ADT7490_16 Datasheet, PDF (28/75 Pages) ON Semiconductor – Remote Thermal Monitor and Fan Controller

ADT7490
Enabling and Disabling THERM on Individual
Channels.
The THERM pin 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
event is not reflected in the status register and the fans do not
go to full speed. 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 5 of Configuration Register 1
(0x40) is set to 1.
Additionally, Bit 3 of Configuration Register 4 (0x7D)
can be used to select PWM speed on 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 ADT7490 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 pullup resistor. In many cases,
the 4-wire fan PWM input has a built-in, pullup resistor.
The ADT7490 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, therefore,
SOT devices can be used where board space is a concern. In
desktops, fans typically draw 250 mA to 300 mA each. If
several fans are driven 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_GS < 3.3 V, for direct interfacing to the PWM output pin.
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 36 shows how to drive a 3-wire fan using PWM
control.
12 V 12 V
TACH
ADT7490
PWM
10 kW
10 kW
4.7 kW
3.3 V
TACH
10 kW
12 V
FAN
Q1
NDT3055L
Figure 36. Driving a 3-wire Fan Using an N-channel
MOSFET
Figure 36 uses a 10 kW pullup resistor for the TACH
signal. This assumes that the TACH signal is an
open-collector from the fan. In all cases, the TACH signal
from the fan must be kept below 3.6 V maximum to prevent
damaging the ADT7490.
Figure 37 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 so that the transistor is
saturated when the fan is powered on.
12 V 12 V
TACH
ADT7490
PWM
10 kW
10 kW
4.7 kW
3.3 V
TACH
470 W
12 V
FAN
Q1
MMBT2222
Figure 37. Driving a 3-wire Fan Using
an NPN Transistor
Because the fan drive circuitry in 4-wire fans is not
switched on or off, as with previous PWM driven/powered
fans, the internal drive circuit is always on and uses the
PWM input as a signal instead of a power supply. This
enables the internal fan drive circuit to perform better than
3-wire fans, especially for high frequency applications.
Figure 38 shows a typical drive circuit for 4-wire fans.
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