Micrel, Inc.

MIC281

April 23, 2014

11

Revision 2.0

Application Information

Remote Diode Section

Most small-signal PNP transistors with characteristics

similar to the JEDEC 2N3906 will perform well as remote

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.

Table 4. Transistors suitable for use as remote diodes

Vendor

Part

Number

Package

Fairchild Semiconductor

MMBT3906

SOT-23

On Semiconductor

MMBT3906L

SOT-23

Philips Semiconductor

SMBT3906

SOT-23

Samsung Semiconductor

KST3906-TF

SOT-23

Minimizing Errors

Self-Heating

One concern when using a part with the temperature

accuracy and resolution of the MIC281 is to avoid errors

understand 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-case operating condition for

dissipated in the part is given in the following equation:

theoretical maximum self-heating is:

4.44mW × 230°C/W = 1.02°C

In most applications, the DATA pin will 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. This is illustrated by the next equation.

In any application, the best approach 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.

0.815mW

typical self-heating is:

0.815mW × 230°C/W = 0.188°C

Series Resistance

The operation of the MIC281 depends upon sensing the

different

current

levels.

For

remote

temperature

measurements, this is done using an external diode

connected

between

T1

and

ground.

Because

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 that 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 (such as digital switching noise), or if

long traces or wires 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 six-to-twelve inches 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.