November 2004

11

MIC281

MIC281

Micrel

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.

REMOTE DIODE (T1)

GUARD/RETURN

1

2

VDD

GND

T1

6

5

4

3

NC

DATA

CLK

GUARD/RETURN

MIC281

Figure 3. Guard Traces/Kelvin Ground Returns

4. Due to the small currents involved in the mea-

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.