RPM-Based Fan Controller with HW Thermal Shutdown
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.
to
to
to
DP
DP
DP
to
DN
to
to
DN
DN
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
Figure 5.7 Diode Connections
SMSC EMC2103
Revision 0.85 (01-29-08)
DATA3S3HEET
SMSC [ SMSC CORPORATION ]
