MIC281
Layout Considerations
The following guidelines should be kept in mind when design-
ing and laying out circuits using the MIC281:
1. Place the MIC281 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,
etc.
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 MIC281’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 MIC281,
thereby providing a Kelvin connection for the
base of the diode. See Figure 3.
3. When using the MIC281 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 MIC281 using the guard traces and
Kelvin return shown in Figure 3. The collector of
the remote diode is typically inaccessible to the
user on these devices.
Micrel
4. Due to the small currents involved in the mea-
surement of the remote diode’s
∆V
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
lines will help reduce susceptibility to radiated
noise (wider traces are less inductive). Use trace
widths and spacing of 10mm wherever possible
and provide a ground plane under the MIC281
and under the connections from the MIC281 to
the remote diode. This will help guard against
stray noise pickup.
6. Always place a good quality power supply
bypass capacitor directly adjacent to, or un-
derneath, the MIC281. This should be a 0.1µF
ceramic capacitor. Surface mount parts provide
the best bypassing because of their low
inductance.
MIC281
1
VDD
2
GND
NC 6
DATA 5
CLK 4
GUARD/RETURN
REMOTE DIODE (T1)
3 T1
GUARD/RETURN
Figure 3. Guard Traces/Kelvin Ground Returns
November 2004
11
MIC281
MICREL [ MICREL SEMICONDUCTOR ]
