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TC9402EJD Datasheet(PDF) 5 Page - TelCom Semiconductor, Inc |
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TC9402EJD Datasheet(HTML) 5 Page - TelCom Semiconductor, Inc |
5 / 13 page 3-291 TELCOM SEMICONDUCTOR, INC. 7 6 5 4 3 1 2 8 Figure 1. 10 Hz to 10 kHz V/F Converter – + +5V +5V 14 V DD +5V RL 10k Ω RL 10k Ω 8 10 9 fOUT fOUT/2 11 3 µsec DELAY SELF- START 12 5 20k Ω 60pF OpAmp CINT 820pF CREF 180pF 12pF RIN 1M Ω VIN INPUT +5V –5V 50k Ω 510k Ω 10k Ω 3 1 OFFSET ADJUST IIN ZERO ADJUST 0V –10V IBIAS VSS 4 –5V 2 OUTPUT COMMON VREF OUT RBIAS 100k Ω AMP OUT TC9400 TC9401 TC9402 GND 6 THRESHOLD DETECTOR THRESHOLD DETECT REFERENCE VOLTAGE (TYPICALLY –5V) ÷2 VREF 7 –3V At the end of the charging period, CREF is shorted out. This dissipates the charge stored on the reference capaci- tor, so that when the output again crosses zero the system is ready to recycle. In this manner, the continued discharg- ing of the integrating capacitor by the input is balanced out by fixed charges from the reference voltage. As the input voltage is increased, the number of reference pulses re- quired to maintain balance increases, which causes the output frequency to also increase. Since each charge in- crement is fixed, the increase in frequency with voltage is linear. In addition, the accuracy of the output pulse width does not directly affect the linearity of the V/F. The pulse must simply be long enough for full charge transfer to take place. VOLTAGE-TO-FREQUENCY (V/F) CIRCUIT DESCRIPTION The TC9400 V/F converter operates on the principal of charge balancing. The operation of the TC9400 is easily understood by referring to Figure 1. The input voltage (VIN) is converted to a current (IIN) by the input resistor. This current is then converted to a charge on the integrating capacitor and shows up as a linearly decreasing voltage at the output of the op amp. The lower limit of the output swing is set by the threshold detector, which causes the reference voltage to be applied to the reference capacitor for a time period long enough to charge the capacitor to the reference voltage. This action reduces the charge on the integrating capacitor by a fixed amount (q = CREF × VREF), causing the op amp output to step up a finite amount. VOLTAGE-TO-FREQUENCY/ FREQUENCY-TO-VOLTAGE CONVERTERS TC9400 TC9401 TC9402 |
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