5
2008 Semtech Corp.
www.semtech.com
SMF3.3
PROTECTION PRODUCTS
Applications Information
SMF Circuit Diagram
Protection of Four Unidirectional Lines
Device Connection for Protection of Four Data Lines
The SMF3.3 is designed to protect up to four unidirec-
tional data lines. The device is connected as follows:
1. Unidirectional protection of four I/O lines is
achieved by connecting pins 1, 3, 4, and 5 to the
data lines. Pin 2 is connected to ground. The
ground connection should be made directly to the
ground plane for best results. The path length is
kept as short as possible to reduce the effects of
parasitic inductance in the board traces.
Due to the “snap-back” characteristics of the low
voltage TVS, it is not recommended that any of the I/O
lines be directly connected to a DC source greater than
snap-back votlage (V
SB
) as the device can latch on as
described below.
EPD TVS Characteristics
The SMF3.3 is constructed using Semtech’s propri-
etary EPD technology. The structure of the EPD TVS is
vastly different from the traditional pn-junction devices.
At voltages below 5V, high leakage current and junction
capacitance render conventional avalanche technology
impractical for most applications. However, by utilizing
the EPD technology, the SMF3.3 can effectively oper-
ate at 3.3V 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.
2
1345
When the TVS is conducting current, it will exhibit a
slight “snap-back” or negative resistance characteris-
tics due to its structures. 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.
