EMC1424 SMSC Corporation, EMC1424 Datasheet - Page 21

• Manufacturer Part Number: EMC1424
• Description: (EMC1423 / EMC1424) Temperature Sensor
• Manufacturer: SMSC Corporation

1°C Temperature Sensor with Hardware Thermal Shutdown
Datasheet
SMSC EMC1423/EMC1424
5.5
5.6
5.7
SYS_SHDN
VDD
DN1
DP1
Beta Compensation
Resistance Error Correction (REC)
Programmable External Diode Ideality Factor
The EMC1423 and EMC1424 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.
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.
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 EMC1423 and EMC1424 automatically correct up
to 100 ohms of series resistance.
The EMC1423 and EMC1424 do not support a programmable Ideality Factor for the External Diode 1
channel. It is hard-wired for an ideality factor of 1.008.
The EMC1423 External Diode 2 channel defaults to measuring external diodes with an ideality factor
of 1.008. Likewise, the EMC1424 External Diode 2 and 3 channels default to measuring 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
1
2
3
4
Figure 5.4 Isolating ALERT and SYS_SHDN Pins
10
9
8
7
SMCLK
SMDATA
GND
ALERT
DATASHEET
21
+3.3V
4.7K -
33K
+2.5 - 5V
Revision 1.16 (03-15-07)
Shared Alert/
22K