LM1819N National Semiconductor, LM1819N Datasheet - Page 4

• 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

ure 1 The coil waveform is filtered squared and limited by
Application Hints
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
where n
the crankshaft in RPM From this formula the maximum fre-
quency normally expected (for an 8 cylinder engine turning
4500RPM) is 300 Hz In certain specialized ignition systems
(motorcycles and some automobiles) where the coil wave-
form 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 Fig-
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 igni-
tion circuit shown in Figure 5 is typical of automotive sys-
tems The switching element ‘‘S’’ is opened and closed in
synchronism with engine rotation When ‘‘S’’ is closed en-
ergy 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 Figure 6 ) can be used as an input to
the LM1819 tachometer circuit This waveform is essentially
constant duty cycle D4 rectifies this waveform thereby pre-
venting negative voltages from reaching the chip C4 and
R5 form a low pass filter which attenuates the high frequen-
cy 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 regu-
lator 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 width and amplitude suitable for driving the
charge pump directly
e
f e
number of cylinders and
REG
n
120
The output at pin 9 is a clean rectangular
(Hz)
(Continued)
e
rotational velocity of
f e
60) These
(5)
4
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 differ-
entiating capacitor) The transfer functions for these two
modes are quite diverse However deflection is always di-
rectly proportional to R2 and ripple is proportional to C2
The following variables are used in the calculation of meter
deflection
symbol
n
V
k
Where the NPN transistor and regulator are used to create a
pulse V
grees (a typical pointer is about 3 degrees wide) depending
on meter damping and the input frequency
The constant pulse width circuit is designed using the fol-
lowing equations
(1)
(2)
(3)
(4)
The constant duty cycle equations are as follows
R
R
C
R
C
The values in Figure 1 were calculated with n
V
mode For distributorless ignitions these same equations will
apply if
f f
IN
REG
REG t
1 s
1 s
Z
2
e
IDLE
e
e
6000RPM
IDLE
b
V
425C
100 A
C
R
C
IN
IN
0 7V and
10(R
1 t
2
2
3 54n C
3 k
e
x10
e
e
description
number of cylinders
engine speed at redline and idle RPM
pointer deflection at redline degrees
charge pump input pulse width seconds
peak to peak input voltages volts
maximum desired ripple degrees
function generator gain degrees volt
input frequency at redline and idle Hz
60 is substituted for
8 5V Acceptable ripple ranges from 3 to 10 de-
1
10
V
R
R
4
REG
R
IN
2
1
k
b
1
V
R1
k
1
R
a
1
IN
f
f
e
REG
IDLE
e
e
R
k
270
e
1
3 mA
V
)
120R
e
3 degrees in the constant duty cycle
IN
425
n
R
2
degrees
1
f
C
k
1
n
1
f
IDLE
e
1
ms
V
IN
e
4
is