1°C Multiple Temperature Sensor with Beta Compensation and Hottest of Thermal Zones
Datasheet
4.2
Temperature Monitoring
The EMC1046/EMC1047 can monitor the temperature of up to six (6) externally connected diodes as
well as the internal or ambient temperature. Each channel is configured with the following features
enabled or disabled based on user settings and system requirements.
APPLICATION NOTE: When measuring a 45nm CPU diode, the reported temperature has an error of approximately
+1.5C at 100°C. This error is related to non-perfect ideality in the CPU diode.
4.2.1
Resistance Error Correction
The EMC1046/EMC1047 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 series
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 EMC1046/EMC1047 eliminates the need to characterize and compensate for parasitic
resistance in the remote diode path.
4.2.2
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. Compensating for this error is also known as
implementing the BJT or transistor 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 Beta Compensation circuitry in the EMC1046/EMC1047 corrects for this beta variation to eliminate
any error which would normally be induced. It automatically detects the appropriate beta setting to use.
4.2.3
4.2.4
Digital Averaging
To reduce the effect of noise and temperature spikes on the reported temperature, all of the external
diode channels use digital averaging. This averaging acts as a running average using the previous four
measured values.
The default setting is to have digital averaging disabled for all channels. It can be enabled for each
channel individually by the Filter Control Register (see Section 5.18).
“Hottest Of” Comparison
At the end of every measurement cycle, the EMC1046/EMC1047 compares all of the user selectable
External Diode channels to determine which of these channels is reporting the hottest temperature.
The hottest temperature is stored in the Hottest Temperature Registers and the appropriate status bit
in the Hottest Status Register is set. As an optional feature, the EMC1046/EMC1047 can also flag an
event if the hottest temperature channel changes. For example, suppose that External Diode channels
1, 3, and 4 are programmed to be compared in the “Hottest Of” Comparison. If the External Diode 1
channel reports the hottest temperature of the three, its temperature is copied into the Hottest
Temperature Registers (in addition to the External Diode 1 Temperature registers) and it is flagged in
the Hottest Status bit. If, on the next measurement, the External Diode 3 channel temperature has
increased such that it is now the hottest temperature, the EMC1046/EMC1047 can flag this event.
SMSC EMC1046/EMC1047
Revision 0.69 (06-29-09)
DATA1S5HEET
SMSC [ SMSC CORPORATION ]
