ADT7476ARQZ ON Semiconductor, ADT7476ARQZ Datasheet - Page 26

• Manufacturer Part Number: ADT7476ARQZ
• Description: IC REMOTE THERMAL CTRLR 24QSOP
• Manufacturer: ON Semiconductor
• Series: dBCool®r

Specifications of ADT7476ARQZ

• Function: Fan Control, Temp Monitor
• Topology: ADC, Comparator, Fan Speed Counter, Multiplexer, Register Bank
• Sensor Type: External & Internal
• Sensing Temperature: -40°C ~ 125°C, External Sensor
• Output Type: SMBus™
• Output Alarm: No
• Output Fan: Yes
• Voltage - Supply: 3 V ~ 3.6 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 24-QSOP
• Full Temp Accuracy: +/- 0.5 C
• Digital Output - Bus Interface: Serial (3-Wire, 4-Wire)
• Maximum Operating Temperature: + 125 C
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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.
THERM output asserts as expected.
THERM Operation in Manual Mode
to full speed, unless Bit 3 of Configuration Register 6 (0x10)
is set to 1.
can be used to select the PWM speed on a THERM event
(100% or maximum PWM).
disable THERM events from affecting the fans.
Fan Drive Using PWM Control
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 might need only a pullup resistor. In many cases,
the 4−wire fan PWM input has a built−in, pullup resistor.
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.
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.
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
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.
GS
If THERM and THERM hysteresis are both enabled, the
In manual mode, THERM events do not cause fans to go
Additionally, Bit 3 of Configuration Register 4 (0x7D)
Bit 2 in Configuration Register 4 (0x7D) can be set to
The ADT7476 uses pulse−width modulation (PWM) to
The ADT7476 PWM frequency can be set to a selection
For 3−wire fans, a single N−channel MOSFET is the only
Typical notebook fans draw a nominal 170 mA, so SOT
< 3.3 V, for direct interfacing to the PWM output pin.
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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 5.5 V maximum to prevent
damaging the ADT7476.
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.
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 35. Driving a 3−Wire Fan Using an N−Channel
Figure 35 uses a 10 kW pullup resistor for the TACH
Figure 36 shows a fan drive circuit using an NPN
Because the fan drive circuitry in 4−wire fans is not
Figure 37 shows a typical drive circuit for 4−wire fans.
ADT7476
Figure 36. Driving a 3−Wire Fan Using an
ADT7476
ADT7476
Figure 37. Driving a 4−Wire Fan
TACH
TACH
PWM
PWM
TACH
PWM
NPN Transistor
4.7kΩ
10kΩ
4.7kΩ
470Ω
10kΩ
MOSFET
10kΩ
4.7kΩ
3.3V
3.3V
2kΩ
10kΩ
10kΩ
10kΩ
3.3V
10kΩ
TACH
TACH
12V
TACH
12V 12V
12V
12V
Q1
NDT3055L
12V, 4−WIRE FAN
12V
Q1
MMBT2222
V
TACH
PWM
CC
12V
FAN
12V
FAN
1N4148
1N4148