MIC384
Micrel
Layout Considerations
4. Due to the small currents involved in the mea-
surement of the remote diode’s ∆V , it is
Thefollowingguidelinesshouldbekeptinmindwhendesign-
ing and laying out circuits using the MIC384:
BE
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.
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.
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.
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.
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
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 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.
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.
MIC384
DATA
CLK
/INT
VDD
A0
1
2
3
4
8
7
6
5
GUARD/RETURN
REMOTE DIODE (T1)
GUARD/RETURN
T1
GND
T2
GUARD/RETURN
REMOTE DIODE (T2)
GUARD/RETURN
Figure 6. Guard Traces/Kelvin Ground Returns
September 2000
19
MIC384
MICREL [ MICREL SEMICONDUCTOR ]
