7

 2008 Semtech Corp.

www.semtech.com

PROTECTION PRODUCTS

PROTECTION PRODUCTS

SRDA3.3-6 and SRDA05-6

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:

V

C

= V

F(RailClamp)

+ V

TVS

This is given in the data sheet as the rated clamping

voltage of the device. For an SRDA05-6 the typical

clamping voltage is <16V at I

PP

=30A. The diodes

internal to the RailClamp are low 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.

(Reference Semtech application note SI99-01 for

further information on board layout)

Matte Tin Lead Finish

Matte tin has become the industry standard lead-free

replacement for SnPb lead finishes. A matte tin finish

is composed of 100% tin solder with large grains.

Since the solder volume on the leads is small com-

pared to the solder paste volume that is placed on the

land pattern of the PCB, the reflow profile will be

determined by the requirements of the solder paste.

Therefore, these devices are compatible with both

lead-free and SnPb assembly techniques. In addition,

unlike other lead-free compositions, matte tin does not

have any added alloys that can cause degradation of

the solder joint.