MIC384 Datasheet, PDF (18/21 Pages) Micrel Semiconductor – Three-Zone Thermal Supervisor Advance Information

MIC384
Micrel
Applications
Remote Diode Selection
Most small-signal PNP transistors with characteristics similar
to the JEDEC 2N3906 will perform well as remote tempera-
ture sensors. Table 6 lists several examples of such parts
that Micrel has tested for use with the MIC384. Other
transistors equivalent to these should also work well.
Minimizing Errors
Self-Heating
One concern when using a part with the temperature accu-
racy and resolution of the MIC384 is to avoid errors in
measuring the local temperature induced by self-heating.
Self-heating is caused by the power naturally dissipated
inside the device due to operating supply current and I/O sink
currents (V_DD × I_DD ) + (V_OL × I_OL). In order to understand
what level of error this represents, and how to reduce that
error, the dissipation in the MIC384 must be calculated and
its effects reduced to a temperature offset.
The worst-case operating condition for the MIC384 is when
V_DD = 5.5V, MSOP-08 package. The maximum power
dissipated 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 MIC384 is negligible. Similarly, the DATA
pin will in all likelihood have a duty cycle of substantially less
than 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. This is illustrated in Equation 2.
If the part is to be used in comparator mode, calculations
similar to those shown above (accounting for the expected
value and duty cycle of I_OL(INT)) will give a good estimate of
the temperature error due to self-heating.
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
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 MIC384 depends upon sensing the
∆V_CB-E of a diode-connected PNP transistor (“diode”) at two
different current levels. For remote temperature measure-
ments, this is done using external diodes connected between
T1, T2 and ground.
Since this technique relies upon measuring the relatively
small voltage difference resulting from two levels of current
through the external diodes, any resistance in series with
those diodes will cause an error in the temperature reading
from the MIC384. A good rule of thumb is this: for each ohm
in series with a zone's external transistor, there will be a 0.9°C
error in the MIC384’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/or T2 pins and the GND pin of the MIC384. The use
of these capacitors 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 diodes. The
maximum recommended total capacitance from the T1 or T2
pin to GND is 2700pF. This typically suggests the use of
2200pF NP0 or C0G ceramic capacitors with a 10% toler-
ance.
If a remote diode is to be at a distance of more than ≈ 6"—12"
from the MIC384, using twisted pair wiring or shielded micro-
phone cable for the connections to the diode can significantly
help reduce noise pickup. If using a long run of shielded cable,
remember to subtract the cable’s conductor-to-shield capaci-
tance from the 2700pF maximum total capacitance.
P_D= [(I_DD × V_DD ) + (I_OL(DATA)) + (I_OL(/INT) × V_OL(/INT))]
P_D= [(0.75mA × 5.5V) + (6mA × 0.8V) + (6mA × 0.8V)]
P_D= 13.73mW
R_q(j−a) of MSOP - 08 package is 206°C / W
Maximum ∆T_J relative to T_A due to self - heating is 13.73mW × 206°C / W = 2.83°C
Equation 1. Worst-Case Self-Heating
[(0.350mA I_DD(typ) × 3.3V) + (25% × 1.5mA I_OL(DATA) × 0.3V) + (1% × 1.5mA I_OL(/INT) × 0.3V)] = 1.27mW
∆T_J= (1.27mW × 206°C / W)
∆T_J = 0.262°C
Equation 2. Real-World Self-heating Example
MIC384
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
18
September 2000