CS8191
http://onsemi.com
8
COSINE
SINE
Air Core
Gauge
Speedometer
V
CC
F/V
OUT
V
REG
CP+
CP–
GND
GND
COS+
SINE+
SQ
OUT
BIAS
GND
GND
COS–
SINE–
FREQ
IN
1
CS8191
Figure 9. Speedometer or Tachometer Application
Notes:
1. The product of C4 and R
T
have a direct effect on gain and therefore directly
affect temperature compensation.
2. C4 Range; 20 pF to 0.2 µF.
3. R4 Range; 100 kΩ to 500 kΩ.
4. The IC must be protected from transients above 60 V and reverse battery conditions.
5. Additional filtering on the FREQ
IN
lead may be required.
6. Gauge coil connections to the IC must be kept as short as possible (≤ 3.0 inch)
for best pointer stability.
Battery
GND
Typical Speedometer
Input
R2
C3
C1
D2
R1D1
0.1 µF
3.9,
500 mW
50 V,
500 mW Zener
1.0 A,
600 PIV
10 kΩ
0.1 µF
C4
0.47 µF
R4
1.0 kΩ
C
CP
0.0033 µF,
+/–30 PPM/°C
R3
3.0 kΩ
R
T
Trim Resistor,
+/–20 PPM/°C
+
Design Example
Maximum meter Deflection = 270°
Maximum Input Frequency = 350 Hz
1. Select R
T
and C
CP
970 FREQ C
CP
R
T
Let C
CP
= 0.0033 µF, find R
T
R
T
270°
970 350 Hz 0.0033 F
R
T
243 k
RT should be a 250 kΩ potentiometer to trim out any
inaccuracies due to IC tolerances or meter movement
pointer placement.
2. Select R3 and R4
Resistor R3 sets the output current from the voltage
regulator. The maximum output current from the voltage
regulator is 10 mA. R3 must ensure that the current does not
exceed this limit.
Choose R3 = 3.3 kΩ
The charge current for C
CP
is
V
REG
0.7 V
3.3 k
1.90 mA
C
CP
must charge and discharge fully during each cycle of
the input signal. Time for one cycle at maximum frequency
is 2.85 ms. To ensure that C
CP
is charged, assume that the
(R3 + R4) C
CP
time constant is less than 10% of the
minimum input period.
T 10%
1
350 Hz
285s
Choose R4 = 1.0 kΩ.
Discharge time: t
DCHG
= R3 × C
CP
= 3.3 kΩ × 0.0033 µF
= 10.9 µs
Charge time: t
CHG
= (R3 + R4)C
CP
= 4.3 kΩ. × 0.0033 µF
= 14.2 µs
3. Determine C4
C4 is selected to satisfy both the maximum allowable
ripple voltage and response time of the meter movement.
C4
C
CP
(V
REG
0.7 V)
V
MAX
With C4 = 0.47 µF, the F/V ripple voltage is 44 mV.
Figure 10 shows how the CS8191 and the CS8441 are
used to produce a Speedometer and Odometer circuit.
