LM1819 Air-Core Meter Driver
February 1995
LM1819 Air-Core Meter Driver
General Description Features
The LM1819 is a function generator/driver for air-core Y Self-centering 20 mA outputs
(moving-magnet) meter movements. A Norton amplifier and Y 12V operation
an NPN transistor are included on chip for signal condition- Y Norton amplifier
ing as required. Driver outputs are self-centering and devel- Y Function generator
op g 4.5V swing at 20 mA. Better than 2% linearity is guar-
anteed over a full 305-degree operating range.
Applications
Y Air-core meter driver
Y Tachometers
Y Ruggedized instruments
Typical Application
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 ( g 100 ppm)
² Components Required for Automotive Load Dump Protection
² ² Available from FARIA Co.
P O Box 983, Uncasville, CT 06382
Tel. 203-848-9271
C1995 National Semiconductor Corporation TL/H/5263 RRD-B30M115/Printed in U. S. A.
Absolute Maximum Ratings
If Military/Aerospace specified devices are required, Operating Temperature b 40§ C to a 85§ C
please contact the National Semiconductor Sales Storage Temperature b 65§ C to b 150§ C
Office/Distributors for availability and specifications.
Lead Temp. (Soldering, 10 seconds) 260§ C
Supply Voltage, V a (pin 13) 20V
BVCEO 20VMIN
Power Dissipation (note 1) 1300 mW
Electrical Characteristics VS e 13.1V TA e 25§ C unless otherwise specified
Symbol Parameter Pin(s) Conditions Min Typ Max Units
IS Supply Current 13 Zero Input Frequency
65 mA
(See Figure 1 )
VREG Regulator Voltage 11 IREG e 0 mA 8.1 8.5 8.9 V
Regulator Output Resistance 11 IREG e 0 mA to 3 mA 13.5 X
VREF Reference Voltage 4 IREF e 0 mA 1.9 2.1 2.3 V
Reference Output Resistance 4 IREF e 0 mA to 50 mA 5.3 kX
Norton Amplifier Mirror Gain 5, 6 IBIAS j 20 mA 0.9 1.0 1.1
hFE NPN Transistor DC Gain 9, 10 125
Function Generator Feedback 1 V1 e 5.1V
1.0 mA
Bias Current
Drive Voltage Extremes, 2, 12 ILOAD e 20 mA
g4 g 4.5 V
Sine and Cosine
Sine Output Voltage 2 V8 e VREF
b 350 0 a 350 mV
with Zero Input
Function Generator Linearity FSD e 305§ g 1.7 %FSD
k Function Generator Gain Meter Deflection/DV8 50.75 53.75 56.75 § /V
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.
Application Hints
AIR-CORE METER MOVEMENTS H fields (Figure 3(c)). H is proportional to the voltage applied
Air-core meters are often favored over other movements as to a coil. Therefore, by varying both the polarity and magni-
a result of their mechanical ruggedness and their indepen- tude of the coil voltages the axle assembly can be made to
dence of calibration with age. A simplified diagram of an air- rotate a full 360§ . The LM1819 is designed to drive the me-
core meter is shown in Figure 2 . There are three basic ter through a minimum of 305§ .
pieces: a magnet and pointer attached to a freely rotating
axle, and two coils, each oriented at a right angle with re-
spect 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
TL/H/5263 – 2
FIGURE 2. Simplified Diagram of an Air Core Meter.
2
Application Hints (Continued)
TL/H/5263 – 3
(a) (b) (c)
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 Comparing [3] to [2] we see that if HSINE varies as the sine
nylon bushings. The torque exerted on the pointer is much of i, and HCOSINE varies as the cosine of i, we will gener-
greater than that found in a typical d’Arsonval movement. In ate a net H field whose direction is the same as i. And since
contrast to a d’Arsonval movement, where calibration is a the axle assembly aligns itself with the net H field, the point-
function of spring and magnet characteristics, air-core me- er will always point in the direction of i.
ter calibration is only affected by the mechanical alignment
THE LM1819
of the drive coils. Mechanical calibration, once set at manu-
facture, can not change. Included in the LM1819 is a function generator whose two
outputs are designed to vary approximately as the sine and
Making pointer position a linear function of some input is a
cosine of an input. A minimum drive of g 20 mA at g 4V is
matter of properly ratioing the drive to each coil. The H field
available at pins 2 (sine) and 12 (cosine). The common side
contributed by each coil is a function of the applied current,
of each coil is returned to a 5.1V zener diode reference and
and the current is a function of the coil voltage. Our desired
fed back to pin 1.
result is to have i (pointer deflection, measured in degrees)
proportional to an input voltage: For the function generator, k j 54§ /V (in equation 1). The
input (pin 8) is internally connected to the Norton amplifier’s
i e kVIN [1]
output. VIN as considered in equation [1] is actually the dif-
where k is a constant of proportionality, with units of de- ference of the voltages at pins 8 (Norton output/function
grees/volt. The vector sum of each coils’ H field must follow generator input) and 4. Typically the reference voltage at pin
the deflection angle i. We know that the axle assembly 4 is 2.1V. Therefore,
always points in the direction of the vector sum of HSINE
i e k(V8bVREF) e 54 (V8b2.1) [4]
and HCOSINE. This direction (see Figure 4 ) is found from the
formula: As V8 varies from 2.1V to 7.75V, the function generator will
drive the meter through the chip’s rated 305§ range.
(i) e arctan À l HSINE l / l HCOSINE l Ó [2]
Air-core meters are mechanically zeroed during manufac-
Recalling some basic trigonometry,
ture such that when only the cosine coil is driven, the point-
(i) e arctan(sin (i) / cos(i )) [3] er indicates zero degrees deflection. However, in some ap-
plications 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 amplifi-
er 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 IMIRROR e 5 mA. The Norton amplifier’s output (8) is
designed to source current into its load. To bypass the Nor-
ton amplifier simply ground the non-inverting input, tie the
inverting input to the reference, and drive pin 8 (Norton out-
put/function generator input) directly.
An NPN transistor is included on chip for buffering and
TL/H/5263 – 4 squaring input signals. Its usefulness is exemplified in Fig-
FIGURE 4. The vector sum of HCOSINE and HSINE points ures 1 & 6 where an ignition pulse is converted to a rectan-
in a direction i measured in a clockwise direction from gular waveform by an RC network and the transistor. The
HCOSINE. emitter is internally connected to ground. It is important not
to allow the base to drop below b5Vdc, 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.
3
Application Hints (Continued)
TACHOMETER APPLICATION
A measure of the operating level of any motor or engine is The charge pump circuit in Figure 7 can be operated in two
the rotational velocity of its output shaft. In the case of an modes: constant input pulse width (C1 acts as a coupling
automotive engine the crankshaft speed is measured using capacitor) and constant input duty cycle (C1 acts as a differ-
the units ‘‘revolutions per minute’’ (RPM). It is possible to entiating capacitor). The transfer functions for these two
indirectly measure the speed of the crankshaft by using the modes are quite diverse. However, deflection is always di-
signal present on the engine’s ignition coil. The fundamental rectly proportional to R2 and ripple is proportional to C2.
frequency of this signal is a function of engine speed and The following variables are used in the calculation of meter
the number of cylinders and is calculated (for a four-stroke deflection:
engine) from the formula:
symbol description
f e n0/120 (Hz) (5)
n number of cylinders
where n e number of cylinders, and 0 e rotational velocity of
0, 0IDLE engine speed at redline and idle, RPM
the crankshaft in RPM. From this formula the maximum fre-
quency normally expected (for an 8 cylinder engine turning i pointer deflection at redline, degrees
4500RPM) is 300 Hz. In certain specialized ignition systems e charge pump input pulse width, seconds
(motorcycles and some automobiles) where the coil wave- VIN peak to peak input voltages, volts
form is operated at twice this frequency ( f e 0/60). These
Di maximum desired ripple, degrees
systems are identified by the fact that multiple coils are used
in lieu of a single coil and distributor. Also, the coils have k function generator gain, degrees/volt
two outputs instead of one. f , f IDLE input frequency at redline and idle, Hz
A typical automotive tachometer application is shown in Fig- Where the NPN transistor and regulator are used to create a
ure 1 . The coil waveform is filtered, squared and limited by pulse VIN e 8.5V. Acceptable ripple ranges from 3 to 10 de-
the RC network and NPN transistor. The frequency of the grees (a typical pointer is about 3 degrees wide) depending
pulse train at pin 9 is converted to a proportional voltage by on meter damping and the input frequency.
the Norton amplifier’s charge pump configuration. The igni- The constant pulse width circuit is designed using the fol-
tion circuit shown in Figure 5 is typical of automotive sys- lowing equations:
tems. The switching element ‘‘S’’ is opened and closed in VIN
(1) 100 mAk k 3 mA
synchronism with engine rotation. When ‘‘S’’ is closed, en- R1
ergy is stored in Lp. When opened, the current in Lp diverts 10e
(2) C1 t
from ‘‘S’’ into C. The high voltage produced in Ls when ‘‘S’’ R1
is opened is responsible for the arcing at the spark plug. R1i 120R1i
(3) R2 e e
The coil voltage (see Figure 6 ) can be used as an input to VINek f VINn0ek
the LM1819 tachometer circuit. This waveform is essentially 1 1
(4) C2 e e
constant duty cycle . D4 rectifies this waveform thereby pre- R2Di f IDLE R2Din0IDLE
venting negative voltages from reaching the chip. C4 and The constant duty cycle equations are as follows:
R5 form a low pass filter which attenuates the high frequen-
RREG t 3 kX
cy ringing, and R7 limits the input current to about 2.5mA.
R6 acts as a base bleed to shut the transistor OFF when R1 s VINx104 bRREG
‘‘S’’ is closed. The collector is pulled up to the internal regu- C1 s e/10(RREG a R1)
lator by RREG. The output at pin 9 is a clean rectangular RZ e i/3.54n0C1 e i/425 f C1
pulse.
C2 e 425C1/Di
Many ignition systems use magnetic, hall effect or optical
The values in Figure 1 were calculated with n e 4,
sensors to trigger a solid state switching element at ‘‘S.’’
0 e 6000RPM, i e 270 degrees, e e 1 ms, VIN is
These systems (see the LM1815) typically generate pulses
VREGb0.7V, and Di e 3 degrees in the constant duty cycle
of constant width and amplitude suitable for driving the
mode. For distributorless ignitions these same equations will
charge pump directly.
apply if 0/60 is substituted for f .
4
Equivalent Schematic
TL/H/5263 – 12
5
Typical Applications
TL/H/5263 – 9
FIGURE 5. Typical Pulse-Squaring Circuit for
Automotive Tachometers.
TL/H/5263 – 11
TL/H/5263–10
FIGURE 6. Waveforms Encountered in Automotive FIGURE 7. Tachometer Charge Pump.
Tachometer Circuit.
Voltage Driven Meter with Norton Amplifier Buffer
TL/H/5263 – 5
Deflection e 54 (VIN b .7)R2/R1 (degrees)
0 to 305§ deflection is obtained with .7 to 5V input.
*Full scale deflection is adjusted by trimming R2.
6
Typical Applications (Continued)
Unbuffered Voltage Driven Meter
TL/H/5263 – 6
Deflection e 54(VIN b 2.1) (degrees)
0 to 305§ deflection is obtained for inputs of 2.1 to 7.75V.
Full scale deflection is adjusted by trimming the input voltage.
Current Driven Meter
TL/H/5263 – 7
Deflection e 54R2IIN (degrees)
Inputs of 0 to 100 mA deflect the meter 0 to 270§ .
*Full scale deflection is adjusted by trimming R2.
7
Typical Applications (Continued)
Level Shifted Voltage Driven Meter
TL/H/5263 – 8
Deflection e 54VIN (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.
8
Physical Dimensions inches (millimeters)
14-Lead (0.150× Wide) Molded Small Outline Package, JEDEC
Order Number LM1819M
NS Package Number M14A
9
LM1819 Air-Core Meter Driver
Physical Dimensions inches (millimeters) (Continued)
Molded Dual-In-Line Package (N)
Order Number LM1819N
NS Package Number N14A
LIFE SUPPORT POLICY
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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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