MSAN-107 Zarlink Semiconductor, Inc., MSAN-107 Datasheet - Page 10

• Manufacturer Part Number: MSAN-107
• Description: Understanding and Eliminating Latch-Up in CMOS Applications
• Manufacturer: Zarlink Semiconductor, Inc.

MSAN-107
these over-voltage conditions need only exist for a
very brief period of time to trigger latch-up. Thus,
even transient over-voltages during power-up may
pose a problem.
To ensure proper power supply sequencing, careful
attention must be paid to the selection of decoupling
components both at the initial design stage and
when design revisions are done. This applies to both
main power supply decoupling as well as local board
decoupling.
evident at main distribution points, local sequencing
can be altered by large capacitors on individual
boards.
requirement are likely to have such decoupling and
hence, must be looked at carefully.
One way of ensuring that power supplies track when
turning on or off is to connect a diode from the lower
supply voltage to the higher one in the case of
unequal supplies (Fig. 15).
supplies to track within one diode drop until they
attain proper levels.
supplies, two diodes can be connected back to back,
forcing supplies to track, independent of which
supply comes up first
Devices Driving Others on Separate PCBs
When integrated circuits in a system drive other
devices on separate PCB’s (via a backplane for
example), then the considerations given in the
previous two sections must be applied globally to the
system. This was already mentioned in the section
on plugging in PCB’s “live”. That is, when a PCB is
plugged into a backplane with the system power
applied, there is the danger that an input or output
pin will contact an active line on the backplane
A-40
Boards which have a large DC power
While proper sequencing may be
.
In the case of two equal
This will cause the
Figure 14 - Power Supply Sequencing
before the power supply connection is made. The
solution to this problem, as mentioned, lies in
indenting the I/O edge terminations with respect to
power supply terminals on the PCB.
Power supply sequencing should be given special
attention
off-board. The same criteria applies here as was
described for multi-supply systems. However, care
must also be taken in single supply systems. In this
case, large amounts of local decoupling can cause
the supply voltages on some boards in the system to
ramp up slower than on others. Devices on boards
whose power supply ramps up quickly, can impress
an over-voltage on devices on other boards. If this
over-voltage is large enough, then latch-up may be
triggered.
Whenever possible, local decoupling should be
equalized on all boards within the system to
minimize these effects.
off-board drivers are three-state devices, a simple
Fig. 15 - Forced Power Supply Tracking with
+5V Supply
CMOS DEVICE A
V
SS
in
V
DD
systems
Clamping Diode
DEVICE A DRIVES DEVICE B
with
In systems where all
Application Note
devices
V
CMOS DEVICE B
DD
+7V Supply
V
that
SS
drive