ADT7460ARQZ-RL7 ONSEMI [ON Semiconductor], ADT7460ARQZ-RL7 Datasheet - Page 23

• Manufacturer Part Number: ADT7460ARQZ-RL7
• Description: dBCOOL Remote Thermal Monitor and Fan Controller
• Manufacturer: ONSEMI [ON Semiconductor]

Configuring the ADT7460 THERM Pin as an Output
THERM as an input, the ADT7460 can optionally drive
THERM low as an output. 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. Since the
temperature for that channel is measured only every
monitoring cycle, once THERM asserts, it is guaranteed to
remain low for at least one monitoring cycle.
Remote 1, local, or Remote 2 temperature THERM limits
are exceeded by 0.25°C. The THERM limit registers are at
Locations 0x6A, 0x6B, and 0x6C, respectively. Setting
Bit 3 of Registers 0x5F, 0x60, and 0x61 enables the THERM
output feature for the Remote 1, local, and Remote 2
temperature channels, respectively. Figure 39 shows how
the THERM pin asserts low as an output in the event of a
critical overtemperature.
Fan Drive Using PWM Control
control fan speed. This relies on varying the duty cycle (or
Figure 39. Asserting THERM as an Output, Based on
THERM
In addition to the ADT7460 being able to monitor
The THERM pin can be configured to assert low if the
The ADT7460 uses pulse width modulation (PWM) to
THERM LIMIT
THERM LIMIT
0.255C
THERM is asserting more frequently on an hourly
basis.
Alternatively, OS or BIOS level software can
time−stamp when the system is powered on. If an
SMBALERT is generated due to the THERM limit
being exceeded, another time−stamp can be taken.
The difference in time can be calculated for a fixed
THERM limit time. For example, if it takes one
week for a THERM limit of 2.914 s to be exceeded
and the next time it takes only one hour, this
indicates a serious degradation in system
performance.
TEMP
Tripping THERM Limits
MONITORING
ADT7460
CYCLE
http://onsemi.com
ADT7460
23
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. A single 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, so SOT devices can be used where board space is
a concern. In desktops, fans can typically draw 250 mA to
300 mA each. If the user drives several fans in parallel from
a single PWM output or drives larger server fans, the
MOSFET needs to handle the higher current requirements.
The only other stipulation is that the MOSFET should have
a gate voltage drive, V
PWM_OUT pin. V
the pullup on the gate is tied to 5.0 V. The MOSFET should
also have a low on resistance to ensure that there is not
significant voltage drop across the FET. This would reduce
the voltage applied across the fan and, therefore, the
maximum operating speed of the fan.
PWM control.
signal. This assumes that the TACH signal is open−collector
from the fan. In all cases, the TACH signal from the fan must
be kept below 5.0 V maximum to prevent damaging the
ADT7460. If in doubt as to whether the fan used has an
open−collector or totem pole TACH output, use one of the
input signal conditioning circuits shown in the Fan Speed
Measurement section.
transistor such as a general−purpose MMBT2222. While
these devices are inexpensive, they tend to have much lower
current handling capabilities and higher on−resistance than
MOSFETs. When choosing a transistor, care should be taken
to ensure that it meets the fan’s current requirements.
transistor is saturated when the fan is powered on.
Figure 40. Driving a 3−Wire Fan Using an N−Channel
Figure 40 shows how a 3−wire fan can be driven using
Figure 40 uses a 10 kW pullup resistor for the TACH
Figure 41 shows a fan drive circuit using an NPN
Ensure that the base resistor is chosen such that the
ADT7460
TACH/AIN
PWM
GS
GS
can be greater than 3.3 V as long as
4.7kΩ
10kΩ
10kΩ
MOSFET
< 3.3 V, for direct interfacing to the
3.3V
10kΩ
TACH
12V
12V
Q1
NDT3055L
12V
FAN
1N4148