S1K50000 Epson Electronics America, Inc., S1K50000 Datasheet - Page 97

• Manufacturer Part Number: S1K50000
• Description: Design Guide S1k50000 Series
• Manufacturer: Epson Electronics America, Inc.

Chapter 8: Pin Arrangement and Simultaneous Operation
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(1) Power-supply currents (I
(2) Noise generated by the operation of output cells
a) Noise generated in power-supply lines
Power-supply currents (I
through the power-supply pins during operation. If a current exceeding this allowable level
flows, the current density in the LSI’s internal power-supply wiring becomes excessively
high, causing the LSI’s reliability to degrade or the device itself to break down. In addition,
the LSI’s internal voltage increases or decreases by the amount of voltage generated by
the current and wiring resistance. This results in device malfunction or adversely affects
the DC and AC characteristics.
To prevent such problems, the current density and the impedance of the power-supply
wiring must be reduced. This can be accomplished by estimating the power consumption
of standard cells in the design of a circuit, and then preparing a sufficient number of power-
supply pins so that the current flowing through each power-supply pin does not exceed the
permissible value. For details, see Section 8.1, “Estimating the Number of Power-Supply
Pins.” In addition, make sure the power-supply pins are placed so as to be separated from
each other.
Note, however, that the number of power-supply pins finally required is not simply the
number of supply pins calculated above, but also includes the power-supply pins that are
added as a means of preventing noise problems. For details on the number of supply pins
added, see Section 8.2, “Simultaneously Operating Buffers and Added Power Supply.”
The noise generated by the operation of output cells can be broadly classified into the two
types specified below. To reduce these types of noise, install as many power supplies as
possible.
The noise generated in power-supply lines presents a problem when multiple outputs are
activated, causing the LSI’s input threshold level to change, which in turn causes
malfunction. The noise in power-supply lines is generated by the simultaneous operation of
output cells, which causes a large current to flow through the power-supply lines.
The power-supply noise is affected by inductance components. Therefore, an LSI’s
equivalent circuit can be expressed as shown in Figure 8-1. In this circuit diagram, when
the output changes state from high to low, a current flows from the output pin into the LSI’s
internal logic, with the current flowing through the equivalent inductance L2 due to the LSI’s
package or the like. At this time, the equivalent inductance L2 causes the voltage in the
LSI’s internal V
supply line is the “noise” generated in power-supply lines. Because this noise is generated
primarily by the equivalent inductance L2, the greater the rapidity of the power-supply
current, the larger the amount of noise generated.
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power-supply line to fluctuate. This voltage fluctuation in the V
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) stipulate the rated amount of current that is allowed to flow
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EPSON
STANDARD CELL S1K50000 SERIES
DESIGN GUIDE
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power-