ADT7470ARQZ Analog Devices Inc, ADT7470ARQZ Datasheet - Page 22

• Manufacturer Part Number: ADT7470ARQZ
• Description: IC SENSOR TEMP FAN CTRLR 16QSOP
• Manufacturer: Analog Devices Inc

Specifications of ADT7470ARQZ

• Function: Fan Control, Temp Monitor
• Topology: ADC, Comparator, Fan Speed Control, Register Bank
• Sensor Type: External
• Sensing Temperature: External Sensor
• Output Type: I²C™
• Output Alarm: No
• Output Fan: Yes
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-QSOP
• Ic Output Type: Digital
• Sensing Accuracy Range: ± 12%
• Supply Current: 500µA
• Supply Voltage Range: 3V To 5.5V
• Sensor Case Style: QSOP
• No. Of Pins: 16
• Msl: MSL 3 - 168 Hours
• Termination Type: SMD
• Filter Terminals: SMD
• Rohs Compliant: Yes
• Accuracy %: 12%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-ADT7470EBZ - BOARD EVALUATION FOR ADT7470
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

ADT7470
FAN SPEED MEASUREMENT
TACH INPUTS
Pin 6, Pin 7, Pin 4, and Pin 9 are open-drain tach inputs
intended for fan speed measurement.
Signal conditioning in the ADT7470 accommodates the slow
rise and fall times typical of fan tachometer outputs. The maxi-
mum input signal range is 0 V to 5 V, even where V
than 5 V. If these inputs are supplied from fan outputs that
exceed 0 V to 5 V, either resistive attenuation of the fan signal
or diode clamping must be included to keep inputs within an
acceptable range. Figure 20 to Figure 23 show circuits for most
common fan tach outputs.
If the fan tach output has a resistive pull-up to V
connected directly to the fan input, as shown in Figure 20.
If the fan output has a resistive pull-up to 12 V (or other voltage
greater than 5 V), the fan output can be clamped with a Zener
diode, as shown in Figure 21. The Zener diode voltage should
be chosen so that it is greater than V
than 5 V, allowing for the voltage tolerance of the Zener. A value
of between 3 V and 5 V is suitable.
If the fan output has a resistive pull-up to 12 V (or other voltage
greater than 5 V), the fan output can be clamped with a Zener
diode, as shown in Figure 21. The Zener diode voltage should
be chosen so that it is greater than V
than 5 V, allowing for the voltage tolerance of the Zener. A value
of between 3 V and 5 V is suitable.
If the fan has a strong pull-up (less than 1 kΩ) to 12 V, or a
totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 22. Alternatively, a resistive
attenuator can be used, as shown in Figure 23.
Pull-Up to voltage > 5 V, for example, 12 V clamped with Zener diode.
12V
12V
PULLUP
*CHOOSE ZD1 VOLTAGE APPROX. 0.8 × V
4.7kΩ
TYP
PULLUP
4.7kΩ
TYP
Figure 20. Fan with Tach Pull-Up to V
Figure 21. Fan with Tach.
OUTPUT
OUTPUT
TACH
TACH
ZENER
ZD1*
TACH
TACH
IH
IH
of the tach input but less
of the tach input but less
ADT7470
ADT7470
FAN SPEED
FAN SPEED
CC
COUNTER
COUNTER
V
CC
CC
V
CC
CC
, it can be
CC
is less
Rev. C | Page 22 of 40
R1 and R2 should be chosen such that
The fan inputs have an input resistance of nominally 160 kΩ to
ground, which should be taken into account when calculating
resistor values.
With a pull-up voltage of 12 V and pull-up resistor less than
1 kΩ, suitable values for R1 and R2 are 100 kΩ and 47 kΩ.
This gives a high input voltage of 3.83 V.
Pulse Stretching
Pulse stretching of the PWM output is performed automatically
in low frequency fan drive mode, to ensure that sufficient tach
readings are taken from the fan.
However, in high frequency fan drive mode, pulse stretching is
disabled. If using 3-wire fans this mode, care should be taken to
ensure that incomplete tach information does not occur at low
PWM duty cycles, or short PWM pulse widths.
Disabling Tach measurement
The tach measurement for each fan can be disabled by writing
to Configuration Register 2 Bits[3:0], at Address 0x74.
Pull-Up to > V
2 V < V
Pull-Up to > V
12V
12V
OR TOTEM-POLE
PULL-UP
<1kΩ
CC
TYP. <1kΩ
PULLUP
*CHOOSE ZD1 VOLTAGE APPROX. 0.8 × V
or Totem-Pole Output, Clamped with Zener and Resistor.
CC
Figure 22. Fan with Strong Tach.
Figure 23. Fan with Strong Tach.
× R2/(R
or Totem-Pole Output, Attenuated with R1/R2.
OUTPUT
TACH
R1*
TACH
*SEE TEXT
10kΩ
O/P
PULL-UP
R1
R2*
ZENER*
+ R1 + R2) < 5 V
TACH
ZD1
TACH
ADT7470
FAN SPEED
ADT7470
FAN SPEED
COUNTER
COUNTER
V
V
CC
CC
CC