EMC1438 datasheet(21/48 Pages) SMSC

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1°C Multiple Temperature Sensor with Hardware Controlled Standby & Hottest of Multiple Zones

Datasheet

SMSC EMC1438

Revision 1.0 (04-29-10)

DATASHEET

5.6

System Configuration Controls

Each channel can be configured to use Resistance Error Correction, Beta Compensation, and Digital

Averaging based on user settings and system requirements. Conversion rates and Dynamic Averaging

are also configurable.

5.6.1

Resistance Error Correction

The EMC1438 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

EMC1438 eliminates the need to characterize and compensate for parasitic resistance in the remote

diode path.

5.6.2

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 EMC1438 corrects for this beta variation to eliminate any error

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

5.6.3

Digital Averaging

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

diode channels can 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 (Section 6.22, "Filter Control Register").

5.6.4

Conversion Rates

The EMC1438 may be configured for different conversion rates based on the system requirements.

The conversion rate is configured as described in Section 6.5, "Conversion Rate Register". The default

conversion rate is 4 conversions per second. Other available conversion rates are shown in Table 6.7.

5.6.5

Dynamic Averaging

Dynamic averaging causes the EMC1438 to measure the external diode channels for an extended time

based on the selected conversion rate. This functionality can be disabled for increased power savings

at the lower conversion rates (see Section 6.4, "Configuration Register"). When dynamic averaging is

enabled, the device will automatically adjust the sampling and measurement time for the external diode

channels. This allows the device to average 2x or 4x longer than the normal 11 bit operation (nominally

21ms per channel) while still maintaining the selected conversion rate. The benefits of dynamic

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