1°C Temperature Sensor with Beta Compensation
Datasheet
5.4
Beta Compensation
The EMC1403 and EMC1404 are 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 for the External Diode 1 channel only. 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.
The External Diode 2 and External Diode 3 channels do not support Beta Compensation.
5.5
5.6
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 EMC1403 and EMC1404 automatically correct up
to 100 ohms of series resistance.
Programmable External Diode Ideality Factor
The EMC1403 and EMC1404 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 EMC1403 and EMC1404 provides a 6-
bit register for each external diode where the ideality factor of the diode used is 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.
5.7
Diode Faults
The EMC1403 and EMC1404 detect an open on the DP and DN pins, and a short across the DP and
DN pins. For each temperature measurement made, the device checks for a diode fault on the external
diode channel(s). When a diode fault is detected, the ALERT pin asserts (unless masked, see
Section 5.8) and the temperature data reads 00h in the MSB and LSB registers (note: the low limit will
not be checked). A diode fault is defined as one of the following: an open between DP and DN, a short
from VDD to DP, or a short from VDD to DN.
If a short occurs across DP and DN or a short occurs from DP to GND, the low limit status bit is set
and the ALERT pin asserts (unless masked). This condition is indistinguishable from a temperature
measurement of 0.000degC (-64°C in extended range) resulting in temperature data of 00h in the MSB
and LSB registers.
If a short from DN to GND occurs (with a diode connected), temperature measurements will continue
as normal with no alerts.
SMSC EMC1403/EMC1404
Revision 2.0 (08-10-12)
DATA2S1HEET
SMSC [ SMSC CORPORATION ]
