User Manual

Functional Description (Continued)

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 60% RPM

(maximum counts)

Mode Select

Nominal

RPM

Time per Revolution

Counts for the 70%

RPM

Time per Revolution

Given Speed in Decimal for 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

DS100072-12

(a) Fan with Tach Pull-Up to +5V

DS100072-13

(b) Fan with Tach Pull-Up to +12V, or Totem-Pole

Output and Resistor Attenuator

DS100072-14

DS100072-15

(d) Fan with Strong Tach Pull-Up or Totem Pole Output

and Diode Clamp

FIGURE 7. Alternatives for Fan Inputs

LM81