CS8190EDWF20 View Datasheet(PDF) - Cherry semiconductor
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CS8190EDWF20 Datasheet PDF : 8 Pages
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Circuit Description and Application Notes
The CS8190 is specifically designed for use with air-core
meter movements. It includes an input comparator for
sensing an input signal from an ignition pulse or speed
sensor, a charge pump for frequency to voltage conver-
sion, a bandgap voltage regulator for stable operation,
and a function generator with sine and cosine amplifiers
to differentially drive the motor coils.
From the simplified block diagram of Figure 5A, the
input signal is applied to the FREQIN lead, this is the
input to a high impedance comparator with a typical pos-
itive input threshold of 2.0V and typical hysteresis of
0.5V. The output of the comparator, SQOUT, is applied to
the charge pump input CP+ through an external capacitor
CT. When the input signal changes state, CT is charged
or discharged through R3 and R4. The charge accumulat-
ed on CT is mirrored to C4 by the Norton Amplifier cir-
cuit comprising of Q1, Q2 and Q3. The charge pump out-
put voltage, F/VOUT, ranges from 2V to 6.3V depending
on the input signal frequency and the gain of the charge
pump according to the formula:
F/VOUT = 2.0V + 2 ´ FREQ ´ CT ´ RT ´ (VREG Ð 0.7V)
RT is a potentiometer used to adjust the gain of the F/V
output stage and give the correct meter deflection. The
F/V output voltage is applied to the function generator
which generates the sine and cosine output voltages. The
output voltage of the sine and cosine amplifiers are
derived from the on-chip amplifier and function genera-
tor circuitry. The various trip points for the circuit (i.e., 0¡,
90¡, 180¡, 270¡) are determined by an internal resistor
divider and the bandgap voltage reference. The coils are
differentially driven, allowing bidirectional current flow
in the outputs, thus providing up to 305¡ range of meter
deflection. Driving the coils differentially offers faster
response time, higher current capability, higher output
voltage swings, and reduced external component count.
The key advantage is a higher torque output for the
pointer.
The output angle, Q, is equal to the F/V gain multiplied
by the function generator gain:
where:
Q = AF/V ´ AFG,
AFG = 77¡/V (typ)
The relationship between input frequency and output
angle is:
Q = AFG ´ 2 ´ FREQ ´ CT ´ RT ´ (VREG Ð 0.7V)
or,
Q = 970 ´ FREQ ´ CT ´ RT
The ripple voltage at the F/V converterÕs output is deter-
mined by the ratio of CT and C4 in the formula:
ÆV =
CT(VREG Ð 0.7V)
C4
Ripple voltage on the F/V output causes pointer or nee-
dle flutter, especially at low input frequencies.
The response time of the F/V is determined by the time
constant formed by RT and C4. Increasing the value of C4
will reduce the ripple on the F/V output but will also
increase the response time. An increase in response time
causes a very slow meter movement and may be unac-
ceptable for many applications.
The CS8190 has an undervoltage detect circuit that dis-
ables the input comparator when VCC falls below
8.0V(typical). With no input signal the F/V output volt-
age decreases and the needle moves towards zero. A sec-
ond undervoltage detect circuit at 6.0V(typical) causes the
function generator to generate a differential SIN drive
voltage of zero volts and the differential COS drive volt-
age to go as high as possible. This combination of volt-
ages (Figure 1) across the meter coil moves the needle to
the 0¡ position. Connecting a large capacitor(> 2000µF) to
the VCC lead (C2 in Figure 6) increases the time between
these undervoltage points since the capacitor discharges
slowly and ensures that the needle moves towards 0¡ as
opposed to 360¡. The exact value of the capacitor depends
on the response time of the system,the maximum meter
deflection and the current consumption of the circuit. It
should be selected by breadboarding the design in the lab.
Design Example
Maximum meter Deflection = 270¡
Maximum Input Frequency = 350Hz
1. Select RT and CT
Q = AGEN ´ ÆF/V
ÆF/V = 2 ´ FREQ ´ CT ´ RT ´ (VREG Ð 0.7V)
Q = 970 ´ FREQ ´ CT ´ RT
Let CT = 0.0033µF, Find RT
270¡
RT = 970 ´ 350Hz ´ 0.0033µF
RT = 243k½
RT should be a 250k½ potentiometer to trim out any inac-
curacies due to IC tolerances or meter movement pointer
placement.
2. Select R3 and R4
Resistor R3 sets the output current from the voltage regu-
lator. The maximum output current from the voltage reg-
ulator is 10mA, R3 must ensure that the current does not
exceed this limit.
Choose R3 = 3.3k½
The charge current for CT is:
VREG Ð 0.7V
3.3k½
= 1.90mA
C1 must charge and discharge fully during each cycle of
the input signal. Time for one cycle at maximum frequen-
cy is 2.85ms. To ensure that CT is discharged, assume that
the (R3 + R4) CT time constant is less than 10% of the
minimum input frequency pulse width.
T = 285µs
Choose R4 = 1k½.
Charge time: T = R3 ´ CT = 3.3k½ ´ 0.0033µF = 10.9µs
Discharge time:T = (R3 + R4)CT = 4.3k½ ´ 0.0033µF = 14.2µs
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