EVAL-ADT7467EBZ ON Semiconductor, EVAL-ADT7467EBZ Datasheet - Page 27

• Manufacturer Part Number: EVAL-ADT7467EBZ
• Description: BOARD EVALUATION FOR ADT7467
• Manufacturer: ON Semiconductor
• Series: dBCool®r

Specifications of EVAL-ADT7467EBZ

• Sensor Type: Temperature
• Sensing Range: -40°C ~ 120°C
• Interface: SMBus (2-Wire/I²C)
• Sensitivity: ±1.5°C
• Voltage - Supply: 3 V ~ 5.5 V
• Embedded: No
• Utilized Ic / Part: ADT7467
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

ACTIVE COOLING
DRIVING THE FAN USING PWM CONTROL
The ADT7467 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 pull-up resistor. In many cases, the 4-wire fan PWM
input has a built-in pull-up resistor.
The ADT7467 PWM frequency can be set to a selection of
low frequencies or a single high PWM frequency. The low fre-
quency options are usually used for 2-wire and 3-wire fans,
and the high frequency option is usually used for 4-wire fans.
For 2-wire or 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. Typical notebook fans draw a nominal 170 mA; therefore,
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 must handle the higher
current requirements. The only other stipulation is that the
MOSFET have a gate voltage drive of V
interfacing to the PWMx pin. V
long as the 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, which would reduce the
voltage applied across the fan and, therefore, the maximum
operating speed of the fan.
Figure 34 shows how to drive a 3-wire fan using PWM control.
Figure 34 uses a 10 kΩ pull-up 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 5 V maximum to prevent damaging the ADT7467. If in
doubt as to whether the fan used has an open-collector or totem
Figure 34. Driving a 3-Wire Fan Using an N-Channel MOSFET
ADT7467
TACHx
PWMx
4.7kΩ
10kΩ
10kΩ
3.3V
10kΩ
GS
12V
can be greater than 3.3 V as
12V
GS
Q1
NDT3055L
< 3.3 V for direct
12V
FAN
1N4148
Rev. 3 | Page 27 of 77 | www.onsemi.com
pole TACH output, use one of the input signal conditioning
circuits shown in the Fan Speed Measurement section.
Figure 35 shows a fan drive circuit using an NPN transistor
such as a general-purpose MMBT2222. Although 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.
Because 4-wire fans are powered continuously, the fan speed is
not switched on or off as with previous PWM driven/powered
fans. This enables it to perform better than 3-wire fans, espe-
cially for high frequency applications. Figure 36 shows a typical
drive circuit for 4-wire fans.
Driving Two Fans from PWM3
The ADT7467 has four TACH inputs available for fan speed
measurement, but only three PWM drive outputs. If a fourth
fan is used in the system, it should be driven from the PWM3
output in parallel with the third fan. Figure 37 shows how to
drive two fans in parallel using low cost NPN transistors.
Figure 38 shows the equivalent circuit using a MOSFET.
ADT7467
Figure 35. Driving a 3-Wire Fan Using an NPN Transistor
ADT7467
TACHx
PWMx
TACHx
PWMx
Figure 36. Driving a 4-Wire Fan
4.7kΩ
665Ω
10kΩ
4.7kΩ
3.3V
2kΩ
10kΩ
10kΩ
3.3V
10kΩ
TACH
12V
TACH
12V 12V
12V
12V, 4-WIRE FAN
Q1
MMBT2222
V
TACH
PWM
CC
12V
FAN
ADT7467
1N4148