参数资料
| 型号： | ADT7468ZEVB |
| 厂商： | ON Semiconductor |
| 文件页数： | 31/81页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR ADT7468 |
| 标准包装： | 1 |
| 其它名称： | EVAL-ADT7468EB EVAL-ADT7468EB-ND |
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ADT7468
If the fan output has a resistive pull-up to 12 V (or other voltage
greater than 5 V) then the fan output can be clamped with a
Zener diode, as shown in Figure 43. The Zener diode voltage
should be chosen so that it is greater than V IH of the TACH
input but less than 5 V, allowing for the voltage tolerance of the
Zener. A value of between 3 V and 5 V is suitable.
FAN SPEED MEASUREMENT
The fan counter does not count the fan TACH output pulses
directly, because the fan speed could be less than 1,000 RPM
and it would take several seconds to accumulate a reasonably
large and accurate count. Instead, the period of the fan revo-
lution is measured by gating an on-chip 90 kHz oscillator into
12V
V CC
the input of a 16-bit counter for N periods of the fan TACH
output (see Figure 46), so the accumulated count is actually
PULL-UP
4.7k ?
TYPICAL
TACH
OUTPUT
TACH
ZD1*
FAN SPEED
COUNTER
ADT7468
proportional to the fan tachometer period and inversely
proportional to the fan speed.
N, the number of pulses counted, is determined by the settings
of Register 0x7B (TACH pulses per revolution register). This
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V CC
Figure 43. Fan with TACH Pull-Up Greater than 5 V, for example, 12 V),
Clamped with Zener Diode
If the fan has a strong pull-up (<1 k?) to 12 V or a totem-pole
output, then a series resistor can be added to limit the Zener
register contains two bits for each fan, allowing one, two
(default), three, or four TACH pulses to be counted.
CLOCK
current, as shown in Figure 44.
5V OR 12V
V CC
PWM
FAN
TACH
1
PULL-UP TYP
<1k ? OR
TOTEM POLE
R1
10k ?
TACH
OUTPUT
TACH
ZD1
ZENER*
FAN SPEED
COUNTER
ADT7468
2
3
4
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V CC
Figure 44. Fan with Strong TACH Pull-Up Greater than V CC or Totem-Pole
Output, Clamped with Zener and Resistor
Alternatively, a resistive attenuator can be used, as shown in
Figure 45. R1 and R2 should be chosen such that
2 V < V PULL-UP × R2 /( R PULL-UP + R1 + R2 ) < 5 V
The fan inputs have an input resistance of nominally 160 k? to
ground, which should be taken into account when calculating
Figure 46. Fan Speed Measurement
Fan Speed Measurement Registers
The fan tachometer readings are 16-bit values consisting of a
2-byte read from the ADT7468.
Reg. 0x28, TACH1 low byte = 0x00 default
Reg. 0x29, TACH1 high byte = 0x00 default
Reg. 0x2A, TACH2 low byte = 0x00 default
resistor values.
Reg. 0x2B, TACH2 high byte = 0x00 default
With a pull-up voltage of 12 V and pull-up resistor less than
1 k?, suitable values for R1 and R2 would be 100 k? and
47 k?, respectively. This gives a high input voltage of 3.83 V.
Reg. 0x2C, TACH3 low byte = 0x00 default
Reg. 0x2D, TACH3 high byte = 0x00 default
12V
V CC
Reg. 0x2E, TACH4 low byte = 0x00 default
<1k ?
R1*
TACH
OUTPUT
TACH
R2*
FAN SPEED
COUNTER
ADT7468
Reg. 0x2F, TACH4 high byte = 0x00 default
Reading Fan Speed from the ADT7468
The measurement of fan speeds involves a 2-register read for
each measurement. The low byte should be read first. This
*SEE TEXT
Figure 45. Fan with Strong TACH Pull-Up Greater than V CC or Totem-Pole
Output, Attenuated with R1/R2
causes the high byte to be frozen until both high and low byte
registers have been read, preventing erroneous TACH readings.
Rev. 3 | Page 31 of 81 | www.onsemi.com
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