LM87

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SNAS034J – APRIL 2000 – REVISED MARCH 2013

Internal Temperature Measurement

The LM87 internal temperature is monitored using a junction type temperature sensor.

Remote Temperature Measurement

The LM87 monitors the temperature of remote semiconductor devices using the p-n junction temperature sensing

principal. Up to two remote IC, diode or bipolar transistor temperatures can be monitored. The remote

measurement channels have been optimized to measure the remote diode of a Pentium II processor. A discrete

diode or bipolar transistor can also be used to sense the temperature of external objects or ambient air. The

2N3904 NPN transistor base emitter junction performs well in this type of application. When using a 2N3904, the

collector should be connected to the base to provide a device that closely approximates the characteristics of the

Pentium II PNP monitoring diode.

When using two external 2N3904 sensors, the D

− inputs should be connected together. This provides the best

possible accuracy by compensating for differences between the 2N3904 and Pentium II sensors.

During each conversion cycle, the remote monitoring inputs perform an external diode fault detection sequence.

If the D+ input is shorted to VCC or floating then the temperature reading will be +127°C, and bit 6 or bit 7 of

Interrupt Status Register 2 will be set. If D+ is shorted to GND or D

−, the temperature reading will be 0°C and bit

6 or 7 of Interrupt Status Register 2 will not be set.

Accuracy Effects of Diode Non-Ideality Factor

The technique used in today's remote temperature sensors is to measure the change in VBE at two different

operating points of a diode. For a bias current ratio of N:1, this difference is given as:

where

•

η is the non-ideality factor of the process the diode is manufactured on,

•

q is the electron charge,

•

k is the Boltzmann's constant,

•

N is the current ratio,

•

T is the absolute temperature in °K.

(4)

The temperature sensor then measures

ΔVBE 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 ΔVBE 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:

TACC = ± 3°C + (±1% of 298°K) = ±6°C

(5)

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.

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