Datasheet

LM2907-N, LM2917-N
www.ti.com SNAS555C –JUNE 2000–REVISED MARCH 2013
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 470Ω will minimize the zener voltage variation to 160 mV. If the resistance goes under 400Ω or
over 600Ω the zener variation quickly rises above 200 mV for the same input variation.
TYPICAL APPLICATIONS
Figure 19. Minimum Component Tachometer
Figure 20. ”Speed Switch”, Load is Energized when f
IN
≥ (1 / ( 2RC))
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