LM87 datasheet(16/32 Pages) NSC

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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

7.0 DAC OUTPUT

The LM87 provides an 8-bit DAC (Digital-to-Analog Con-

verter) with an output range of 0 to 2.5 volts (9.80 mV LSB).

This DAC can be used in any way, but in most applications of

the LM87 the DAC will be used for fan control. Typically the

DAC output would be amplified to provide the up to 12 volt

drive required by the fan. At power-on the DAC provides full

output, insuring that full fan speed is the default condition.

Care should be taken such that the analog circuitry tied to

this pin does not drive this pin above 2.5 V. Doing so will

place the LM87 in NAND tree test mode which will make all

pins inputs. After the first SMBus communication with the

LM87, it will leave NAND tree test mode and all inputs/

outputs will function normally.

Fans do not start reliably at reduced voltages, so operation

at a reduced voltage should be preceded by a brief (typically

10099512

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

10099513

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

and Resistor Attenuator

10099514

10099515

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

and Diode Clamp

FIGURE 7. Alternatives for Fan Inputs

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