MIC280_06 Datasheet, PDF (21/23 Pages) Micrel Semiconductor – Precision IttyBitty Thermal Supervisor

MIC280
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 8 lists several examples of such parts that
Micrel has tested for use with the MIC280. Other transistors
equivalent to these should also work well.
Vendor
Part Number
Package
Fairchild Semiconductor MMBT3906
SOT-23
On Semiconductor
MMBT3906L
SOT-23
Philips Semiconductor
PMBT3906
SOT-23
Samsung Semiconductor KST3906-TF
SOT-23
Table 8: Transistors suitable for use as remote diodes
Minimizing Errors
Self-Heating
One concern when using a part with the temperature accuracy
and resolution of the MIC280 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 MIC280 must be calculated
and its effects reduced to a temperature offset. The worst-
case operating condition for the MIC280 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))+(I_OL(/INT)×V_OL(/INT)]
P_D = [(0.4mA× 3.6V)+(6mA× 0.5V)+(6mA× 0.5V)]
P_D = 7.44mW
R_θ(J-A) of SOT23-6 package is 230°C/W
Theoretical Maximum ∆T_J due to self-heating is:
7.44mW × 230°C/W = 1.7112°C
Worst-case self-heating
In most applications, the /INT output will be low for at most
a few milliseconds before the host resets it back to the high
state, making its duty cycle low enough that its contribution to
self-heating of the MIC280 is negligible. Similarly, the DATA
pin will in all likelihood 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 in interrupt-mode
usage given in the following equation:
(0.23mA I_DD(typ) × 3.3V) + (25% × 1.5mA I_OL(DATA) × 0.15V)
+ (1% × 1.5mA I_OL(/INT) × 0.15V) = 0.817mW
∆T_J = (0.8175mW × 230°C/W) = 0.188°C
Real-world self-heating example
In any application, the best test is to verify performance
against calculation in the final application environment. This
is especially true when dealing with systems for which tem-
perature data may be poorly defined or unobtainable except
by empirical means.
Micrel
Series Resistance
The operation of the MIC280 depends upon sensing the V_CB-E
of a diode-connected PNP transistor (“diode “) at two differ-
ent 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 MIC280. A good rule of thumb is this: for
each ohm in series with the external transistor, there will be
a 0.8°C error in the MIC280’s temperature measurement. It is
not difficult 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 filter capacitor between the
T1 and GND pins of the MIC280. The use of this capacitor is
recommended in environments with a lot of high frequency
noise (such as digital switching noise), or if long wires are
used to conect to the remote diode. The maximum recom-
mended total capacitance from the T1 pin to GND is 2200pF.
This typically suggests the use of a 1800pF NP0 or C0G
ceramic capacitor with a 10% tolerance. If the remote diode
is to be at a distance of more than 6"-12" from the MIC280,
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 maximum total capacitance.
Layout Considerations
The following guidelines should be kept in mind when design-
ing and laying out circuits using the MIC280:
1. Place the MIC280 as close to the remote diode
as possible, while taking care to avoid severe
noise sources such as high frequency power
transformers, CRTs, memory and data busses,
and the like.
2. Since any conductance from the various volt-
ages on the PC Board and the T1 line can in-
duce serious errors, it is good practice to guard
the remote diode's emitter trace with a pair of
ground traces. These ground traces should be
returned to the MIC280's own ground pin. They
should not be grounded at any other part of their
run. However, it is highly desirable to use these
guard traces to carry the diode's own ground
return back to the ground pin of the MIC280,
thereby providing a Kelvin connection for the
base of the diode. See Figure 6.
3. When using the MIC280 to sense the tempera-
ture of a processor or other device which has an
integral thermal diode, e.g., Intel's Pentium II, III,
IV, AMD Athlon CPU, Xilinx Virtex FPGAs, con-
nect the emitter and base of the remote sensor
to the MIC280 using the guard traces and Kelvin
return shown in Figure 6. The collector of the
remote diode is typically inaccessible to the user
May 2006
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MIC280