MIC281
Micrel
Series Resistance
Application Information
Remote Diode Selection
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.
Vendor
Fairchild Semiconductor
On Semiconductor
Infineon Technologies
Samsung Semiconductor
Part Number
MMBT3906
MMBT3906L
SMBT3906
KST3906-TF
Package
SOT-23
SOT-23
SOT-23
SOT-23
Table 4. Transistors Suitable for Use as Remote
Diodes
Minimizing Errors
Self-Heating
The operation of the MIC281 depends upon sensing the
V
CB-E
of a diode-connected PNP transistor (“diode “) at two dif-
ferent current levels. For remote temperature measurements,
this is done using an external diode connected between T1
and ground. Since 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 this: 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
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
Equation 1 below.
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 Equation 2. In any application, the best ap-
proach 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
= [(0.4mA
×
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
Equation 1. Worst-Case Self-Heating
P
D
= [(I
DD
×
V
DD
)+(I
OL(DATA)
×
V
OL(DATA)
)]
P
D
= [(0.23mA
×
3.3V)+(25%
×
1.5mA
×
0.15V)]
P
D
= 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
Equation 2. Real-World Self-Heating Example
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 6”-12” 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.
MIC281
10
November 2004
MICREL [ MICREL SEMICONDUCTOR ]
