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below the thermal limit. Because the temperature for that
channel is measured only once for every monitoring cycle,
after 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 THERM temperature limits
are exceeded by 0.25°C. The THERM temperature limit
registers are at Register 0x6A, Register 0x6B, and
Register 0x6C,
Configuration Register 5 (0x7C) enables the THERM
output feature for the Remote 1, local, and Remote 2
temperature channels, respectively. Figure 34 shows how
the THERM pin asserts low as an output in the event of a
critical overtemperature.
THERM LIMIT
0.255C
THERM LIMIT
TEMP
THERM
Figure 34. Asserting THERM as an Output, Based on
An alternative method of disabling THERM is to program
the THERM temperature limit to –63°C or less in Offset 64
mode, or −128°C or less in twos complement mode; that is,
for THERM temperature limit values less than –63°C or
–128°C, respectively, THERM is disabled.
Enabling and Disabling THERM on individual Channels
THERM can be enabled/disabled for individual or
combinations of temperature channels using Bits [7:5] of
Configuration Register 5 (0x7C).
THERM Hysteresis
Setting Bit 0 of Configuration Register 7 (0x11) disables
THERM hysteresis.
If THERM hysteresis is enabled and THERM is disabled
(Bit 2 of Configuration Register 4, 0x7D), the THERM pin
does not assert low when a THERM event occurs. If
THERM hysteresis is disabled and THERM is disabled
(Bit 2 of Configuration Register 4, 0x7D) and assuming the
appropriate pin is configured as THERM), the THERM pin
asserts low when a THERM event occurs.
If THERM and THERM hysteresis are both enabled, the
THERM output asserts as expected.
THERM Operation in Manual Mode
In manual mode, THERM events do not cause fans to go
to full speed, unless Bit 3 of Configuration Register 6
(0x10) is set to 1.
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respectively.
Setting
Bits [5:7]
MONITORING
CYCLE
Tripping THERM Limits
ADT7476A
Additionally, Bit 3 of Configuration Register 4 (0x7D)
can be used to select the PWM speed on a THERM event
(100% or maximum PWM).
Bit 2 in Configuration Register 4 (0x7D) can be set to
disable THERM events from affecting the fans.
Fan Drive Using PWM Control
The ADT7476A uses pulse-width modulation (PWM) to
control fan speed. This relies on varying the duty cycle (or
of
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 might need only a pullup resistor. In many cases,
the 4-wire fan PWM input has a built-in, pullup resistor.
The ADT7476A PWM frequency can be set to a selection
of low frequencies or a single high PWM frequency. The
low frequency options are used for 3-wire fans, while the
high frequency option is usually used with 4-wire fans.
For 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 and the input capacitance of the FET. Because a
10 kW (or greater) resistor must be used as a PWM pullup,
an FET with large input capacitance can cause the PWM
output to become distorted and adversely affect the fan
control range. This is a requirement only when using high
frequency PWM mode.
Typical notebook fans draw a nominal 170 mA, so SOT
devices can be used where board space is a concern. In
desktops, fans 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 should have a gate voltage drive,
V
< 3.3 V, for direct interfacing to the PWM output pin.
GS
The MOSFET should also have a low on resistance to ensure
that there is not a 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 35 shows how to drive a 3-wire fan using PWM
control.
TACH
ADT7476A
PWM
Figure 35. Driving a 3-wire Fan Using an N-channel
www.onsemi.com
27
12 V
12 V
10 kW
10 kW
12 V
FAN
TACH
4.7 kW
3.3 V
10 kW
Q1
NDT3055L
MOSFET
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