Datasheet
DATASHEET
SMT172
last update
juni 1, 2015
reference
V10.0
page
7/9
To obtain the inaccuracy given above, a couple of aspects must be taken into account, like the way of
measuring, energy efficiency and package induced errors. Please find below an overview.
Measuring the duty cycle
In general, measuring the duty cycle is relatively simple. The signal is sampled over a certain number of
periods, and after that the duty cycle is calculated as the total high time divided by the total time interval.
The duty can then be used in the formula to calculate the temperature. This method has advantage of less
calculation load for a microcontroller (MCU) and it has been used in the evaluation boards of the SMT160.
In the SMT172, it is a little bit different.
For the accurate measurement with the SMT172 the following aspects has to be taken in account:
• Always start the measurement on the negative edge
• Measure over 8 periods (or a multiple of eight)
The measurement can start at any negative edge.
After each period (fi. DC1, see above) the duty cycle has to be calculated using the equation given
above. After eight periods there are DC
1
,…. DC
8
Therefore the valid duty cycle is: :
This duty cycle has to be used in the conversion formula between DC and temperature.
In case the sampling frequency is not high enough, extra measurement noise is introduced. In that
case, a measurement can be performed over a multiple of eight periods, and the valid duty cycle is
the average of all the duty cycles:
(N should be a multiple of 8)
• Formula between Duty Cycle and temperature
The temperature can be calculated based on the second order formula below:
This second order equation can better interpret the averaged duty cycle to temperature, and thus a more
accurate result can be achieved. The equation corrects for the typical error curve versus the temperature
as in the graph on the previous page.
Application Information
Temperature measurement
The SMT172 measures the temperature of its bipolar transistors with high precision. Due to the great
thermal conducting property of single crystalline silicon, we can assume the temperature difference within
the sensor die to be negligible. However the thermal property of the package material, the shape and the
size of soldering pads, the neighbouring components on the PCB as well as the presence of dedicated
thermal sinks are all affecting the die temperature that the sensor is measuring. Therefore a good thermal
path between the die and the objects under measurement should be carefully designed and considered.
t
Hi
: time interval of high
t
Li
: time interval of low
DC
1
DC
2
DC
3
DC
4
DC
5
DC
6
DC
7
DC
8
Started from
any period
t
H
1
t
L1