MIC384 datasheet(18/21 Pages) MICREL

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MIC384

Micrel

MIC384

September 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 MIC384.

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 MIC384 is to avoid errors in

measuring the local temperature induced by self-heating.

Self-heating is caused by the power naturally dissipated

inside the device due to operating supply current and I/O sink

currents (V

DD × IDD ) + (VOL × IOL). In order to understand

what level of error this represents, and how to reduce that

error, the dissipation in the MIC384 must be calculated and

its effects reduced to a temperature offset.

The worst-case operating condition for the MIC384 is when

DD = 5.5V, MSOP-08 package.

The 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 MIC384 is negligible. Similarly, the DATA

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

than 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. This is illustrated in Equation 2.

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

similar to those shown above (accounting for the expected

value and duty cycle of I

OL(INT)) will give a good estimate of

the temperature error due to self-heating.

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

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

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 MIC384 depends upon sensing the

∆V

CB-E of a diode-connected PNP transistor (“diode”) at two

different current levels. For remote temperature measure-

ments, this is done using external diodes connected between

T1, T2 and ground.

Since this technique relies upon measuring the relatively

small voltage difference resulting from two levels of current

through the external diodes, any resistance in series with

those diodes will cause an error in the temperature reading

from the MIC384. A good rule of thumb is this: for each ohm

in series with a zone's external transistor, there will be a 0.9

°C

error in the MIC384’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/or T2 pins and the GND pin of the MIC384. The use

of these capacitors 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 diodes. The

maximum recommended total capacitance from the T1 or T2

pin to GND is 2700pF. This typically suggests the use of

2200pF NP0 or C0G ceramic capacitors with a 10% toler-

ance.

If a remote diode is to be at a distance of more than

≈ 6"—12"

from the MIC384, using twisted pair wiring or shielded micro-

phone 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 capaci-

tance from the 2700pF maximum total capacitance.

[(I

) (I

)]

[(0.75mA

5.5V) (6mA

0.8V) (6mA

0.8V)]

13.73mW

of MSOP - 08 package is 206 C / W

Maximum T relative to T due to self - heating is 13.73mW

206 C / W = 2.83 C

DDD

OL(DATA)

OL(/INT)

q(j a)

=×+

+×

∆×

−

Equation 1. Worst-Case Self-Heating

[(0.350mA I

3.3V) (25% 1.5mA I

0.3V) (1% 1.5mA I

0.3V)] = 1.27mW

(1.27mW

206 C / W)

DD(typ)

OL(DATA)

OL(/INT)

×+

∆=

0.262 C

Equation 2. Real-World Self-heating Example

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