EMC1428-1-AP-TR SMSC, EMC1428-1-AP-TR Datasheet - Page 20

• Manufacturer Part Number: EMC1428-1-AP-TR
• Description: Board Mount Temperature Sensors Octal Temp Sensor
• Manufacturer: SMSC

Specifications of EMC1428-1-AP-TR

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Revision 0.69 (06-29-09)
4.5.2
4.5.3
4.5.4
4.5.5
4.5.6
EMC1428 eliminates the need to characterize and compensate for parasitic resistance in the remote
diode path.
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 EMC1428 corrects for this beta variation to eliminate any error
which would normally be induced. It automatically detects the appropriate beta setting to use.
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
“Hottest Of” Comparison
At the end of every measurement cycle, the EMC1428 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 EMC1428 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 EMC1428 can flag this event as an interrupt condition and assert the
ALERT pin.
Conversion Rates
The EMC1428 may be configured for different conversion rates based on the system requirements.
The conversion rate is configured as described in
conversions per second. Other available conversion rates are shown in
Dynamic Averaging
Dynamic averaging causes the EMC1428 to measure the external diode channels for an extended time
based on the selected conversion rate. This functionality can be disabled for increased power savings
at the lower conversion rates (see
automatically adjust the sampling and measurement time for the external diode channels. This allows
the device to average 2x or 4x longer than the normal 11 bit operation (nominally 21ms per channel)
while still maintaining the selected conversion rate. The benefits of dynamic averaging are improved
1°C Multiple Temperature Sensor with HW Thermal Shutdown & Hottest of Thermal Zones
DATASHEET
Section
20
5.5). When dynamic averaging is enabled, the device will
Section
Section
5.5. The default conversion rate is 4
5.24).
Table
5.7.
SMSC EMC1428
Datasheet