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ADM1024ARUZ Datasheet(PDF) 17 Page - ON Semiconductor

Part No. ADM1024ARUZ
Description  System Hardware Monitor with Remote Diode Thermal Sensing
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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ADM1024ARUZ Datasheet(HTML) 17 Page - ON Semiconductor

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17
If the fan has a strong pullup (less than 1 k
W) to 12 V, or
a totem−pole output, then 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. 7)
2.0 V t VPULLUP
R2
RPULLUP ) R1 ) R2
t 5.0 V
The fan inputs have an input resistance of nominally
160 k
W to ground, so this should be taken into account when
calculating resistor values.
With a pullup voltage of 12 V and pullup resistor less than
1 k
W, suitable values for R1 and R2 would be 100 kW and
47 k
W. This will give a high input voltage of 3.83 V.
Figure 27. Fan with Strong Tach Pullup to >VCC or
Totem Pole Output, Clamped with Zener and Resistor
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 y VCC
PULLUP
TOTEM−POLE
12V
VCC
FAN SPEED
COUNTER
FAN1 OR
FAN2
TACH
OUTPUT
ZD1*
ZENER
160kW
TYP <1k OR
W
R1
10kW
Figure 28. Fan with Strong Tach Pullup to > VCC or
Totem Pole Output, Attenuated with R1/R2
*SEE TEXT.
12V
VCC
FAN SPEED
COUNTER
FAN1 OR
FAN2
TACH
OUTPUT
R1*
R2*
160k
<1k W
W
Fan Speed Measurement
The fan counter does not count the fan tachometer output
pulses directly because the fan speed may be less than
1000 rpm and 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
22.5 kHz oscillator into the input of an 8−bit counter for two
periods of the fan tachometer output, as shown in Figure 29;
the accumulated count is actually proportional to the fan
tachometer period and inversely proportional to the fan
speed.
Figure 29. Fan Speed Measurement
22.5kHz
CLOCK
CYCLE
PERIOD
START OF
MONITORING
MEASUREMENT
FAN1
PERIOD
MEASUREMENT
FAN2
INPUT
FAN1
INPUT
FAN2
REG. 1 BIT 0
CONFIG.
The monitoring cycle begins when a one is written to the
Start Bit (Bit 0), and a zero to the INT_Clear Bit (Bit 3) of
the Configuration Register. INT_Enable (Bit 1) should be
set to one to enable the INT output. The measurement begins
on the rising edge of a fan tachometer pulse, and ends on the
next−butone rising edge. The fans are monitored
sequentially, so if only one fan is monitored, the monitoring
time is the time taken after the Start Bit for it to produce two
complete tachometer cycles or for the counter to reach full
scale, whichever occurs sooner. If more than one fan is
monitored, the monitoring time depends on the speed of the
fans and the timing relationship of their tachometer pulses.
This is illustrated in Figure 30. Once the fan speeds have
been measured, they will be stored in the Fan Speed Value
Registers and the most recent value can be read at any time.
The measurements will be updated as long as the monitoring
cycle continues.
To accommodate fans of different speed and/or different
numbers of output pulses per revolution, a prescaler
(divisor) of 1, 2, 4, or 8 may be added before the counter. The
default value is 2, which gives a count of 153 for a fan
running at 4400 rpm, producing two output pulses per
revolution.
The count is calculated by the equation:
(eq. 8)
Count + 22.5
103
60
RPM
Divisor
For constant speed fans, fan failure is normally considered
to have occurred when the speed drops below 70% of
nominal, which would correspond to a count of 219. Full
scale (255) would be reached if the fan speed fell to 60% of
its nominal value. For temperature−controlled variable
speed fans, the situation will be different.
Table 5 shows the relationship between fan speed and time
per revolution at 60%, 70%, and 100% of nominal rpm for
fan speeds of 1100, 2200, 4400, and 8800 rpm, and the
divisor that would be used for each of these fans, based on
two tachometer pulses per revolution.
FAN1 and FAN2 Divisors are programmed into Bits 4 to 7
of the VID0–3/Fan Divisor Register.


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