MSAN-107 Zarlink Semiconductor, Inc., MSAN-107 Datasheet - Page 12

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

MSAN-107
Minimizing intertrack capacitance by interleaving
signal and ground tracks should be done wherever
board
clamping diodes can be connected on tracks
exhibiting these voltage excursions. The diodes may
need be Schottky diodes if regular ones do not
clamp soon enough toprevent current flow through
I/O structures. Regular silicon diodes may still be
used if they are referenced to voltages inset by 0.7V
from the supply rails. The clamping circuit shown in
Fig. 18 should be quite effective, but as can be seen,
this circuit will dissipate power. This may or may not
be a problem depending on the overall system
requirements.
absorb the high frequency energy.
values shown are selected for a 5V supply and
should be scaled for other supply voltages.
Ribbon Cables - A Special Case
A ribbon cable is a special case of long bus
structure. The problems mentioned in the previous
section also apply here. However, if the ribbon cable
is of sufficient length, then its inductive properties
become significant. The distributed inductance and
capacitance form a second order circuit which can
“ring” when driven by fast, digital signals. The result
is the generation of damped oscillations centered
about the positive and negative supply rails (Fig. 19).
The positive and negative excursions outside of the
supply rails impress over-voltages on inputs and
outputs connected to the ribbon cable. If of sufficient
amplitude, these over-voltages may trigger latch-up.
Solving the problem can be as simple as terminating
each end of such cables with resistors to reduce the
ringing voltages.
dissipate extra power. An alternative is to connect
external protection diodes as shown in Fig. 20.
A-42
Fig. 17 - Local Decoupling to Offset Load
System Supply
System Ground
space
0.01 to
0.1 F
The decoupling capacitors help to
permits.
Switching Effects
However, these resistors will
V
SS
CMOS
Device
Alternatively,
V
DD
+
The resistor
Loads
Other
To
external
10 F
Fig. 19-Ringing Effect Due to Driving Ribbon Cable
These
over-voltages.
necessary to use Schottky diodes to ensure that the
external diodes conduct before the input/output
structures do
Systems with End-User Accessible Inputs/
Outputs
An extreme condition of input/output over-voltage
can develop in systems which have end user
accessible I/O ports. The user may apply signals to
these ports when the system power supply is not
turned on. Devices in the system connected to these
ports are likely to latch-up when the power is turned
on due to the current flowing through the I/O
structures.
with these I/O’s to limit the current during these
periods.
direct effect on the speed and noise performance of
these ports
end user applies voltages to the I/O ports which
exceed the system power supply voltages.
protection resistors suggested above may provide
adequate protection against this hazard as well.
Fig. 18 - Clamping Circuit for Long Buses
0.1 F
0.1 F
diodes
As mentioned, these resistors will have
.
.
Resistors can be connected in series
610
Latch-up may also be triggered if the
610
If the problem persists, it may be
5V
will
3.0K
(0.7V)
(4.3V)
clamp
Application Note
TO BUS TRACK
any
generated
The