MIC281

Micrel

MIC281

10

November 2004

Application Information

Remote Diode Selection

Most small-signal PNP transistors with characteristics similar

totheJEDEC2N3906willperformwellasremote temperature

sensors. Table 4 lists several examples of such parts that

Micrel has tested for use with the MIC281. Other transistors

equivalent to these should also work well.

Vendor

Part Number

Package

Fairchild Semiconductor

MMBT3906

SOT-23

On Semiconductor

MMBT3906L

SOT-23

Infineon Technologies

SMBT3906

SOT-23

Samsung Semiconductor

KST3906-TF

SOT-23

Table 4. Transistors Suitable for Use as Remote

Diodes

Minimizing Errors

Self-Heating

Oneconcernwhenusingapartwiththetemperatureaccuracy

and resolution of the MIC281 is to avoid errors induced by

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

that error, the dissipation in the MIC281 must be calculated

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

3.6V. The maximum power dissipated in the part is given in

Equation 1 below.

In most applications, the DATA pin will have a duty cycle of

substantiallybelow25%inthelowstate.Theseconsiderations,

combinedwithmoretypicaldeviceandapplicationparameters,

give a better system-level view of device self-heating. This

is illustrated by Equation 2. In any application, the best ap-

proach is to verify performance against calculation in the final

application environment. This is especially true when dealing

with systems for which some temperature data may be poorly

defined or unobtainable except by empirical means.

the theoretical maximum self-heating is:

4.44mW×230°C/W = 1.02°C

Equation 1. Worst-Case Self-Heating

the typical self-heating is:

0.815mW×230°C/W = 0.188°C

Equation 2. Real-World Self-Heating Example

Series Resistance

The operation of the MIC281 depends upon sensing the

ferent 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 MIC281. 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 MIC281’s temperature measurement. It is

not difficult to keep the series resistance well below an ohm

(typically < 0.1Ω), 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 MIC281. The use of this capacitor is

recommended in environments with a lot of high frequency

noise(suchasdigitalswitchingnoise),oriflongtracesorwires

are used to connect to the remote diode. The recommended

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

remote diode is to be at a distance of more than 6”-12” from

the MIC281, 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 total capacitance.