LM80

SNIS102F – JUNE 1999 – REVISED MARCH 2013

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Voltage Measurements

R1 or R3

R2 or R4

Voltage at Analog Inputs

( ADC code 190)

+2.5V

23.7 k

Ω

75 k

Ω

+1.9V

+3.3V

22.1 k

Ω

30 k

Ω

+1.9V

+5.0V

24 k

Ω

14.7 k

Ω

+1.9V

+12V

160 k

Ω

30.1 k

Ω

+1.9V

−12V

160 k

Ω

35.7 k

Ω

+1.9V

−5V

36 k

Ω

16.2 k

Ω

+1.9V

LAYOUT AND GROUNDING

Analog inputs will provide best accuracy when referred to the AGND pin or a supply with low noise. A separate,

low-impedance ground plane for analog ground, which provides a ground point for the voltage dividers and

analog components, will provide best performance but is not mandatory. Analog components such as voltage

dividers should be located physically as close as possible to the LM80.

The power supply bypass, the parallel combination of 10

μF (electrolytic or tantalum) and 0.1 μF (ceramic)

bypass capacitors connected between pin 9 and ground, should also be located as close as possible to the

LM80.

FAN INPUTS

Inputs are provided for signals from fans equipped with tachometer outputs. These are logic-level inputs with an

from fan outputs which exceed 0 to V+, either resistive division or diode clamping must be included to keep

inputs within an acceptable range, as shown in Alternatives for Fan Inputs. R2 is selected so that it does not

develop excessive error voltage due to input leakage. R1 is selected based on R2 to provide a minimum input of

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 current 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, either a larger pull up resistor should be used or resistors connected 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 Divisor Register, is set to 2 (choices are 1, 2,

4, and 8) providing 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:

(3)

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

pulses, thus maintaining a nominal fan count of 153. Therefore the divisor should be set to 4 for a fan that

provides 1 pulse per revolution with a nominal RPM of 4400.

16

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