EMC1187 datasheet(21/47 Pages) SMSC

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Triple Channel 1°C Temperature Sensor with Hardware Thermal Shutdown and 1.8V SMBus Communications

Datasheet

SMSC EMC1187

Revision 1.0 (07-11-13)

DATASHEET

5.7

Temperature Measurement

The EMC1187 can monitor the temperature of up to two externally connected diodes.

The device contains programmable High, Low, and Therm limits for all measured temperature

channels. If the measured temperature goes below the Low limit or above the High limit, the ALERT

pin can be asserted (based on user settings).

5.7.1

Beta Compensation

The EMC1187 is 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.

Compensating for this error is also known as implementing the transistor or BJT model for temperature

measurement.

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.

5.7.2

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

Figure 5.3 Isolating ALERT and SYS_SHDN Pin

EMC1187

SMDATA

SMCLK

ALERT

VDD

DP1

DN1

SYS_SHDN

GND

+3.3V

22K

4.7K -

33K

+2.5 - 5V

DP2

DN2

+3.3V

22K

4.7K -

33K

+2.5 - 5V

Shared SYS_SHDN

Shared Alert

EMC1187 Datasheet(PDF) 21 Page - SMSC Corporation

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