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參數(shù)資料
| 型號： | LM64 |
| 廠商： | National Semiconductor Corporation |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | 【1∑C Remote Diode Temperature Sensor with PWM Fan Control and 5 GPIO’s |
| 中文描述： | 【1ΣC遠程二極管溫度傳感器，PWM風扇控制和5個GPIO的 |
| 文件頁數(shù)： | 28/29頁 |
| 文件大?。?/td> | 363K |
| 代理商： | LM64 |
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3.0 Application Notes
(Continued)
3.5 PCB LAYOUT FOR MINIMIZING NOISE
In a noisy environment, such as a processor mother board,
layout considerations are very critical. Noise induced on
traces running between the remote temperature diode sen-
sor and the LM64 can cause temperature conversion errors.
Keep in mind that the signal level the LM64 is trying to
measure is in microvolts. The following guidelines should be
followed:
1.
Place a 0.1 μF power supply bypass capacitor as close
as possible to the V
pin and the recommended 2.2 nF
capacitor as close as possible to the LM64’s D+ and D
pins. Make sure the traces to the 2.2 nF capacitor are
matched.
2.
Ideally, the LM64 should be placed within 10 cm of the
Processor diode pins with the traces being as straight,
short and identical as possible. Trace resistance of 1
can cause as much as 1C of error. This error can be
compensated by using the Remote Temperature Offset
Registers, since the value placed in these registers will
automatically be subtracted from or added to the remote
temperature reading.
3.
Diode traces should be surrounded by a GND guard ring
to either side, above and below if possible. This GND
guard should not be between the D+ and D lines. In the
event that noise does couple to the diode lines it would
be ideal if it is coupled common mode. That is equally to
the D+ and D lines.
Avoid routing diode traces in close proximity to power
supply switching or filtering inductors.
Avoid running diode traces close to or parallel to high
speed digital and bus lines. Diode traces should be kept
at least 2 cm apart from the high speed digital traces.
If it is necessary to cross high speed digital traces, the
diode traces and the high speed digital traces should
cross at a 90 degree angle.
The ideal place to connect the LM64’s GND pin is as
close as possible to the Processor’s GND associated
with the sense diode.
Leakage current between D+ and GND should be kept
to a minimum. One nano-ampere of leakage can cause
as much as 1C of error in the diode temperature read-
ing. Keeping the printed circuit board as clean as pos-
sible will minimize leakage current.
Noise coupling into the digital lines greater than 400 mVp-p
(typical hysteresis) and undershoot less than 500 mV below
GND, may prevent successful SMBus communication with
the LM64. SMBus no acknowledge is the most common
symptom, causing unnecessary traffic on the bus. Although
the SMBus maximum frequency of communication is rather
low (100 kHz max), care still needs to be taken to ensure
proper termination within a system with multiple parts on the
bus and long printed circuit board traces. An RC lowpass
filter with a 3 dB corner frequency of about 40 MHz is
included on the LM64’s SMBCLK input.Additional resistance
can be added in series with the SMBData and SMBCLK lines
to further help filter noise and ringing. Minimize noise cou-
pling by keeping digital traces out of switching power supply
areas as well as ensuring that digital lines containing high
speed data communications cross at right angles to the
SMBData and SMBCLK lines.
4.
5.
6.
7.
8.
20065521
FIGURE 11. Ideal Diode Trace Layout
L
www.national.com
28
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