LM2907

TL H 7942LM2907 LM2917Frequency to Voltage ConverterFebruary 1995LM2907 LM2917Frequency to Voltage ConverterGeneral DescriptionThe LM2907 LM2917series are monolithic frequency to voltage converters with a high gain op amp comparator de-signed to operate a relay lamp or other load when the input frequency reaches or exceeds a selected rate The tachom-eter 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 inputAdvantagesY Output swings to ground for zero frequency input Y Easy to use V OUT e f IN c V CC c R1c C1Y Only one RC network provides frequency doublingYZener regulator on chip allows accurate and stable fre-quency to voltage or current conversion (LM2917)FeaturesY Ground referenced tachometer input interfaces directly with variable reluctance magnetic pickupsY Op amp comparator has floating transistor outputY50mA sink or source to operate relays solenoids me-ters or LEDsY Frequency doubling for low rippleYTachometer has built-in hysteresis with either differen-tial input or ground referenced input Y Built-in zener on LM2917Y g 0 3%linearity typicalYGround referenced tachometer is fully protected from damage due to swings above V CC and below groundApplicationsY Over under speed sensingY Frequency to voltage conversion (tachometer)Y SpeedometersY Breaker point dwell meters Y Hand-held tachometer Y Speed governors Y Cruise controlY Automotive door lock control Y Clutch control Y Horn controlYTouch or sound switchesBlock and Connection Diagrams Dual-In-Line and Small Outline Packages Top ViewsTL H 7942–1Order Number LM2907M-8or LM2907N-8See NS Package Number M08A or N08E TL H 7942–2Order Number LM2917M-8or LM2917N-8See NS Package Number M08A or N08ETL H 7942–3Order Number LM2907N See NS Package Number N14ATL H 7942–4Order Number LM2917M or LM2917N See NS Package Number M14A or N14AC 1995National Semiconductor CorporationRRD-B30M115 Printed in U S AAbsolute Maximum Ratings(Note1)If Military Aerospace specified devices are required please contact the National Semiconductor Sales Office Distributors for availability and specifications Supply Voltage28V Supply Current(Zener Options)25mA Collector Voltage28V Differential Input VoltageTachometer28V Op Amp Comparator28V Input Voltage RangeTachometer LM2907-8 LM2917-8g28V LM2907 LM29170 0V to a28V Op Amp Comparator0 0V to a28V Power DissipationLM2907-8 LM2917-81200mW LM2907-14 LM2917-141580mW (See Note1)Operating Temperature Range b40 C to a85 C Storage Temperature Range b65 C to a150 C Soldering InformationDual-In-Line PackageSoldering(10seconds)260 C Small Outline PackageVapor Phase(60seconds)215 C Infrared(15seconds)220 C See AN-450‘‘Surface Mounting Methods and Their Effect on Product Reliability’’for other methods of soldering sur-face mount devicesElectrical Characteristics V CC e12V DC T A e25 C see test circuitSymbol Parameter Conditions Min Typ Max Units TACHOMETERInput Thresholds V IN e250mVp-p 1kHz(Note2)g10g25g40mVHysteresis V IN e250mVp-p 1kHz(Note2)30mVOffset Voltage V IN e250mVp-p 1kHz(Note2)LM2907 LM29173 510mVLM2907-8 LM2917-8515mVInput Bias Current V IN e g50mV DC0 11m A V OH Pin2V IN e a125mV DC(Note3)8 3V V OL Pin2V IN e b125mV DC(Note3)2 3V I2 I3Output Current V2e V3e6 0V(Note4)140180240m A I3Leakage Current I2e0 V3e00 1m A K Gain Constant(Note3)0 91 01 1 Linearity f IN e1kHz 5kHz 10kHz(Note5)b1 00 3a1 0% OP AMP COMPARATORV OS V IN e6 0V310mV I BIAS V IN e6 0V50500nAInput Common-Mode Voltage0V CC b1 5V VVoltage Gain200V mVOutput Sink Current V C e1 04050mAOutput Source Current V E e V CC b2 010mASaturation Voltage I SINK e5mA0 10 5VI SINK e20mA1 0VI SINK e50mA1 01 5V2Electrical Characteristics V CC e12V DC T A e25 C see test circuit(Continued)Symbol Parameter Conditions Min Typ Max Units ZENER REGULATORRegulator Voltage R DROP e470X7 56VSeries Resistance10 515XTemperature Stability a1mV CTOTAL SUPPLY CURRENT3 86mA Note1 For operation in ambient temperatures above25 C the device must be derated based on a150 C maximum junction temperature and a thermal resistance of101 C W junction to ambient for LM2907-8and LM2917-8 and79 C W junction to ambient for LM2907-14and LM2917-14Note2 Hysteresis is the sum a V TH b(b V TH) offset voltage is their difference See test circuitNote3 V OH is equal to c V CC b1V BE V OL is equal to c V CC b1V BE therefore V OH b V OL e V CC 2 The difference V OH b V OL and the mirror gain I2 I3 are the two factors that cause the tachometer gain constant to vary from1 0Note4 Be sure when choosing the time constant R1c C1that R1is such that the maximum anticipated output voltage at pin3can be reached with I3c R1 The maximum value for R1is limited by the output resistance of pin3which is greater than10M X typicallyNote5 Nonlinearity is defined as the deviation of V OUT( pin3)for f IN e5kHz from a straight line defined by the V OUT 1kHz and V OUT 10kHz C1e1000pF R1e68k and C2e0 22mFdGeneral Description(Continued)The op amp comparator is fully compatible with the ta-chometer and has a floating transistor as its output This feature allows either a ground or supply referred load of up to50mA The collector may be taken above V CC up to a maximum V CE of28VThe two basic configurations offered include an8-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 The more versatile configurations provide differential ta-chometer 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 outputBoth of these configurations are available with an active shunt regulator connected across the power leads The reg-ulator clamps the supply such that stable frequency to volt-age and frequency to current operations are possible with any supply voltage and a suitable resistorTest Circuit and WaveformTL H 7942–6Tachometer Input Threshold MeasurementTL H 7942–7 3Typical Performance CharacteristicsTotal Supply CurrentTemperatureZener Voltage vs Output vs TemperatureNormalized Tachometer Output vs TemperatureNormalized Tachometer and I 3vs Supply Voltage Tachometer Currents I 2and I 3vs TemperatureTachometer Currents I 2vs Temperature Tachometer Linearity vs TemperatureTachometer Linearity Tachometer Linearity vs R1vs Temperature Tachometer Input Hysteresis Characteristics Op Amp Output Transistor CharacteristicsOp Amp Output Transistor TL H 7942–54Applications InformationThe LM2907series of tachometer circuits is designed for minimum external part count applications and maximum ver-satility In order to fully exploit its features and advantages let’s examine its theory of operation The first stage of oper-ation 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 pro-vide a single-ended ac output This single input is also fully protected against voltage swings to g28V which are easily attained with these types of pickupsThe 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 diodeFollowing 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 dis-charged linearly between two voltages whose difference is V CC 2 Then in one half cycle of the input frequency or a time equal to1 2f IN the change in charge on the timing capacitor is equal to V CC 2c C1 The average amount of current pumped into or out of the capacitor then isD Q T e i c(AVG)e C1cV CC2c(2f IN)e V CC c f IN c C1The 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 V O e i c c R1 and the total conversion equation becomes V O e V CC c f IN c C1c R1c KWhere K is the gain constant typically1 0 The size of C2is dependent only on the amount of ripple voltage allowable and the required response time CHOOSING R1AND C1There are some limitations on the choice of R1and C1 which should be considered for optimum performance The timing capacitor also provides internal compensation for the charge pump and should be kept larger than500pF for very accurate operation Smaller values can cause an error cur-rent on R1 especially at low temperatures Several consid-erations must be met when choosing R1 The output current at pin3is internally fixed and therefore V O R1must be less than or equal to this value If R1is too large it can become a significant fraction of the output impedance at pin3which degrades linearity Also output ripple voltage must be con-sidered and the size of C2is affected by R1 An expression that describes the ripple content on pin3for a single R1C2 combination isV RIPPLE eV CC2cC1C2c 1b V CC c f IN c C1I2J pk-pk It appears R1can be chosen independent of ripple howev-er response time or the time it takes V OUT to stabilize at a new voltage increases as the size of C2increases so a compromise between ripple response time and linearity must be chosen carefullyAs a final consideration the maximum attainable input fre-quency is determined by V CC C1and I2f MAX eI2C1c V CCUSING ZENER REGULATED OPTIONS(LM2917)For those applications where an output voltage or current must be obtained independent of supply voltage variations the LM2917is 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 about3mA at the voltage level provided by the zener At low supply voltages there must be some current flowing in the resistor above the3mA circuit current to op-erate the regulator As an example if the raw supply varies from9V to16V a resistance of470X will minimize the ze-ner voltage variation to160mV If the resistance goes un-der400X or over600X the zener variation quickly rises above200mV for the same input variationTypical ApplicationsMinimum Component TachometerTL H 7942–85Typical Applications(Continued)‘‘Speed Switch’’Load is Energized When f IN t1 2RCTL H 7942–9 Zener Regulated Frequency to Voltage ConverterTL H 7942–10Breaker Point Dwell MeterTL H 7942–116Typical Applications(Continued)Voltage Driven Meter Indicating Engine RPMV O e6V 400Hz or6000ERPM(8Cylinder Engine)TL H 7942–12Current Driven Meter Indicating Engine RPMI O e10mA 300Hz or6000ERPM(6Cylinder Engine)TL H 7942–13Capacitance MeterV OUT e1V–10V for C X e0 01to0 1mFd(R e111k)TL H 7942–147Typical Applications (Continued)Two-Wire Remote Speed SwitchTL H 7942–15100Cycle Delay SwitchV3steps up in voltage by the amountV CC c C1C2for each complete input cycle (2zero crossings)TL H 7942–16ExampleIf C2e 200C1after 100consecutive input cycles V3e 1 2V CC8Typical Applications (Continued)Variable Reluctance Magnetic Pickup Buffer CircuitsPrecision two-shot output frequency equals twice input frequency Pulse width e V CC 2C1I2Pulse height e V ZENERTL H 7942–39TL H 7942–17Finger Touch or Contact SwitchTL H 7942–18TL H 7942–19Flashing LED Indicates OverspeedFlashing begins when f IN t 100HzFlash rate increases with input frequency increase beyond trip pointTL H 7942–209Typical Applications(Continued)Frequency to Voltage Converter with2Pole Butterworth Filter to Reduce Ripplef POLE e 0 707 2q RCu RESPONSE e2 572q f POLETL H 7942–21Overspeed LatchTL H 7942–22Output latches when TL H 7942–23f IN eR2R1a R21RCReset by removing V CC10Typical Applications(Continued)Some Frequency Switch Applications May Require Hysteresis in theComparator Function Which can be Implemented in Several WaysTL H 7942–24TL H 7942–25TL H 7942–26TL H 7942–27TL H 7942–2811Typical Applications (Continued)Changing the Output Voltage for an Input Frequency of ZeroTL H 7942–29TL H 7942–30Changing Tachometer Gain Curve or Clamping the Minimum Output VoltageTL H 7942–31TL H 7942–3212Anti-Skid Circuit Functions‘‘Select-Low’’CircuitTL H 7942–33TL H 7942–34V OUT is proportional to the lower of the two input wheel speeds‘‘Select-High’’CircuitTL H 7942–35TL H 7942–36V OUT is proportional to the higher of the two input wheel speeds‘‘Select-Average’’CircuitTL H 7942–3713Equivalent Schematic DiagramT L H 7942–38T h i s c o n n e c t i o n m a d e o n L M 2907-8a n d L M 2917-8o n l yT h i s c o n n e c t i o n m a d e o n L M 2917a n d L M 2917-8o n l y1415Physical Dimensions inches(millimeters)8-Lead(0 150 Wide)Molded Small Outline Package JEDECOrder Number LM2907M-8or LM2917M-8NS Package Number M08A16Physical Dimensions inches(millimeters)(Continued)Molded SO Package(M)Order Number LM2917MNS Package Number M14AMolded Dual-In-Line Package(N)Order Number LM2907N-8or LM2917N-8NS Package Number N08E17L M 2907 L M 2917F r e q u e n c y t o V o l t a g e C o n v e r t e rPhysical Dimensions inches (millimeters)(Continued)Molded Dual-In-Line Package (N)Order Number LM2907N or LM2917NNS Package Number N14ALIFE SUPPORT POLICYNATIONAL’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 2 A critical component is any component of a life systems which (a)are intended for surgical implant support device or system whose failure to perform can into the body or (b)support or sustain life and whose be reasonably expected to cause the failure of the life failure to perform when properly used in accordance support device or system or to affect its safety or with instructions for use provided in the labeling can effectivenessbe reasonably expected to result in a significant injury to the userNational Semiconductor 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