EVAL-ADT7463EB ON Semiconductor, EVAL-ADT7463EB Datasheet - Page 25

• Manufacturer Part Number: EVAL-ADT7463EB
• Manufacturer: ON Semiconductor

Specifications of EVAL-ADT7463EB

• Lead Free Status / RoHS Status: Supplier Unconfirmed

Configuring the ADT7463 THERM Pin as an Output
In addition to the ADT7463 being able to monitor THERM as
an input, the ADT7463 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 mea-
sured only every monitoring cycle, once THERM asserts it is
guaranteed to remain low for at least one monitoring cycle.
The THERM pin can be configured to assert low if the
Remote 1, Local, or Remote 2 Temperature THERM limits get
exceeded by 0.25°C. The THERM limit registers are at loca-
tions 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 28 shows how the THERM pin
asserts low as an output in the event of a critical overtemperature.
FAN DRIVE USING PWM CONTROL
The ADT7463 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. A single NMOSFET 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, and 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 you drive several fans
in parallel from a single PWM output or drive 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
REV. C
THERM LIMIT
THERM LIMIT
+0.25 C
Figure 28. Asserting THERM as an Output, Based
on Tripping THERM Limits
THERM
TEMP
GS
GS
can be greater than 3.3 V as long as the
< 3.3 V for direct interfacing to the
MONITORING
ADT7463
CYCLE
–25–
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. This reduces the voltage applied across
the fan and therefore the maximum operating speed of the fan.
Figure 29 shows how a 3-wire fan may be driven using PWM
control.
Figure 29 uses a 10 kΩ pull-up resistor for the TACH 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 V
maximum to prevent damaging the ADT7463. 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 of the data sheet.
Figure 30 shows a fan drive circuit using an NPN 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.
Ensure that the base resistor is chosen such that the transistor is
saturated when the fan is powered on.
Figure 30. Driving a 3-Wire Fan Using an NPN Transistor
Figure 29. Driving a 3-Wire Fan Using an
N-Channel MOSFET
ADT7463
ADT7463
TACH/AIN
TACH/AIN
PWM
PWM
4.7k
4.7k
10k
470
10k
10k
3.3V
3.3V
10k
10k
TACH
TACH
12V
12V
12V
12V
Q1
NDT3055L
Q1
MMBT2222
12V
FAN
12V
FAN
ADT7463
1N4148
1N4148