Rev. 3.0 October 2010 www.aosmd.com Page 5 of 10
AOZ8001A
Application Information
The AOZ8001A TVS is design to protect two data lines
from fast damaging transient over-voltage by clamping it
to a reference. When the transient on a protected data
line exceed the reference voltage the steering diode is
forward bias thus, conducting the harmful ESD transient
away from the sensitive circuitry under protection.
PCB Layout Guidelines
Printed circuit board layout is the key to achieving the
highest level of surge immunity on power and data lines.
The location of the protection devices on the PCB is the
simplest and most important design rule to follow. The
AOZ8001A devices should be located as close as possi-
ble to the noise source. The placement of the AOZ8001A
devices should be used on all data and power lines that
enter or exit the PCB at the I/O connector. In most
systems, surge pulses occur on data and power lines
that enter the PCB through the I/O connector. Placing
the AOZ8001A devices as close as possible to the noise
source ensures that a surge voltage will be clamped
before the pulse can be coupled into adjacent PCB
traces. In addition, the PCB should use the shortest
possible traces. A short trace length equates to low
impedance, which ensures that the surge energy will be
dissipated by the AOZ8001A device. Long signal traces
will act as antennas to receive energy from fields that are
produced by the ESD pulse. By keeping line lengths as
short as possible, the efficiency of the line to act as an
antenna for ESD related fields is reduced. Minimize inter-
connecting line lengths by placing devices with the most
interconnect as close together as possible. The protec-
tion circuits should shunt the surge voltage to either the
reference or chassis ground. Shunting the surge voltage
directly to the IC’s signal ground can cause ground
bounce. The clamping performance of TVS diodes on a
single ground PCB can be improved by minimizing the
impedance with relatively short and wide ground traces.
The PCB layout and IC package parasitic inductances
can cause significant overshoot to the TVS’s clamping
voltage. The inductance of the PCB can be reduced by
using short trace lengths and multiple layers with
separate ground and power planes. One effective
method to minimize loop problems is to incorporate a
ground plane in the PCB design. The AOZ8001A ultra-
low capacitance TVS is designed to protect four high
speed data transmission lines from transient over-volt-
ages by clamping them to a fixed reference. The low
inductance and construction minimizes voltage over-
shoot during high current surges. When the voltage on
the protected line exceeds the reference voltage the
internal steering diodes are forward biased, conducting
the transient
current away from the sensitive circuitry.
Good circuit board layout is critical for the suppression
of ESD induced transients. The following guidelines are
recommended:
1. Place the TVS near the IO terminals or connectors to
restrict transient coupling.
2. Fill unused portions of the PCB with ground plane.
3. Minimize the path length between the TVS and the
protected line.
4. Minimize all conductive loops including power and
ground loops.
5. The ESD transient return path to ground should be
kept as short as possible.
6. Never run critical signals near board edges.
7. Use ground planes whenever possible.
8. Avoid running critical signal traces (clocks, resets,
etc.) near PCB edges.
9. Separate chassis ground traces from components
and signal traces by at least 4mm.
10. Keep the chassis ground trace length-to-width ratio
<5:1 to minimize inductance.
11. Protect all external connections with TVS diodes.
