SMBus Fan Control with 1°C Accurate Temperature Monitoring
Datasheet
5.5.3
Beta Compensation
The EMC2101 is software configurable to monitor the temperature of basic diodes (e.g. 2N3904), or
CPU thermal diodes. It automatically detects the type of external diode (CPU diode, diode connected
transistor, or PN diode) and determines the optimal setting to reduce temperature errors introduced by
beta variation.
5.5.4
Resistance Error Correction (REC)
Parasitic resistance in series with the external diode limits the accuracy obtainable from temperature
measurement devices. The voltage developed across this resistance by the switching diode currents
cause the temperature measurement to read higher than the true temperature. Contributors to series
resistance are PCB trace resistance, on die (i.e. on the processor) metal resistance, bulk resistance
in the base and emitter of the temperature transistor. Typically, the error caused by series resistance
is +0.7°C per ohm. Temperature errors caused by up to 100Ω of series resistance are automatically
corrected.
5.5.5
Programmable Ideality Factor
The EMC2101 is designed for an external diode with an ideality factor of 1.008. When an external
diode, processor or discrete, has a different ideality factor, an error is introduced in the temperature
measurement which must be corrected. This is typically done using programmable offset registers but
this correction is only accurate at one temperature since an ideality factor mismatch introduces an error
that is a linear function of temperature. To provide maximum flexibility to the user, the EMC2101
provides a 6-bit register to set the ideality factor for the external diode which eliminates errors across
all temperatures. (See Table 6.13.)
APPLICATION NOTE: This feature is only required in rare circumstances. The majority of errors introduced are
correced with the Beta Compensation and Resistance Error Correction circuitry.
5.5.6
Diode Faults
The EMC2101 detects the major types of diode faults; an open input DP-DN, a short across DP-DN,
short to GND, and short to VDD. For each temperature measurement made, the device checks for a
diode fault on the external diode.
If an open fault or a short of the DP pin to VDD is detected, then the temperature data is changed to
+127C and the Fault bit in the Status Register will bet set. If the high and / or TCRIT limits are set
below this value, and they are not masked, then the ALERT / TACH pin will be asserted. In addition,
the HIGH and TCRIT status bits will be set accordingly.
If a short between the diode pins or a short to GND is detected, then the temperature data is changed
to +127.875°C. If the high and / or TCRIT limits are set below this value, and they are not masked,
then the ALERT / TACH pin will be asserted. In addition, the HIGH and TCRIT status bits will be set
accordingly. The FAULT bit will not be set.
APPLICATION NOTE: If the Temperature Filter is enabled and a diode fault occurs, the diode fault status bit will be
set and the temperature data is updated immediately. The Filter will stop accumulating data
so long as the diode fault remains in effect.
APPLICATION NOTE: When a Diode Fault is detected, the ALERT / TACH pin behavior is still subject to the Fault
Queue.
SMSC EMC2101
Revision 2.53 (03-13-07)
DATA2S5HEET
SMSC [ SMSC CORPORATION ]
