Functional Description (Continued)
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
(c) Fan with Tach Pull-Up to +12V and Diode Clamp
FIGURE 7. Alternatives for Fan Inputs
Counts are based on 2 pulses per revolution tachometer
outputs.
RPM
Time per Revolution
Counts for “Divide by 2”
(Default) in Decimal
153 counts
Comments
4400
3080
2640
13.64 ms
19.48 ms
22.73 ms
Typical RPM
70% RPM
60% RPM
219 counts
255 counts
(maximum counts)
Mode Select
Nominal RPM
Time per Revolution
Counts for the
70% RPM
Time per Revolution
for 70% RPM
9.74 ms
Given Speed in Decimal
Divide by 1
Divide by 2
Divide by 4
Divide by 8
8800
4400
2200
1100
6.82 ms
13.64 ms
27.27 ms
54.54 ms
153
153
153
153
6160
3080
1540
770
19.48 ms
38.96 ms
77.92 ms
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.
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
Fans do not start reliably at reduced voltages, so operation
at a reduced voltage should be preceded by a brief (typically
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