Functional description – Rainbow Electronics LM81 User Manual

Page 18

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 provide a 0 to 11.5 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