EMC1428 datasheet(20/53 Pages) MICROCHIP

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1°C Multiple Temperature Sensor with HW Thermal Shutdown & Hottest of Thermal Zones

Data Sheet

DS20005275A-page 20

 2014 Microchip Technology Inc.

4.5.1

Resistance Error Correction

The EMC1428 includes active Resistance Error Correction to remove the effect of up to 100 ohms of

series resistance. Without this automatic feature, voltage developed across the parasitic resistance in

the remote diode path causes the temperature to read higher than the true temperature is. The error

induced by parasitic resistance is approximately +0.7°C per ohm. Sources of series resistance include

bulk resistance in the remote temperature transistor junctions, series resistance in the CPU, and

resistance in the printed circuit board traces and package leads. Resistance error correction in the

EMC1428 eliminates the need to characterize and compensate for parasitic resistance in the remote

diode path.

4.5.2

Beta Compensation

The forward current gain, or beta, of a transistor is not constant as emitter currents change. As well,

it is not constant over changes in temperature. The variation in beta causes an error in temperature

reading that is proportional to absolute temperature. Compensating for this error is also known as

implementing the BJT or transistor 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 Beta Compensation circuitry in the EMC1428 corrects for this beta variation to eliminate any error

which would normally be induced. It automatically detects the appropriate beta setting to use.

4.5.3

Digital Averaging

To reduce the effect of noise and temperature spikes on the reported temperature, all of the external

diode channels use digital averaging. This averaging acts as a running average using the previous four

measured values.

The default setting is to have digital averaging disabled for all channels. It can be enabled for each

channel individually by the Filter Control Register (see Section 5.24).

4.5.4

“Hottest Of” Comparison

At the end of every measurement cycle, the EMC1428 compares all of the user selectable External

Diode channels to determine which of these channels is reporting the hottest temperature. The hottest

temperature is stored in the Hottest Temperature Registers and the appropriate status bit in the Hottest

Status Register is set. As an optional feature, the EMC1428 can also flag an event if the hottest

temperature channel changes. For example, suppose that External Diode channels 1, 3, and 4 are

programmed to be compared in the “Hottest Of” Comparison. If the External Diode 1 channel reports

the hottest temperature of the three, its temperature is copied into the Hottest Temperature Registers

(in addition to the External Diode 1 Temperature registers) and it is flagged in the Hottest Status bit.

If, on the next measurement, the External Diode 3 channel temperature has increased such that it is

now the hottest temperature, the EMC1428 can flag this event as an interrupt condition and assert the

ALERT pin.

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