SRDA3.3-4 查看數據表（PDF） - Semtech Corporation

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SRDA3.3-4

RailClamp® Low Capacitance TVS Diode Array

Semtech Corporation 

SRDA3.3-4 through SRDA12-4

PROTECTION PRODUCTS

Applications Information (continued)

the current will be directed through the integrated TVS

diode to ground. The total clamping voltage seen by

the protected IC due to this path will be:

technology, the SRDA3.3-4 can effectively operate at

3.3V while maintaining excellent electrical characteris-

tics.

V =V

+V

C

F(RailClamp)

TVS

This is given in the data sheet as the rated clamping

voltage of the device. For an SRDA05-4 the typical

clamping voltage is <16V at I =30A. The diodes

internal to the RailClamp arePlPow capacitance, fast

switching devices that are rated to handle high tran-

sient currents and maintain excellent forward voltage

characteristics.

Using the RailClamp does not negate the need for good

board layout. All other inductive paths must be consid-

ered. The connection between the positive supply and

the SRDA and from the ground plane to the SRDA

must be kept as short as possible. The path between

the SRDA and the protected line must also be mini-

mized. The protected lines should be routed directly to

the SRDA. Placement of the SRDA on the PC board is

also critical for effective ESD protection. The device

should be placed as close as possible to the input

connector. The reason for this is twofold. First,

inductance resists change in current flow. If a signifi-

cant inductance exists between the connector and the

TVS, the ESD current will be directed elsewhere (lower

resistance path) in the system. Second, the effects of

radiated emissions and transient coupling can cause

upset to other areas of the board even if there is no

direct path to the connector. By placing the TVS close

to the connector it will divert the ESD current immedi-

ately and absorb the ESD energy before it can be

coupled into nearby traces.

The IV characteristic curve of the EPD device is shown

in Figure 4. The device represents a high impedance

to the circuit up to the working voltage (VRWM). During a

transient event, the device will begin to conduct as it is

biased in the reverse direction. When the punch-

through voltage (VPT) is exceeded, the device enters a

low impedance state, diverting the transient current

away from the protected circuit. When the device is

conducting current, it will exhibit a slight “snap-back” or

negative resistance characteristic due to its structure.

This must be considered when connecting the device

to a power supply rail. To return to a non-conducting

state, the current through the device must fall below

the snap-back current (approximately < 50mA) to allow

it to travel back through the negative resistance

region. If this is a concern, a 10kΩ current limiting

resistor can be placed between the supply rail and the

positive reference pins (2 and 3) to prevent device

latch-up.

RailClamp is a registered trademark of Semtech corporation

IPP

VBRR

ISB

IPT

IR

VRWM VSB VPT VC

IBRR

(Reference Semtech application note SI99-01 for

further information on board layout)

SRDA3.3-4 EPD TVS Characteristics

The internal TVS of the SRDA3.3-4 is constructed using

Semtech’s proprietary EPD technology. The structure

of the EPD TVS is vastly different from the traditional

pn-junction devices that are internal to the SRDA05-4

and SRDA12-4 devices. At voltages below 5V, high

leakage current and junction capacitance render

conventional avalanche technology impractical for

most applications. However, by utilizing the EPD

 2003 Semtech Corp.

7

Figure 4 - EPD TVS IV Characteristic Curve

www.semtech.com

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