LM79 datasheet(19/31 Pages) NSC

19 / 31 page

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

DS100036-14

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

DS100036-15

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

Output and Resistor Attenuator

DS100036-16

DS100036-17

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

and Diode Clamp

FIGURE 9. Alternatives for Fan Inputs

LM79 Datasheet(PDF) 19 Page - National Semiconductor (TI)

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