millimeters) (Continued)

LM1819 Air-Core Meter Driver

Molded Dual-In-Line Package (N)

Order Number LM1819N

NS Package Number N14A

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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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