MIC184YM View Datasheet(PDF) - Micrel

Part Name

Description

Manufacturer

MIC184YM

Local/Remote Thermal Supervisor

Micrel 

MIC184

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.

Micrel

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 ΔVBE, 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 VDD 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 VDD pin, and a 4.7µF, 6.3V electrolytic

capacitor from VDD to GND. See Figure 9.

May 2006

17

MIC184

Share Link: