LM1270NA datasheet(13/27 Pages) NSC

13 / 27 page

Emphasis (Continued)

Bits 0 to 3 in register 0C control the center frequency. When

an “8” is programmed into the register the t

Center Freq is

typically 80 ns. An “F” will give about 145 ns. A “0” will give no

emphasis in the Hi-Brite window.

ESD and Arc-Over Protection

The LM1270 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 LM1270 is 3.5 kV, however, many monitor manu-

facturers are now testing their monitors to level 4 of the IEC

801-2 specification which requires the monitor to survive an

8 kV discharge. External ESD protection is needed to sur-

vive this level of ESD. The LM1270 provides excellent pro-

tection against both ESD and arc-over, but this is not a

substitute for good PCB layout.

Figure 9 shows the recommended input protection for the

LM1270. This provides the best protection against ESD.

When this protection is combined with a good PCB layout

the LM1270 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 9. 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 LM1270 also provide good protection

against arc-over, however good PCB layout is necessary.

The LM1270 should not be exposed directly to the voltages

that may occur during arc-over. The main vulnerability of the

LM1270 to arc-over is through the ground traces on the

PCB. For proper protection all ground connections associ-

ated with the LM1270, 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

LM1270 grounds. Figure 12, which shows the demo board

layout, is an excellent example of an effective ground plane.

Figures 10, 11 are the schematic to the demo board. The list

below should be followed to ensure a PCB with good

grounding:

•

All grounds associated with the LM1270 should be con-

nected together through a large ground plane.

•

CRT driver ground is connected to the LM1270 and video

pre-amp ground area 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 LM1270 grounds.

•

Input signal traces for SDA, SCL, H Flyback, and Sync

inputs should be kept away from the CRT driver and all

traces that could carry the arc current.

•

Output signal traces of the LM1270 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

LM1270 may become more vulnerable to arc-over. Improper

grounding is by far the most common cause of a video

pre-amp failure during arc-over.

20063527

FIGURE 9. Recommended Video Input ESD Protection

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