MIC281_14 Datasheet, PDF (11/14 Pages) Micrel Semiconductor – Low-Cost IttyBitty™ Thermal Sensor

Micrel, Inc.
MIC281
Application Information
Remote Diode Section
Most small-signal PNP transistors with characteristics
similar to the JEDEC 2N3906 will perform well as remote
temperature sensors. Table 4 lists several examples of
such parts that Micrel has tested for use with the MIC281.
Other transistors equivalent to these should also work well.
Table 4. Transistors suitable for use as remote diodes
Vendor
Part
Number
Package
Fairchild Semiconductor MMBT3906
SOT-23
On Semiconductor
MMBT3906L
SOT-23
Philips Semiconductor
SMBT3906
SOT-23
Samsung Semiconductor KST3906-TF
SOT-23
Minimizing Errors
Self-Heating
One concern when using a part with the temperature
accuracy and resolution of the MIC281 is to avoid errors
induced by self-heating (V_DD × I_DD) + (V_OL × I_OL). In order to
understand what level of error this might represent, and
how to reduce that error, the dissipation in the MIC281
must be calculated and its effects reduced to a
temperature offset. The worst-case operating condition for
the MIC281 is when V_DD = 3.6V.The maximum power
dissipated in the part is given in the following equation:
P_D = [(I_DD × V_DD)+(I_OL(DATA) × V_OL(DATA))]
P_D = [(0.4mA× 3.6V)+(6mA× 0.5V)]
P_D = 4.44mW
R_θ(J-A) of SOT23-6 package is 230°C/W, therefore the
theoretical maximum self-heating is:
4.44mW × 230°C/W = 1.02°C
In most applications, the DATA pin will have a duty cycle
of substantially below 25% in the low state. These
considerations, combined with more typical device and
application parameters, give a better system-level view of
device self-heating. This is illustrated by the next equation.
In any application, the best approach is to verify
performance against calculation in the final application
environment. This is especially true when dealing with
systems for which some temperature data may be poorly
defined or unobtainable except by empirical means.
P_D = [(I_DD × V_DD)+(I_OL(DATA) × V_OL(DATA))]
P_D = [(0.23mA× 3.3V)+(25% × 1.5mA× 0.15V)] PD =
0.815mW
R_θ(J-A) of SOT23-6 package is 230°C/W, therefore the
typical self-heating is:
0.815mW × 230°C/W = 0.188°C
Series Resistance
The operation of the MIC281 depends upon sensing the
V_CB-E of a diode-connected PNP transistor (diode) at two
different current levels. For remote temperature
measurements, this is done using an external diode
connected between T1 and ground. Because this
technique relies upon measuring the relatively small
voltage difference resulting from two levels of current
through the external diode, any resistance in series with
the external diode will cause an error in the temperature
reading from the MIC281. A good rule of thumb is that for
each ohm in series with the external transistor, there will
be a 0.9°C error in the MIC281’s temperature
measurement. It is not difficult to keep the series
resistance well below an ohm (typically <0.1Ω), so this will
rarely be an issue.
Filter Capacitor Selection
It is usually desirable to employ a filter capacitor between
the T1 and GND pins of the MIC281. The use of this
capacitor is recommended in environments with a lot of
high frequency noise (such as digital switching noise), or if
long traces or wires are used to connect to the remote
diode. The recommended total capacitance from the T1
pin to GND is 2200pF. If the remote diode is to be at a
distance of more than six-to-twelve inches from the
MIC281, using twisted pair wiring or shielded microphone
cable for the connections to the diode can significantly
reduce noise pickup. If using a long run of shielded cable,
remember to subtract the cable’s conductor-to-shield
capacitance from the 2200pF total capacitance.
April 23, 2014
11
Revision 2.0