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LM2917N Datasheet(PDF) 5 Page - National Semiconductor (TI) |
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LM2917N Datasheet(HTML) 5 Page - National Semiconductor (TI) |
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5 / 18 page  Applications Information The LM2907 series 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 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 dis- charged linearly between two voltages whose difference is VCC 2 Then in one half cycle of the input frequency or a time equal to 12 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 D Q T e ic(AVG) e C1 c VCC 2 c (2fIN) e VCC c fIN c C1 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 constanttypically 10 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 cur- rent on R1 especially at low temperatures Several consid- erations 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 con- sidered 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 VRIPPLE e VCC 2 c C1 C2 c 1bVCCcfINcC1 I2 J pk-pk It appears R1 can be chosen independent of ripple howev- er 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 fre- quency is determined by VCC C1 and I2 fMAX e I2 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 op- erate the regulator As an example if the raw supply varies from 9V to 16V a resistance of 470X will minimize the ze- ner voltage variation to 160 mV If the resistance goes un- der 400X or over 600X the zener variation quickly rises above 200 mV for the same input variation Typical Applications Minimum Component Tachometer TLH7942 – 8 5 |
Similar Part No. - LM2917N |
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Similar Description - LM2917N |
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