SMF3.3.TC datasheet(5/8 Pages) SEMTECH

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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

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 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

(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.

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