LM1269, 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.

Note: The 5V supply must remain on for proper operation. Since the LM1269

is a CMOS device its power consumption will be minimal.

External DACs: Four DACs with external outputs are pro-

vided in the LM1269. 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.

ESD and Arc-Over Protection

The LM1269 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 LM1269 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 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 protection 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.

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:

•

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

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 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.

DS200099-29

FIGURE 7. Recommended Video Input ESD Protection

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