Datasheet

LM95245
www.ti.com SNIS148G –OCTOBER 2007–REVISED MARCH 2013
Solving Equation 11 for R
PCB
equal to -1.5Ω to 2.5Ω results in the additional error due to the spread in this series
resistance of -0.93°C to +1.55°C. The spread in error cannot be canceled out, as it would require measuring
each individual thermal diode device. This is quite difficult and impractical in a large volume production
environment.
Equation 11 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 cancelled out by subtracting it from the output readings of the LM95245.
Processor Family Transistor Equation η
T
, non-ideality Series R,Ω
min typ max
Intel Processor on 45 nm process 0.997 1.001 1.008 4.5
Intel Processor on 65 nm process 0.997 1.001 1.005 4.52
Note: NA = Not Available at publication of this document.
PCB LAYOUT FOR MINIMIZING NOISE
Figure 20. Ideal Diode Trace Layout
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. V
DD
should be bypassed with a 0.1 µF capacitor in 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.
2. A 100 pF diode bypass capacitor is recommended to filter high frequency noise but may not be necessary.
The LM95245 can handle capacitance up to 3.3 nF (see Typical Performance Curve "Remote Temperature
Reading Sensitivity to Thermal Diode Filter Capacitance"). Place the filter capacitors close to the LM95245
pins and make sure the traces to this capacitor are matched.
3. 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.
4. 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.
5. Avoid routing diode traces in close proximity to power supply switching or filtering inductors.
6. 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.
7. 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.
8. The ideal place to connect the LM95245's GND pin is as close as possible to the Processors GND
associated with the sense diode.
9. 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.
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