SP505AN Datasheet, PDF (16/30 Pages) Sipex Corporation – Multi-Protocol Serial Transceivers

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:
1) Reverse Standoff Voltage - normal circuit operating
voltage. For RS-232, the maximum V_RWM = 15V.
2) Peak Pulse or Transient Current - expected transient
current. (I_PP)
3) Reverse Breakdown Voltage - device begins to
avalanche and becomes a low impedance path to
ground for the transient. (V_BR)
4) Maximum Junction Capacitance - loading capacitance
of the diode structure. More capacitance will affect the
total AC performance. (C_J)
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.
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
V_RWM rating will affect the input current measurement
and thus fail the impedance test.
I
I_pp
V_br
V_c
V_rwm
I_t
I_r
I_r
I_t
V
V_rwm
V_br
V_c
I_pp
Figure 28. I-V Curve of a TVS diode
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.
TransZorb is a trademark of General Semiconductor Industries.
SP505/6/7APN/03
SP505, SP506, SP507 Application Note
16
© Copyright 2000 Sipex Corporation