RPM-Based Fan Controller with HW Thermal Shutdown
Datasheet
5.1
Temperature Monitoring
External diode channels one and two can be configured to monitor either discrete thermal diodes or a
CPU / GPU thermal diode. External diode channel three is always configured to monitor a discrete
diode-connected transistor (such as a 2N3904) or an AMD thermal diode. Each channel can enable
the Resistance Error Correction functionality and external diode channels one and two can adjust the
Beta Compensation settings (disabling it if desired). The disabling of these features is only
recommended in two situations:
1. An AMD thermal diode is being monitored. The AMD thermal diode is physically a 2-terminal diode
and will not function with either Beta Compensation or Resistance Error Correction. Because of
this, when an EMC2102 temperature channel is interfacing an AMD thermal diode, both Beta
Compensation and Resistance Error Correction must be disabled.
2. A discrete diode connected transistor (such as 2N3904) is used. In this configuration, Beta
Compensation must be disabled, but Resistance Error Correction should remain enabled.
5.1.1
Resistance Error Correction
The EMC2102 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
EMC2102 eliminates the need to characterize and compensate for parasitic resistance in the remote
diode path.
5.1.2
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.
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 EMC2102 corrects for this beta variation to eliminate any error
which would normally be induced.
5.1.3
Fault Queue
To avoid spurious interrupts and Critical/Thermal Trip events induced by thermal spikes and noise
injection, the selected Thermal / Critical Shutdown Temperature channel (see Section 5.7.2) is filtered
through a fault queue. This fault queue requires that a user-defined number of consecutive out-of-limit
errors be recorded before it will cause an interrupt or trigger the Critical/Thermal trip event.
The fault queue only applies to the measurement channels that will cause the SYS_SHDN# pin to be
asserted including any software configured channels (see Section 5.7). In addition, the fault queue
applies to all enabled channels simultaneously and will trigger the SYS_SHDN# pin if there are the
desired number of consecutive measurements with any or all channels exceeding their corresponding
limits.
Revision 1.95 (10-19-06)
SMSC EMC2102
DATA2S0HEET
SMSC [ SMSC CORPORATION ]
