icrel, Inc.
MIC281
pril 23, 2014
11
Revision 2.0
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
?J-A)
of SOT23-6 package is 230癈/W, therefore the
theoretical maximum self-heating is:
4.44mW ?230癈/W = 1.02癈
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 nal application
environment. This is especially true when dealing with
systems for which some temperature data may be poorly
dened 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
?J-A)
of SOT23-6 package is 230癈/W, therefore the
typical self-heating is:
0.815mW ?230癈/W = 0.188癈
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癈   error   in   the   MIC281s   temperature
measurement. It is not difcult to keep the series
resistance well below an ohm (typically <0.1&), so this will
rarely be an issue.
Filter Capacitor Selection
It is usually desirable to employ a lter 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 signicantly
reduce noise pickup. If using a long run of shielded cable,
remember to subtract the cables conductor-to-shield
capacitance from the 2200pF total capacitance.