ADT7476ARQH ONSEMI [ON Semiconductor], ADT7476ARQH Datasheet - Page 28

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ADT7476ARQH

Manufacturer Part Number
ADT7476ARQH
Description
dBCOOL Remote Thermal Controller and Voltage Monitor
Manufacturer
ONSEMI [ON Semiconductor]
Datasheet
voltage greater than 5.5 V, the fan output can be clamped
with a Zener diode, as shown in Figure 42. The Zener diode
voltage should be chosen so that it is greater than V
TACH input but less than 5.5 V, allowing for the voltage
tolerance of the Zener. A value between 5.0 V and 5.5 V is
suitable.
a totem−pole output, a series resistor can be added to limit
the Zener current, as shown in Figure 43.
in Figure 44. R1 and R2 should be chosen such that:
160 kW to ground, which should be taken into account when
calculating resistor values.
1 kW, suitable values for R1 and R2 are 100 kW and 40 kW,
respectively. This gives a high input voltage of 3.42 V.
2 V t V
If the fan output has a resistive pullup to 12 V, or other
If the fan has a strong pullup (less than 1 kW) to 12 V or
Alternatively, a resistive attenuator can be used, as shown
The fan inputs have an input resistance of nominally
With a pullup voltage of 12 V and pullup resistor less than
Figure 44. Fan with Strong TACH Pullup to >V
Figure 42. Fan with Strong TACH Pullup to > 5.5 V,
Figure 43. Fan with Strong TACH Pullup to >V
FAN
5V OR 12V
(for Example, 12 V) Clamped with Zener Diode
Totem−Pole Output, Attenuated with R1/R2
12V
12V
or Totem−Pole Output, Clamped with
PULLUP
PULLUP TYP
TOTEM POLE
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8
<1kΩ OR
Zener Diode and Resistor
PULLUP
TYPICAL
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8
<1kΩ
4.7kΩ
R2 R
TACH
OUTPUT
TACH
OUTPUT
TACH
OUTPUT
10kΩ
R1*
R1
PULLUP
*SEE TEXT
ZD1*
R2*
TACH
) R1 ) R2 t 5.5 V
ZD1
ZENER*
TACH
TACH
ADT7476A
FAN SPEED
ADT7476A
COUNTER
FAN SPEED
ADT7476
FAN SPEED
COUNTER
COUNTER
V
V
CC
CC
V
×
CC
IH
×
V
CC
CC
V
(eq. 4)
http://onsemi.com
CC
CC
of the
or
28
pulses directly because the fan speed could be less than
1000 RPM, and it takes several seconds to accumulate a
reasonably large and accurate count. Instead, the period of
the fan revolution is measured by gating an on−chip 90 kHz
oscillator into the input of a 16−bit counter for N periods of
the fan TACH output (Figure 45), so the accumulated count
is actually proportional to the fan tachometer period and
inversely proportional to the fan speed.
settings of TACH pulses per revolution register (0x7B). This
register contains two bits for each fan, allowing one, two
(default), three, or four TACH pulses to be counted.
Fan TachometerReading Registers
of a 2−byte read from the ADT7476.
Register 0x28, TACH1 Low Byte = 0x00 default
Register 0x29, TACH1 High Byte = 0x00 default
Register 0x2A, TACH2 Low Byte = 0x00 default
Register 0x2B, TACH2 High Byte = 0x00 default
Register 0x2C, TACH3 Low Byte = 0x00 default
Register 0x2D, TACH3 High Byte = 0x00 default
Register 0x2E, TACH4 Low Byte = 0x00 default
Register 0x2F, TACH4 High Byte = 0x00 default
Reading Fan Speed from the ADT7476
for each measurement. The low byte should be read first.
This causes the high byte to be frozen until both high and
low byte registers have been read, preventing erroneous
TACH readings. The fan tachometer reading registers report
back the number of 11.11 ms period clocks (90 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
are being counted).
period, the higher the count value, the slower the fan is
actually running. A 16−bit fan tachometer reading of
0xFFFF indicates that either the fan has stalled or is running
very slowly (<100 RPM).
CLOCK
The fan counter does not count the fan TACH output
N, the number of pulses counted, is determined by the
The fan tachometer readings are 16−bit values consisting
The measurement of fan speeds involves a 2−register read
Because the device is essentially measuring the fan TACH
TACH
PWM
High Limit: > Comparison Performed
1
Figure 45. Fan Speed Measurement
2
3
4

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