SR2.8 Datasheet, PDF (4/7 Pages) Semtech Corporation – RailClamp® Low Capacitance TVS Diode Array

SR2.8
PROTECTION PRODUCTS
Applications Information
Device Connection Options for Protection of
Two High-Speed Data Lines
Data Line Protection Using Internal TVS Diode
as Reference
The SR2.8 TVS is designed to protect two data lines
from transient over-voltages by clamping them to a
fixed reference. When the voltage on the protected
line exceeds the reference voltage (plus diode V_F) the
steering diodes are forward biased, conducting the
transient current away from the sensitive circuitry.
Data lines are connected at pins 2 and 3. The nega-
tive reference (REF1) is connected at pin 1. This pin
should be connected directly to a ground plane on the
board for best results. The path length is kept as short
as possible to minimize parasitic inductance.
The positive reference (REF2) is connected at pin 4.
The options for connecting the positive reference are
as follows:
Note that pins 4 is connected internally to the cathode
of the low voltage TVS. It is not recommended that this
IPP
pin be directly connected to a DC source greater than
the snap-back votlage (V_SB) as the device can latch on
as described below.
I_SB
EPD TVS Characteristics
These devices are constructed using Semtech’s
proprietary EPD technology. By utilizing the EPD tech-
nology, the SR2.8 can effectively operate at 2.8V while
maintaining excellent electrical characteristics.
The EPD TVS employs a complex nppn structure in
contrast to the pn structure normally found in tradi-
tional silicon-avalanche TVS diodes. Since the EPD
TVS devices use a 4-layer structure, they exhibit a
slightly different IV characteristic curve when compared
to conventional devices. During normal operation, the
device represents a high-impedance to the circuit up to
the device working voltage (V_RWM). During an ESD
event, the device will begin to conduct and will enter a
low impedance state when the punch through voltage
(V_PT) is exceeded. Unlike a conventional device, the low
voltage TVS will exhibit a slight negative resistance
characteristic as it conducts current. This characteris-
tic aids in lowering the clamping voltage of the device,
but must be considered in applications where DC
voltages are present.
When the TVS is conducting current, it will exhibit a
slight “snap-back” or negative resistance
VBRR
IPT
I_R
I_BRR
VRWM VSB VPT VC
Figure 1 - EPD TVS IV Characteristic Curve
characteristics due to its structure. This point is
defined on the curve by the snap-back voltage (V_SB)
and snap-back current (I_SB). To return to a non-
conducting state, the current through the device must
fall below the I_SB (approximately <50mA) and the
voltage must fall below the V_SB (normally 2.8 volts for a
3.3V device). If a 3.3V TVS is connected to 3.3V DC
source, it will never fall below the snap-back voltage of
2.8V and will therefore stay in a conducting state.
 2008 Semtech Corp.
4
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