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LM1208 データシート(PDF) 9 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM1208
部品情報  130 MHz/85 MHz RGB Video Amplifier System with Blanking
Download  23 Pages
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メーカー  NSC [National Semiconductor (TI)]
ホームページ  http://www.national.com
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LM1208 データシート(HTML) 9 Page - National Semiconductor (TI)

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ESD and Arc-Over Protection
The ESD cells of the LM1208 are improved over the original
LM1205/LM1207 parts. The monitor designer must still use
good PC board layout techniques when designing with the
LM1208. The human body model ESD susceptibility of these
parts is rated at 3 kV (Note 4) . This is a 50% improvement
over the original parts. However, many monitor manufactur-
ers are now testing their monitors to the level 4 of the IEC
801-2 specification. This requires the inputs to the monitor to
survive an 8 kV discharge. If the monitor designer expects to
survive such levels he MUST provide external ESD protec-
tion to the video pre-amp inputs! PC board layout is just as
important with these new parts as with the original LM1205/
LM1207 parts. The advantage given by the LM1208 will
provide better protection against ESD and arc-over, but
these improved parts are not a substitute for good PCB
layout.
Figure 5 shows the recommended input protection for a
video pre-amp. The two diodes at the video pre-amp input
and after the 30
Ω series resistor offers the best protection
against ESD. When this protection is combined with a good
PCB layout, the video pre-amp will easily survive the IEC
801-2 level 4 (8 kV ESD) testing commonly done by monitor
manufacturers. If the protection diodes are moved to the
video input side of the 30
Ω resistor, then the ESD protection
will be less effective. There is also the risk of damaging the
diodes since there is no resistor for current limiting. In such
a design a heavier duty diode, such as the FDH400, should
be used. It is strongly recommended that the protection
diodes be added as shown in
Figure 5. The 1N4148 diode
has a maximum capacitance of 4 pF. This would have little
effect on the response of the video system due to the low
impedances of the input video.
Many monitor designers prefer to use a single zener diode
instead of the recommended two diodes at the video
pre-amp input. The required location of the zener diode is
shown in
Figure 5. It is shown as a dashed line, indicating an
alternative to the two diode solution. The zener diode does
give the savings of one component, but now the protection is
less effective. To minimize capacitance, the zener diode
needs to have a zener voltage of 24V or higher. This is well
above the V
CC voltage of the LM1208. The zener diode must
be located at the video input for protection against a low
voltage surge. The 30
Ω resistor is needed to limit the current
of such a voltage surge, protecting the video pre-amp. Pro-
tection against ESD by using a zener diode is about as
effective as having the two diodes at the video input (same
location as the zener diode). A higher series resistor may be
necessary for protection against the zener voltage, but this
may also have an effect on the system bandwidth.
The new ESD cells of the LM1208 also increases its toler-
ance against arc-over. Once again the monitor designer
must be careful in his PCB layout for good ESD protection. In
the video chain only the outputs of the CRT driver are
directly exposed to the voltages that may occur during
arc-over. A good PCB layout is the best protection for the
video pre-amp against arc-over. The pre-amp vulnerability is
mainly through the ground traces on the PCB. For proper
operation all ground connections associated with the video
pre-amp, including the grounds to the bypass capacitors,
must have short returns to the ground pins. A significant
ground plane should be used to connect all the pre-amp
grounds.
Figure 16, the demo board layout, is an excellent
example of an effective ground plane, yet using only a single
sided PCB layout. Here is a check list to make sure a PC
board layout has good grounding:
All associated grounds with the video pre-amp are con-
nected together through a large ground plane.
CRT driver ground is connected to the video pre-amp
ground at one point.
CRT and arc protection grounds are connected directly to
chassis, or the main ground. There is no arc-over current
flow from these grounds through the pre-amp or CRT
driver grounds.
If any one of the above suggestions are not followed, then
the LM1208 may become vulnerable to arc-overs. Improper
grounding is by far the most common cause of a video
pre-amp failure during arc-over.
DS100083-10
FIGURE 5. Recommended Video Input ESD Protection
www.national.com
9


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