MIC384_05 Datasheet, PDF (19/22 Pages) Micrel Semiconductor – Three-Zone Thermal Supervisor

MIC384
Layout Considerations
The following guidelines should be kept in mind when design-
ing and laying out circuits using the MIC384:
1. Place the MIC384 as close to the remote diodes
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 to the T1 or T2 line can
induce serious errors, it is good practice to guard
the remote diodes’ emitter traces with pairs of
ground traces. These ground traces should be
returned to the MIC384’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 diodes’ own ground
return back to the ground pin of the MIC384,
thereby providing a Kelvin connection for the
base of the diodes. See Figure 6.
3. When using the MIC384 to sense the tempera-
ture of a processor or other device which has an
integral thermal diode, e.g., Intel’s Pentium III,
connect the emitter and base of the remote sen-
sor to the MIC384 using the guard traces and
Kelvin return shown in Figure 6. The collector
of the remote diode is typically inaccessible to
the user on these devices. To allow for this, the
MIC384 has superb rejection of noise appearing
from collector to GND, as long as the base to
ground connection is relatively quiet.
Micrel
4. Due to the small currents involved in the mea-
surement of the remote diode’s ΔV_BE, it is
important to adequately clean the PC board after
soldering to prevent current leakage. This is
most likely to show up as an issue in situations
where water-soluble soldering fluxes are used.
5. In general, wider traces for the ground and
T1/T2 lines will help reduce susceptibility to
radiated noise (wider traces are less inductive).
Use trace widths and spacing of 10 mils wher-
ever possible and provide a ground plane under
the MIC384 and under the connections from
the MIC384 to the remote diodes. This will help
guard against stray noise pickup.
6. Always place a good quality 0.1µF power supply
bypass capacitor directly adjacent to, or under-
neath, the MIC384. Surface-mount capacitors
are preferable because of their low inductance.
7. When the MIC384 is being powered from par-
ticularly noisy power supplies, or from supplies
which may have sudden high-amplitude spikes
appearing on them, it can be helpful to add ad-
ditional power supply filtering. This should be
implemented as a 100Ω resistor in series with
the part’s VDD pin, and an additional 4.7µF,
6.3V electrolytic capacitor from VDD to GND.
See Figure 7.
MIC384
1 DATA
2 CLK
3 /INT
4 GND
VDD 8
A0 7
T1 6
T2 5
GUARD/RETURN
REMOTE DIODE (T1)
GUARD/RETURN
GUARD/RETURN
REMOTE DIODE (T2)
GUARD/RETURN
Figure 6. Guard Traces/Kelvin Ground Returns
September 2005
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MIC384