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ADT7466ZEVB

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ADT7466
Rev. 2 | Page 28 of 48 | www.onsemi.com
If the fan output has a resistive pull-up to 12 V (or other voltage
greater than 6.5 V), the fan output can be clamped with a Zener
diode, as shown in Figure 37. The Zener diode voltage should
be greater than V
IH
of the tach input but less than 6.5 V,
allowing for the voltage tolerance of the Zener. A value of
between 3 V and 5 V is suitable.
04711-036
FAN DRIVE
V
CC
ADT7466
TACH
ZD1*
ZENER
*CHOOSE ZD1 VOLTAGE APPROX. 0.8
×
V
CC
TACH
OUTPUT
FAN SPEED
COUNTER
PULL-UP
4.7kΩ
TYP.
Figure 37. Fan with Tach.
Pull-Up to Voltage >6.5 V, for Example, 12 V Clamped with Zener Diode.
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 38. Alternatively, a resistive
attenuator can be used, as shown in Figure 39.
R1 and R2 should be chosen such that
2 V < V
PULLUP
× R2/(R
PULLUP
+ R1 + R2) < 5 V
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.
04711-037
FAN DRIVE
V
CC
ADT7466
TACH
ZD1*
ZENER
*CHOOSE ZD1 VOLTAGE APPROX. 0.8
×
V
CC
TACH
OUTPUT
R1
10kΩ
FAN SPEED
COUNTER
PULL-UP
TYP. < 1kΩ
OR TOTEM POLE
Figure 38. Fan with Strong Tach.
Pull-Up to >V
CC
or Totem Pole Output, Clamped with Zener and Resistor.
04711-038
FAN DRIVE
V
CC
ADT7466
TACH
R1*
R2*
*SEE TEXT
TACH
OUTPUT
FAN SPEED
COUNTER
<1kΩ
Figure 39. Fan with Strong Tach.
Pull-Up to >VCC or Totem Pole Output, Attenuated with R1/R2.
Fan Speed Registers
The fan counter does not count the fan tach output pulses
directly because the fan speed can be less than 1000 rpm; it
would take several seconds to accumulate a reasonably large
and accurate count. Instead, the period of the fan revolution is
measured by gating an on-chip 82 kHz oscillator into the input
of a 16-bit counter for N periods of the fan tach output, as
shown in Figure 40. The accumulated count is actually
proportional to the fan tachometer period and inversely
proportional to the fan speed.
04711-039
C
LOC
K
TACH
1
2
Figure 40. Fan Speed Measurement
N, the number of pulses counted, is determined by the settings
of Register 0x39 (fan pulses per revolution register). This
register contains 2 bits for each fan, allowing 1, 2 (default), 3 or
4 tach pulses to be counted.
The fan tachometer readings are 16-bit values consisting of a
2-byte read from the ADT7466.
Table 24. Fan Speed Measurement Registers
Register Description Default
0x48 TACH1 low byte 0xFF
0x49 TACH1 high byte 0xFF
0x4A TACH2 low byte 0xFF
0x4B TACH2 high byte 0xFF
Reading Fan Speed from the ADT7466
Measuring fan speeds involves a 2-register read for each
measurement. The low byte should be read first, which causes
the high byte to be frozen until both high and low byte registers
are read. This prevents erroneous tach readings.
The fan tachometer reading registers report the number of
12.2 μs period clocks (82 kHz oscillator) gated to the fan speed
counter from the rising edge of the first fan tach pulse to the
rising edge of the third fan tach pulse, assuming two pulses per
revolution is being counted. Since the device is essentially
measuring the fan tach period, the higher the count value, the
slower the fan is actually running. A 16-bit fan tachometer
reading of 0xFFFF indicates either that the fan has stalled or
that it is running very slowly (<75 rpm).
A greater than comparison is performed when comparing with
the high limit.
ADT7466
Rev. 2 | Page 29 of 48 | www.onsemi.com
The actual fan tach period is being measured in this case.
Therefore, when the fan tach limit is exceeded, a 1 is set for the
appropriate status bit and can be used to generate an
ALERT
.
The fan tach limit registers are 16-bit values consisting of 2 bytes.
Table 25. Fan Tach Limit Registers
Register Description Default
0x4C TACH1 minimum low byte 0xFF
0x4D TACH1 minimum high byte 0xFF
0x4E TACH2 minimum low byte 0xFF
0x4F TACH2 minimum high byte 0xFF
Fan Speed Measurement Rate
The fan tach readings are normally updated once every second.
The FAST bit (Bit 3) of Configuration Register 3 (0x02) updates
the fan tach readings every 250 ms, when set to 1. If any of the
fans are not being driven by a fan drive output, but are powered
directly from 5 V or 12 V, its associated dc bit in Configuration
Register 3 should be set. This allows tach readings to be taken
on a continuous basis for fans connected directly to a dc source.
Calculating Fan Speed
Assuming a fan with two pulses/revolution (and two
pulses/revolution being measured) fan speed is calculated by
Fan Speed (rpm) = (82000 × 60)/Fan Tach Reading
where Fan Tach Reading is the 16-bit fan tachometer reading.
For example, if
TACH1 High Byte (Reg. 0x49) = 0x17
TACH1 Low Byte (Reg. 0x48) = 0xFF
then fan speed in rpm is
Fan 1 TACH reading = 0x17FF = 6143 decimal
rpm = (82000 × 60)/Fan 1 TACH reading
rpm = (82000 × 60)/6143 = 800 = fan speed
Fan Pulses Per Revolution
Different fan models can output either 1, 2, 3, or 4 tach pulses
per revolution. Once the number of fan tach pulses is
determined, it can be programmed into the fan pulses per
revolution register (0x39) for each fan. Alternatively, this
register can be used to determine the number of
pulses/revolution output by a given fan. By plotting fan speed
measurements at 100% speed with different pulses/revolution
settings, the smoothest graph with the lowest ripple determines
the correct pulses/revolution value.
Table 26. Fan Pulses Per Revolution Register
Fan Default
1:0 FAN1 2 pulses per revolution
3:2 FAN2 2 pulses per revolution
Table 27. Fan Pulses Per Revolution Values
Code Pulses per Revolution
00 1
01 2
10 3
11 4
The ADT7466 has a unique fan spin-up function. It spins the
fan with the fan start-up voltage until two tach pulses are
detected on the tach input. Once two pulses are detected, the
fan drive goes to the expected running value. The advantage of
this is that fans have different spin-up characteristics and take
different times to overcome inertia. The ADT7466 runs the fans
just fast enough to overcome inertia and is quieter on spin-up
than fans programmed to spin-up for a given spin-up time.
FAN START-UP TIMEOUT
To prevent false interrupts being generated as a fan spins up
(since it is below running speed), the ADT7466 includes a fan
start-up timeout function. This is the time limit allowed for two
tach pulses to be detected on spin-up. For example, if a
2-second fan start-up timeout is chosen, and no tach pulses
occur within two seconds of the start of spin-up, a fan fault is
detected and flagged in Interrupt Status Register 1.
Start-Up Timeout Configuration (Reg. 0x38)
Bits 2:0 control the start-up timeout for DRIVE1. Bits 5:3
control the start-up timeout for DRIVE2.
Table 28. Start-Up Timeout Configuration
Code Timeout
000 No start-up timeout
001 100 ms
010 250 ms
011 400 ms
100 667 ms
101 1 second
110 2 seconds
111 4 seconds
ADT7466
Rev. 2 | Page 30 of 48 | www.onsemi.com
AUTOMATIC FAN SPEED CONTROL
The ADT7466 has a local temperature sensor and a remote
temperature channel, which can be connected to an on-chip
diode-connected transistor on a CPU. In addition, the two
analog input channels can be reconfigured for temperature
measurement. Any or all of these temperature channels can be
used as the basis for automatic fan speed control to drive fans
according to system temperature. By running the fans at only
the speed needed to maintain a desired temperature, acoustic
noise is reduced. Reducing fan speed can also decrease system
current consumption.
To use automatic fan control (AFC), a number of parameters
must be set up.
Which Temperature Channel Controls Which Fan?
This is determined by the AFC configuration registers (0x05
and 0x06). AFC1 configuration register controls Fan 1, and
AFC2 configuration register controls Fan 2. Setting bits in these
registers decides which temperature channels controls the fan.
Table 29. AFC Configuration Registers
Bit Description
Bit 0 Fan controlled by TH1 or REM2
Bit 1 Fan controlled by TH2
Bit 2 Fan controlled by Remote Temperature 1
Bit 3 Fan controlled by local temperature
Bit 4 Fan under manual control
Bit 5 Fan at minimum speed
Bit 6 Fan at start-up speed
Bit 7 Fan at maximum speed
If more than one of the temperature channel Bits 0:3 are set, the
channel that demands the highest fan speed takes control.
When TH1 and TH2 are set up as AIN1 and AIN2, these pins
still control the AFC loop if Bits 0:1 in the AFC configuration
register are set. Bits 0:1 should not be set in analog input mode.
If the manual control bit is set, AFC is switched off and the
DRIVE registers can be programmed manually. This overrides
any setting of the temperature channel bits. The maximum
RPM registers, 0x34 and 0x35, should be set to 0x00 when the
fans are under manual control.
If the minimum speed bit is set, AFC is switched off and the fan
runs at minimum speed. This overrides any setting of Bits 4:0.
If the start-up speed bit is set, AFC is switched off and the fan
runs at start-up speed. This overrides any setting of Bits 5:0.
If the maximum speed bit is set, AFC is switched off and the fan
runs at maximum speed. This overrides any setting of Bits 6:0.
Fan Start Voltage (V_FAN_ON)
This is the minimum drive voltage from the DAC at which a fan
starts running. This depends on the parameters of the fan and
the characteristics of the fan drive circuit.
Minimum Fan Speed (V_FAN_MIN)
This is the minimum drive voltage from the DAC at which a fan
keeps running, which is lower than the voltage required to start
it. This depends on the parameters of the fan and the
characteristics of the fan drive circuit.
Maximum Fan Speed
For acoustic reasons it may be desirable to limit the maximum
rpm of the fans. These values are programmed into the
maximum fan speed registers (0x34 and 0x35). During AFC,
the fan speed is monitored and is never allowed to exceed the
programmed limit, even if the AFC loop demands it. However,
the maximum fan speed limit can be overridden by a
THERM
event, which sets the fan drive to full scale (full speed) for
emergency cooling.
Operating Temperature Range
The temperature range over which AFC operates can be
programmed by using the TMIN and TRANGE registers.
TMIN is the temperature at which a fan starts and runs at
minimum speed when in AFC mode. TRANGE is the
temperature range over which AFC operates. Thus, if TMIN is
set to 40°C and TRANGE is set to 20°C, the fan starts when the
temperature exceeds 40°C and the fan reaches maximum speed
at a temperature of 60°C.
Enhanced Acoustics
When fan speed is controlled automatically, a temperature event
can cause the fan drive output to change instantaneously to a new
value. The sudden subsequent change in fan speed can cause an
audible noise pulse. To avoid this problem, the ADT7466 can be
programmed so that the drive value changes in a series of small
steps, using the enhanced acoustics register (0x36).
Bits 2:0 of this register allow eight step sizes from 1 to 48 bits to
be selected for Fan 1. Bits 5:3 do the same for Fan 2. When
automatic fan control requires a change in drive value, the value
changes by the step size once every 250 ms until the final value
is reached. For example, if the step size is 3 and the drive value
changes from 137 to 224, the drive value takes 29 ms × 250 ms
to reach its final value.
Enhanced acoustics for the Fan 1 output (DRIVE1) can be
enabled by setting Bit 6 of the enhanced acoustics register, and
by setting Bit 7 for Fan 2 (DRIVE2).
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ADT7466ZEVB

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ON Semiconductor
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BOARD EVALUATION ADT7466
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