Multiple Channel 1°C Temperature Sensors with Beta Compensation
Datasheet
The ALERT pin is used as an interrupt signal or as an SMBus Alert signal that allows an SMBus slave
to communicate an error condition to the master. One or more ALERT outputs can be hard-wired
together.
6.3.2
ALERT Pin Comparator Mode
When the ALERT pin is configured to operate in comparator mode, it will be asserted if any of the
measured temperatures exceeds the respective high limit. The ALERT pin will remain asserted until
all temperatures drop below the corresponding high limit minus the Therm Hysteresis value.
When the ALERT pin is asserted in comparator mode, the corresponding high limit status bits will be
set. Reading these bits will not clear them until the ALERT pin is deasserted. Once the ALERT pin is
deasserted, the status bits will be automatically cleared.
The MASK_ALL bit will not block the ALERT pin in this mode; however, the individual channel masks
(see Section 7.11) will prevent the respective channel from asserting the ALERT pin.
6.4
Temperature Measurement
The EMC1413 / EMC1414 can monitor the temperature of up to two / three externally connected
diodes. Each external diode channel is configured with Resistance Error Correction and Beta
Compensation based on user settings and system requirements.
The device contains programmable High, Low, and Therm limits for all measured temperature
channels. If the measured temperature goes below the Low limit or above the High limit, the ALERT
pin can be asserted (based on user settings). If the measured temperature meets or exceeds the
Therm Limit, the THERM pin is asserted unconditionally, providing two tiers of temperature detection.
6.4.1
Beta Compensation
The EMC1413 / EMC1414 is configured to monitor the temperature of basic diodes (e.g., 2N3904) or
CPU thermal diodes. It automatically detects the type of external diode (CPU diode or diode connected
transistor) and determines the optimal setting to reduce temperature errors introduced by beta
variation. Compensating for this error is also known as implementing the transistor or BJT 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.
For the EMC1414, the External Diode 2 and External Diode 3 channels do not support Beta
Compensation.
6.4.2
6.4.3
Resistance Error Correction (REC)
Parasitic resistance in series with the external diodes will limit 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. The EMC1413 / EMC1414 automatically corrects up
to 100 ohms of series resistance.
Programmable External Diode Ideality Factor
The EMC1413 / EMC1414 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
SMSC EMC1413 / EMC1414
Revision 1.41 (02-23-12)
DATA2S1HEET
SMSC [ SMSC CORPORATION ]
