参数资料
| 型号： | ADT7473ZEVB |
| 厂商： | ON Semiconductor |
| 文件页数： | 28/73页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION FOR ADT7473 |
| 产品变化通告： | MFG CHG Notification ADI to ON Semi |
| 标准包装： | 1 |
| 系列： | dBCool® |
| 类型： | 温度传感器 |
| 适用于相关产品： | ADT7473 |
| 所含物品： | 评估板 |
| 其它名称： | EVAL-ADT7473EBZ EVAL-ADT7473EBZ-ND |
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ADT7473
pulse (assuming two pulses per revolution are being
counted). Because the device is essentially measuring the
fan TACH period, the higher the count value, the slower the
fan is actually running. A 16-bit fan tachometer reading of
0xFFFF indicates either the fan has stalled or is running very
slowly (<100 RPM).
High Limit > Comparison Performed
Because the actual fan TACH period is measured, falling
below a fan TACH limit by 1 sets the appropriate status bit
and can be used to generate an SMBALERT.
Measuring Fan TACH
When the ADT7473/ADT7473 ? 1 starts up, TACH
measurements are locked. In effect, an internal read of the
low byte has been made for each TACH input. The net result
of this is that all TACH readings are locked until the high
byte is read from the corresponding TACH registers. All
TACH related interrupts are also ignored until the
appropriate high byte is read.
Once the corresponding high byte has been read, TACH
measurements are unlocked and interrupts are processed as
normal.
Fan TACH Limit Registers
The fan TACH limit registers are 16-bit values consisting
of two bytes.
Table 35. FAN TACH LIMIT REGISTERS
Fan Speed (RPM) = (90,000 ? 60)/Fan TACH Reading
where Fan TACH Reading is the 16-bit fan tachometer
reading.
Example
TACH1 High Byte (Register 0x29) = 0x17
TACH1 Low Byte (Register 0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TACH Reading = 0x17FF = 6143 (decimal)
RPM = (f ? 660)/Fan 1 TACH Reading
RPM = (90000 ? 660)/6143
Fan Speed = 879 RPM
Fan Pulses per Revolution
Different fan models can output either one, two, three, or
four TACH pulses per revolution. Once the number of fan
TACH pulses has been determined, it can be programmed
into the fan pulses per revolution register (Register 0x7B)
for each fan. Alternatively, this register can be used to
determine the number or pulses per revolution output by a
given fan. By plotting fan speed measurements at a 100%
speed with different pulses per revolution setting, the
smoothest graph with the lowest ripple determines the
correct pulses per revolution value.
Table 36. TACH PULSES/REVOLUTION REGISTER
(REG. 0X7B)
Register
0x54
0x55
0x56
0x57
0x58
Description
TACH1 Minimum Low Byte
TACH1 Minimum High Byte
TACH2 Minimum Low Byte
TACH2 Minimum High Byte
TACH3 Minimum Low Byte
Default
0xFF
0xFF
0xFF
0xFF
0xFF
Bit
<1:0>
<3:2>
<5:4>
<7:6>
Mnemonic
FAN1 Default
FAN2 Default
FAN3 Default
FAN4 Default
Description
2 Pulses per Revolution
2 Pulses per Revolution
2 Pulses per Revolution
2 Pulses per Revolution
0x59
0x5A
TACH3 Minimum High Byte
TACH4 Minimum Low Byte
0xFF
0xFF
Table 37. TACH PULSES/REVOLUTION REGISTER
BIT VALUES
0x5B
TACH4 Minimum High Byte
0xFF
Value
Description
Fan Speed Measurement Rate
The fan TACH readings are normally updated once every
second.
The FAST bit (Bit 3) of Configuration Register 3 (0x78),
00
01
10
11
1 Pulse per Revolution
2 Pulses per Revolution
3 Pulses per Revolution
4 Pulses per Revolution
when set, updates the fan TACH readings every 250 ms.
If any of the fans are not being driven by a PWM channel
but are powered directly from 5.0 V or 12 V, their associated
dc bit in Configuration Register 3 should be set. This allows
TACH readings to be taken on a continuous basis for fans
connected directly to a dc source. For optimal results, the
associated dc bit should always be set when using 4-wire
fans.
Calculating Fan Speed
Assuming a fan has two pulses per revolution (and two
pulses per revolution being measured), fan speed is
Fan Spin-up
The ADT7473/ADT7473 ? 1 has a unique fan spin-up
function. It spins the fan at 100% PWM duty cycle until two
TACH pulses are detected on the TACH input. Once two
TACH pulses are detected, the PWM duty cycle goes to the
expected running value, for example, 33%. The advantage
is that fans have different spin-up characteristics and take
different times to overcome inertia. The ADT7473/
ADT7473 ? 1 runs the fans just fast enough to overcome
inertia and is quieter on spin-up than fans programmed for
a given spin-up time.
calculated by:
http://onsemi.com
28
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