MIC3000BML datasheet(66/68 Pages) MICREL

66 / 68 page

MIC3000

Micrel

M9999-101204

October 2004

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.

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.

GNDA

XPN

MIC3000

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.

9 10 11 12

24 23 22 21 20 19

VCCT

TXFAULT

TXDISABLE

DATA

CLOCK

MOD-DEF (0)

RATESEL

LOS

VCCR

TOP VIEW

Figure 35. Typical SFP Control and Status I/O

Signal Routing (not to scale)

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