LM1207AN National Semiconductor, LM1207AN Datasheet - Page 10

• Manufacturer Part Number: LM1207AN
• Manufacturer: National Semiconductor

Specifications of LM1207AN

• Power Supply Requirement: Single
• Dual Supply Voltage (typ): Not RequiredV
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Mounting: Through Hole
• Lead Free Status / RoHS Status: Not Compliant

www.national.com
ESD and Arc-Over Protection
The ESD cells of the LM1205A/LM1207A are improved over
the original LM1205/LM1207 parts. The monitor designer
must still use good PC board layout techniques when de-
signing with the LM1205A/LM1207A. 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. How-
ever, many monitor manufacturers are now testing their
monitors to the level 4 of the IEC 801-2 specification. This re-
quires 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 protection 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 advan-
tage given by the LM1205A/LM1207A will provide better pro-
tection 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
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
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 di-
odes 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 imped-
ances 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
diode must be located at the video input for protection
against a low voltage surge. The 30
limit the current of such a voltage surge, protecting the video
pre-amp. Protection 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 LM1205A/LM1207A also in-
creases its tolerance against arc-over. Once again the moni-
tor 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 vulnerabil-
ity 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 capaci-
tors, 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
CC
voltage of the LM1205A/LM1207A. The zener
series resistor offers the best protection
resistor, then the ESD protection
resistor is needed to
10
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-
• CRT driver ground is connected to the video pre-amp
• CRT and arc protection grounds are connected directly to
If any one of the above suggestions are not followed, then
the
arc-overs. Improper grounding is by far the most common
cause of a video pre-amp failure during arc-over.
FIGURE 5. Recommended Video Input ESD Protection
nected together through a large ground plane.
ground at one point.
chassis, or the main ground. There is no arc-over current
flow from these grounds through the pre-amp or CRT
driver grounds.
LM1205A/LM1207A may
become
vulnerable
DS012866-30
to