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LM2422TE Scheda tecnica(PDF) 8 Page - Texas Instruments |
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LM2422TE Scheda tecnica(HTML) 8 Page - Texas Instruments |
8 / 15 page RTH = 110 oC - 60oC 22W = 2.3 oC/W OBSOLETE LM2422TE SNOSAL1D – JANUARY 2005 – REVISED APRIL 2013 www.ti.com EFFECT OF LOAD CAPACITANCE Figure 7 shows the effect of increased load capacitance on the speed of the device. This demonstrates the importance of knowing the load capacitance in the application. Increasing the load capacitance from 10 pF to 20 pF adds about 4.5 ns to the rise time and 3.5 ns to the fall time. It is important to keep the board capacitance as low as possible to maximize the speed of the driver. EFFECT OF OFFSET Figure 8 shows the variation in rise and fall times when the output offset of the device is varied from 120V to 130VDC. Offset has little effect on the LM2422. The rise time increases less than 0.5 ns as the offset is increased in voltage and the fall time decreases by about 0.5 ns with the same offset adjustment. THERMAL CONSIDERATIONS Figure 9 shows the performance of the LM2422 in the test circuit shown in Figure 3 as a function of case temperature. The figure shows that the rise time of the LM2422 increases by about 2ns as the case temperature increases from 30°C to 110°C. Over the same case temperature range the fall time increased by about 2.5 ns. Figure 10 shows the maximum power dissipation of the LM2422 vs. Frequency when all three channels of the device are driving into a 10 pF load with a 110VP-P alternating one pixel on, one pixel off. Note that the frequency given in Figure 10 is half of the pixel frequency. 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 (190V in this case). A TV picture will not have frequency content over the whole picture exceeding 20 MHz. It is important to establish the worst case condition under normal viewing to give a realistic worst-case power dissipation for the LM2422. One test is a 1 to 30 MHz sine wave sweep over the active line. This would give a slightly lower power than taking the average of the power between 1 and 30 MHz. This average is 23.5 W. A sine wave will dissipate slightly less power, probably about 21 W or 22 W of power dissipation. All of this information is critical for the designer to establish the heat sink requirement for his application. The designer should note that if the load capacitance is increased the AC component of the total power dissipation will also increase. The LM2422 case temperature must be maintained below 110°C given the maximum power dissipation estimate of 22W. If the maximum expected ambient temperature is 60°C and the maximum power dissipation is 22W then a maximum heat sink thermal resistance can be calculated: (1) This example assumes a capacitive load of 10 pF 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. OPTIMIZING TRANSIENT RESPONSE Referring to 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 13 can be used as a good starting point for the evaluation of the LM2422. 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. Due to arc over considerations it is recommended that the values shown in Figure 13 not be changed by a large amount. Figure 12 shows the typical cathode pulse response with an output swing of 110VPP inside a modified production TV set using the LM1237 pre-amp. 8 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated Product Folder Links: LM2422TE |
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