LM95241EB National Semiconductor, LM95241EB Datasheet - Page 19

• Manufacturer Part Number: LM95241EB
• Description: BOARD EVALUATION LM95241
• Manufacturer: National Semiconductor
• Series: PowerWise®, TruTherm®r

Specifications of LM95241EB

• Sensor Type: Temperature
• Sensing Range: 0°C ~ 140°C
• Interface: SMBus (2-Wire/I²C)
• Sensitivity: ±1°C
• Voltage - Supply: 3 V ~ 3.6 V
• Embedded: No
• Utilized Ic / Part: LM95241
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

lected, to measure a AMD Athlon processor, with a typical
non-ideality of 1.008, for a temperature range of 60 °C to 100
°C the correction factor would calculate to:
Therefore, 1.75°C should be subtracted from the temperature
readings of the LM95241 to compensate for the differing typ-
ical non-ideality target.
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 LM95241 can cause temperature conversion errors.
Keep in mind that the signal level the LM95241 is trying to
measure is in microvolts. The following guidelines should be
followed:
1.
2.
3.
V
with 100pF. The 100pF capacitor should be placed as
close as possible to the power supply pin. A bulk
capacitance of approximately 10µF needs to be in the
near vicinity of the LM95241.
A 100pF diode bypass capacitor is recommended to filter
high frequency noise but may not be necessary. Make
sure the traces to the 100pF capacitor are matched.
Place the filter capacitors close to the LM95241 pins.
Ideally, the LM95241 should be placed within 10cm 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 1°C of error. This error can be
DD
T
CF
should be bypassed with a 0.1µF capacitor in parallel
=[(1.003−1.008)÷1.003]×(80+273) =−1.75°C
FIGURE 4. Ideal Diode Trace Layout
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4.
5.
6.
7.
8.
9.
Noise coupling into the digital lines greater than 400mVp-p
(typical hysteresis) and undershoot less than 500mV below
GND, may prevent successful SMBus communication with
the LM95241. SMBus no acknowledge is the most common
symptom, causing unnecessary traffic on the bus. Although
the SMBus maximum frequency of communication is rather
low (100kHz 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 3db corner frequency of about 40MHz is included on
the LM95241'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.
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 2cm 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 LM95241's GND pin is as
close as possible to the Processor's GND that is
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.
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