EFFECT OF LOAD CAPACITANCE

The output rise and fall times will be slower than specified if

the load capacitance at the output is more than 8 pF as

shown in

Figure 8

TLH12683 – 9

FIGURE 8 Effect of Load Capacitance on

RiseFall Time

The monitor designer should ensure that stray capacitance

applied to the LM2408 is as low as possible

THERMAL CONSIDERATIONS

Power supply current increases as the input signal increas-

es and consequently power dissipation also increases

The LM2408 cannot be used without heat sinking Typical

‘‘average’’ power dissipation with the device output voltage

at one half the supply voltage is 19W per channel for a total

dissipation of 57W package dissipation The power dissipa-

tion does not vary much as output voltage varies The

LM2408 case temperature must be maintained below

100 C If the maximum expected ambient temperature is

50 C then a maximum heat sink thermal resistance can be

calculated

57W

e

88 CW

This example assumes a typical CRT capacitive load and is

without a resistive load Note that this thermal resistance

must be achieved when the heat sink is operating in the

monitor

PC BOARD LAYOUT CONSIDERATIONS

For optimum performance an adequate ground plane isola-

tion between channels good supply bypassing and minimiz-

ing unwanted feedback are necessary Also the length of

the signal traces from the preamplifier to the LM2408 and

from the LM2408 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 and Sons New York 1976

‘‘Guide to CRT Video Design’’ National Semiconductor Ap-

plication Note 861

‘‘Video Amplifier Design for Computer Monitors’’ National

Semiconductor Application Note 1013

Because of its high small signal bandwidth the part may

oscillate when it is used in a typical application with a

preamp in a monitor if feedback occurs around the video

amplifier through the chassis wiring To prevent this leads

to the input circuit should be shielded and input circuit wir-

ing should be spaced as far as possible from output circuit

wiring Power should be removed as quickly as possible

from an amplifier that is oscillating since power dissipation

in the part is very high in this mode and the part may be

damaged if oscillations continue and the power supply can

supply more than 250 mA

Capacitive loading on the output will cause some overshoot

and peaking This can be controlled by placing a resistor in

series with the output of the part Because of differences in

stray capacitance in different pc board layouts the best val-

ue of resistance to use must be determined separately for

each application Typical values between 50X and 200X

provide good performance with the larger values resulting

in less peaking and slower rise and fall times

Driving the output voltage of the part outside of its linear

range will cause distorted signal waveforms and recovery

times that are very much longer than the specified rise and

fall times When the amplifier output voltage is being driven

from positive saturation into the linear range an overshoot

of several volts for up to 50 ns may occur In a typical moni-

tor design this may occur if blanking pulses are applied to

the video signal The output voltage range should be limited

so this does not happen and will be approximately no lower

TYPICAL APPLICATION

A typical application of the LM2408 is shown in

Figure 9

Used in conjunction with an LM1205 a complete video

channel from monitor input to CRT cathode can be

achieved Performance is satisfactory for all applications up

up to 160 MHz

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