- 您现在的位置：买卖IC网 > PDF目录378403 > AN-5031 (Fairchild Semiconductor Corporation) GTLP Power Configuration PDF资料下载
参数资料
| 型号： | AN-5031 |
| 厂商： | Fairchild Semiconductor Corporation |
| 英文描述： | GTLP Power Configuration |
| 中文描述： | GTLP电源配置 |
| 文件页数： | 4/5页 |
| 文件大小： | 1188K |
| 代理商： | AN-5031 |
www.fairchildsemi.com
4
A
Termination Summary
When is the V
T
source at fault for signal integrity issues
This is a difficult task since R
T
selection is often the largest
culprit relating to signal integrity problems. The first sign of
an inadequate termination is GTLP signals not reaching
the proper V
OH
level of 1.5V. It is true that R
T
can be at
fault here, but to further isolate the problem down to the ter-
mination supply, adjust the operating frequency to a very
low rate thus minimizing line reflections. At 1MHz, a signal
not reaching V
OH
is more than likely due to improper resis-
tor selection, for Thevenin applications, or current limiting.
Note what other events may be simultaneously occurring
as well. The transmission of a word consisting of 16 LOWs
on the same bus but through a different device may be
where all the power is distributed. If a required power level
cannot be met by a supply it will sacrifice voltage levels to
meet current demands. Pay attention to current specifica-
tions on supplies and note at what voltage they are speci-
fied.
Insufficient capacitor selection for V
T
can also affect signal
integrity. This is noticed on the falling edge of GTLP sig-
nals, which is when the current is sourced from the termi-
nation supply. Adjusting the value of the capacitor(s) on the
supply will result in minor edge rate changes and even
affect the V
OL
and V
OH
levels. After calculating one capaci-
tor or set of capacitors, venture to each side of the spec-
trum and observe the results in signal integrity. Parasitics
and underestimated surge currents can exceed the vari-
ables used in calculations.
Variable Threshold Voltage V
REF
Capable of adjusting the threshold level at which a GTLP
edge results in a TTL edge when operating in the BA direc-
tion, V
REF
has the smallest power requirement of the vari-
ous GTLP voltage levels. Requiring less than 1
μ
A, a
simple voltage divider circuit is recommended. Sizing the
resistors accordingly for this consistent, small current will
limit the amount of power constantly drawn to ground.
There is a trade-off between power and noise immunity.
Although larger value resistors will require less power,
lower impedances will better dampen noise spikes. Using a
linear regulator for V
REF
would be functional but it is over
design. Although V
REF
requires such a small current, do
not underestimate the importance of the voltage.
Datasheet specifications recommend
±
2% control on V
REF
,
a normally 1V source. Any shifts on this voltage level at the
input pin to the GTLP device can introduce unwanted skew
and possibly cause false glitches if the shifts are excessive.
Sufficient bypass capacitors placed near the device on the
V
REF
line can help a great deal in eliminating false glitches.
Since adjusting V
REF
can improve noise margin and TTL
output duty cycle, it is common practice to implement a
potentiometer for minor tweaking in an engineering model.
High accuracy discrete resistors are typically used in the
final model. If any transceiver is dedicated as a driver only,
it is not necessary to connect the V
REF
pin to the1V supply.
A connection to ground will suffice for this application. The
V
REF
pin is only required for the receiving function of GTLP
devices.
While adjusting V
REF
, focus should be on the following
parameters:
1. TTL Output Duty Cycle
On a non-inverting GTLP device, TTL output duty cycle
will increase as the V
REF
level is decreased. Select a
level that provides the optimum duty cycle for system
timing.
2. Signal Integrity
While adjusting V
REF
will not improve the shape of sig-
nal edges, it can be adjusted to shift the threshold level
above or below particularly degraded sections. If the
GTLP signal passed through the threshold region
twice, i.e. fell below 1V and experienced a slight reflec-
tion bringing the signal back up to 1.01V then continu-
ing to fall, a glitch would be evident in the TTL output. A
minor V
REF
adjustment to 1.05V could be a simple fix.
3. Noise Margin Associated with V
REF
Adjust V
REF
such that upper and lower noise margin,
i.e., the distance from V
REF
to V
OH
and V
OL
, are equal.
This adjustment better prepares the bus for an unantic-
ipated EMI or noise event.
After adjusting V
REF
, it is important to observe the changes
in all possible configurations so that the backplane can
eliminate undesired side effects. Difficulty can sometimes
arise when V
REF
conflicts occur between various card con-
figurations. V
REF
is typically set at one level but adjusting it
through a dynamic control that can sense backplane load-
ing during operation is possible.
Capacitor Selection
Various types and configurations of capacitors are avail-
able and necessary in all backplane designs. In any appli-
cation, they are used to apply control through filtering or as
local energy storage devices. On the power node, such as
V
CC
, the bypass capacitor is used to maintain a set voltage
bias for the device during current transients. While current
transients are not HIGH on the V
CC
pins, the voltage they
condition is crucial to operation of the chip. A V
CC
drop
below 3V for most 3.3V devices degrades the delays for
the device to specifications outside those guaranteed in the
datasheet. Capacitors selected to protect a voltage drop of
300mV are not the only decoupling to be designed in. High
frequency noise on the V
CC
line should also be filtered as
best as possible.
The largest current transients on any GTLP backplane will
occur at the V
T
node, as discussed earlier. At the V
T
node
there is no crucial level that must be maintained. Each
application will have different noise margin requirements
that dictate the V
T
level. It is common to maintain V
T
within
100mV, since noise margins are often greater than 200mV.
In either of these cases, to include V
REF
decoupling,
selecting the value and number of capacitors is an art that
varies between applications. Items to consider include:
supply impedance, allowable voltage delta, trace size, lead
inductance, equivalent series resistance, resonant fre-
quency, etc. Cookbook equations can be used as a starting
point but text references should be consulted prior to final
design. The purpose of this application note is to present
the various power requirements and configurations of a
backplane, which will aide in the selection of decoupling
capacitors.
相关PDF资料 |
PDF描述 |
|---|---|
| AN-5058 | Family Frequently Asked Questions (FAQs) |
| AN-5061 | Layout Guidelines |
| AN-53 | Implementing an RC5051 DC-DC Converter on Pentium II Motherboards |
| AN-6003 | Shoot-through in Synchronous Buck Converters |
| AN-6006 | LJT 19C 19#20 PIN WALL RECP |
相关代理商/技术参数 |
参数描述 |
|---|---|
| AN503-10-26 | 制造商: 功能描述: 制造商:undefined 功能描述: |
| AN5033 | 制造商:未知厂家 制造商全称:未知厂家 功能描述:TV TUNING CONTROL CIRCUIT |
| AN5036 | 制造商:未知厂家 制造商全称:未知厂家 功能描述:TV Channel Selector Circuit |
| AN503-6-4 | 制造商:AN# - MILITARY 功能描述: |
| AN503-6-6 | 制造商:AN# - MILITARY 功能描述: 制造商: 功能描述: 制造商:undefined 功能描述: |
发布紧急采购，3分钟左右您将得到回复。