EMC1403 datasheet(21/55 Pages) MICROCHIP

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1°C Temperature Sensor with Beta Compensation

Data Sheet

 2014 Microchip Technology Inc.

DS20005272A-page 21

5.4

Beta Compensation

The EMC1403 and EMC1404 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. 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.

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

5.5

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 EMC1403 and EMC1404 automatically correct up

to 100 ohms of series resistance.

5.6

Programmable External Diode Ideality Factor

The EMC1403 and EMC1404 is designed for 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

temperatures. To provide maximum flexibility to the user, the EMC1403 and EMC1404 provides a 6-

bit register for each external diode where the ideality factor of the diode used is programmed to

eliminate errors across all temperatures.

APPLICATION NOTE:

When monitoring a substrate transistor or CPU diode and beta compensation is enabled, the

Ideality Factor should not be adjusted. Beta Compensation automatically corrects for most

ideality errors.

5.7

Diode Faults

The EMC1403 and EMC1404 detect an open on the DP and DN pins, and a short across the DP and

DN pins. For each temperature measurement made, the device checks for a diode fault on the external

diode channel(s). When a diode fault is detected, the ALERT pin asserts (unless masked, see

Section 5.8) and the temperature data reads 00h in the MSB and LSB registers (note: the low limit will

not be checked). A diode fault is defined as one of the following: an open between DP and DN, a short

from VDD to DP, or a short from VDD to DN.

If a short occurs across DP and DN or a short occurs from DP to GND, the low limit status bit is set

and the ALERT pin asserts (unless masked). This condition is indistinguishable from a temperature

measurement of 0.000degC (-64°C in extended range) resulting in temperature data of 00h in the MSB

and LSB registers.

If a short from DN to GND occurs (with a diode connected), temperature measurements will continue

as normal with no alerts.

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