ADT7460ARQ-REEL ON Semiconductor, ADT7460ARQ-REEL Datasheet - Page 27

• Manufacturer Part Number: ADT7460ARQ-REEL
• Description: IC SENSOR TEMP FAN-CTRL 16QSOP
• Manufacturer: ON Semiconductor
• Series: dBCool®r

Specifications of ADT7460ARQ-REEL

• Rohs Status: RoHS non-compliant
• Function: Fan Control, Temp Monitor
• Topology: ADC, Comparator, Multiplexer, Register Bank
• Sensor Type: External & Internal
• Sensing Temperature: -40°C ~ 120°C, External Sensor
• Output Type: SMBus™
• Output Alarm: No
• Output Fan: Yes
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: -40°C ~ 120°C
• Mounting Type: Surface Mount
• Package / Case: 16-QSOP

FAN DRIVE USING PWM CONTROL
The ADT7460 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, so SOT devices can be
used where board space is a concern. In desktops, fans can
typically draw 250 mA to 300
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
dir
than 3.3 V as long as the pull-up on the gate is tied to 5 V. The
MO
there is
reduc
max
Figure 41 shows how a 3
control.
Figure 41 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 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.
ect interfacing to the PWM_OUT pin. V
SFET sh
imum operating speed of the fan.
e th
Figure 41. Driving a 3-Wire Fan by Using an N-Channel MOSFET
ADT7460
no
e voltag
t signif
TACH/AIN
ould also ha
PWM
e applied
icant v
4.7kΩ
10kΩ
olt
ve a lo
-wire fan can be driven using PWM
10kΩ
age drop across the FET. This woul
across the fan and, therefore, the
3.3V
10kΩ
voltage drive, V
mA each. If the user drives
w on resis
TACH
12V
12V
tance to ensure that
Q1
NDT3055L
GS
12V
FAN
GS
can be greater
< 3.3 V, for
1N4148
Rev. C | Page 27 of 52
d
Figure 42 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.
Driving Two Fans from PWM3
Note that the ADT7460 has four TACH inputs available for fan
speed measurement, but only three PWM drive outputs. If a
fourth fan is being used in the system, it should be driven from
the PWM3 output in parallel with the third fan. Figure 43 shows
how to drive two fans in parallel using low cost NPN transistors.
Figure 44 is the equivalent circuit using the NDT3055L MOSFET.
Note that since the MOSFET can handle up to 3.5 A, it is simply
a matter of connecting another fan directly in parallel with the
first.
Care should be take
and FETs to ensure tha
current and that they sink less th n the 8 mA maximum current
specified
Driving Up to Three Fans from PWM2
TACH measurements for fans are synchronized to particular
PWM channels, for example, TACH1 is synchronized to PWM1.
TACH3 and TACH4 are both synchronized to PWM3, so
PWM3 can drive two fans. Alternatively, PWM3 can be
grammed to synchronize TACH2, TACH3, and TACH4 to the
PWM3 output. This allows PWM3 to drive two or three fans. In
this case, the drive circuitry looks the same as shown in Figure 42,
Figure 43, and Figure 44. The SYNC bit in Register 0x62 enables
this function.
ADT7460
Figure 42. Driving a 3-Wire Fan by Using an NPN Transistor
on the data sheet.
TACH/AIN
PWM
n in designing drive circui
t the PWM pins are not
4.7kΩ
470Ω
10kΩ
3.3V
10kΩ
TACH
12V
a
12V
Q1
MMBT2222
12V
FAN
required to source
ts with transistors
ADT7460
1N4148
pro-