LM1819N National Semiconductor, LM1819N Datasheet - Page 3

• Manufacturer Part Number: LM1819N
• Description: IC AIR CORE MTER DVR(TACH)14DIP
• Manufacturer: National Semiconductor
• Type: Driverr

Specifications of LM1819N

• Number Of Drivers/receivers: 0/1
• Voltage - Supply: 12V
• Mounting Type: Through Hole
• Package / Case: *
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Protocol: -
• Other names: *LM1819N

Application Hints
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 me-
ter calibration is only affected by the mechanical alignment
of the drive coils Mechanical calibration once set at manu-
facture 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
proportional to an input voltage
where k is a constant of proportionality with units of de-
grees volt The vector sum of each coils’ H field must follow
the deflection angle
always points in the direction of the vector sum of H
and H
formula
Recalling some basic trigonometry
FIGURE 4 The vector sum of H
in a direction
H
COSINE
COSINE
( )
( )
e
FIGURE 3 Magnet and pointer position are controlled by the H field generated by the two drive coils
e
e
kV
arctan
arctan(sin ( ) cos( ))
IN
This direction (see Figure 4 ) is found from the
measured in a clockwise direction from
(pointer deflection measured in degrees)
(a)
l
H
SINE
We know that the axle assembly
(Continued)
l l
COSINE
H
COSINE
and H
l
SINE
3
TL H 5263 – 4
1
2
points
SINE
(b)
3
ures 1
Comparing 3 to 2 we see that if H
of
ate a net H field whose direction is the same as
the axle assembly aligns itself with the net H field the point-
er will always point in the direction of
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
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 k j 54 V (in equation 1) The
input (pin 8) is internally connected to the Norton amplifier’s
output V
ference of the voltages at pins 8 (Norton output function
generator input) and 4 Typically the reference voltage at pin
4 is 2 1V Therefore
As V
drive the meter through the chip’s rated 305 range
Air-core meters are mechanically zeroed during manufac-
ture such that when only the cosine coil is driven the point-
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 I
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
squaring input signals Its usefulness is exemplified in 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
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
8
and H
varies from 2 1V to 7 75V the function generator will
MIRROR
IN
6 where an ignition pulse is converted to a rectan-
e
COSINE
as considered in equation 1 is actually the dif-
k(V
8
e
b
5 mA The Norton amplifier’s output (8) is
V
varies as the cosine of
REF
)
e
54 (V
8
b
b
5V
2 1)
SINE
(c)
dc
g
varies as the sine
20 mA at
as damage may
we will gener-
TL H 5263– 3
And since
g
4V is
4