RPM-Based Fan Controller with Multiple Temperature Zones & Hardware Thermal Shutdown
Datasheet
6.13.2
Resistance Error Correction
The EMC2113 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 parasitic 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
EMC2113 eliminates the need to characterize and compensate for parasitic resistance in the remote
diode path.
6.13.3
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 EMC2113 corrects for this beta variation to eliminate any error
which would normally be induced. It automatically detects the appropriate beta setting to use.
6.13.4
Ideality Configuration
The EMC2113 is designed for external diodes with an ideality factor of 1.008. Not all external diodes,
processor or discrete, will have this exact value. This variation of the ideality factor introduces error in
the temperature measurement which must be corrected for. This correction is typically done using
programmable offset registers. Since an ideality factor mismatch introduces an error that is a function
of temperature, this correction is only accurate within a small range of temperatures. To provide
maximum flexibility to the user, the EMC2113 provides a register for each external diode where the
ideality factor of the diode used may be programmed to eliminate errors across all temperatures.
APPLICATION NOTE: When monitoring a substrate transistor or CPU diode and beta compensation is enabled, the
Ideality Factor should not be adjusted. Beta Compensation automatically corrects for most
ideality errors.
6.13.5
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 7.25, "Fan Configuration 2 Register").
6.14
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 6.6, "Diode Connections".
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
SMSC EMC2113
Revision 1.2 (10-08-09)
DATA3S5HEET
SMSC [ SMSC CORPORATION ]
