MIC284

Micrel

MIC284

18

September 29, 2000

Applications

Remote Diode Selection

Most small-signal PNP transistors with characteristics similar

to the JEDEC 2N3906 will perform well as remote tempera-

ture sensors. Table 6 lists several examples of such parts

that Micrel has tested for use with the MIC284.

Other

transistors equivalent to these should also work well.

Minimizing Errors

Self-Heating

One concern when using a part with the temperature accu-

racy and resolution of the MIC284 is to avoid errors induced

by self-heating (V

In order to

understand what level of error this might represent, and how

to reduce that error, the dissipation in the MIC284 must be

calculated and its effects reduced to a temperature offset.

The worst-case operating condition for the MIC284 is when

V

T he maximum power

dissipated in the part is given in Equation 1 below.

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 MIC284 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. This is illustrated by Equation 2.

If the part is to be used in comparator mode, calculations

similar to those shown in Equation 2 (accounting for the

expected value and duty cycle of I

will give a good estimate of the device’s self-heating error.

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 some

Vendor

Part Number

Package

Fairchild

MMBT3906

SOT-23

On Semiconductor

MMBT3906L

SOT-23

Phillips Semiconductor

PMBT3906

SOT-23

Samsung

KST3906-TF

SOT-23

Table 6. Transistors Suitable for Remote Temperature Sensing Use

of the thermal data (e.g., PC board thermal conductivity and

ambient temperature) may be poorly defined or unobtainable

except by empirical means.

Series Resistance

The operation of the MIC284 depends upon sensing the

∆V

different current levels. For remote temperature measure-

ments, this is done using an external diode connected be-

tween 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 MIC284. 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 MIC284’s temperature measurement. It isn’t difficult to

keep the series resistance well below an ohm (typically <

0.1

Ω), so this will rarely be an issue.

Filter Capacitor Selection

It is sometimes desirable to use a filter capacitor between the

T1 and GND pins of the MIC284. 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 attach to the remote diode. The maximum recom-

mended total capacitance from the T1 pin to GND is 2700pF.

This typically suggests the use of a 2200pF 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 MIC284, using twisted pair wiring or shielded

microphone cable for the connections to the diode can

significantly help reduce noise pickup. If using a long run of

shielded cable, remember to subtract the cable’s conductor-

to-shield capacitance from the 2700pF maximum total ca-

pacitance.

P

[(I

V

) (I

V

) (I

V

) (I

V

)]

P

[(0.75mA

5.5V) + (6mA

0.8V) + (6mA

0.8V) + (6mA

0.8V)

P

18.53mW

R

of MSOP - 08 package is 206 C / W

Maximum T relative to T due to self heating is 18.53mW 206 C / W = 3.82 C

DDD

DD

OL(DATA)

OL(DATA)

OL(/INT)

OL(/INT)

OL(/CRIT)

OL(/CRIT)

D

D

(j-a)

JA

=×+

×

+

×

+

×

=

×

×××

=

°

∆×

°

°

θ

Equation 1. Worst-case self-heating

[(0.35mA I

3.3V) (25% 1.5mA I

0.3V) (1% 1.5mA I

0.3V) + (25% 1.5mA I

0.3V) = 1.38mW

T

(1.38mW

206 C / W)

DD(typ)

OL(DATA)

OL(/INT)

OL(/CRIT)

J

×+

×

×

+

×

×

×

×

∆=

×

°

=

°

0.29 C

Equation 2. Real-world self-heating example