Datasheet

LM82
www.ti.com SNIS113D –JANUARY 2000–REVISED MARCH 2013
• q is the electron charge,
• k is the Boltzmann's constant,
• N is the current ratio,
• T is the absolute temperature in °K. (1)
The temperature sensor then measures ΔV
BE
and converts to digital data. In this equation, k and q are well
defined universal constants, and N is a parameter controlled by the temperature sensor. The only other
parameter is η, which depends on the diode that is used for measurement. Since ΔV
BE
is proportional to both η
and T, the variations in η cannot be distinguished from variations in temperature. Since the non-ideality factor is
not controlled by the temperature sensor, it will directly add to the inaccuracy of the sensor. For the Pentium II
Intel specifies a ±1% variation in η from part to part. As an example, assume a temperature sensor has an
accuracy specification of ±3 °C at room temperature of 25 °C and the process used to manufacture the diode
has a non-ideality variation of ±1%. The resulting accuracy of the temperature sensor at room temperature will
be:
T
ACC
= ± 3°C + (±1% of 298 °K) = ±6 °C (2)
The additional inaccuracy in the temperature measurement caused by η, can be eliminated if each temperature
sensor is calibrated with the remote diode that it will be paired with.
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 LM82 can cause temperature
conversion errors. The following guidelines should be followed:
1. Place a 0.1 μF power supply bypass capacitor as close as possible to the V
CC
pin and the recommended 2.2
nF capacitor as close as possible to the D+ and D− pins. Make sure the traces to the 2.2nF capacitor are
matched.
2. The recommended 2.2nF diode bypass capacitor actually has a range of 200pF to 3.3nF. The average
temperature accuracy will not degrade. Increasing the capacitance will lower the corner frequency where
differential noise error affects the temperature reading thus producing a reading that is more stable.
Conversely, lowering the capacitance will increase the corner frequency where differential noise error affects
the temperature reading thus producing a reading that is less stable.
3. Ideally, the LM82 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.
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.(See Figure 15)
5. Avoid routing diode traces in close proximity to power supply switching signals 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 2cm. 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 LM82's GND pin is as close as possible to the Processors GND associated
with the sense diode.
9. Leakage current between D+ and GND should be kept to a minimum. One nano-ampere of leakage can
cause as much as 1°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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