LM87CIMTX National Semiconductor, LM87CIMTX Datasheet - Page 16

• Manufacturer Part Number: LM87CIMTX
• Description: IC,Data Acquisition System,8-CHANNEL,8-BIT,TSSOP,24PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM87CIMTX

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Divide by 1
Divide by 2
Divide by 4
Divide by 8
Mode Select
Functional Description
Counts are based on 2 pulses per revolution tachometer
outputs.
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
(c) Fan with Tach Pull-Up to +12V and Diode Clamp
RPM
4400
3080
2640
(a) Fan with Tach Pull-Up to +5V
Nominal RPM
8800
4400
2200
1100
Time per Revolution
13.64 ms
19.48 ms
22.73 ms
Time per Revolution
(Continued)
13.64 ms
27.27 ms
54.54 ms
6.82 ms
FIGURE 7. Alternatives for Fan Inputs
10099512
10099514
Given Speed in Decimal
16
Counts for “Divide by 2”
(b) Fan with Tach Pull-Up to +12V, or Totem-Pole Output
(d) Fan with Strong Tach Pull-Up or Totem Pole Output
Counts for the
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
(Default) in Decimal
(maximum counts)
153
153
153
153
153 counts
219 counts
255 counts
and Resistor Attenuator
and Diode Clamp
70% RPM
6160
3080
1540
770
Time per Revolution
Typical RPM
Comments
for 70% RPM
70% RPM
60% RPM
19.48 ms
38.96 ms
77.92 ms
9.74 ms
10099515
10099513