MIC184BMM datasheet(16/21 Pages) MICREL

16 / 21 page

MIC184

Micrel

MIC184

November 2000

{START Polling-Based Test and Initialization

Routine}

1. Temporarily disable the host’s interrupt input

from the device under test.

2. Write 0000 0010b (02h) to the CONFIG register.

3. Write 1100 1000 1000 0000b = C880h to T_SET

and T_HYST. This corresponds to -55.5

°C.

4. Wait t

conv (160ms max.) for the part to finish at

least one A/D conversion.

5. Read the contents of the CONFIG register:

a) If the part is an MIC184, the MSB will be set

to one (CONFIG = 82h).

b) If the part is a conventional LM75-type part,

the MSB will be zero (CONFIG = 02h).

6. Write 0111 1111 1000 0000b = 7F80h to T_SET

and T_HYST. This corresponds to +127.5

°C.

7. Wait an additional t

conv for the part to finish a

second conversion.

8. Read CONFIG again, to clear the interrupt

request from step (7). This will also clear STS, if

the part under test is an MIC184.

9. Based on the results of the test in step (4), do

the following four steps within 50ms total:

a) Set the CONFIG register as required.

b) Load T_HYST with its operational value.

Load T_SET with its operational value.

d) Re-enable the host’s interrupt handling input

from the part under test.

{END}

These routines force the device under test to generate an

overtemperature fault (steps 3 and 4), followed by an

undertemperature fault (steps 6 through 8). This sequence

causes the device under test to exit the routine prepared to

respond to an overtemperature condition. If being immedi-

ately prepared to detect an undertemperature condition upon

exit is desired, swap steps 3 and 6 in each routine.

Remote Diode Selection

Most small-signal PNP transistors with characteristics similar

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

ture sensors. Table 3 lists several examples of such parts.

Micrel has tested those marked with a bullet for use with the

MIC184.

Vendor

Part Number

Package

Tested

Fairchild

MMBT3906

SOT-23

On Semiconductor

MMBT3906L

SOT-23

Phillips Semiconductor

PMBT3906

SOT-23

Rohm Semiconductor

SST3906

SOT-23

Samsung

KST3906-TF

SOT-23

Zetex

FMMT3906

SOT-23

Table 5. Transistors Suitable for Remote Temperature Sensing Use

Minimizing Errors

Self-Heating

One concern when using a part with the temperature accu-

racy and resolution of the MIC184 is to avoid errors induced

by self-heating (V

DD × IDD). In order to understand what level

of error this might represent, and how to reduce that error, the

dissipation in the MIC184 must be calculated, and its effects

examined as a temperature error.

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

few milliseconds before the host sets it back to the high state,

making its duty cycle low enough that its contribution to self-

heating of the MIC184 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, allow the

following calculation of typical device self-heating in interrupt-

mode:

D = (IDD(typ.) 3.3V + 25% IOL(data)0.3V +

1% I

OL(int)0.3V)

D = (0.3mA × 3.3V + 25% × 1.5mA × 0.3V +

× 1.5mA × 0.3V)

∆T

J = 1.11mW × 206°C/W

∆T

J relative to TA is 0.23°C

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

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

of the thermal data, (for example, PC board thermal conduc-

tivity and/or ambient temperature), may be poorly defined or

unavailable except by empirical means.

Series Resistance

The operation of the MIC184 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 an external diode connected be-

tween A2/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

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