OBSOLETE

LM2422TE

www.ti.com

SNOSAL1D – JANUARY 2005 – REVISED APRIL 2013

PC BOARD LAYOUT CONSIDERATIONS

For optimum performance, an adequate ground plane, isolation between channels, good supply bypassing and

minimizing unwanted feedback are necessary. Also, the length of the signal traces from the signal inputs to the

LM2422 and from the LM2422 to the CRT cathode should be as short as possible. The following references are

recommended:

Ott, Henry W., “Noise Reduction Techniques in Electronic Systems”, John Wiley & Sons, New York, 1976.

“Video Amplifier Design for Computer Monitors”, TI Application Note 1013.

Pease, Robert A., “Troubleshooting Analog Circuits”, Butterworth-Heinemann, 1991.

Because of its high small signal bandwidth, the part may oscillate in a TV if feedback occurs around the video

channel through the chassis wiring. To prevent this, leads to the video amplifier input circuit should be shielded,

and input circuit wiring should be spaced as far as possible from output circuit wiring.

TYPICAL APPLICATION

A typical application of the LM2422 is shown in Figure 14. Used in conjunction with a pre-amp with a 1.2V black

level output no buffer transistors are required to obtain the correct black level at the cathodes. If the pre-amp has

a black level closer to 2V, then an NPN transistor should be used to drop the video black level voltage closer to

1.2V.

The neck board in Figure 14 has two transistors in each channel enabling this board to work with pre-amps with

a black level output as high as 2.5V. Some popular AVPs do have a black level of 2.5V. For lower black levels

either one or both transistors would not be used.

It is important that the TV designer use component values for the driver output stage close to the values shown

in Figure 14. These values have been selected to protect the LM2422 from arc over. Diodes D1–D6 must also be

used for proper arc over protection. The TI demonstration board can be used to evaluate the LM2422 in a TV.

TI DEMONSTRATION BOARD

Figure 15 shows the routing and component placement on the TI LM2422 demonstration board. This board

provides a good example of a layout that can be used as a guide for future layouts. Note the location of the

following components:

•

•

•

The routing of the LM2422 outputs to the CRT is very critical to achieving optimum performance. Figure 16

the components are placed so that they almost line up from the output pin of the LM2422 to the blue cathode pin

of the CRT connector. This is done to minimize the length of the video path between these two components.

Note also that D1, D2 and R3 are placed to minimize the size of the video nodes that they are attached to. This

minimizes parasitic capacitance in the video path and also enhances the effectiveness of the protection diodes.

The anode of protection diode D2 is connected directly to a section of the ground plane that has a short and

to decoupling capacitor C5 which is connected to the same area of the ground trace as D2. The diode placement

and routing is very important for minimizing the voltage stress on the LM2422 during an arc over event.

This demonstration board uses large PCB holes to accommodate socket pins, which function to allow for multiple

insertions of the LM2422 in a convenient manner. To benefit from the enhanced LM2422 package with thin

leads, the device should be secured in small PCB holes to optimize the metal-to-metal spacing between the

leads.

Copyright © 2005–2013, Texas Instruments Incorporated

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