CS8191
http://onsemi.com
8
COSINE
SINE
Air Core
Gauge
Speedometer
VCC
F/VOUT
VREG
CP+
CP–
GND
COS+
SINE+
SQOUT
BIAS
GND
COS–
SINE–
FREQIN
1
CS8191
Figure 9. Speedometer or Tachometer Application
Notes:
1. The product of C4 and RT have a direct effect on gain and therefore directly
affect temperature compensation.
2. C4 Range; 20 pF to 0.2
F.
3. R4 Range; 100 k
to 500 k.
4. The IC must be protected from transients above 60 V and reverse battery conditions.
5. Additional filtering on the FREQIN 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
R1
D1
0.1
F
3.9,
500 mW
50 V,
500 mW Zener
1.0 A,
600 PIV
10 k
0.1
F
C4
0.47
F
R4
1.0 k
CCP
0.0033
F,
+/–30 PPM/
°C
R3
3.0 k
RT
Trim Resistor,
+/–20 PPM/
°C
+
Design Example
Maximum meter Deflection = 270
°
Maximum Input Frequency = 350 Hz
1. Select RT and CCP
Q + 970
FREQ
CCP
RT
Let CCP = 0.0033 F, find RT
RT +
270
°
970
350 Hz
0.0033
mF
RT + 243 kW
RT should be a 250 k
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 k
The charge current for CCP is
VREG * 0.7 V
3.3 k
W
+ 1.90 mA
CCP 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 CCP is charged, assume that the
(R3 + R4) CCP time constant is less than 10% of the
minimum input period.
T
+ 10%
1
350 Hz
+ 285ms
Choose R4 = 1.0 k
.
Discharge time: tDCHG= R3 × CCP = 3.3 k × 0.0033 F
= 10.9
s
Charge time: tCHG = (R3 + R4)CCP = 4.3 k. × 0.0033 F
= 14.2
s
3. Determine C4
C4 is selected to satisfy both the maximum allowable
ripple voltage and response time of the meter movement.
C4
+
CCP(VREG * 0.7 V)
DVMAX
With C4 = 0.47
F, 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.