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CS4121EDWF20G Datasheet(PDF) 7 Page - ON Semiconductor |
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CS4121EDWF20G Datasheet(HTML) 7 Page - ON Semiconductor |
7 / 12 page CS4121 http://onsemi.com 7 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 FREQIN 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, SQOUT, is applied to the charge pump input CP+ through an external capacitor CCP. When the input signal changes state, CCP is charged or discharged through R3 and R4. The charge accumulated on CCP is mirrored to C4 by the Norton Amplifier circuit comprising of Q1, Q2 and Q3. The charge pump output voltage, F/VOUT, 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: F VOUT + 2.0 V ) 2.0 FREQ CCP RT (VREG * 0.7 V) 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 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 V AFG, where: AFG + 77° V(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 CCP and C4 in the formula: DV + CCP(VREG * 0.7 V) C4 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 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 unacceptable for many applications. Design Example Maximum meter Deflection = 270 ° Maximum Input Frequency = 350 Hz 1. Select RT and CCP q + 970 FREQ CCP RT + 270° Let CT = 0.0033 mF, find RT RT + 270 ° 970 350 Hz 0.0033 mF RT + 243 kW RT should be a 250 k W 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 W 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 + 285 ms Choose R4 = 1.0 k W. Discharge time: tDCHG = R3 × CCP = 3.3 k W × 0.0033 mF = 10.9 ms Charge time: tCHG = (R3 + R4)CCP = 4.3 k W. × 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 * 0.7 V) DVMAX With C4 = 0.47 mF, the F/V ripple voltage is 44 mV. |
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