EVAL-ADT7462EBZ ON Semiconductor, EVAL-ADT7462EBZ Datasheet - Page 27

• Manufacturer Part Number: EVAL-ADT7462EBZ
• Description: BOARD EVALUATION FOR ADT7462
• Manufacturer: ON Semiconductor
• Type: Temperature Sensorr

Specifications of EVAL-ADT7462EBZ

• Contents: Evaluation Board
• For Use With/related Products: ADT7462
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Using the ADT7462 with 3−Wire Fans
3−wire fan.
MOSFET, such as the NDT3055L, is used as the pass device.
The specifications of the MOSFET depend on the maximum
current required by the fan being driven. A typical PC fan
can draw a nominal current ranging from a few hundred
milliamps to over an amp of current. Depending on the
current rating of the fan, a SOT device can be used where
board space is a concern. If several fans in parallel are driven
from a single PWM output or if larger server fans are driven,
the MOSFET must handle the higher current requirements.
The only other stipulation is that the MOSFET should have
a gate voltage drive, V
PWM pins. V
pullup on the gate is tied to 5.0 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 reduce the full speed
of the fan.
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 5.0 V maximum to prevent
damaging the ADT7462. 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.
Figure 40 shows the most typical circuit used with a
The external circuitry required is very simple. A
Figure 40 uses a 10 kW pullup resistor for the TACH
ADT7462
ADT7462
TACH/AIN
Figure 39. Driving a 2−Wire Fan
Figure 40. Driving a 3−Wire Fan
TACH
PWM
GS
PWM
can be greater than 3.3 V as long as the
3.3 V
GS
10 kΩ
10 kΩ
TYPICAL
4.7 kΩ
10 kΩ
< 3.3 V, for direct interfacing to the
3.3 V
0.01 mF
10 kΩ
12 V FAN
5.0 V or
12 V
+V
Q1
NDT3055L
Q1
NDT3055L
12 V
R
2 Ω
TYPICAL
SENSE
12 V
FAN
1N4148
1N4148
http://onsemi.com
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speed measurement more difficult because the TACH signal
is chopped by the PWM drive signal. Pulse stretching is
required in this case to make accurate fan speed
measurements. For more information, see the Fan Speed
Measurement section.
Using the ADT7462 with 4−Wire Fans
fans.
continuously, unlike previous PWM driven/powered fans,
4−wire fans tend to perform better than 3−wire fans,
especially for high frequency applications. 4−wire frames
also eliminate the requirement for pulse stretching, because
the TACH signal is always available.
Driving Two Fans from Each PWM
available for fan speed measurement, but only four PWM
drive outputs. If all eight fans are being used in the system,
two fans should be driven in parallel from each PWM
output. Figure 42 shows how to drive two fans in parallel
using the NDT3055L MOSFET. This information is
relevant for low frequency mode only (2−wire and 3−wire
fans), because the PWM and TACHs need to be
synchronized to obtain accurate fan speed measurements
using pulse stretching (see the Fan Speed Measurement with
Pulse Stretching section). In high frequency mode and when
using 4−wire fans, the TACH signal is always valid because
the fan is always powered on.
is simply a matter of connecting another fan directly in
parallel with the first. Care should be taken in designing
drive circuits with transistors and FETs to ensure that the
PWM pins are not required to source current and that they
sink less than the 8 mA maximum current specified on the
MOSFET data sheet.
Driving a 3−wire fan with a PWM signal makes the fan
Figure 41 shows the most typical circuit used with 4−wire
Because the electronics in a 4−wire fan are powered
Note that the ADT7462 has up to eight TACH inputs
Note that because the MOSFET can handle up to 3.5 A, it
ADT7462
Figure 41. Driving a 4−Wire Fan
TACH
PWM
4.7 kΩ
3.3 V or 5.0 V
10 kΩ
10 kΩ
12 V
2 kΩ
TACH
12 V
12 V, 4−WIRE FAN
V
TACH
PWM
CC