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LM2429 Datenblatt(PDF) 9 Page - Texas Instruments |
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LM2429 Datenblatt(HTML) 9 Page - Texas Instruments |
9 / 15 page RTH = 117°C - 70°C = 4.1°C 11.4W OBSOLETE LM2429 www.ti.com SNOSA76D – JULY 2003 – REVISED APRIL 2013 THERMAL CONSIDERATIONS Figure 9 shows the performance of the LM2429 in the test circuit shown in Figure 3 as a function of case temperature. The figure shows that the rise and fall times of the LM2429 increase by approximately 15% and 30%, respectively, as the case temperature increases from 50°C to 90°C. This corresponds to a speed degradation of 3.75% and 7.5% for every 10°C rise in case temperature. Figure 10 shows the power dissipation of the LM2429 vs. Frequency when all three channels of the device are driving an 8pF load with a 130VPP alternating one pixel on, one pixel off signal. The graph assumes a 72% active time (device operating at the specified frequency) which is typical in a TV application. The other 28% of the time the device is assumed to be sitting at the black level (165V in this case). Table 1 also shows the typical power dissipation of the LM2429 for various video patterns in the 480i and 480p video formats. Figure 10, Figure 11, and Table 1 give the designer the information needed to determine the heatsink requirement for the LM2429. For example, if an HDTV application uses the 480p format and "Vertical Lines 2 On 2 Off" is assumed to be the worst-case pattern to be displayed, then the power dissipated will be 11.4W (from Table 1). Figure 11 shows that the maximum allowed case temperature is 117°C when 11.4W is dissipated. If the maximum expected ambient temperature is 70°C, then a maximum heatsink thermal resistance can be calculated: (1) This example assumes a capacitive load of 8pF 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. NOTE A LM126X preamplifier, with rise and fall times of about 2 ns, was used to drive the LM2429 for these power measurements. Using a preamplifier with rise and fall times slower than the LM126X will cause the LM2429 to dissipate less power than shown in Table 1. OPTIMIZING TRANSIENT RESPONSE In Figure 13, there are three components (R1, R2 and L1) that can be adjusted to optimize the transient response of the application circuit. Increasing the values of R1 and R2 will slow the circuit down while decreasing overshoot. Increasing the value of L1 will speed up the circuit as well as increase overshoot. It is very important to use inductors 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 TI application board. The values shown in Figure 14 and Figure 15 can be used as a good starting point for the evaluation of the LM2429. Using a variable resistor for R1 will simplify finding the value needed for optimum performance in a given application. Once the optimum value is determined, the variable resistor can be replaced with a fixed value. Figure 12 shows the typical cathode pulse response with an output swing of 130VPP using a LM1269 preamplifier. PC BOARD LAYOUT CONSIDERATIONS For optimum performance, an adequate ground plane, isolation between channels, good supply bypassing and minimizing unwanted feedback are necessary. Also, the length of the signal traces from the preamplifier to the LM2429 and from the LM2429 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”, Texas Instruments 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. Copyright © 2003–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM2429 |
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