MIC384 datasheet(19/21 Pages) MICREL

19 / 21 page

September 2000

MIC384

Micrel

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

sensor to the MIC384 using the guard traces

and Kelvin return shown in Figure 6. The

collector of the remote diode is typically inacces-

sible 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.

REMOTE DIODE (T1)

GUARD/RETURN

DATA

CLK

/INT

GND

VDD

GUARD/RETURN

REMOTE DIODE (T2)

GUARD/RETURN

MIC384

Figure 6. Guard Traces/Kelvin Ground Returns

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/

T2 lines will help reduce susceptibility to radi-

ated noise (wider traces are less inductive). Use

trace widths and spacing of 10 mils wherever

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 0.1

µF 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

particularly noisy power supplies, or from

supplies which may have sudden high-amplitude

spikes appearing on them, it can be helpful to

add additional power supply filtering. This

should be implemented as a 100

Ω resistor in

series with the part’s VDD pin, and an additional

4.7

µF, 6.3V electrolytic capacitor from VDD to

GND. See Figure 7.

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