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 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 FREQ
IN
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, SQ
OUT
, is applied to
the charge pump input CP+ through an external capacitor
C
T
. When the input signal changes state, CT is charged
or discharged through R3 and R4. The charge accumulat-
ed on C
T
is mirrored to C4 by the Norton Amplifier cir-
cuit comprising of Q1, Q2 and Q3. The charge pump out-
put voltage, F/V
OUT
, ranges from 2V to 6.3V depending
on the input signal frequency and the gain of the charge
pump according to the formula:
F/V
OUT
= 2.0V + 2
´
FREQ
´
C
T
´
R
T
´
(V
REG
Ð 0.7V)
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 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:
Q
= A
F/V
´
A
FG
,
where:
A
FG
= 77¡ (typ)
/V
The relationship between input frequency and output
angle is:
Q
= A
FG
´
2
´
FREQ
´
C
T
´
R
T
´
(V
REG
Ð 0.7V)
or,
Q
= 970
´
FREQ
´
C
T
´
R
T
The ripple voltage at the F/V converterÕs output is deter-
mined by the ratio of C
T
and C4 in the formula:
ÆV =
C
T
(V
REG
Ð 0.7V)
C4
Design Example
Maximum meter Deflection = 270¡
Maximum Input Frequency = 350Hz
1. Select R
T
and C
T
Q
= A
GEN
´
Æ
F/V
Æ
F/V
= 2
´
FREQ
´
C
T
´
R
T
´
(V
REG
Ð 0.7V)
Q
= 970
´
FREQ
´
C
T
´
R
T
Let C
T
= 0.0033µF, Find R
T
270¡
R
T
= 970
´
350Hz
´
0.0033µF
R
T
= 243k�½
R
T
should be a 250k�½ 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 = 3.3k�½
The charge current for C
T
is
V
REG
Ð 0.7V
= 1.90mA
3.3k�½
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 C
T
is discharged, assume that
the (R3 + R4) C
T
time constant is less than 10% of the
minimum input frequency pulse width.
T = 285µs
Choose R4 = 1k�½.
Charge time:
T = R3
´
C
T
= 3.3k�½
´
0.0033µF = 10.9µs
Discharge time:T = (R3 + R4)C
T
= 4.3k�½
´
0.0033µF = 14.2µs
3. Determine C4
C4 is selected to satisfy both the maximum allowable rip-
ple voltage and response time of the meter movement.
C4 =
C
T
(V
REG
Ð 0.7V)
V
RIPPLE(MAX)
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 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 unac-
ceptable for many applications.
5
With C4 = 0.47µF, the F/V ripple voltage is 44mV.
Figure 7 shows how the CS4121 and the CS-8441 are used
to produce a Speedometer and Odometer circuit.
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
