TMPR4955AF datasheet(29/168 Pages) TOSHIBA

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General Safety Precautions and Usage Considerations

3-9

3.3.10

Decoupling

Spike currents generated during switching can cause Vcc (Vdd) and GND (Vss) voltage levels to

fluctuate, causing ringing in the output waveform or a delay in response speed. (The power supply

and GND wiring impedance is normally 50

Ω to 100 Ω.) For this reason, the impedance of power

supply lines with respect to high frequencies must be kept low. This can be accomplished by using

thick and short wiring for the Vcc (Vdd) and GND (Vss) lines and by installing decoupling

capacitors (of approximately 0.01

µF to 1 µF capacitance) as high-frequency filters between Vcc

(Vdd) and GND (Vss) at strategic locations on the printed circuit board.

For low-frequency filtering, it is a good idea to install a 10- to 100-

µF capacitor on the printed

circuit board (one capacitor will suffice). If the capacitance is excessively large, however, (e.g.

several thousand

µF) latch-up can be a problem. Be sure to choose an appropriate capacitance

value.

An important point about wiring is that, in the case of high-speed logic ICs, noise is caused mainly

by reflection and crosstalk, or by the power supply impedance. Reflections cause increased signal

delay, ringing, overshoot and undershoot, thereby reducing the device’s safety margins with

respect to noise. To prevent reflections, reduce the wiring length by increasing the device

mounting density so as to lower the inductance (L) and capacitance (C) in the wiring. Extreme

care must be taken, however, when taking this corrective measure, since it tends to cause

crosstalk between the wires. In practice, there must be a trade-off between these two factors.

3.3.11

External noise

Printed circuit boards with long I/O or signal pattern lines are

vulnerable to induced noise or surges from outside sources.

Consequently, malfunctions or breakdowns can result from

overcurrent or overvoltage, depending on the types of device

used. To protect against noise, lower the impedance of the

pattern line or insert a noise-canceling circuit. Protective

measures must also be taken against surges.

For details of the appropriate protective measures for a

particular device, consult the relevant databook.

3.3.12

Electromagnetic interference

Widespread use of electrical and electronic equipment in recent years has brought with it radio

and TV reception problems due to electromagnetic interference. To use the radio spectrum

effectively and to maintain radio communications quality, each country has formulated

regulations limiting the amount of electromagnetic interference which can be generated by

individual products.

Electromagnetic interference includes conduction noise propagated through power supply and

telephone lines, and noise from direct electromagnetic waves radiated by equipment. Different

measurement methods and corrective measures are used to assess and counteract each specific

type of noise.

Difficulties in controlling electromagnetic interference derive from the fact that there is no

method available which allows designers to calculate, at the design stage, the strength of the

electromagnetic waves which will emanate from each component in a piece of equipment. For this

reason, it is only after the prototype equipment has been completed that the designer can take

measurements using a dedicated instrument to determine the strength of electromagnetic

interference waves. Yet it is possible during system design to incorporate some measures for the

prevention of electromagnetic interference, which can facilitate taking corrective measures once

the design has been completed. These include installing shields and noise filters, and increasing

Input/Output

Signals

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