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VFC320 Datasheet(PDF) 5 Page - Texas Instruments |
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VFC320 Datasheet(HTML) 5 Page - Texas Instruments |
5 / 11 page VFC320 5 SBVS017A (8) In the time t1 + t2 the integrator capacitor C2 charges and discharges but the net voltage change is zero. Thus ∆Q = 0 = I IN t1 + (IIN – IA) t2 So that IIN (t1 + t2) = IA t2 But since t1 + t2 = and IIN = fOUT = In the time t1, IB charges the one-shot capacitor C1 until its voltage reaches –7.5V and trips comparator B. Thus t2 = Using in yield f V RC I I OUT IN B A () () . 76 75 11 =• Since IA = IB the result is fOUT = Since the integrating capacitor, C2, affects both the rising and falling segments of the ramp voltage, its tolerance and temperature coefficient do not affect the output frequency. It should, however, have a leakage current that is small com- pared to IIN, since this parameter will add directly to the gain error of the VFC. C1, which controls the one-shot period, should be very precise since its tolerance and temperature coefficient add directly to the errors in the transfer function. begins to ramp down again before the input amplifier has a chance to saturate. In effect the comparators and flip-flop form a one-shot whose period is determined by the internal reference and a 1mA current sink plus the external capacitor, C1. After the one-shot resets, fOUT changes back to logic 0 and the cycle begins again. The transfer function for the VFC320 is derived for the circuit shown in Figure 4. Detailed waveforms are shown in Figure 5. fOUT = 1 t 1 + t2 0V –7.5V ∆V OUT t 1 t 2 FIGURE 5. Integrator and VFC Output Timing. (1) FIGURE 4. Functional Block Diagram of the VFC320. f OUT V IN 1 R 1 I A R2 R2 V IN C IN 7.5 I B 7.5 R 1 C1 V IN (3) (2) (4), (5) (6) (7) (9) Comparators Flip- flop Common f OUT f IN One-shot V OUT 1 5 411 I A I IN e 1 e 2 Switch C 1 One-shot Capacitor –V CC 12 +V CC 7 Pull-up Resitor R 2 10 13 Input Amp Constant Current Sinks (1mA) –7.5V Ref B Integrating Capacitor R 1 Input Resistor Q 1 C 2 I B A For Postive Input Voltages use e 1, short e2. For Negative Input Voltages use e 2, short e1. For Differental Input Voltages use e 1 and e2. V IN: f OUT = V IN 7.5 R 1 C1 +V PULL-UP (VPU) (5V to 15V Typically) 14 |
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