Datasheet

LM81

SNAS011E –JUNE 1999–REVISED FEBRUARY 2002

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ALTERNATIVES FOR FAN INPUTS

Figure 12. Fan with Tach Pull-Up to +5V Figure 13. Fan with Tach Pull-Up to +12V, or

Totem-Pole Output and Resistor Attenuator

Figure 14. Fan with Tach Pull-Up to +12V and Figure 15. Fan with Strong Tach Pull-Up or Totem

Diode Clamp Pole Output and Diode Clamp

NOTE

Counts are based on 2 pulses per revolution tachometer outputs.

RPM Time per Revolution Counts for “Divide by 2” Comments

(Default) in Decimal

4400 13.64 ms 153 counts Typical RPM

3080 19.48 ms 219 counts 70% RPM

2640 22.73 ms 255 counts (maximum counts) 60% RPM

Mode Select Nominal RPM Time per Counts for the Given Speed 70% RPM Time per Revolution for

Revolution in Decimal 70% RPM

Divide by 1 8800 6.82 ms 153 6160 9.74 ms

Divide by 2 4400 13.64 ms 153 3080 19.48 ms

Divide by 4 2200 27.27 ms 153 1540 38.96 ms

Divide by 8 1100 54.54 ms 153 770 77.92 ms

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