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LM331N データシート(PDF) 6 Page - National Semiconductor (TI) |
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LM331N データシート(HTML) 6 Page - National Semiconductor (TI) |
6 / 16 page Typical Applications (Continued) PRINCIPLES OF OPERATION OF A SIMPLIFIED VOLTAGE-TO-FREQUENCY CONVERTER The LM131 is a monolithic circuit designed for accuracy and versatile operation when applied as a voltage-to-frequency (V-to-F) converter or as a frequency-to-voltage (F-to-V) con- verter A simplified block diagram of the LM131 is shown in Figure 2 and consists of a switched current source input comparator and 1-shot timer The operation of these blocks is best understood by going through the operating cycle of the basic V-to-F converter Figure 2 which consists of the simplified block diagram of the LM131 and the various resistors and capacitors con- nected to it The voltage comparator compares a positive input voltage V1 at pin 7 to the voltage Vx at pin 6 If V1 is greater the comparator will trigger the 1-shot timer The output of the timer will turn ON both the frequency output transistor and the switched current source for a period te11 RtCt During this period the current i will flow out of the switched current source and provide a fixed amount of charge Qei c t into the capacitor CL This will normally charge Vx up to a higher level than V1 At the end of the timing period the current i will turn OFF and the timer will reset itself Now there is no current flowing from pin 1 and the capaci- tor CL will be gradually discharged by RL until Vx falls to the level of V1 Then the comparator will trigger the timer and start another cycle The current flowing into CL is exactly IAVE e i c (11cRtCt) c f and the current flowing out of CL is exactly Vx RL j VIN RL IfVIN is doubled the frequency will double to main- tain this balance Even a simple V-to-F converter can pro- vide a frequency precisely proportional to its input voltage over a wide range of frequencies TLH5680 – 4 FIGURE 2 Simplified Block Diagram of Stand-Alone Voltage-to-Frequency Converter Showing LM131 and External Components DETAIL OF OPERATION FUNCTIONAL BLOCK DIAGRAM ( FIGURE 1a ) The block diagram shows a band gap reference which pro- vides a stable 19 VDC output This 19 VDC is well regulated over a VS range of 39V to 40V It also has a flat low tem- perature coefficient and typically changes less than % over a 100 C temperature change The current pump circuit forces the voltage at pin 2 to be at 19V and causes a current ie190VRS to flow For Rse14k ie135 mA The precision current reflector pro- vides a current equal to i to the current switch The current switch switches the current to pin 1 or to ground depending on the state of the RS flip-flop The timing function consists of an RS flip-flop and a timer comparator connected to the external RtCt network When the input comparator detects a voltage at pin 7 higher than pin 6 it sets the RS flip-flop which turns ON the current switch and the output driver transistor When the voltage at pin 5 rises to VCC the timer comparator causes the RS flip-flop to reset The reset transistor is then turned ON and the current switch is turned OFF However if the input comparator still detects pin 7 higher than pin 6 when pin 5 crosses VCC the flip-flop will not be reset and the current at pin 1 will continue to flow in its attempt to make the voltage at pin 6 higher than pin 7 This condition will usually apply under start-up conditions or in the case of an overload voltage at signal input It should be noted that during this sort of overload the output frequency will be 0 as soon as the signal is restored to the working range the output frequency will be resumed The output driver transistor acts to saturate pin 3 with an ON resistance of about 50X In case of overvoltage the output current is actively limited to less than 50 mA The voltage at pin 2 is regulated at 190 VDC for all values of i between 10 mAto500 mA It can be used as a voltage reference for other components but care must be taken to ensure that current is not taken from it which could reduce the accuracy of the converter PRINCIPLES OF OPERATION OF BASIC VOLTAGE- TO-FREQUENCY CONVERTER ( FIGURE 1 ) The simple stand-alone V-to-F converter shown in Figure 1 includes all the basic circuitry of Figure 2 plus a few compo- nents for improved performance A resistor RINe100 kXg10% has been added in the path to pin 7 so that the bias current at pin 7 (b80 nA typical) will cancel the effect of the bias current at pin 6 and help provide minimum frequency offset The resistance RS at pin 2 is made up of a 12 kX fixed resistor plus a 5 kX (cermet preferably) gain adjust rheo- stat The function of this adjustment is to trim out the gain tolerance of the LM131 and the tolerance of Rt RL and Ct 6 |
同様の部品番号 - LM331N |
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同様の説明 - LM331N |
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