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| 型号： | ADT7490ARQZ-R7 |
| 厂商： | ON Semiconductor |
| 文件页数： | 30/75页 |
| 文件大小： | 0K |
| 描述： | IC THERM MON FAN CTLR 24-QSOP |
| 产品变化通告： | MFG CHG Notification ADI to ON Semi QSOP 24ld Pkg (MSL) Change 17/Jun/2010 Product Discontinuation 30/Sept/2011 |
| 标准包装： | 1,000 |
| 系列： | dBCool® |
| 功能： | 风扇控制，温度监控器 |
| 传感器类型： | 内部和外部 |
| 感应温度： | -40°C ~ 125°C |
| 精确度： | ±1.5°C（最小值） |
| 拓扑： | ADC，比较器，风扇速度计数器，多路复用器，寄存器库 |
| 输出类型： | SMBus? |
| 输出警报： | 无 |
| 输出风扇： | 是 |
| 电源电压： | 3 V ~ 3.6 V |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 24-SSOP（0.154"，3.90mm 宽） |
| 供应商设备封装： | 24-QSOP |
| 包装： | 带卷 (TR) |
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ADT7490
If the fan output has a resistive pullup to 12 V, or other
voltage greater than 3.6 V, 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 3.6 V, allowing for the voltage
tolerance of the Zener. A value of between 3.0 V and 3.6 V
is suitable.
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 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 (see Figure 46), so the accumulated
12 V
V CC
count is actually proportional to the fan tachometer period
and inversely proportional to the fan speed.
PULLUP
4.7 k W
TYP
TACH
OUTPUT
ZD1*
TACH
ADT7490
FAN SPEED
COUNTER
N, the number of pulses counted, is determined by the
settings of the 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.
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 ? V CC
Figure 43. Fan with Strong TACH Pullup to > 3.6 V,
(for Example, 12 V) Clamped with Zener Diode
CLOCK
PWM
If the fan has a strong pullup (less than 1 k W ) to 12 V or
a totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 44.
5 V or12 V V CC
FAN
TACH
1
2
3
4
Figure 46. Fan Speed Measurement
PULLUP
TYP < 1 k W
R1
10 k W
TACH
OUTPUT
TACH
ZD1*
ZENER
ADT7490
FAN SPEED
COUNTER
Fan Speed Measurement Registers
The fan tachometer registers are 16-bit values consisting
of a 2-byte read from the ADT7490.
OR TOTEM-POLE
Table 30. FAN TACHOMETER REGISTERS
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 ? V CC
Figure 44. Fan with Strong TACH. Pullup to > V CC
or Totem - Pole Output, Clamped with
Zener Diode and Resistor
Alternatively, a resistive attenuator can be used, as shown
in Figure 45. R1 and R2 should be chosen such that
Register
0x28
0x29
0x2A
0x2B
0x2C
Description
TACH1 Low Byte
TACH1 High Byte
TACH2 Low Byte
TACH2 High Byte
TACH3 Low Byte
Default
0x00
0x00
0x00
0x00
0x00
2 V t V PULLUP
R2 R PULLUP ) R1 ) R2 t 3.6 V (eq. 6)
0x2D
TACH3 High Byte
0x00
The fan inputs have an input resistance of nominally
160 k W to ground, which should be taken into account when
calculating resistor values.
0x2E
0x2F
TACH4 Low Byte
TACH4 High Byte
0x00
0x00
With a pullup voltage of 12 V and pullup resistor less than
1 k W , suitable values for R1 and R2 are 100 k W and 40 k W ,
respectively. This gives a high input voltage of 3.42 V.
12 V V CC
Reading Fan Speed from the ADT7490
The measurement of fan speeds involves a 2-register read
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
< 1 k W
R1
100 k W
TACH
OUTPUT
TACH
R2
40 k W
ADT7490
FAN SPEED
COUNTER
TACH readings. The fan tachometer reading registers report
back the number of 11.11 m s 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).
Because the device is essentially measuring the fan TACH
Figure 45. Fan with Strong TACH. Pullup to > V CC or
Totem-Pole Output, Attenuated with R1/R2
period, the higher the count value, the slower the fan is
actually running. A 16-bit fan tachometer reading of
http://onsemi.com
30
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