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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 / 15 page Typical Performance Characteristics (Continued) Typical Applications PRINCIPLES OF OPERATION OF A SIMPLIFIED VOLTAGE-TO-FREQUENCY CONVERTER The LM231/331 are monolithic circuits designed for accu- racy and versatile operation when applied as voltage-to-frequency (V-to-F) converters or as frequency-to-voltage (F-to-V) converters. A simplified block diagram of the LM231/331 is shown in Figure 3 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 3, which consists of the simplified block diagram of the LM231/331 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, V x, 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 t=1.1 R tCt. During this period, the current i will flow out of the switched current source and provide a fixed amount of charge, Q=i x t, into the capacitor, C L. 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 capacitor C L will be gradually discharged by RL until Vx falls to the level of V1. Then the comparator will trigger the timer and start an- other cycle. The current flowing into C L is exactly IAVE = i x (1.1xRtCt)x f, and the current flowing out of C L is exactly Vx/RL ≅ VIN/RL. If V IN is doubled, the frequency will double to maintain this balance. Even a simple V-to-F converter can provide a fre- quency precisely proportional to its input voltage over a wide range of frequencies. DETAIL OF OPERATION, FUNCTIONAL BLOCK DIAGRAM ( Figure 2) The block diagram shows a band gap reference which pro- vides a stable 1.9 V DC output. This 1.9 VDC is well regulated over a V S range of 3.9V to 40V. It also has a flat, low tem- perature coefficient, and typically changes less than 1⁄2% over a 100˚C temperature change. The current pump circuit forces the voltage at pin 2 to be at 1.9V, and causes a current i=1.90V/R S to flow. For Rs=14k, i=135 µA. The precision current reflector provides 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 R S flip-flop. The timing function consists of an R S flip-flop, and a timer comparator connected to the external R tCt network. When the input comparator detects a voltage at pin 7 higher than pin 6, it sets the R S flip-flop which turns ON the current switch and the output driver transistor. When the voltage at pin 5 rises to 2⁄3 V CC, 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 2⁄3 V CC, the flip-flop will not be reset, and the current at pin 1 will continue to flow, in its at- tempt to make the voltage at pin 6 higher than pin 7. This Power Drain vs V SUPPLY DS005680-34 Output Saturation Voltage vs I OUT (Pin 3) DS005680-35 Nonlinearity Error, Precision F-to-V Converter ( Figure 7) DS005680-36 DS005680-4 FIGURE 3. Simplified Block Diagram of Stand-Alone Voltage-to-Frequency Converter and External Components www.national.com 6 |
同様の部品番号 - LM331N |
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同様の説明 - LM331N |
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