MSAN-107 Zarlink Semiconductor, Inc., MSAN-107 Datasheet - Page 7

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

Application Note
1)
2)
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4)
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8)
Each of the above entries will now be examined in
terms of its potential for triggering latch-up. The first
four items are very interdependent. While each of
these will be given consideration in separate
sections, cross referencing will be extensive. The
remaining items are relatively independent and thus,
will be looked at in relative isolation.
Insertion/Removal of System PBC’s “Live”
Inserting or removing printed circuit cards from a
powered-up system can trigger latch-up in several
different ways if certain precautions are not taken.
One potential hazard that can occur is for an input or
output edge terminal to make contact before the
power supply pins are connected.
device on another circuit card, this input/output pin
could have a voltage applied to it with no supply
voltage to the device. Even if this situation exists for
only a short period of time, then latch-up may be
triggered when the power supply pin is connected. It
is important to note that three-state outputs are also
vulnerable in this situation. Such output drivers only
present a high impedance to voltages within the
device supply rails.
exceeding the supply can indeed trigger latch-up.
System
The system is powered by multiple supply
Circuits utilize complex capactive decoupling
Integrated circuits on one system PCB drive
Devices drive high capacitive loads such as
System contains high speed address and/or
System has electronic inputs that are directly
Digital devices are driven from analog devices
allow insertion or removal of printed circuit
cards with system power applied.
voltages (e.g.
multi-supply
regulated, +5V unregulated).
techniques particularly associated with multiple
power supply voltages.
other
backplane, ribbon cable, etc.
long data or address busses.
data buses of sufficient length to cause their
inductive properties to become significant at the
frequencies in question (ribbon cables are a
prime example).
accessible by the end user of the system.
powered from higher supply voltages, utilizing
input diodes for clamping.
devices
operation/maintenance
at
12V, + 5V, and Gnd) or has a
on
same
Voltages on these outputs
different
voltage
PCB’s
If driven by a
procedures
(e.g.
via
+5V
a
One solution to this problem is to slightly extend the
power supply terminals with respect to the remaining
edge terminals on the PCB (Fig. 9). This will ensure
that power supply connections are the first made and
last broken on insertion and removal of the PCB
respectively.
Plugging a circuit card live into a system with
multi-power supply voltages can result in the
application of power supply over-voltages to certain
devices.
shown in Fig. 10.
decoupling was plugged into a system live, then the
following situation could result.
capacitors are discharged and that C
greater than C
inserted, the +12V terminal makes connection first,
then the ground, and lastly the +5V connection is
made. In this situation, C
connected in series. The +12 volts applied to C
causes the voltage at the ground point to increase in
Figure 9 - PCB with Inset I/O Edge Terminals
Figure 10 - Local Decoupling Scheme in
PRINTED CIRCUIT
INPUTS/OUTPUTS
Consider the local decoupling scheme
C
C
1
BOARD
1
2
TO CIRCUIT
. It is possible that when the PCB is
C
Multi-Supply System
2
If a PCB containing such
V
V
SS
DD
1
and C
C
2
MSAN-107
2
Assume that all
are momentarily
+12V Supply
+5V Supply
1
Ground
is much
A-37
1