Rainbow Electronics LM87 - Manual (Page 16)

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
(c) Fan with Tach Pull-Up to +12V and Diode Clamp
10099515
(d) Fan with Strong Tach Pull-Up or Totem Pole Output
and Diode Clamp
FIGURE 7. Alternatives for Fan Inputs
LM87
www.national.com 16