Dual RPM-Based PWM Fan Controller with Hardware 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 EMC2104 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 1 channel 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 6.10).
5.12
Thermistor Support
The External Diode 1, External Diode 2, and External Diode 3 channels can be configured to monitor
a thermistor. When this function is enabled, the data on the VIN1, VIN2, or VIN3 channels can be
configured to measure a simple voltage input or a ground-connected thermistor circuit (see Appendix
A for more information).
The External Diode 1 channel can only be configured as a voltage input if the SHDN_SEL pin is set
to a logic ‘1’.
5.13
Diode Connections
The diode connection for the External Diode 1 channel is determined at power-up based on the
SHDN_SEL pin (see Section 5.1.1). This channel can support a diode-connected transistor (such as
a 2N3904) or a substrate transistor (such as those found in an CPU or GPU) as shown in Figure 5.7.
The External Diode 3 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 fourth temperature channel is
available that shares the DP3 and DN3 pins. When in this mode, both the external diode 3 channel
and external diode 4 channel thermal diodes must be connected as a diode.
Revision 1.74 (05-08-08)
SMSC EMC2104
DATA3S6HEET
SMSC [ SMSC CORPORATION ]
