maximum capacitance of 4 pF, which will have little effect on
the response of the video system due to the low impedance
of the input video.
Functional Description (Continued)
LM1267, giving a high impedance at the output pin. After bits
0 and 1 of register 9 are set to a 1, the power supplies to the
CRT driver and CRT can be turned off.
The ESD cells of the LM1267 also provide good protection
against arc-over, however good PCB layout is necessary.
The LM1267 should not be exposed directly to the voltages
that may occur during arc-over. The main vulnerability of the
LM1267 to arc-over is though the ground traces on the PCB.
For proper protection all ground connections associated with
the LM1267, 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 LM1267
grounds. Figure 10, which shows the demo board layout, is
an excellent example of an effective ground plane. The list
below should be followed to ensure a PCB with good
grounding:
Note: The 5V supply must remain on for proper operation. Since the LM1267
is a CMOS device its power consumption will be minimal.
External DACs: Four DACs with external outputs are pro-
vided in the LM1267. Normally these DACs will be used for
color balance and brightness control. If the brightness con-
trol is done at G1, then three DACs would be used for color
balance and the last DAC would be used for controlling the
G1 voltage.
There is also a provision to set the brightness at the cath-
odes. DAC 4 can be set to vary the outputs of the other three
DACs after the color balance is completed. This is accom-
plished by adding the output of DAC 4 to the other 3 DACs.
Bits 3 and 4 of register 9 are set to a 1 for brightness control
at the cathodes. Bit 3 sets the output range of DAC 1–3 to
50% of their full range. Bit 4 adds 50% of DAC 4 to the other
three DACs. These two adjustments keeps the overall output
voltage of DAC 1–3 in the proper range and still allows
brightness control. For either mode of brightness control, the
DACs are ideally set to work with the LM2479 or LM2480 for
DC restoration at the cathodes of the CRT.
•
•
•
All grounds associated with the LM1267 should be 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
the chassis or main ground. There is no arc-over current
flow from these grounds through the LM1267 grounds.
•
•
Input signal traces for SDA, SCL, H Flyback, and Clamp
should be kept away from the CRT driver and all traces
that could carry the arc current.
ESD and Arc-Over Protection
Output signal traces of the LM1267 should be kept away
from the traces that carry the output signals of the CRT
driver.
The LM1267 incorporates full ESD protection with special
consideration given to maximizing arc-over robustness. The
monitor designer must still use good circuit design and PCB
layout techniques. The human body model ESD susceptibil-
ity of the LM1267 is 3.5 kV, however many monitor manu-
facturers are now testing their monitors to the level 4 of the
IEC 801-2 specification which requires the monitor to survive
an 8 kV discharge. External ESD protection is needed to
survive this level of ESD. The LM1267 provides excellent
protection against both ESD and arc-over, but this is not a
substitute for good PCB layout.
If any one of the above suggestions is not followed the
LM1267 may become more vulnerable to arc-over. Improper
grounding is by far the most common cause of video pre-
amp failure during arc-over.
Figure 7 shows the recommended input protection for the
LM1267. This provides the best protection against ESD.
When this protection is combined with good PCB layout the
LM1267 will easily survive the IEC 801-2 level 4 testing (8 kV
ESD). It is strongly recommended that the protection diodes
be added as shown in Figure 7. The 1N4148 diode has a
DS200050-29
FIGURE 7. Recommended Video Input ESD Protection
13
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