CS289
Tachometer Application
RPM x # OF CYL. = Frequency
60
2
V
F/V
OUT
= 2.1 + Frequency x C
T
x R
T
(V
REG
-0 .7)
The above equations were used in calculating the follow-
ing values, where V
F/V
OUT
= 7.1V at =270° and C
T
= 0.01 F.
4 cylinder: Freq = 200Hz, R
T
= 320kΩ
6 cylinder: Freq = 300Hz, R
T
= 220kΩ
8 cylinder: Freq = 400Hz, R
T
= 150kΩ
Typical values shown above apply to a nominal value of
V
REG
of 8.5 volts. It must be realized that trimming of R
T
will be necessary to compensate for variations in regula-
tor voltage from one unit to another.
An alternative to this adjustment is to replace R
2
with a
potentiometer, as shown in Figure 2.
Partial schematic shown in Figure 3 represents one met-
hod for use with DC applications instead of frequency.
10kΩ
TRIM
I
OUT/EΘ
S
Q
OUT
F-V
OUT
V
COS
V
REG
S
Q
IN
F-V
OUT
R
T
R
T
C
P
Gnd
C
P
+
C
P
–
C
P
V
BIAS
Gnd
C
P
+
C
P
–
NC
2.1V
0.7V
C
T
I
IN
1kΩ
Figure 2: Alternate Trimming Method
Figure 3: DC Application
5
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
