MIC184YM-TR Datasheet MIC184YMM-TR, MIC184YM-TR, MIC184YM | P16-P18

MIC184 Micrel

MIC184 16 May 2006

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.

c) 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 under-

temperature 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 immediately pre-

pared 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.

Minimizing Errors

Self-Heating

One concern when using a part with the temperature accuracy

and resolution of the MIC184 is to avoid errors induced by

self-heating (V

× I

). 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 inter-

rupt-mode:

= (I

DD(typ.)

3.3V + 25% I

OL(data)

0.3V +

1% I

OL(int)

0.3V)

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

1% × 1.5mA × 0.3V)

ΔT

= 1.11mW × 206°C/W

ΔT

relative to T

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

ΔV

CB-E

of a diode-connected PNP transistor ("diode") at

two different current levels. For remote temperature mea-

surements, this is done using an external diode connected

between 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 from the

MIC184. 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

MIC184's temperature measurement. It is not difﬁcult to keep

the series resistance well below an ohm (typically 0.1Ω), so

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

May 2006 17 MIC184

MIC184 Micrel

in most systems this will not be an issue.

Filter Capacitor Selection

When using a remote diode for temperature sensing, it is

sometimes desirable to use a ﬁlter capacitor between the

A2/T1 and GND pins of the MIC184. The use of this capaci-

tor is recommended in environments with a signiﬁcant high

frequency noise (such as digital switching noise), or if long

wires are used to connect to the remote diode. The maximum

recommended total capacitance from the A2/T1 pin to GND

is 2700pF. This usually 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 MIC184, using a shielded cable (solid foil shield

microphone cable is a good choice) for the connections to the

diode can signiﬁcantly help reduce noise pickup. Remember

to subtract the cable's conductor-to-shield capacitance from

the 2700pF maximum total capacitance.

Layout Considerations

Local Mode Only Applications:

If the MIC184 is not going to be used with an external diode,

the best layout is one which keeps it thermally coupled to the

subsystem(s) whose temperature it must monitor, while avoid-

ing any strong sources of EMI, RFI, or electrostatically coupled

noise. Two of the most common examples of such sources

are switching power supply transformers and CRTs.

Remote Mode Applications:

1. If the remote sensing capability of the

MIC184 will be used in an application, place the

MIC184 as close to the remote diode as pos-

sible, while taking care to avoid severe noise

sources (high frequency power transformers,

CRTs, memory and data busses, and the like).

2. Since any conductance from the various volt-

ages on the PC Board and the A2/T1 pin can

induce serious errors, it is good practice to guard

the remote diode’s emitter trace with a pair of

ground traces. These ground traces should be

returned to the MIC184’s own ground pin. They

should not be grounded at any other part of their

run. However, it is highly desirable to use these

guard traces to carry the diode’s own ground

return back to the ground pin of the MIC184,

thereby providing a Kelvin connection for the

base of the diode. See Figure 8.

3. When using the MIC184 to sense the tempera-

ture of a processor or other device which has an

integral on-board “diode” (e.g., Intel’s Pentium

III), connect the emitter and base of the remote

sensor to the MIC184 using the guard traces

and Kelvin return shown in Figure 8. The col-

lector of the remote “diode” is inaccessible to

the user on these types of chips. To allow for

this, the MIC184 has superb rejection of noise

appearing from collector to GND, as long as the

base to ground connection is relatively quiet.

4. Due to the small currents involved in the mea-

surement of the remote diode’s ΔV

, it is

important to adequately clean the PC board after

soldering. This is most likely to show up as an

issue in some situations where water-soluble

soldering ﬂuxes are used.

5. In general, wider traces for the ground and

A2/T1 pins will help reduce susceptibility to radi-

ated noise (wider traces are less inductive). Use

trace widths and spacing of 10 mils wherever

possible. Wherever possible, place a ground

plane under the MIC184, and under the connec-

tions from the MIC184 to the remote diode. This

will help guard against stray noise pickup.

6. Always place a good quality V

bypass ca-

pacitor directly adjacent to, or underneath, the

MIC184. This part should be a 0.1µF ceramic

capacitor. Surface-mount parts provide the best

bypassing because of their low inductance.

7. When the MIC184 is being powered from par-

ticularly noisy power supplies, or from supplies

which may have sudden high-amplitude spikes

appearing on them, it can be helpful to add ad-

ditional power supply ﬁltering. This should be

implemented as a 100Ω resistor in series with

the part’s V

pin, and a 4.7µF, 6.3V electrolytic

capacitor from V

to GND. See Figure 9.

MIC184 Micrel

MIC184 18 May 2006

REMOTE DIODE (A2/T1)

GUARD/RETURN

DATA

CLK

INT

GND

VDD

A2/T1

GUARD/RETURN

Figure 8. Guard Traces/Kelvin Ground Returns

DATA

FROM

SERIAL BUS

HOST

2N3906

2200pF

MIC184

CLK

INT

VDD

100

3.0V to 3.6V

10k Pull-ups

A2/T1

GND

4.7µF0.1F

Figure 9. V

Decoupling for Very Noisy Supplies

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