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
µ
F to 1
µ
F 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-
µ
F capacitor on the printed
circuit board (one capacitor will suffice). If the capacitance is excessively large, however, (e.g.
several thousand
µ
F) 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
Содержание TMPR7901
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