CS4121EDWFR20G Datasheet, PDF (7/12 Pages) ON Semiconductor – Low Voltage Precision Air-Core Tach/Speedo Driver

CS4121
CIRCUIT DESCRIPTION and APPLICATION NOTES
The CS4121 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 conversion,
a bandgap voltage regulator for stable operation, and a
function generator with sine and cosine amplifiers to
differentially drive the meter coils.
From the partial schematic of Figure 7, the input signal is
applied to the FREQ_IN lead, this is the input to a high
impedance comparator with a typical positive input
threshold of 2.0 V and typical hysteresis of 0.5 V. The output
of the comparator, SQ_OUT, is applied to the charge pump
input CP+ through an external capacitor C_CP. When the
input signal changes state, C_CP is charged or discharged
through R3 and R4. The charge accumulated on C_CP is
mirrored to C4 by the Norton Amplifier circuit comprising
of Q1, Q2 and Q3. The charge pump output voltage, F/V_OUT,
ranges from 2.0 V to 6.3 V depending on the input signal
frequency and the gain of the charge pump according to the
formula:
FńVOUT + 2.0 V ) 2.0 FREQ CCP RT (VREG * 0.7 V)
R_T 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 generator 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:
q + AF_ńV AFG,
where:
AFG + 77°ńV(typ)
The relationship between input frequency and output
angle is:
q + AFG 2.0 FREQ CCP RT (VREG * 0.7 V)
or,
q + 970 FREQ CCP RT
The ripple voltage at the F/V converter’s output is
determined by the ratio of C_CP and C4 in the formula:
DV
+
CCP(VREG
C4
*
0.7
V)
Ripple voltage on the F/V output causes pointer or needle
flutter especially at low input frequencies.
The response time of the F/V is determined by the time
constant formed by R_T 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 unacceptable for
many applications.
Design Example
Maximum meter Deflection = 270°
Maximum Input Frequency = 350 Hz
1. Select R_T and C_CP
q + 970 FREQ CCP RT + 270°
Let C_T = 0.0033 mF, find R_T
RT + ₉₇₀
270°
350 Hz 0.0033 mF
RT + 243 kW
R_T should be a 250 kW 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 kW
The charge current for C_CP is
VREG * 0.7
3.3 kW
V
+
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
+
285
ms
Choose R4 = 1.0 kW.
Discharge time: t_DCHG = R3 × C_CP = 3.3 kW × 0.0033 mF
= 10.9 ms
Charge time: t_CHG = (R3 + R4)C_CP = 4.3 kW. × 0.0033 mF
= 14.2 ms
3. Determine C4
C4 is selected to satisfy both the maximum allowable
ripple voltage and response time of the meter movement.
C4
+
CCP(VREG *
DVMAX
0.7
V)
With C4 = 0.47 mF, the F/V ripple voltage is 44 mV.
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