S1X60000 Datasheet, PDF (223/358 Pages) Epson Company – EMBEDDED ARRAY

Chapter 9 Circuit Design
It should be noted that the number of power supply pins ultimately required for the
LSI is not simply the number of power supply pins determined above, but must also
include the power supply pins that are added for noise protection purposes or the like.
For details on the added power supply pins, refer to Section 9.11.2, “Simultaneous
Operation and Adding Power Supplies.”
(2) Noise generated by the operation of output cells
The noise generated by the operation of output cells is broadly classified into the two
types specified below. To reduce these types of noise, it is helpful to install as many
power supply pins as possible.
a) Noise generated in the power supply lines
The noise generated in the power supply lines presents a problem when there are
multiple operating outputs. It causes the LSI’s input threshold level to change,
which in turn causes the LSI to operate erratically. This type of noise is generated
by a large current flowing into the power supply lines due to the simultaneous
operation of output cells.
Power supply noise in particular is affected by the inductance component of the
circuit. Therefore, the LSI’s equivalent circuit can be expressed as shown in
Figure 9-15. When the output in this circuit diagram changes state from High to
Low, a current flows from the output pin into the LSI, with the current flowing
through the equivalent inductance L2 (due to the LSI package or the like) to the
ground. At this time, the equivalent inductance L2 causes the voltage of the LSI’s
internal VSS power supply line to change. A voltage fluctuation occurring in this
VSS power supply line is referred to here as the noise generated in the power
supply line. Because this type of noise is caused mainly by the equivalent
inductance L2, there is a tendency that the greater the surge of the power supply
current, the greater the magnitude of the noise generated.
VDD
Input pin
L1
VDD（internal）
Output pin
L3
V1
VSS（i nter nal）
L2
Figure 9-15 LSI Equivalent Circuit
214
EPSON
EMBEDDED ARRAY S1X60000 SERIES
DESIGN GUIDE