EMC2103 datasheet(33/84 Pages) SMSC

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RPM-Based Fan Controller with HW Thermal Shutdown

Datasheet

SMSC EMC2103

Revision 0.85 (01-29-08)

DATASHEET

5.11.3

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. This correction is done by 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 EMC2103 corrects for this beta variation to eliminate any error

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

5.11.4

Digital Averaging

The external diode channels support a 4x digital averaging filter. Every cycle, this filter updates the

temperature data based an a running average of the last 4 measured temperature values. The digital

averaging reduces temperature flickering and increases temperature measurement stability.

The digital averaging can be disabled by setting the DIS_AVG bit in the Configuration 2 Register (see

Section 6.11).

5.12

Diode Connections

The External Diode 1 channel can support a diode-connected transistor (such as a 2N3904) or a

substrate transistor requiring the BJT or transistor model (such as those found in a CPU or GPU) as

shown in Figure 5.7.

The External Diode 2 channel supports any diode connection shown or it can be configured to operate

in anti-parallel diode (APD) mode. When configured in APD mode, a third temperature channel is

available that shares the DP2 and DN2 pins. When in this mode, both the external diode 2 channel

and external diode 3 channel thermal diodes must be connected as diodes.

Figure 5.7 Diode Connections

Local Ground

Typical remote

substrate transistor

i.e. CPU substrate PNP

Typical remote

discrete PNP transistor

i.e. 2N3906

Typical remote

discrete NPN transistor

i.e. 2N3904

EMC2103 Datasheet(PDF) 33 Page - SMSC Corporation

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