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LM2433 Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM2433 Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 13 page Application Hints (Continued) EFFECT OF OFFSET Figure 7 shows the variation in rise and fall times when the DC offset of the 110V PP output swing is varied between 120V and 150V DC. The rise time and fall time show a maxi- mum variation of about 6% relative to the center data point (135V DC), which is a relatively small variation in speed over the 30V DC offset range. THERMAL CONSIDERATIONS Figure 8 shows the performance of the LM2433 as a function of case temperature. The figure shows that the rise and fall times of the LM2433 increase by approximately 4.5% and 6.5%, respectively, as the case temperature increases from 40˚C to 90˚C. This corresponds to a speed degradation of about 0.9% and 1.3% for every 10˚C rise in case tempera- ture, which is very stable performance over the temperature range. POWER DISSIPATION AND HEATSINK CALCULATION Worst-Case Power Dissipation Figure 9 shows the maximum power dissipation of the LM2433 vs. square wave frequency when the device uses V CC of 220V and is driving a 10 pF load with 110VPP swing alternating one pixel on, one pixel off signal. Note that the frequency range shown in the power dissipation figure is one-half the actual pixel frequency. The graph assumes 80% active time (device operating at the specified frequency), which is typical in an EDTV application. The other 20% of the time the device is assumed to be sitting at the black level (190V in this case). Under this worst-case condition, the maximum power dissipated by the LM2433 is about 6.8W at around 40 MHz. It is important to note that this power dissi- pation is a result of a high frequency square wave input, which is unrealistic in practical TV applications. The band- width of the input source used to drive the LM2433 was over 300 MHz. Using a RGB video processor or preamplifier with less bandwidth will cause the LM2433 to dissipate less power than shown in Figure 9 at the same conditions. A Practical Approach to Power Dissipation The power curve (Figure 9) mentioned previously shows the LM2433 power dissipation for square wave frequencies ranging from 1 to 50 MHz at 110V PP swing. In practice, it is uncommon for a TV to display average frequency content over the entire picture exceeding 20 MHz. Therefore, it is important to establish the worst-case picture condition under normal viewing to give a realistic maximum power dissipa- tion for the LM2433. Here is one approach: An EDTV signal generator pattern that yields a practical worst-case picture condition is a “multi-burst” pattern that consists of a 1-to-30 MHz sine wave sweep over each of the active lines. The power dissipated by the LM2433 as a result of this picture condition can be approximated by taking the average of the power between 1 to 30 MHz in Figure 9. This average is 5.1W. Because a square wave input was used to generate this power curve, a sine wave would cause the LM2433 to dissipate slightly less power, say 5.0W. This is one common way to determine a practical figure for maxi- mum power dissipation. It is the system designer’s respon- sibility to establish the worst-case picture condition for his particular application and measure dissipation under that condition to choose a proper heatsink. Heatsink Calculation Example Once the maximum dissipation is known, Figure 10 can be used to determine the heatsink requirement for the LM2433. If the 1-to-30 MHz multi-burst test described previously is assumed to be worst-case picture condition that yields maxi- mum dissipation, then the LM2433 will dissipate about 5.0W. The power derating curve shows that the maximum allowed case temperature is 127.5˚C when 5.0W is dissipated. If the maximum expected ambient temperature is 65˚C, then the maximum thermal resistance from device case-to-air ( θ CA) can be calculated: θ CA =(TCMAX –TAMAX)/PDMAX = θCS + θSA θ CA = (127.5˚C – 65˚C) / 5.0W = 12.5˚C/W. θ CS is the thermal resistance of the thermal compound at the case-to-heatsink interface and θ SA is the thermal resistance of the heatsink at the rated conditions. This example assumes a capacitive load of 10 pF and no resistive load. The designer should note that if the V CC supply voltage, output swing, input bandwidth, or load ca- pacitance is increased, then the power dissipation will also increase. 20146210 FIGURE 12. Recommended Application Circuit www.national.com 7 |
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