MIC284_05 Datasheet, PDF (18/20 Pages) Micrel Semiconductor – Two-Zone Thermal Supervisor

MIC284
Applications
Remote Diode Selection
Most small-signal PNP transistors with characteristics similar
to the JEDEC 2N3906 will perform well as remote temperature
sensors. Table 6 lists several examples of such parts that
Micrel has tested for use with the MIC284. Other transistors
equivalent to these should also work well.
Minimizing Errors
Self-Heating
One concern when using a part with the temperature accuracy
and resolution of the MIC284 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 MIC284 must be calculated
and its effects reduced to a temperature offset.
The worst-case operating condition for the MIC284 is when
V_DD = 5.5V, MSOP-08 package. T he maximum power dis-
sipated in the part is given in Equation 1 below.
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 MIC284 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. This is illustrated by Equation 2.
If the part is to be used in comparator mode, calculations
similar to those shown in Equation 2 (accounting for the
expected value and duty cycle of I_OL(/INT) and I_OL(/CRIT))
will give a good estimate of the device’s self-heating error.
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 some
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of the thermal data (e.g., PC board thermal conductivity and
ambient temperature) may be poorly defined or unobtainable
except by empirical means.
Series Resistance
The operation of the MIC284 depends upon sensing the
ΔV_CB-E of a diode-connected PNP transistor (“diode”) at
two different current levels. For remote temperature mea-
surements, 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 MIC284. 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 MIC284’s temperature measurement. It isn’t 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 sometimes desirable to use a filter capacitor between the
T1 and GND pins of the MIC284. 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 attach to the remote diode. The maximum recommended
total capacitance from the T1 pin to GND is 2700pF. This
typically suggests the use of a 2200pF 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 MIC284, using twisted pair wiring or shielded
microphone cable for the connections to the diode can signifi-
cantly help reduce noise pickup. If using a long run of shielded
cable, remember to subtract the cable’s conductor-to-shield
capacitance from the 2700pF maximum total capacitance.
Layout Considerations
The following guidelines should be kept in mind when design-
ing and laying out circuits using the MIC284:
P_D = [(I_DD x V_DD) + (I_OL(DATA)) x V_OL(DATA) + (I_OL(/INT) x V_OL(/INT)) + (I_OL(/CRIT) x V_OL(/CRIT))]
P_D = [(0.75mA x 5.5V) + (6mA x 0.8V) + (6mA x 0.8V) + (6mA x 0.8V)]
P_D = 18.53mW
R_θ(j-a) of MSOP - 08 package is 206°C/W
Maximum ∆T_J relative to T_A due to self heating is 18.53mW x 206°C/W = 3.82°C
Equation 1. Worst-case self-heating
[(0.35mA I_DD(typ) x 3.3V) + (25% x 1.5mA I_OL(DATA)) x 0.3V) + (1% x 1.5mA I_OL(/INT) x 0.3V) + (25% x 1.5mA I_OL(/CRIT) x 0.3V) = 1.38mW
∆T_J = (1.38mW x 206°C/W) = 0.29°C
Equation 2. Real-world self-heating example
Vendor
Part Number
Package
Fairchild
MMBT3906
SOT-23
On Semiconductor
MMBT3906L
SOT-23
Phillips Semiconductor
PMBT3906
SOT-23
Samsung
KST3906-TF
SOT-23
Table 6. Transistors Suitable for Remote Temperature Sensing Use
MIC284
18
September 2005