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ADM1029
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15
Fan Speed Limits
Fans generally do not overspeed if run from the correct
voltage, so the failure condition of interest is under-speed
due to electrical or mechanical failure. For this reason only
low-speed limits are programmed into the Tach Limit
Registers for the fans. These registers are at address 78h for
Fan 1 and 79h for Fan 2. It should be noted that, since fan
period rather than speed is being measured, the fan speed
count will be larger the slower the fan speed. Therefore a fan
failure fault will occur when the measurement exceeds the
limit value.
For the most accurate fan failure indication, the oscillator
frequency should be chosen to give as large a limit value as
possible without the counter overranging. A count close to
?full-scale or 191 is the optimum value.
For example, if a fan produces two tach pulses per
revolution and the fan failure speed is to be 600 rpm, the
oscillator frequency should be set to 940 Hz. This will give
a count at the fail speed of:
(eq. 3)
940 4 606002 + 188
If the oscillator frequency were only 470 Hz, the count
would be 94, while an oscillator frequency of 1880 Hz
cannot be used because the count would be 376 and the
counter would overrange.
Fan Monitoring Cycle Time
Five complete tach periods are required to carry out a fan
speed measurement Therefore, if the start of a fan
measurement just misses a rising edge, the measurement can
take almost six tach periods for each fan.
The worst-case monitoring cycle time is when both fans
are under speed and the fan speed counter counts up to its
maximum value. The actual count takes 256 oscillator
pulses over four tach periods, plus a further two tach periods
or 128 oscillator pulses before the count starts. The total
monitoring cycle time is therefore:
(eq. 4)
t
MEAS
+ 384f
OSC(FAN 1)
) 384f
OSC(FAN 2)
In order to read a valid result from the Fan Tach Value
Registers, the total monitoring time allowed after starting
the monitoring cycle should be greater than this.
Tach Signal Conditioning
Signal conditioning in the ADM1029 accommodates the
slow rise and fall times typical of fan tachometer outputs.
The maximum input signal range is 0 V to 5 V, even if V
CC
is less than 5 V. In the event that 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.
Figures 25 a to 28 show circuits for most common fan tach
outputs.
If the fan tach output has a resistive pull-up to V
CC
, it can
be connected directly to the fan input, as shown in Figure 25.
Figure 25. Fan with Tach Pull-up to +V
CC
12 V
V
CC
FAN SPEED
COUNTER
TACH
OUTPUT
TACH1
OR TACH 2
PULL-UP
4.7 kW
TYP
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 26. The Zener
voltage should be chosen so that it is greater than V
IH
 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.
Figure 26. Fan with Tach. Pull-up to Voltage > 6.5 V
(e.g., 12 V) Clamped with Zener Diode
12 V
V
CC
FAN SPEED
COUNTER
TACH
OUTPUT
TACH1
OR
TACH 2
PULL-UP
4.7 kW
TYP
ZD1*
ZENER
* Choose ZD1 Voltage Approx. 0.8 ?V
CC
If the fan has a strong pull-up (less than 1 kW) to 12 V, or
a totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 27. Alternatively, a
resistive attenuator may be used, as shown in Figure 28.
R1 and R2 should be chosen such that:
(eq. 5)
2 V t V
PULLUP
  R2(R
PULLUP
) R1 ) R2) t 5 V
The fan inputs have an input resistance of nominally
160 kW to ground, so this should be taken into account when
calculating resistor values.
With a pull-up voltage of 12 V and pull-up resistor less
than 1 kW, suitable values for R1 and R2 would be 100 kW
and 47 kW. This will give a high input voltage of 3.83 V.
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