LM2907, LM2917

FREQUENCY-TO-VOLTAGE CONVERTERS

SLFS011A – MARCH 1986 – REVISED JULY 1993

6

POST OFFICE BOX 655303

APPLICATION INFORMATION

level increases as the size of C2 increases. A compromise between ripple, response time, and linearity must

C1

V

CC

hertz

•

where:

C1 is in farads, and

zener regulator options (LM2917)

For those applications in which an output voltage or current must be obtained independent of supply voltage

variations, the LM2917 can be used. The most important factor in selecting a dropping resistor for the

unregulated supply is that the frequency-to-voltage converter circuit and the operational amplifier alone require

approximately 3 mA at the voltage level set by the zener diode. 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 supply

voltage varies between 9 V and 16 V, a resistance of 470

Ω minimizes the zener voltage variation to typically

160 mV. If the resistance goes under 400

Ω or above 600 Ω, the zener variation quickly rises above 200 mV

for the same input variation.

Charge

Pump

C1

TACH +

R1

CAP1

IN –

10 k

Ω

VCC

C2

CAPO/IN +

Emitter/Follower

Output

E

+

–

Figure 2. Minimum-Component Tachometer