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LM1819 Scheda tecnica(PDF) 3 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Il numero della parte LM1819
Spiegazioni elettronici  Air-Core Meter Driver
Download  10 Pages
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Produttore elettronici  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM1819 Scheda tecnica(HTML) 3 Page - National Semiconductor (TI)

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Application Hints (Continued)
TLH5263 – 3
(c)
(b)
(a)
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 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 i (pointer deflection measured in degrees)
proportional to an input voltage
ie
kVIN
1
where k is a constant of proportionality with units of de-
greesvolt 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 HSINE
and HCOSINE This direction (see Figure 4 ) is found from the
formula
(i)earctan
l HSINE l l HCOSINE l
2
Recalling some basic trigonometry
(i)earctan(sin (i)
cos(i ))
3
TLH5263 – 4
FIGURE 4 The vector sum of HCOSINE and HSINE points
in a direction i measured in a clockwise direction from
HCOSINE
Comparing 3 to 2 we see that if HSINE varies as the sine
of i and HCOSINE varies as the cosine of i we will gener-
ate a net H field whose direction is the same as i And since
the axle assembly aligns itself with the net H field the point-
er 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 g20 mA at g4V is
available at pins 2 (sine) and 12 (cosine) The common side
of each coil is returned to a 51V zener diode reference and
fed back to pin 1
For the function generator kj54 V (in equation 1) The
input (pin 8) is internally connected to the Norton amplifier’s
output VIN as considered in equation 1 is actually the dif-
ference of the voltages at pins 8 (Norton outputfunction
generator input) and 4 Typically the reference voltage at pin
4 is 21V Therefore
ie
k(V8bVREF) e 54 (V8b21)
4
As V8 varies from 21V to 775V the function generator will
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 21V
equal to the pin 4 reference Mirror gain remains essentially
flat to IMIRRORe5 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-
putfunction generator input) directly
An NPN transistor is included on chip for buffering and
squaring input signals Its usefulness is exemplified in
Fig-
ures 1
6 where an ignition pulse is converted to a rectan-
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 b5Vdc as damage may
occur The 21V reference previously described is derived
from an 85V 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


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