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LM2426 Datenblatt(PDF) 7 Page - National Semiconductor (TI) |
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LM2426 Datenblatt(HTML) 7 Page - National Semiconductor (TI) |
7 / 12 page Application Hints (Continued) POWER SUPPLY BYPASS Since the LM2426TE is a wide bandwidth amplifier, proper power supply bypassing is critical for optimum performance. Improper power supply bypassing can result in large over- shoot, ringing or oscillation. 0.1µF capacitors should be con- nected from the supply pins, V CC and VBB, to ground, as close to the LM2426TE as is practical. Additionally, a 22µF or larger electrolytic capacitor should be connected from both supply pins to ground reasonably close to the LM2426TE. ARC PROTECTION During normal CRT operation, internal arcing may occasion- ally occur. Spark gaps, in the range of 300V, connected from the CRT cathodes to CRT ground will limit the maximum voltage, but to a value that is much higher than allowable on the LM2426TE. This fast, high voltage, high energy pulse can damage the LM2426TE output stage. The application circuit shown in Figure 13 is designed to help clamp the voltage at the output of the LM2426TE to a safe level. The clamp diodes, D1 and D2, should have a fast transient response, high peak current rating, low series impedance and low shunt capacitance. 1SS83 or equivalent diodes are recommended. D1 and D2 should have short, low imped- ance connections to V CC and ground respectively. The cath- ode of D1 should be located very close to a separately decoupled bypass capacitor (C3 in Figure 13). The ground connection of D2 and the decoupling capacitor should be very close to the LM2426TE ground. This will significantly reduce the high frequency voltage transients that the LM2426TE would be subjected to during an arcover condi- tion. Resistor R2 limits the arcover current that is seen by the diodes while R1 limits the current into the LM2426TE as well as the voltage stress at the outputs of the device. R2 should be a 1⁄2W solid carbon type resistor. R1 can be a 1⁄4W metal or carbon film type resistor. Having large value resistors for R1 and R2 would be desirable, but this has the effect of increasing rise and fall times. Inductor L1 is critical to reduce the initial high frequency voltage levels that the LM2426TE would be subjected to. The inductor will not only help protect the device but it will also help minimize rise and fall times as well as minimize EMI. For proper arc protection, it is impor- tant to not omit any of the arc protection components shown in Figure 13. 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. EFFECT OF OFFSET Figure 8 shows the variation in rise and fall times when the output offset of the device is varied from 105 to 115V DC. The rise time shows a variation of less than 7% relative to the center data point (110V DC). The fall time shows a variation of less than 2% relative to the center data point. THERMAL CONSIDERATIONS Figure 9 shows the performance of the LM2426TE 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 LM2426TE increase by approximately 10% and 4%, respec- tively, as the case temperature increases from 50˚C to 90˚C. This corresponds to a speed degradation of 2.5% and 1% for every 10˚C rise in case temperature. Figure 10 shows the power dissipation of the LM2426TE vs. Frequency when all three channels of the device are driving an 8pF load with a 110V PP 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 LM2426TE for various video patterns in the 480i, 480p, 720p, and 1080i video formats. Figure 10, Figure 11, and Table 1 give the designer the information needed to determine the heatsink requirement for the LM2426TE. For example, if an HDTV application uses the 720p 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 14.4W (from Table 1). Figure 11 shows that the maximum allowed case temperature is 108˚C when 14.4W is dissipated. If the maximum expected ambient temperature is 70˚C, then a maximum heatsink thermal re- sistance can be calculated: 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: An LM126X preamplifier, with rise and fall times of about 2 ns, was used to drive the LM2426TE for these power measurements. Using a preamplifier with rise and fall times slower than the LM126X will cause the LM2426TE to dissi- pate 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. In- creasing 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 NSC application board. The values shown in Figure 14 and Figure 15 can be used as a good starting point for the evaluation of the LM2426TE. 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. 20066410 FIGURE 13. One Channel of the LM2426TE with the Recommended Application Circuit www.national.com 7 |
Ähnliche Teilenummer - LM2426 |
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Ähnliche Beschreibung - LM2426 |
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