THERMAL CONSIDERATIONS

LM2419’s transfer characteristic and power dissipation ver-

sus DC input voltage is shown in

Figure 5 Power supply

current increases as the input voltage increases conse-

quently power dissipation increases For the LM2419 the

worst case power dissipation occurs when a white screen is

displayed on the CRT Considering a 20% black retrace

time in a 1024 x 768 display resolution application the aver-

age power dissipation for continuous white screen is less

at 75V and white level at 25V) Although the total power

dissipation is less than 12W for a continuous white screen

the heat sink should be selected for 13W power dissipation

because of the variation in power dissipation from part to

part

For thermal and gain linearity considerations the output low

voltage (white level) should be maintained above 20V If the

device is operated at an output low voltage below 20V the

power dissipation might exceed 47W per channel (ie 14W

power dissipation for the device) Note that the device can

be operated at lower power by reducing the peak to peak

video output voltage to less than 50V and clamping the vid-

eo black level close to the supply voltage

TLH11442 – 10

The LM2419 requires that the package be properly heat

sunk under all operating conditions

Maximum ratings re-

quire that the device case temperature be limited to 90 C

maximum Thus for 50 C maximum ambient temperature

and 13W maximum power dissipation the thermal resist-

ance of the heat sink should be

SHORT CIRCUIT PROTECTION

The output of LM2419 is not short circuit protected Short-

device The minimum DC load resistance the LM2419 can

driving a 16 kX load for an extended period of time is not

recommended because of power dissipation considera-

tions If the LM2419 is used to drive a resistive load then the

load should be 10 kX or greater

RGB Video Application

A complete video section for an RGB CRT monitor is shown

in

Figure 6 The LM1203 video preamplifier and the LM2419

include almost all the circuitry required between the video

input connection and the CRT’s cathodes However an ex-

ternally generated back porch clamp signal is required to

accomplish DC restoration of the video signal

Figure 6 ’s circuit is excellent choice for a non-interlaced

put swing and 12 pF load the risefall time for

Figure 6 ’s

circuit was measured at 75 ns In this application feedback

is local to the LM1203 For detailed information on the

LM1203 please refer to the LM1203 data sheet

PC BOARD LAYOUT CONSIDERATIONS

For optimum performance adequate ground plane isolation

between channels good supply bypassing and minimizing

unwanted feedback are necessary Moreover the length of

the signal trace from the preamplifier to the LM2419 and

from the LM2419 to the cathode should be as short as is

practical The following book is highly recommended

Ott Henry W

Noise Reduction Techniques in Electronic

Systems John Wiley

Sons New York 1976

5