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LM2907 Tachometer Speed Switch Building Block Applications LM2907 Tachometer Speed Switch Building Block Applications INTRODUCTION Frequency to voltage converters are available in a number of forms from a number of sources but invariably require significant additional components before they can be put to use in a given situation The LM2907 LM2917 series of devices was developed to overcome these objections Both input and output interface circuitry is included on chip so that a minimum number of additional components is required to complete the function In keeping with the systems building block concept these devices provide an output voltage which is proportional to input frequency and provide zero output at zero frequency In addition the input may be referred to ground The devices are designed to operate National Semiconductor Application Note 162 June 1976 from a single supply voltage which makes them particularly suitable for battery operation PART 1 GENERAL OPERATION PRINCIPLES Circuit Description Referring to Figure 1 the family of devices all include three basic components an input amplifier with built-in hysteresis a charge pump frequency to voltage converter and a versatile op amp comparator with an uncommitted output transistor LM2917 incorporates an active zener regulator on-chip LM2907 deletes this option Both versions are obtainable in 14-pin and in 8-pin dual-in-line molded packages and to special order in other packages LM2907N-8 LM2917N-8 TL H 7451 ­ 1 TL H 7451 ­ 2 LM2907N LM2917N AN-162 TL H 7451 ­ 3 TL H 7451 ­ 4 FIGURE 1 Block Diagrams C1995 National Semiconductor Corporation TL H 7451 RRD-B30M115 Printed in U S A Input Hysteresis Amplifier The equivalent schematic diagram is shown in Figure 2 Q1 through Q11 comprise the input hysteresis amplifier Q1 through Q4 comprise an input differential amplifier which by virtue of PNP level shifting enables the circuit to operate with signals referenced to ground Q7 Q8 D4 and D5 comprise an active load with positive feedback This load behaves as a bi-stable flip-flop which may be set or reset depending upon the currents supplied from Q2 and Q3 Consider the situation where Q2 and Q3 are conducting equally i e the input differential voltage is zero Assuming Q7 to be conducting it will be noted that the current from Q3 will be drawn by Q7 and Q8 will be in the ``OFF'' state This allows the current from Q2 to drive Q7 in parallel with D4 and a small resistor D4 and Q7 are identical geometry devices so that the resistor causes Q7 to be biased at a higher level than D4 Thus Q7 will be able to conduct more current than Q3 provides In order to reverse the state of Q7 and Q8 it will be necessary to reduce the current from Q2 below that provided by Q3 by an amount which is established by R1 It can be shown that this requires a differential input to Q1 and Q4 of approximately 15mV Since the circuit is symmetrical the threshold voltage to reverse the state is 15 mV in the other dire