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 op-

eration 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 thresh-

olds 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

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 V

one half cycle of the input frequency or a time equal to 1/2 f

IN

the change in charge on the timing capacitor is equal to

V

out of the capacitor then is:

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 V

R1, and the total conversion equation becomes:

V

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 accu-

rate 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 V

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:

It appears R1 can be chosen independent of ripple, however

response time, or the time it takes V

voltage increases as the size of C2 increases, so a compro-

mise between ripple, response time, and linearity must be

chosen carefully.

As a final consideration, the maximum attainable input fre-

quency is determined by V

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 470

Ω will minimize the

zener voltage variation to 160 mV. If the resistance goes

under 400

Ω or over 600Ω the zener variation quickly rises

above 200 mV for the same input variation.

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