LM80 datasheet(14/29 Pages) NSC

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Functional Description (Continued)

6.0 FAN INPUTS

Inputs are provided for signals from fans equipped with ta-

chometer outputs. These are logic-level inputs with an ap-

proximate threshold of V

+/2. Signal conditioning in the LM80

accommodates the slow rise and fall times typical of fan ta-

chometer outputs. The maximum input signal range is 0 to

+. 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 accept-

able range, as shown in

Figure 7. R2 is selected so that it

does not develop excessive error voltage due to input leak-

age. 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 LM80 is off,

the LM80 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 Fan Divisor/RST_OUT/OS Register.

FAN1 and FAN2 inputs can also be programmed to be level

sensitive digital inputs.

Fans that provide only one pulse per revolution would re-

quire a divisor set twice as high as fans that provide two

pulses, thus maintaining a nominal fan count of 153. There-

fore the divisor should be set to 4 for a fan that provides 1

pulse per revolution with a nominal RPM of 4400.

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