THMC50_08 Datasheet, PDF (15/34 Pages) Texas Instruments – REMOTE/LOCAL TEMPERATURE MONITOR AND FAN CONTROLLER WITH SMBus INTERFACE

THMC50
REMOTE/LOCAL TEMPERATURE MONITOR AND
FAN CONTROLLER WITH SMBus INTERFACE
SLIS090 – JULY 1999
PRINCIPLES OF OPERATION
analog output - FAN_SPD (continued)
NOTE:
R5 in Figure 9 and R6 in Figure 11 are required to insure FAN_SPD/NTEST_IN is below the
NTEST_IN enable threshold during power up. Without these pulldown resistors, the external fan
drive circuit would cause the THMC50 to always power up in the NAND tree test mode. If an external
fan drive circuit other than the ones shown above is used, insure that the FAN_SPD/NTEST_IN
terminal is held below 0.3 × VCC3AUX during power up to prevent the THMC50 from entering the
NAND tree test mode inadvertently.
The circuits shown in Figure 6 through Figure 11 should be implemented with the following considerations:
D The output transistor or FET used must be capable of sourcing a current greater than the maximum fan
current value, and it must be capable of handling the power dissipation generated by the voltage drop across
the device when the fan operates at less than full speed.
D It may be necessary to add clamping diodes to protect the output transistor or FET from voltage transients
caused by a sudden drop in fan speed from full scale to 0 V.
D All of the circuits shown in Figure 6 through Figure 11 deliver an output voltage swing from near 0 V to near
12 V. The exception is the circuit in Figure 6, which is only able to supply a voltage up to a V_BE drop down
from 12 V due to the emitter-follower drive stage.
D If a different circuit is implemented, the gain of the amplifier should be around 4.8 to amplify the FAN_SPD
output from 0 V-to-2.5 V to 0 V-to-12 V.
D When selecting an operational amplifier, insure that its input common mode range accounts for the range
of the FAN_SPD output (0 V to 2.5 V) and that the output voltage swing of the operational amplifier is
suitable.
D The operational amplifier can either be powered from 12 V alone or from a ±12 V dual supply. If the
operational amplifier is powered from 12 V only, then the input common mode range of the operational
amplifier should include ground to accommodate the FAN_SPD output voltage range (0 V to 2.5 V). The
output voltage swing of the operational amplifier should be less than 0.6 V, if it is desired to have the fan
turn off when the DAC is commanded to 0x00.
D Insure that the FAN_SPD/NTEST_IN terminal is held below 0.3 × VCC3AUX during power up to prevent
the THMC50 from entering the NAND tree test mode inadvertently. A pulldown resistor, such as R5 in Figure
9 and R6 in Figure 11, may be needed for external circuits to source current into the FAN_SPD/NTEST_IN
terminal to satisfy this requirement.
D Care should also be taken when using a dual supply for the operational amplifier. Clamping diodes from
the output of the operational amplifier to ground should be used to prevent the base-emitter junction of the
external transistor from being reversed bias if the output of the operational amplifier swings below ground.
FAN_OFF output
The open drain FAN_OFF output of the THMC50 provides a way to turn off the cooling fan regardless of the
voltage on the FAN_SPD analog output terminal. Setting bit 5 in the configuration register (0x40) to a 1 causes
this output to float. Setting bit 5 in the configuration register (0x40) to a 0 causes the output to sink current (go
low). This output can be used in conjunction with an external FET or transistor to gate the fan on or off as shown
in Figure 12.
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