MIC3000 Datasheet, PDF (66/68 Pages) Micrel Semiconductor – SFP Management IC

MIC3000
In any application, the best and often easiest approach is to
measure performance in the final application environment.
This is especially true when dealing with systems for which
some temperature data may be poorly defined or unobtain-
able except by empirical means. If desired, the external
calibration constants may be used to correct the temperature
readings.
Series Resistance with External Temperature Sensor
The operation of the MIC3000 depends upon sensing the
VCB-E of a diode-connected PNP transistor (“diode”) at two
different current levels. For remote temperature measure-
ments, this is done using an external diode connected be-
tween XPN and ground. Since this technique relies upon
measuring the relatively small voltage difference resulting
from two levels of current through the external diode, any
resistance in series with the external diode will cause an error
in the temperature reading from the MIC3000. A good rule of
thumb is this: for each ohm in series with the external
transistor, there will be a 0.9°C error in the MIC3000’s
temperature measurement. It is not difficult to keep the series
resistance well below an ohm (typically <0.1), so this will
rarely be an issue.
XPN Filter Capacitor Selection
It is desirable to employ a filter capacitor between XPN and
GNDA. The use of this capacitor is especially recommended
in environments with a lot of high frequency noise (such as
digital switching noise), or if long wires are used to connect to
the remote diode. The maximum recommended total capaci-
tance from the XPN pin-to-GND is 2000pF. The recom-
mended typical capacitor is a 1000pF NP0 or C0G ceramic
capacitor with a 10% tolerance. If the remote diode is to be at
a distance of more than 6" to 12" from the MIC3000, using
twisted pair wiring or shielded microphone cable for the
connections to the diode can significantly reduce noise
pickup. If using a long run of shielded cable, remember to
subtract the cable’s conductor-to-shield capacitance from
the 2000pF maximum total capacitance.
XPN Layout Considerations
The following guidelines should be kept in mind when design-
ing and laying out circuits using the MIC3000 and a remote
thermal diode:
1. Place the MIC3000 as close to the remote diode as
possible, while taking care to avoid severe noise
sources such as high speed data busses, and the
like.
2. Since any conductance from the various voltages
on the PC board and the XPN line can induce
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
MIC3000’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 MIC3000, thereby providing a
Kelvin connection for the base of the diode.
Micrel
3. When using the MIC3000 to sense the tempera-
ture of a processor or other device which has an
integral thermal diode, connect the emitter and
base of the remote sensor to the MIC3000 using
the guard traces and Kelvin return, shown in Figure
34. The collector of the remote diode is typically
inaccessible to the user on these devices.
4. Due to the small currents involved in the measure-
ment of the remote diode’s ∆V_BE, it is important to
adequately clean the PC board after soldering to
prevent current leakage. This phenomenon will
most likely 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 MIC3000 and
under the connections from the MIC3000 to the
remote diode. This will help guard against stray
noise pickup.
MIC3000
GNDA
XPN
GUARD/RETURN
REMOTE DIODE (XPN)
GUARD/RETURN
Figure 34. Guard Traces and Kelvin Return
for Remote Thermal Diode
Layout Considerations
Small Form-Factor Pluggable (SFP) Transceivers
The pinout of the MIC3000 digital control and status signals
was optimized for use in small form-factor pluggable (SFP
MSP) optical transceivers. If the MIC3000 is mounted on the
bottom of the PC board with the correct rotation, the control
and status I/O can be routed to the host connector without
changing the order. This is shown in Figure 35 below.
24 23 22 21 20 19
1
18
2
17
3
16
4
TOP VIEW ₁₅
5
14
6
13
7 8 9 10 11 12
V_CCR
V_CCR
LOS
RATESEL
MOD-DEF (0)
CLOCK
DATA
TXDISABLE
TXFAULT
V_CCT
Figure 35. Typical SFP Control and Status I/O
Signal Routing (not to scale)
M9999-101204
66
October 2004