MIC280 datasheet(21/23 Pages) MICREL

21 / 23 page

November 2004

MIC280

Micrel

Application Information

Remote Diode Selection

Most small-signal PNP transistors with characteristics similar

totheJEDEC2N3906willperformwellasremote temperature

sensors. Table 8 lists several examples of such parts that

Micrel has tested for use with the MIC280. Other transistors

equivalent to these should also work well.

Vendor

Part Number

Package

Fairchild Semiconductor

MMBT3906

SOT-23

On Semiconductor

MMBT3906L

SOT-23

Philips Semiconductor

PMBT3906

SOT-23

Samsung Semiconductor

KST3906-TF

SOT-23

Table 8: Transistors suitable for use as remote diodes

Minimizing Errors

Self-Heating

Oneconcernwhenusingapartwiththetemperatureaccuracy

and resolution of the MIC280 is to avoid errors induced by

self-heating (VDD × IDD) + (VOL × IOL). In order to understand

what level of error this might represent, and how to reduce

that error, the dissipation in the MIC280 must be calculated

and its effects reduced to a temperature offset. The worst-

case operating condition for the MIC280 is when VDD = 3.6V.

The maximum power dissipated in the part is given in the

following equation:

PD = [(IDD×VDD)+(IOL(DATA)×VOL(DATA))+(IOL(/INT)×VOL(/INT)]

PD = [(0.4mA×3.6V)+(6mA×0.5V)+(6mA×0.5V)]

PD = 7.44mW

Rθ(J-A) of SOT23-6 package is 230°C/W

Theoretical Maximum ∆TJ due to self-heating is:

7.44mW×230°C/W = 1.7112°C

Worst-case self-heating

In most applications, the /INT output will be low for at most

a few milliseconds before the host resets it back to the high

state, making its duty cycle low enough that its contribution to

self-heating of the MIC280 is negligible. Similarly, the DATA

pin will in all likelihood have a duty cycle of substantially below

25% in the low state. These considerations, combined with

more typical device and application parameters, give a better

system-level view of device self-heating in interrupt-mode

usage given in the following equation:

(0.23mA IDD(typ)×3.3V)+(25%×1.5mA IOL(DATA)×0.15V)

+ (1%×1.5mA IOL(/INT)×0.15V) = 0.817mW

∆

TJ = (0.8175mW×230°C/W) = 0.188°C

Real-world self-heating example

In any application, the best test is to verify performance

against calculation in the ﬁnal application environment. This

is especially true when dealing with systems for which tem-

perature data may be poorly deﬁned or unobtainable except

by empirical means.

Series Resistance

TheoperationoftheMIC280dependsuponsensingtheVCB-E

of a diode-connected PNP transistor (“diode “) at two differ-

ent current levels. For remote temperature measurements,

this is done using an external diode connected between T1

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 MIC280. A good rule of thumb is this: for

each ohm in series with the external transistor, there will be

a 0.8°C error in the MIC280’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.

Filter Capacitor Selection

It is usually desirable to employ a ﬁlter capacitor between the

T1 and GND pins of the MIC280. The use of this capacitor is

recommended in environments with a lot of high frequency

noise (such as digital switching noise), or if long wires are

used to conect to the remote diode. The maximum recom-

mended total capacitance from the T1 pin to GND is 2200pF.

This typically suggests the use of a 1800pF NP0 or C0G

ceramic capacitor with a 10% tolerance. If the remote diode

is to be at a distance of more than 6"-12" from the MIC280,

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

2200pF maximum total capacitance.

Layout Considerations

The following guidelines should be kept in mind when design-

ing and laying out circuits using the MIC280:

1. Place the MIC280 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 in-

duce 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 MIC280'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 MIC280,

thereby providing a Kelvin connection for the

base of the diode. See Figure 6.

3. When using the MIC280 to sense the tempera-

ture of a processor or other device which has an

integral thermal diode, e.g., Intel's Pentium II, III,

IV, AMD Athlon CPU, Xilinx Virtex FPGAs, con-

nect the emitter and base of the remote sensor

to the MIC280 using the guard traces and Kelvin

return shown in Figure 6. The collector of the

remote diode is typically inaccessible to the user

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