OBSOLETE

LM2402

www.ti.com

SNOS445B – AUGUST 1999 – REVISED APRIL 2013

Figure 7 shows the total power dissipation of the LM2402 vs. Frequency when all three channels of the device

are driving both an 8 pF load and a 20 pF load. This graph gives the designer the information needed to

determine the heat sink requirement for his application. The designer should note that if the load capacitance is

increased the AC component of the total power dissipation will also increase as shown in Figure 7. The designer

should also remember that the actual video signal has a period of around 70% to 75%. The remainder of the

time the video signal is inactive, or at the black level (below the black level if blanked). During this time the

LM2402 will be at the black level, or below, dissipating under 4W. Referring to Figure 15 and using an input

black level voltage of 1.9V, the power dissipation during the inactive video time is 3.8W, including both the 80V

and 12V supplies.

The LM2402 case temperature must be maintained below 100°C. Assume the worst case operating condition is

a 100 MHz square wave during active video (a pixel clock of 200 MHz with one pixel on, one pixel off). From

Figure 7 one can see that the power dissipation of the LM2402 is 28W if the 100 MHz square wave is applied all

the time. One must also compensate for the inactive period of video. From Figure 15 it has been calculated that

the power dissipation during the inactive video is 4W. Therefore there is an additional 24W of power dissipation

due to the AC signal. Assume that the AC signal is active 72% of the time. Now the AC power dissipation is:

24W × 0.72 = 17W

(1)

The total power dissipation for 72% active video time is:

17W + 4W = 21W

(2)

If the maximum expected ambient temperature is 50°C and using the maximum power dissipation of 21W (video

being active only 72% of the frame), then a maximum heat sink thermal resistance can be calculated:

(3)

This example assumes a capacitive load of 8 pF and no resistive load.

Typical Application

A typical application of the LM2402 is shown in Figure 11. Used in conjunction with three LM2202s, a complete

video channel from monitor input to CRT cathode can be achieved. Performance is excellent for resolutions up to

1600 × 1200 and pixel clock frequencies at 200 MHz. Figure 11 is the schematic for the TI demonstration board

that can be used to evaluate the LM2202/2402 combination in a monitor.

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 preamplifier to the

LM2402 and from the LM2402 to the CRT cathode should be as short as possible. The red video trace from the

buffer transistor to the LM2402 input is about the absolute maximum length one should consider on a PCB

layout. If possible the traces should actually be shorter than the red video trace. The following references are

recommended for video board designers:

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

“Guide to CRT Video Design”, Texas Instruments Application Note 861 (SNOA268).

“Video Amplifier Design for Computer Monitors”, Texas Instruments Application Note 1013 (SNVA031).

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

Because of its high small signal bandwidth, the part may oscillate in a monitor 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.

Copyright © 1999–2013, Texas Instruments Incorporated

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