Mitsubishi DPLUS 74SB -BKA Service Manual page 143

ance and brightness control. If the brightness control 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 cathodes.
DAC 4 can be set to vary the outputs of the other three DACs
after the color balance is completed. This is accomplished 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.
ESD and Arc-Over Protection
The LM1269 incorporates full ESD protection with special con-
sideration given to maximizing arc-over robustness. The moni-
tor designer must still use good circuit design and PCB layout
techniques. The human body model ESD susceptibility of the
LM1269 is 3.5 kV, however many monitor manufacturers are
now testing their monitors to the level 4 of the IEC 801-2 specifi-
cation which requires the monitor to survive an 8kV discharge.
External ESD protection is needed to survive this level of ESD.
The LM1269 provides excellent protection against both ESD
and arc-over, but this is not a substitute for good PCB layout.
Figure 7 shows the recommended input protection for the LM1269.
This provides the best protection against ESD. When this protec-
tion is combined with good PCB layout the LM1269 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 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.
The ESD cells of the LM1269 also provide good protection
against arc-over, however good PCB layout is necessary. The
LM1269 should not be exposed directly to the voltages that
may occur during arc-over. The main vulnerability of the LM1269
to arc-over is though the ground traces on the PCB. For proper
protection all ground connections associated with the LM1269,
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 LM1269 grounds 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:
All grounds associated with the LM1269 should be con-
nected together through a large ground plane.
CRT driver ground is connected to the video pre-amp group
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 LM1269 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.
Output signal traces of the LM1269 should be kept away from
the traces that carry the output signals of the CRT driver. If any
one of the above suggestions is not followed the LM1269 may
become more vulnerable to arc-over. Improper grounding is by
far the most common cause of video pre-amp failure during
arc-over.
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