TMPR3916 datasheet(24/225 Pages) TOSHIBA

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3 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

Input/Output

Signals

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