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ADT7467
BIOS could read the THERM timer once an hour
to determine the cumulative THERM assertion
time. If, for example, the total THERM assertion
time is <22.76 ms in Hour 1, >182.08 ms in
Hour 2, and >2.914 sec in Hour 3, this can indicate
that system performance is degrading significantly,
because THERM is asserting more frequently on
an hourly basis.
Alternatively, OS or BIOS level software can
timestamp when the system is powered on. If an
SMBALERT is generated because the THERM
timer limit has been exceeded, another timestamp
can be taken. The difference in time can be
calculated for a fixed THERM timer limit. For
example, if it takes one week for a THERM timer
limit of 2.914 sec to be exceeded and the next time
it takes only 1 hour, this is an indication of a
serious degradation in system performance.
Configuring the THERM Pin as an Output
In addition to monitoring THER M as an input, the
ADT7467 can optionally drive THERM low as an output. In
cases where PROCHOT is bidirectional, THERM can be
used to throttle the processor by asserting PROCHOT. The
user can preprogram system-critical thermal limits. If the
temperature exceeds a thermal limit by 0.25 ? C, THERM
asserts low. If the temperature is still above the thermal limit
on the next monitoring cycle, THERM stays low. THERM
remains asserted low until the temperature is equal to or
below the thermal limit. Because the temperature for that
channel is measured only once for every monitoring cycle,
it is guaranteed to remain low for at least one monitoring
cycle after THERM is asserted.
The THERM pin can be configured to assert low if the
Remote 1, local, or Remote 2 T HERM temperature limits
are exceeded by 0.25 ? C. The THERM temperature limit
registers are at Register 0x6A, Register 0x6B, and Register
0x6C, respectively. Setting Bit 3 of Register 0x5F, Register
0x60, and Register 0x61 enables the THERM output feature
for the Remote 1, local, and Remote 2 temperature channels,
An alternative method of disabling THERM is to program
the THERM temperature limit to ? 64 ? C or less in Offset 64
mode, or to ? 128 ? C or less in twos complement mode;
therefore, for THERM temperature limit values less than
? 64 ? C or ? 128 ? C, respectively, THERM is disabled.
Active Cooling
Driving the Fan using PWM Control
The ADT7467 uses pulse-width modulation (PWM) to
control fan speed. This relies on varying the duty cycle (or
on/off ratio) of a square wave applied to the fan to vary the
fan speed. The external circuitry required to drive a fan using
PWM control is extremely simple. For 4-wire fans, the
PWM drive may need only a pull-up resistor. In many cases,
the 4-wire fan PWM input has a built-in pull-up resistor.
The ADT7467 PWM frequency can be set to a selection
of low frequencies or a single high PWM frequency. The low
frequency options are usually used for 2-wire and 3-wire
fans, and the high frequency option is usually used for
4-wire fans.
For 2-wire or 3-wire fans, a single N-channel MOSFET is
the only drive device required. The specifications of the
MOSFET depend on the maximum current required by the
fan being driven. Typical notebook fans draw a nominal
170 mA; therefore, SOT devices can be used where board
space is a concern. In desktops, fans can typically draw
250 mA to 300 mA each. If you drive several fans in parallel
from a single PWM output or drive larger server fans, the
MOSFET must handle the higher current requirements. The
only other stipulation is that the MOSFET have a gate
voltage drive of V GS < 3.3 V for direct interfacing to the
PWMx pin. V GS can be greater than 3.3 V as long as the
pull-up on the gate is tied to 5 V. The MOSFET should also
have a low on resistance to ensure that there is not significant
voltage drop across the FET, which would reduce the
voltage applied across the fan and, therefore, the maximum
operating speed of the fan.
Figure 33 shows how to drive a 3-wire fan using PWM
control.
respectively. Figure 32 shows how the THERM pin asserts
low as an output in the event of a critical overtemperature.
12 V
12 V
THERM LIMIT
+0.25 ? C
TACHx
10 k W
10 k W
12 V
FAN
THERM LIMIT
ADT7467
4.7 k W
3.3 V
TEMP
10 k W
THERM
PWMx
Q1
NDT3055L
ADT7467
MONITORING CYCLE
Figure 33. Driving a 3-wire Fan Using an N-channel
MOSFET
Figure 32. Asserting THERM as an Output, Based on
Tripping THERM Limits
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