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2011 Microchip Technology Inc. DS01420A-page 1
AN1420
INTRODUCTION
For many years, the 8-lead SOIC package has been
the most popular package for serial EEPROMs, but
now smaller packages are becoming more and more
commonplace. This offers a number of benefits; the
reductions in footprint size and component height are
some of the more obvious ones. Smaller packages also
generally offer a cost advantage over their larger
counterparts.
Currently, the SOT-23 package is one of the smallest
packages available for serial EEPROMs, and its popu-
larity has been growing significantly. However, while
the SOIC is an industry-standard package available
from all major EEPROM vendors, the SOT-23 package
is still relatively new, especially in larger memory den-
sities. When a new, smaller package is first released by
a vendor, there is always an inherent lag before the
package becomes available from all vendors. For
example, Microchip was the first vendor to offer 32- and
64-Kbit I
2
C™ serial EEPROMs in a 5-lead SOT-23
package and it will take time before other vendors offer
similar devices.
System designers can use the SOT-23 package and
enjoy the associated benefits, or they can maintain a
broad list of possible vendors. Now, by combining
footprints, both can be done at the same time.
METHODOLOGY
By simply including multiple footprints for the serial
EEPROM on your board, you can solve this issue. Nor-
mally, this would waste board space since only one
footprint would be used for a given assembly. However,
due to the SOT-23 package’s extremely small size, you
can fit a SOT-23 footprint within the same area as an
SOIC footprint, as shown in Figure 1. This allows you
to combine multiple footprints with little to no wasted
space. By following this technique, you gain the cost
advantage associated with SOT-23 packages, but can
still qualify a variety of different vendors. In addition,
there are also a few other benefits that open up when
this approach is employed.
This idea can also be applied to other 8-lead packages,
such as the TSSOP and MSOP, if the application
allows for a small amount of extra board space to be
used.
FIGURE 1: COMBINED 6-LEAD SOT-23
AND 8-LEAD PACKAGE
BLOCK DIAGRAM
BENEFITS
Smaller packages offer a cost advantage while larger
packages allow for a broader approved vendor list. So,
naturally, including both footprints on your board gives
you the opportunity to reap both benefits. But there is
something else you gain only by combining footprints:
flexibility.
The added flexibility is seen in two ways. First, with
multiple footprints, your supply options are greatly
expanded since you can now use multiple packages
from each of many approved vendors. Second, your
inventory management will become more flexible.
It is common to try to use the same devices in multiple
applications. This simplifies inventory-related issues,
and combining device orders results in higher volumes
and thus, lower costs per device. Perhaps SOIC is suf-
ficient for all of your current applications, but in the
future you may need to use SOT-23. If you included a
SOT-23 footprint on your current boards, you can
seamlessly migrate them to SOT-23 as well, thereby
reducing how many part numbers you need to order
and store in inventory.
Author: Chris Parris
Microchip Technology Inc.
CS
CLK
DI
DO
V
CC
PE*
ORG*
V
SS
6-ld SOT-23
Narrow, 3.90 mm SOIC,
4.4 mm TSSOP, or MSOP
Note: Pins PE and ORG are not available on some
devices.
Adding Flexibility by Using Multiple
Footprints for Microwire Serial EEPROMs
