SP506CF datasheet(16/30 Pages) SIPEX

16 / 30 page

SP505/6/7APN/03

SP505, SP506, SP507 Application Note

ESD Protection and EMI Filtering

It is now a requirement for networking equipment,

in order to receive the European "CE" mark, to

withstand a certain amount of environmental

hazards. Among these are ESD and EMI immunity as

well as EMI emissions, which is the equipment's

own generation of electromagnetic interference.

Electrostatic discharge and overvoltage transients

are important to suppress in any system. The

specification generally used for ESD immunity is

EN61000-4-2 (formerly IEC1000-4-2), which

specifies Air Discharge and Contact Discharge

Methods. For "CE" approval, the acceptance level

is generally "Level 2" per the IEC1000-4-2

specification, which is 4kV Air Discharge and 4kV

Contact Discharge. While the SP505, SP506, and

SP507 has reasonable handling withstand voltages

built in the I/O structures of the device, external

protection is always a good idea.

One method of protection is incorporating

TransZorbs™ or transient voltage suppression ICs,

which are back-to-back Zener diodes connected on

the line to ground. There are a variety of manufacturers

such as Motorola, Siemens, Semtech, Protek Devices,

and more. The key specifications are:

Reverse Standoff Voltage - normal circuit operating

voltage. For RS-232, the maximum V

RWM = 15V.

Peak Pulse or Transient Current - expected transient

current. (I

PP)

Reverse Breakdown Voltage - device begins to

avalanche and becomes a low impedance path to

ground for the transient. (V

BR)

Maximum Junction Capacitance - loading capacitance

of the diode structure. More capacitance will affect the

total AC performance. (C

A variety of transzorbs were tested and all perform

well in the presence of ESD transients. For faster data

rates such as V.11 and V.35 signals, low capacitance

is important since an additional 50pF load could add

5ns to the transition time and affect the overall

transmission rate. The Semtech LCDA15C-6 and

Protek Devices SM16LC15C are especially designed

for data communications because of the multichannel

line support and the low junction capacitance.

Figure 29 illustrates a TVS configuration using the

Semtech LCDA15C-6 connected to the clock and

data signals of the SP505, SP506 and SP507.

The LCDAC-6 was chosen due to its low junction

capacitance of 20pF, which are important for high

speed clock and data lines. Protek's SM16LC15C can

also be used as the junction capacitance is 25pF.

However, the two TVS devices are not pin compatible.

Protek's SM16LC15C contains protection for eight

lines and has a straight-through pinout. One side of

the SM16LC15C is grounded. The LCDAC-6 uses a

8-pin SOIC package as opposed to the 16-pin

package with the SM16LC15C. Since two ICs are

needed anyway for clock and data, the smaller

package is usually preferred. Refer to each of the

manufacturer's datasheet for details. Figure 30

illustrates a TVS configuration to the handshaking

signals. As these signals are for control and indication,

they do not usually switch at high speed. The junction

capacitance for these devices are less critical.

Figure 28. I-V Curve of a TVS diode

Lower V

RWM values can be selected instead of 15V.

If the configuration is straightforward, using 5V to

8V V

RWM values is fine for the driver outputs and

receiver inputs. Using 5V V

RWM on the driver is fine

since the clamping occurs at the reverse breakdown

voltage(V

BR), which is 6V for most 5V transzorbs.

However, during compliancy testing, the V.28

receiver may be subjected to 15V in order to test the

input impedance. Applying a voltage exceeding the

RWM rating will affect the input current measurement

and thus fail the impedance test.

TransZorb is a trademark of General Semiconductor Industries.

Vrwm

Vbr

Ipp

SP506CF Folha de dados(PDF) 16 Page - Sipex Corporation

SP506CF Folha de dados(HTML) 16 Page - Sipex Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Folha de dados Download

Ligação URL