LM2907 datasheet(5/18 Pages) NSC

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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

ic(AVG) e C1 c

VCC

(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

1bVCCcfINcC1

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

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

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