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| 型號(hào)： | T7264 |
| 廠商： | Lineage Power |
| 英文描述： | U-Interface 2B1Q Transceiver(U接口 2B1Q收發(fā)器) |
| 中文描述： | U型接口2B1Q收發(fā)器（ü接口2B1Q收發(fā)器） |
| 文件頁數(shù)： | 42/54頁 |
| 文件大?。?/td> | 876K |
| 代理商： | T7264 |
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42
Lucent Technologies Inc.
Data Sheet
April 1998
T7264 U-Interface 2B1Q Transceiver
Appendix A. Questions and Answers
(continued)
Q12
: Why must secondary protection, such as the
521A protection diode, be used
A12
: The purpose of the 521A is to protect against
metallic surges below the breakdown level of the
primary protector.
Such metallic surges will be coupled through the
transformer and could cause device damage if
the currents are high. The 521A does not provide
absolute protection for the device, but rather
works in conjunction with the built-in protection
on the device leads.
The breakdown voltage level for secondary pro-
tection devices must be chosen to be above the
normal working voltage of the signal and typically
below the breakdown voltage level of the next
stage of protection. The 521A has a minimum
breakdown voltage level of 6.95 V and a maxi-
mum breakdown voltage of 8.0 V.
The chip pins which the 521A protects are
pins 36 (HP), 31 (HN), 32 (LOP), and 35 (LON).
The 16.9
resistors will help to protect pins 32
and 35, but pins 31 and 36 will be directly
exposed to the voltage across the 521A. The on-
chip protection on these pins consists of output
diodes and a pair of polysilicon resistors. These
pins have been thoroughly tested to ensure that
an 8 V level will not damage them; therefore, no
third level of protection is needed between the
521A and the HP and HN pins.
The 521A has a maximum reverse surge voltage
level of 10 V at 50 A. Sustained currents this
large are not a concern in this application, since it
is assumed that some form of primary protection
is being used, such as the Teccor* P2103AA
SIDACtor*. Thus, there should never be more
than 8.0 V across the 521A, except for possibly
an ESD or lightning hit. In these cases, the T7264
is able to withstand at least
±
500 V (human-body
model) on its pins.
Another consideration is the capacitive loading
that the protection device presents to the target
device. This is generally required to be negligible
at the operating frequency.
From a practical point of view, the device is
chosen to meet not only voltage, current, and
capacitance requirements, but also to meet price,
availability, manufacturing, and second-
sourcing requirements. The 521A device is a
general-purpose device that meets system
requirements for voltage, current, capacitance,
price, etc., for some of Lucent’s customers. Other
devices which may be acceptable alternates are
as follows: Motorola
SA6.0C (through-hole and
surface-mount), Microsemi SMSJ6.0C-SMB (sur-
face-mount), and SGS-Thomson
SM6T6V8C
(surface-mount).
Q13
: Bellcore TR-TSY-000078, Section 3.2.4.1, prohib-
its using silver metallization, but the 521A protec-
tion diodes have silver-plated leads. Does this
indicate an incompatibility problem
A13
: Bellcore TR-TSY-000078, Section 3.2.4.1 prohib-
its silver “. . . when electromigration is a problem.”
The 521A diodes are only used as secondary
protection, and, therefore, almost never carry any
current. Electromigration occurs only when there
is a high current flow for an extended period of
time. Since the diodes are never subjected to this
condition, electromigration is not a problem, and
the silver plating is acceptable.
Q14
: Where can information be obtained on lightning
and surge protection requirements for 2B1Q
products
A14
: ANSI T1.601, Appendix B, provides a list of
applicable specifications to which you may refer.
Also, there are many manufacturers of overvolt-
age protection devices who are familiar with the
specifications and would be willing to assist in
surge protection design. The ITU-T K series
recommendations are also a good source of
information on protection, especially Recommen-
dation K.11, “Principles of Protection Against
Overvoltages and Overcurrents,” which presents
an overview of protection principles.
* Teccor and SIDACtor are trademarks of Teccor, Inc.
Motorola is a registered trademark of Motorola, Inc.
SGS-Thomson is a registered trademark of SGS-Thomson
Microelectronics, Inc.
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