1°C Temperature Sensor with Hardware Thermal Shutdown
Datasheet
5.5
Beta Compensation
The EMC1422 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.
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 EMC1422 automatically corrects up to 100 ohms
of series resistance.
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 EMC1422 detects 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.
5.8
Consecutive Alerts
The EMC1422 contains multiple consecutive alert counters. One set of counters applies to the ALERT
pin and the second set of counters applies to the SYS_SHDN pin. Each temperature measurement
channel has a separate consecutive alert counter for each of the ALERT and SYS_SHDN pins. All
counters are user programmable and determine the number of consecutive measurements that a
temperature channel(s) must be out-of-limit or reporting a diode fault before the corresponding pin is
asserted.
See Section 6.13 for more details on the consecutive alert function.
SMSC EMC1422
Revision 1.24 (02-05-08)
DATA1S9HEET
SMSC [ SMSC CORPORATION ]
