Datasheet

LM87

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SNAS034J –APRIL 2000–REVISED MARCH 2013

Note that Fan 1 and Fan 2 Divisors are programmable via the VID/Fan Divisor Register.

Fan tachometer outputs that provide one pulse per revolution should use a divisor setting twice that of outputs

that provide two pulses per revolution. For example, a 4400 RPM fan that provides one pulse per revolution

should have the divisor set to 4 such that the nominal counter output is 153.

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

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

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

Figure 10. Alternatives for Fan Inputs

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

(3)

DAC OUTPUT

The LM87 provides an 8-bit DAC (Digital-to-Analog Converter) 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.

Product Folder Links: LM87