lm95245eb National Semiconductor Corporation, lm95245eb Datasheet - Page 22

• Manufacturer Part Number: lm95245eb
• Description: Precision Remote Diode Digital Temperature Sensor With Trutherm? Bjt Beta Compensation Technology For 45nm Process
• Manufacturer: National Semiconductor Corporation

www.national.com
Solving Equation 6 for R
additional error due to the spread in the series resistance of
±1.07°C. The spread in error cannot be canceled out, as it
would require measuring each individual thermal diode de-
vice. This is quite difficult and impractical in a large volume
production environment.
Equation 6 can also be used to calculate the additional error
caused by series resistance on the printed circuit board. Since
the variation of the PCB series resistance is minimal, the bulk
of the error term is always positive and can simply be can-
celled out by subtracting it from the output readings of the
LM95245.
Note: NA= Not Available at publication of this document.
3.2 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 sensor
and the LM95245 can cause temperature conversion errors.
Keep in mind that the signal level the LM95245 is trying to
measure is in microvolts. The following guidelines should be
followed:
1.
Processor Family
Intel Processor on
45 nm process
Intel Processor on
65 nm process
V
parallel with 100 pF. The 100 pF capacitor should be
placed as close as possible to the power supply pin. A
bulk capacitance of approximately 10 µF needs to be in
the near vicinity of the LM95245.
DD
should be bypassed with a 0.1 µF capacitor in
FIGURE 8. Ideal Diode Trace Layout
0.997
Transistor Equation η
PCB
min
NA
equal to ±1.73Ω results in the
non-ideality
1.001
typ
NA
1.005
max
NA
T
30015117
,
Series
4.52
R,Ω
NA
(6)
22
2.
3.
4.
5.
6.
7.
8.
9.
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 LM95245. 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 LM95245's SMBCLK input. Additional resistance can be
added in series with the SMBDAT and SMBCLK lines to fur-
ther help filter noise and ringing. Minimize noise coupling 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 SMBDAT and
SMBCLK lines.
A 100 pF diode bypass capacitor is recommended to filter
high frequency noise but may not be necessary. Make
sure the traces to the 100 pF capacitor are matched.
Place the filter capacitors close to the LM95245 pins.
Ideally, the LM95245 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 1Ω can cause as much as 0.62°C of error. This error
can be compensated by using simple software offset
compensation.
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 LM95245's GND pin is as
close as possible to the Processors GND associated with
the sense diode.
Leakage current between D+ and GND and between D+
and D− should be kept to a minimum. Thirteen nano-
amperes of leakage can cause as much as 0.2°C of error
in the diode temperature reading. Keeping the printed
circuit board as clean as possible will minimize leakage
current.