LM81 datasheet(18/33 Pages) NSC

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Functional Description (Continued)

7.0 DAC OUTPUT

The LM81 provides an 8-bit DAC (Digital-to-Analog Con-

verter) with an output range of 0 to 1.25 volts (4.88 mV LSB).

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

the LM81 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 2.5V. Doing so will

place the LM81 in NAND tree test mode which will make all

pins inputs, thus disabling any response from the LM81.

Fans do not start reliably at reduced voltages, so operation

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

1 second) excursion to full operating voltage, then reduce

the voltage. Most fans do not operate at all below 5 to 7

volts. At those lower voltages the fan will simply consume

current, dissipate power, and not operate and such condi-

tions should be avoided.

The output of the amplifier can be configured to provide a

high or low side pass transistor. A high side pass transistor

simplifies the coupling of tachometer outputs to the tachom-

eter inputs of the LM81 since the fan remains grounded. Low

side drive will require AC coupling along with clamping at the

LM81 input to prevent negative excursions.

A typical circuit for fan drive is shown in

Figure 8. R4 is used

when a negative power supply is available to eliminate offset

in the amplifier and providea0to11.5 volt output (actually

12 volts less saturation). Omitting R4 will create a “dead

zone” between approximately 0 to 6 volts output (a poten-

tially unusable region anyway). In many applications protect-

ing the pass transistor Q2 from faults such as a shorted fan

can be accomplished by taking advantage of the current limit

already existing on the 12 volt supply. Q2 will have to be

heat-sunk accordingly. Otherwise, use the suggested current

limit circuit as shown.

DS100072-28

FIGURE 8. Amplifier circuit for connection between DAC output and fan.

LM81 Datasheet(PDF) 18 Page - National Semiconductor (TI)