LM81 datasheet(15/32 Pages) NSC

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Functional Description (Continued)

The power supply bypass, the parallel combination of 10 µF

(electrolytic or tantalum) and 0.1 µF (ceramic) bypass ca-

pacitors connected between pin 12 and ground, should also

be located as close as possible to the LM81.

6.0 FAN INPUTS

The FAN1 and FAN2 inputs accept signals from fans

equipped with tachometer outputs. These are logic-level in-

puts with an approximate threshold of V

+/2. Signal condition-

ing in the LM81 accommodates the slow rise and fall times

typical of fan tachometer outputs. The maximum input signal

range is 0 to V

+. In the event these inputs are supplied from

fan outputs which exceed 0 to V

+, either resistive division or

diode clamping must be included to keep inputs within an ac-

ceptable range, as shown in

Figure 7. R2 is selected so that

it does not develop excessive voltage due to input leakage.

R1 is selected based on R2 to provide a minimum input of 2V

and a maximum of V

+. R1 should be as low as possible to

provide the maximum possible input up to V

+ for best noise

immunity. Alternatively, use a shunt reference or zener diode

to clamp the input level.

If fans can be powered while the power to the LM81 is off,

the LM81 inputs will provide diode clamping. Limit input cur-

rent to the Input Current at Any Pin specification shown in the

ABSOLUTE MAXIMUM RATINGS section. In most cases,

open collector outputs with pull-up resistors inherently limit

this current. If this maximum current could be exceeded, ei-

ther a larger pull up resistor should be used or resistors con-

nected in series with the fan inputs.

The Fan Inputs gate an internal 22.5 kHz oscillator for one

period of the Fan signal into an 8-bit counter (maximum

count = 255). The default divisor, located in the VID/Fan Di-

visor Register, is set to 2 (choices are 1, 2, 4, and 8) provid-

ing a nominal count of 153 for a 4400 rpm fan with two

pulses per revolution. Typical practice is to consider 70% of

normal RPM a fan failure, at which point the count will be

219.

Determine the fan count according to:

Note that Fan 1 and Fan 2 Divisors are programmable via

the VID/Fan Divisor Register.

Fan tachometer outputs that provide one pulse per revolu-

tion should use a divisor setting twice that of outputs that

provide two pulses per revolution. For example, a 4400 RPM

fan that provides one pulse per revolution should have the

divisor set to 4 such that the nominal counter output is 153.

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