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LM2432 bảng dữ liệu(PDF) 7 Page - National Semiconductor (TI) |
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LM2432 bảng dữ liệu(HTML) 7 Page - National Semiconductor (TI) |
7 / 13 page Application Hints (Continued) EFFECT OF LOAD CAPACITANCE Figure 7 shows the effect of increased load capacitance on the speed of the device. The rise and fall time increase by 7% and 7.5%, respectively, per additional pF above 10 pF. EFFECT OF OFFSET Figure 8 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 shows a variation of less than 10% relative to the center data point (135V DC). The fall time shows a variation of 14% relative to the center data point. THERMAL CONSIDERATIONS Figure 9 shows the performance of the LM2432 as a function of case temperature. The figure shows that the rise and fall times of the LM2432 increase by approximately 17% and 20%, respectively, as the case temperature increases from 40˚C to 90˚C. This corresponds to a speed degradation of only 3.5% and 4.0% for every 10˚C rise in case temperature. POWER DISSIPATION AND HEATSINK CALCULATION Worst-Case Power Dissipation Figure 10 shows the maximum power dissipation of the LM2432 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 a HDTV application. The other 20% of the time the device is assumed to be sitting at the black level (190V in this case). Under these worst-case condition, the maximum power dissipated by the LM2432 is about 8.9W at around 30 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 LM2432 was over 300 MHz. Using a RGB video processor or preamplifier with less bandwidth may cause the LM2432 to dissipate less power than shown in Figure 10 at the same conditions. A Practical Approach to Power Dissipation The power curve (Figure 10) mentioned previously shows the LM2432 power dissipation for square wave frequencies ranging from 1 to 50 MHz at 110V PP swing. In practice, it is unrealistic 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 LM2432. Here is one approach: A HDTV 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 LM2432 as a result of this picture condition can be approximated by taking the average of the power between 1 to 30 MHz in Figure 10. This average is 7W. Because a square wave input was used to generate this power curve, a sine wave would cause the LM2432 to dissipate slightly less power, probably about 6.7W. This is one common way to determine a practical figure for maximum power dissipation. It is the system de- signer’s responsibility to establish the worst-case picture condition for his particular application and measure dissipa- tion under that condition to choose a proper heatsink. Heatsink Calculation Example Once the maximum dissipation is known, Figure 11 can be used to determine the heatsink requirement for the LM2432. 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 LM2432 will dissipate about 6.7W. The power derating curve shows that the maximum allowed case temperature is 120˚C when 6.7W is dissipated. If the maximum expected ambient temperature is 65˚C, then the maximum thermal resistance from device case-to-sink ( θ CS) can be calculated: θ CS =(TCMAX –TAMAX)/PDMAX θ CS = (120˚C – 65˚C) / 6.7W = 8.2˚C/W This example assumes a capacitive load of 10 pF and no resistive load. The designer should note that if output swing, V CC supply voltage, input bandwidth, or load capacitance is increased, then the AC component of the total power dissi- pation will also increase. 20137710 FIGURE 13. Recommended Application Circuit www.national.com 7 |
Số phần tương tự - LM2432 |
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Mô tả tương tự - LM2432 |
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