MIC284 Datasheet, PDF (19/20 Pages) Micrel Semiconductor – Two-Zone Thermal Supervisor Advance Information

MIC284
Layout Considerations
The following guidelines should be kept in mind when design-
ing and laying out circuits using the MIC284:
1. Place the MIC284 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. Since any conductance from the various volt-
ages 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 MIC284’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
MIC284, thereby providing a Kelvin connection
for the base of the diode. See Figure 6.
3. When using the MIC284 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 MIC284 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 MIC284 has superb rejection of noise
appearing from collector to GND, as long as the
base to ground connection is relatively quiet.
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 10 mils wherever
possible and provide a ground plane under the
MIC284 and under the connections from the
MIC284 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 under-
neath, the MIC284. This should be a 0.1µF
ceramic capacitor. Surface-mount parts provide
the best bypassing because of their low induc-
tance.
7. When the MIC284 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 a 4.7µF,
6.3V electrolytic capacitor from VDD to GND.
See Figure 7.
MIC284
1 DATA
2 CLK
3 /INT
4 GND
VDD 8
A0 7
T1 6
/CRIT 5
GUARD/RETURN
REMOTE DIODE (T1)
GUARD/RETURN
Figure 6. Guard Traces/Kelvin Ground Returns
3.3V
10k pull-ups
FROM
SERIAL BUS
HOST
OVER-TEMP
SHUTDOWN
100
MIC284
DATA
VDD
CLK
T1
/INT
A0
/CRIT
GND
0.1µF 4.7µF
2200pF
Figure 7. V_DD Decoupling for Very Noisy Supplies
Remote
Diode
September 29, 2000
19
MIC284