Figure 10 shows the maximum power dissipation of the

LM2422 vs. Frequency when all three channels of the device

are driving into a 10 pF load with a 110V

pixel on, one pixel off. Note that the frequency given in

Figure 10 is half of the pixel frequency. The graph assumes

a 72% active time (device operating at the specified fre-

quency), which is typical in a TV application. The other 28%

of the time the device is assumed to be sitting at the black

level (190V in this case). A TV picture will not have frequency

content over the whole picture exceeding 20 MHz. It is

important to establish the worst case condition under normal

viewing to give a realistic worst-case power dissipation for

the LM2422. One test isa1to30MHz sine wave sweep

over the active line. This would give a slightly lower power

than taking the average of the power between 1 and 30 MHz.

This average is 23.5 W. A sine wave will dissipate slightly

less power, probably about 21 W or 22 W of power dissipa-

tion. All of this information is critical for the designer to

establish the heat sink requirement for his application. The

designer should note that if the load capacitance is in-

creased the AC component of the total power dissipation will

also increase.

The LM2422 case temperature must be maintained below

110˚C given the maximum power dissipation estimate of

22W. If the maximum expected ambient temperature is 60˚C

and the maximum power dissipation is 22W then a maximum

heat sink thermal resistance can be calculated:

This example assumes a capacitive load of 10 pF and no

resistive load. The designer should note that if the load

capacitance is increased the AC component of the total

power dissipation will also increase.

OPTIMIZING TRANSIENT RESPONSE

Referring to Figure 13, there are three components (R1, R2

and L1) that can be adjusted to optimize the transient re-

sponse of the application circuit. Increasing the values of R1

and R2 will slow the circuit down while decreasing over-

shoot. Increasing the value of L1 will speed up the circuit as

well as increase overshoot. It is very important to use induc-

tors with very high self-resonant frequencies, preferably

above 300 MHz. Ferrite core inductors from J.W. Miller

Magnetics (part # 78FR--K) were used for optimizing the

performance of the device in the NSC application board. The

values shown in Figure 13 can be used as a good starting

point for the evaluation of the LM2422. Using a variable

resistor for R1 will simplify finding the value needed for

optimum performance in a given application. Once the opti-

mum value is determined the variable resistor can be re-

placed with a fixed value. Due to arc over considerations it is

recommended that the values shown in Figure 13 not be

changed by a large amount.

Figure 12 shows the typical cathode pulse response with an

output swing of 110V

using the LM1237 pre-amp.

PC BOARD LAYOUT CONSIDERATIONS

For optimum performance, an adequate ground plane, iso-

lation between channels, good supply bypassing and mini-

mizing 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”, National

Semiconductor 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 NSC demonstration board can be

used to evaluate the LM2422 in a TV.

NSC DEMONSTRATION BOARD

Figure 15 shows the routing and component placement on

the NSC 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 compo-

nents:

•

C4—V

and ground pins

•

C6—V

ground

•

C5, C8 — V

clamp diodes. Very important for arc protection.

The routing of the LM2422 outputs to the CRT is very critical

to achieving optimum performance. Figure 16 shows the

routing and component placement from pin 10 (V

LM2422 to the blue cathode. Note that 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 pro-

tection diodes. The anode of protection diode D2 is con-

nected directly to a section of the ground plane that has a

short and direct path to the heater ground and the LM2422

ground pins. The cathode of D1 is connected to V

close to decoupling capacitor C5 which is connected to the

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