MIC281_14 Datasheet, PDF (12/14 Pages) Micrel Semiconductor – Low-Cost IttyBitty™ Thermal Sensor

Micrel, Inc.
MIC281
Layout Considerations
The following guidelines should be kept in mind when
designing 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, and the like.
2. Because any conductance from the various voltages
on the PC board and the T1 line can induce 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 temperature 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 sensor 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.
4. Due to the small currents involved in the measurement
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 underneath, the
MIC281. This should be a 0.1µF ceramic capacitor.
Surface-mount parts provide the best bypassing
because of their low inductance.
Figure 3. Guard Traces/Kelvin Ground Returns
April 23, 2014
12
Revision 2.0