ITT - Index of

SAY115
SPEEDOMETER INTEGRATED CIRCUIT
CHARACTERISTICS TA = 25° C
Supply voltage ........... 11 to 16V
Quiescent Current
(without stepping motor) ..... 16mA
Input trigger thresholds
VII. "low" ..................... 2.5V
VIH "high" .................... 3.5V
Input Current
1, at V, VIH ••••••••••••••••••• =0
NOTE-The monostable will trigger on the
trailing edge of the input signal if V'Lis exceeded.
V'/13 Reference voltage ............ 6.5V
V'113 Current source voltage
range ..................... 0 to 5V
10 Instrument current,
peak value ................... 2.3V
R,/s
tp6 Output pulse period
(Pin 6) .......... 0.67 X R'/ll X Cll",
Analog output voltage D.C.
V,O f = 0 ..................... 6.5V
V,O f = fmax, tp
-T = 0.9 ......... 2.1V
period
R"ut 10 Output resistance of
analog output ............... 10K{J
fi/fo Divider ratio of binary divider
Note (1) ................ 32 or 64
V"at, Saturation voltage of motor
V,sat outputs at l,=b=200mA ....... 1.2V
NOTE (1)-Ordering Instructions:
Order SAY115X for -:- 32 (5 stage).
Order SAY115,Y for -:- 64 (6 stage).
Application Information
The speedometer SAY115 is usually driven from
the gearbox (transmission) of an automobile
vi, for example, a reed-relay contact, a proximity
switch or an inductive sensor. The monostable
is triggered by the trailing edge (negative
going) of the input signal and is therefore not
susceptible to contact bounce of a reed relay.
This assumes that the input signal meets trigger
threshold limits shown in the data. The
pulse width of the monostable is set by RC
components R'/ll and Cll/12 and can be adapted
to a wide input frequency range.
The current source output at Pin 6 is proportional
to the input frequency and can be adjusted
by a resistor or potentiometer, R,/s. The resistors
R'/8 or R'/ll can be chosen for calibration of
the speedometer system. Due to the current
source, temperature variation of the resistance of
the moving-coil instruments do not affect the
reading. The current source is essentially thermally
neutral. Thus, temperature effects are limited
to the external components R'/ll, Rus and
Cll/12. Therefore, virtually all temperature drift
effects on the reading are eliminated when
the supply voltage is turned on.
One side .of the instrument is connected to
ground thus simplifying mechanical construction.
If required, a filter capacitor can be connected
to the analog output in order to generate a
d.c. voltage as a linear function of speed.
Thus by using a simple comparator, K, a warning
signal can be set off when the speed
exceeds or falls below a pre-set value. For
convenience, a stabilized reference voltage is
available at pin 2 of 6.5 volts. A potentiometer
may be used and the temperature dependence
of the comparator switching point is therefore,
a function of the comparator, potentiometer,
and the RC components (R'/Il and Cll",).
The binary divider divides the input frequency
by 2' or 2" and drives an output stage consisting
of two NPN Darlingtons. Both outputs
change alternately to "high" and "low" during
half periods of the output signal. Both outputs
have a freewheeling diode with a common
cathode output at Pin 4.
In order to protect the circuit from high voltage
spikes generated in an automobile electrical
system an external filter network Rs, Cs should
be used as shown. The stepping motor current
does not flow through the filter resistor, Rs,
thus allowing sure starting of the stepping
motor even under low battery voltages. The
cooling pins of the package are the ground
connection pins of the SAY115 and should be
soldered to the PC board to allow good thermal
conduction.
8-107