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1°C Temperature Sensor with Hardware Thermal Shutdown

Datasheet

SMSC EMC1423/EMC1424

Revision 1.16 (03-15-07)

DATASHEET

5.5

Beta Compensation

The EMC1423 and EMC1424 are configured to monitor the temperature of basic diodes (e.g. 2N3904),

or CPU thermal diodes. It automatically detects the type of external diode (CPU diode or diode

connected transistor) and determines the optimal setting to reduce temperature errors introduced by

beta variation for the External Diode 1 channel only.

For discrete transistors configured with the collector and base shorted together, the beta is generally

sufficiently high such that the percent change in beta variation is very small. For example, a 10%

variation in beta for two forced emitter currents with a transistor whose ideal beta is 50 would contribute

approximately 0.25°C error at 100°C. However for substrate transistors where the base-emitter junction

is used for temperature measurement and the collector is tied to the substrate, the proportional beta

variation will cause large error. For example, a 10% variation in beta for two forced emitter currents

with a transistor whose ideal beta is 0.5 would contribute approximately 8.25°C error at 100°C.

The External Diode 2 and External Diode 3 channels do not support Beta Compensation.

5.6

Resistance Error Correction (REC)

Parasitic resistance in series with the external diodes will limit the accuracy obtainable from

temperature measurement devices. The voltage developed across this resistance by the switching

diode currents cause the temperature measurement to read higher than the true temperature.

Contributors to series resistance are PCB trace resistance, on die (i.e. on the processor) metal

resistance, bulk resistance in the base and emitter of the temperature transistor. Typically, the error

caused by series resistance is +0.7°C per ohm. The EMC1423 and EMC1424 automatically correct up

to 100 ohms of series resistance.

5.7

Programmable External Diode Ideality Factor

The EMC1423 and EMC1424 do not support a programmable Ideality Factor for the External Diode 1

channel. It is hard-wired for an ideality factor of 1.008.

The EMC1423 External Diode 2 channel defaults to measuring external diodes with an ideality factor

of 1.008. Likewise, the EMC1424 External Diode 2 and 3 channels default to measuring external

diodes with an ideality factor of 1.008.

Not all external diodes, processor or discrete, will have this exact value. This variation of the ideality

factor introduces error in the temperature measurement which must be corrected for. This correction

is typically done using programmable offset registers. Since an ideality factor mismatch introduces an

error that is a function of temperature, this correction is only accurate within a small range of

Figure 5.4 Isolating ALERT and SYS_SHDN Pins

SMDATA

SMCLK

ALERT

VDD

DP1

DN1

SYS_SHDN

GND

+3.3V

Shared Alert/

22K

4.7K -

33K

+2.5 - 5V

EMC1423 Datenblatt(PDF) 21 Page - SMSC Corporation

EMC1423 Datenblatt(HTML) 21 Page - SMSC Corporation

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