LM2907

LM2907 LM2917 Frequency to Voltage Converter February 1995 LM2907 LM2917 Frequency to Voltage Converter General Description The LM2907 LM2917 series are monolithic frequency to voltage converters with a high gain op amp comparator designed to operate a relay lamp or other load when the input frequency reaches or exceeds a selected rate The tachometer uses a charge pump technique and offers frequency doubling for low ripple full input protection in two versions (LM2907-8 LM2917-8) and its output swings to ground for a zero frequency input Y Y Y Y Y Frequency doubling for low ripple Tachometer has built-in hysteresis with either differential input or ground referenced input Built-in zener on LM2917 g 0 3% linearity typical Ground referenced tachometer is fully protected from damage due to swings above VCC and below ground Applications Y Y Y Y Y Y Y Y Advantages Y Y Y Y Output swings to ground for zero frequency input Easy to use VOUT e fIN c VCC c R1 c C1 Only one RC network provides frequency doubling Zener regulator on chip allows accurate and stable frequency to voltage or current conversion (LM2917) Features Y Y Y Y Y Y Ground referenced tachometer input interfaces directly with variable reluctance magnetic pickups Op amp comparator has floating transistor output 50 mA sink or source to operate relays solenoids meters or LEDs Over under speed sensing Frequency to voltage conversion (tachometer) Speedometers Breaker point dwell meters Hand-held tachometer Speed governors Cruise control Automotive door lock control Clutch control Horn control Touch or sound switches Block and Connection Diagrams Dual-In-Line and Small Outline Packages Top Views TL H 7942 – 1 TL H 7942 – 2 Order Number LM2907M-8 or LM2907N-8 See NS Package Number M08A or N08E Order Number LM2917M-8 or LM2917N-8 See NS Package Number M08A or N08E TL H 7942 – 3 TL H 7942 – 4 Order Number LM2907N See NS Package Number N14A C1995 National Semiconductor Corporation TL H 7942 Order Number LM2917M or LM2917N See NS Package Number M14A or N14A RRD-B30M115 Printed in U S A Absolute Maximum Ratings (Note 1) If Military Aerospace specified devices are required please contact the National Semiconductor Sales Office Distributors for availability and specifications Supply Voltage Supply Current (Zener Options) Collector Voltage Differential Input Voltage Tachometer Op Amp Comparator Input Voltage Range Tachometer LM2907-8 LM2917-8 LM2907 LM2917 Op Amp Comparator 28V 25 mA 28V 28V 28V g 28V 0 0V to a 28V 0 0V to a 28V Power Dissipation LM2907-8 LM2917-8 LM2907-14 LM2917-14 (See Note 1) Operating Temperature Range Storage Temperature Range Soldering Information Dual-In-Line Package Soldering (10 seconds) Small Outline Package Vapor Phase (60 seconds) Infrared (15 seconds) 1200 mW 1580 mW b 40 C to a 85 C b 65 C to a 150 C 260 C 215 C 220 C See AN-450 ‘‘Surface Mounting Methods and Their Effect on Product Reliability’’ for other methods of soldering surface mount devices Electrical Characteristics VCC e 12 VDC Symbol TACHOMETER Input Thresholds Hysteresis Offset Voltage LM2907 LM2917 LM2907-8 LM2917-8 Input Bias Current VOH VOL I2 I3 I3 K Pin 2 Pin 2 Output Current Leakage Current Gain Constant Linearity OP AMP COMPARATOR VOS IBIAS Input Common-Mode Voltage Voltage Gain Output Sink Current Output Source Current Saturation Voltage VC e 1 0 VIN e 6 0V VIN e 6 0V Parameter TA e 25 C see test circuit Conditions Min Typ Max Units VIN e 250 mVp-p VIN e 250 mVp-p VIN e 250 mVp-p 1 kHz (Note 2) 1 kHz (Note 2) 1 kHz (Note 2) g 10 g 25 g 40 mV mV 30 35 5 10 15 1 mV mV mA V V VIN e g 50 mVDC VIN e a 125 mVDC (Note 3) VIN e b125 mVDC (Note 3) V2 e V3 e 6 0V (Note 4) I2 e 0 V3 e 0 (Note 3) fIN e 1 kHz 5 kHz 10 kHz (Note 5) 09 b1 0 01 83 23 140 180 240 01 mA mA 10 03 11 a1 0 % 3 50 0 200 40 50 10 01 10 500 VCCb1 5V mV nA V V mV mA mA VE e VCC b2 0 ISINK e 5 mA ISINK e 20 mA ISINK e 50 mA 05 10 V V V 10 15 2 Electrical Characteristics VCC e 12 VDC Symbol ZENER REGULATOR Regulator Voltage Series Resistance Temperature Stability TOTAL SUPPLY CURRENT Parameter TA e 25 C see test circuit (Continued) Min Typ Max Units Conditions RDROP e 470X 7 56 10 5 a1 V 15 X mV C 6 mA 38 Note 1 For operation in ambient temperatures above 25 C the device must be derated based on a 150 C maximum junction temperature and a thermal resistance of 101 C W junction to ambient for LM2907-8 and LM2917-8 and 79 C W junction to ambient for LM2907-14 and LM2917-14 Note 2 Hysteresis is the sum a VTH b ( b VTH) offset voltage is their difference See test circuit c VCC b 1 VBE VOL is equal to c VCC b 1 VBE therefore VOH b VOL e VCC 2 The difference VOH b VOL and the mirror gain Note 3 VOH is equal to I2 I3 are the two factors that cause the tachometer gain constant to vary from 1 0 Note 4 Be sure when choosing the time constant R1 c C1 that R1 is such that the maximum anticipated output voltage at pin 3 can be reached with I3 c R1 The maximum value for R1 is limited by the output resistance of pin 3 which is greater than 10 MX typically Note 5 Nonlinearity is defined as the deviation of VOUT ( C1 e 1000 pF R1 e 68k and C2 e 0 22 mFd pin 3) for fIN e 5 kHz from a straight line defined by the VOUT 1 kHz and VOUT 10 kHz General Description (Continued) The op amp comparator is fully compatible with the tachometer and has a floating transistor as its output This feature allows either a ground or supply referred load of up to 50 mA The collector may be taken above VCC up to a maximum VCE of 28V The two basic configurations offered include an 8-pin device with a ground referenced tachometer input and an internal connection between the tachometer output and the op amp non-inverting input This version is well suited for single speed or frequency switching or fully buffered frequency to voltage conversion applications Test Circuit and Waveform The more versatile configurations provide differential tachometer input and uncommitted op amp inputs With this version the tachometer input may be floated and the op amp becomes suitable for active filter conditioning of the tachometer output Both of these configurations are available with an active shunt regulator connected across the power leads The regulator clamps the supply such that stable frequency to voltage and frequency to current operations are possible with any supply voltage and a suitable resistor Tachometer Input Threshold Measurement TL H 7942 – 7 TL H 7942 – 6 3 Typical Performance Characteristics Total Supply Current Zener Voltage vs Temperature Normalized Tachometer Output vs Temperature Normalized Tachometer Output vs Temperature Tachometer Currents I2 and I3 vs Supply Voltage Tachometer Currents I2 and I3 vs Temperature Tachometer Linearity vs Temperature Tachometer Linearity vs Temperature Tachometer Linearity vs R1 Tachometer Input Hysteresis vs Temperature Op Amp Output Transistor Characteristics Op Amp Output Transistor Characteristics TL H 7942 – 5 4 Applications Information The LM2907 series of tachometer circuits is designed for minimum external part count applications and maximum versatility In order to fully exploit its features and advantages let’s examine its theory of operation The first stage of operation is a differential amplifier driving a positive feedback flip-flop circuit The input threshold voltage is the amount of differential input voltage at which the output of this stage changes state Two options (LM2907-8 LM2917-8) have one input internally grounded so that an input signal must swing above and below ground and exceed the input thresholds to produce an output This is offered specifically for magnetic variable reluctance pickups which typically provide a single-ended ac output This single input is also fully protected against voltage swings to g 28V which are easily attained with these types of pickups The differential input options (LM2907 LM2917) give the user the option of setting his own input switching level and still have the hysteresis around that level for excellent noise rejection in any application Of course in order to allow the inputs to attain common-mode voltages above ground input protection is removed and neither input should be taken outside the limits of the supply voltage being used It is very important that an input not go below ground without some resistance in its lead to limit the current that will then flow in the epi-substrate diode Following the input stage is the charge pump where the input frequency is converted to a dc voltage To do this requires one timing capacitor one output resistor and an integrating or filter capacitor When the input stage changes state (due to a suitable zero crossing or differential voltage on the input) the timing capacitor is either charged or discharged linearly between two voltages whose difference is VCC 2 Then in one half cycle of the input frequency or a time equal to 1 2 fIN the change in charge on the timing capacitor is equal to VCC 2 c C1 The average amount of current pumped into or out of the capacitor then is DQ V e ic(AVG) e C1 c CC c (2fIN) e VCC c fIN c C1 T 2 The output circuit mirrors this current very accurately into the load resistor R1 connected to ground such that if the pulses of current are integrated with a filter capacitor then VO e ic c R1 and the total conversion equation becomes VO e VCC c fIN c C1 c R1 c K Where K is the gain constant typically 1 0 The size of C2 is dependent only on the amount of ripple voltage allowable and the required response time CHOOSING R1 AND C1 There are some limitations on the choice of R1 and C1 which should be considered for optimum performance The timing capacitor also provides internal compensation for the charge pump and should be kept larger than 500 pF for very accurate operation Smaller values can cause an error current on R1 especially at low temperatures Several considerations must be met when choosing R1 The output current at pin 3 is internally fixed and therefore VO R1 must be less than or equal to this value If R1 is too large it can become a significant fraction of the output impedance at pin 3 which degrades linearity Also output ripple voltage must be considered and the size of C2 is affected by R1 An expression that describes the ripple content on pin 3 for a single R1C2 combination is VCC C1 c c VRIPPLE e 2 C2 It appears R1 can be chosen independent of ripple however response time or the time it takes VOUT to stabilize at a new voltage increases as the size of C2 increases so a compromise between ripple response time and linearity must be chosen carefully As a final consideration the maximum attainable input frequency is determined by VCC C1 and I2 I2 fMAX e C1 c VCC USING ZENER REGULATED OPTIONS (LM2917) For those applications where an output voltage or current must be obtained independent of supply voltage variations the LM2917 is offered The most important consideration in choosing a dropping resistor from the unregulated supply to the device is that the tachometer and op amp circuitry alone require about 3 mA at the voltage level provided by the zener At low supply voltages there must be some current flowing in the resistor above the 3 mA circuit current to operate the regulator As an example if the raw supply varies from 9V to 16V a resistance of 470X will minimize the zener voltage variation to 160 mV If the resistance goes under 400X or over 600X the zener variation quickly rises above 200 mV for the same input variation 1 V c fIN c C1 b CC I2 J pk-pk Typical Applications Minimum Component Tachometer TL H 7942 – 8 5 Typical Applications (Continued) ‘‘Speed Switch’’ Load is Energized When fIN t 1 2RC TL H 7942 – 9 Zener Regulated Frequency to Voltage Converter TL H 7942 – 10 Breaker Point Dwell Meter TL H 7942 – 11 6 Typical Applications (Continued) Voltage Driven Meter Indicating Engine RPM VO e 6V 400 Hz or 6000 ERPM (8 Cylinder Engine) TL H 7942 – 12 Current Driven Meter Indicating Engine RPM IO e 10 mA 300 Hz or 6000 ERPM (6 Cylinder Engine) TL H 7942 – 13 Capacitance Meter VOUT e 1V – 10V for CX e 0 01 to 0 1 mFd (R e 111k) TL H 7942 – 14 7 Typical Applications (Continued) Two-Wire Remote Speed Switch TL H 7942 – 15 100 Cycle Delay Switch VCC c C1 C2 for each complete input cycle (2 zero crossings) V3 steps up in voltage by the amount Example If C2 e 200 C1 after 100 consecutive input cycles V3 e 1 2 VCC TL H 7942 – 16 8 Typical Applications (Continued) Variable Reluctance Magnetic Pickup Buffer Circuits Precision two-shot output frequency equals twice input frequency VCC C1 Pulse width e 2 I2 Pulse height e VZENER TL H 7942 – 39 TL H 7942 – 17 Finger Touch or Contact Switch TL H 7942 – 19 TL H 7942 – 18 Flashing LED Indicates Overspeed Flashing begins when fIN t 100 Hz Flash rate increases with input frequency increase beyond trip point TL H 7942 – 20 9 Typical Applications (Continued) Frequency to Voltage Converter with 2 Pole Butterworth Filter to Reduce Ripple fPOLE e 0 707 2qRC 2 57 2qfPOLE uRESPONSE e TL H 7942 – 21 Overspeed Latch Output latches when fIN e 1 R1 a R2 RC R2 TL H 7942 – 23 Reset by removing VCC TL H 7942–22 10 Typical Applications (Continued) Some Frequency Switch Applications May Require Hysteresis in the Comparator Function Which can be Implemented in Several Ways TL H 7942 – 24 TL H 7942 – 25 TL H 7942 – 26 TL H 7942 – 27 TL H 7942 – 28 11 Typical Applications (Continued) Changing the Output Voltage for an Input Frequency of Zero TL H 7942 – 30 TL H 7942 – 29 Changing Tachometer Gain Curve or Clamping the Minimum Output Voltage TL H 7942 – 32 TL H 7942–31 12 Anti-Skid Circuit Functions ‘‘Select-Low’’ Circuit TL H 7942 – 34 VOUT is proportional to the lower of the two input wheel speeds TL H 7942 – 33 ‘‘Select-High’’ Circuit TL H 7942 – 36 VOUT is proportional to the higher of the two input wheel speeds TL H 7942 – 35 ‘‘Select-Average’’ Circuit TL H 7942 – 37 13 Equivalent Schematic Diagram 14 TL H 7942 – 38 This connection made on LM2907-8 and LM2917-8 only This connection made on LM2917 and LM2917-8 only 15 Physical Dimensions inches (millimeters) 8-Lead (0 150 Wide) Molded Small Outline Package JEDEC Order Number LM2907M-8 or LM2917M-8 NS Package Number M08A 16 Physical Dimensions inches (millimeters) (Continued) Molded SO Package (M) Order Number LM2917M NS Package Number M14A Molded Dual-In-Line Package (N) Order Number LM2907N-8 or LM2917N-8 NS Package Number N08E 17 LM2907 LM2917 Frequency to Voltage Converter Physical Dimensions inches (millimeters) (Continued) Molded Dual-In-Line Package (N) Order Number LM2907N or LM2917N NS Package Number N14A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION As used herein 1 Life support devices or systems are devices or systems which (a) are intended for surgical implant into the body or (b) support or sustain 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not assume any responsibility for use of any circuitry described no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications