TMPR4955AF datasheet(37/168 Pages) TOSHIBA

37 / 168 page

General Safety Precautions and Usage Considerations

3-17

3.6

Protecting Devices in the Field

3.6.1

Temperature

Semiconductor devices are generally more sensitive to temperature than are other electronic

components. The various electrical characteristics of a semiconductor device are dependent on the

ambient temperature at which the device is used. It is therefore necessary to understand the

temperature characteristics of a device and to incorporate device derating into circuit design. Note

also that if a device is used above its maximum temperature rating, device deterioration is more

rapid and it will reach the end of its usable life sooner than expected.

3.6.2

Humidity

Resin-molded devices are sometimes improperly sealed. When these devices are used for an

extended period of time in a high-humidity environment, moisture can penetrate into the device

and cause chip degradation or malfunction. Furthermore, when devices are mounted on a regular

printed circuit board, the impedance between wiring components can decrease under high-

humidity conditions. In systems which require a high signal-source impedance, circuit board

leakage or leakage between device lead pins can cause malfunctions. The application of a

moisture-proof treatment to the device surface should be considered in this case. On the other

hand, operation under low-humidity conditions can damage a device due to the occurrence of

electrostatic discharge. Unless damp-proofing measures have been specifically taken, use devices

only in environments with appropriate ambient moisture levels (i.e. within a relative humidity

range of 40% to 60%).

3.6.3

Corrosive gases

Corrosive gases can cause chemical reactions in devices, degrading device characteristics.

For example, sulphur-bearing corrosive gases emanating from rubber placed near a device

(accompanied by condensation under high-humidity conditions) can corrode a device’s leads. The

resulting chemical reaction between leads forms foreign particles which can cause electrical

leakage.

3.6.4

Radioactive and cosmic rays

Most industrial and consumer semiconductor devices are not designed with protection against

radioactive and cosmic rays. Devices used in aerospace equipment or in radioactive environments

must therefore be shielded.

3.6.5

Strong electrical and magnetic fields

Devices exposed to strong magnetic fields can undergo a polarization phenomenon in their plastic

material, or within the chip, which gives rise to abnormal symptoms such as impedance changes

or increased leakage current. Failures have been reported in LSIs mounted near malfunctioning

deflection yokes in TV sets. In such cases the device’s installation location must be changed or the

device must be shielded against the electrical or magnetic field. Shielding against magnetism is

especially necessary for devices used in an alternating magnetic field because of the electromotive

forces generated in this type of environment.

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