5

The CS4101 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 to a high impedance comparator with a typical pos-

itive input threshold of 3.4V and typical hysteresis of

the charge pump input CP+ through an external capaci-

charged or discharged through R3 and R4. The charge

Amplifier circuit comprising Q1, Q2 and Q3. The charge

depending on the input signal frequency and the gain of

the charge pump according to the formula:

output stage and give the correct meter deflection. The

F/V output voltage is applied to the function generator

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,

Θ, is equal to the F/V gain multiplied

by the function generator gain:

Θ = A

where:

The relationship between input frequency and output

angle is:

Θ = A

or,

Θ = 970 × FREQ × C

The ripple voltage at the F/V converter’s output is deter-

∆V =

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

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.

Design Example

Maximum meter Deflection = 270°

Maximum Input Frequency = 350Hz

Θ = A

∆

Θ = 970 × FREQ × C

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-

exceed this limit.

Choose R3 = 3.3k

Ω

= 1.90mA

the input signal. Time for one cycle at maximum frequen-

minimum input frequency pulse width.

T = 285µs

Choose R4 = 1k

Ω.

Charge time:

T = R3

× C

3. Determine C4

C4 is selected to satisfy both the maximum allowable rip-

ple voltage and response time of the meter movement.

C4 =

With C4 = 0.47µF, the F/V ripple voltage is 44mV.

Figure 7 shows how the CS4101 and the CS-8441 are used

to produce a speedometer and odometer circuit.

3.3k

Ω

270°

970

× 350Hz × 0.0033µF

C4

Circuit Description and Application Notes