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LM2481 Fiches technique(PDF) 3 Page - National Semiconductor (TI) |
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LM2481 Fiches technique(HTML) 3 Page - National Semiconductor (TI) |
3 / 8 page Typical Performance Characteristics (V CC = +145V, VBB = +8V, VIN = +2.5V, Test Circuit - Figure 2) Theory of Operation The simplified circuit diagram of the LM2481 is shown in Figure 1. The DC clamp circuit amplifies the input signal by -20 and the gain is set by the resistor ratio of 20R and R. The output will require a pull-up resistor to 145V. Figure 2 shows the test circuit for evaluation of the LM2481 Clamp Amplifier. A high impedance voltmeter (>100M Ω)is used for DC measurements at the output. Application Hints INTRODUCTION National Semiconductor (NSC) is committed to provide ap- plication information that assists our customers in obtaining the best performance possible from our products. The fol- lowing information is provided in order to support this com- mitment. The reader should be aware that the optimization of performance was done using a specific printed circuit board designed at NSC. Variations in performance can be realized due to physical changes in the printed circuit board and the application. Therefore, the designer should know that com- ponent value changes may be required in order to optimize performance in a given application. The values shown in this document can be used as a starting point for evaluation purposes. POWER SUPPLY BYPASS The LM2481 should have proper power supply bypassing for optimum performance. A 0.1 µF capacitor should be con- nected from the supply pins, VCC and VBB, to ground, as close to the supply and ground pins as is practical. Addition- ally, a 1 µF electrolytic capacitor should be connected from the supply pins to ground. The electrolytic capacitor should also be placed reasonably close to the LM2481’s supply and ground pins. ARC PROTECTION During normal CRT operation, internal arcing may occasion- ally occur. To protect the LM2481 against arcing the follow- ing steps should be done (See Figure 7): • The traces connecting the LM126X preamp DACs and the LM2481 input pins (pins 1, 13, and 14) should be run around the top of the LM2421. They should not go through the output circuit of the LM2421. • C43, C44, and C45 should be located close to the LM126X preamp. • R44, R45, and R46 should be located close to the LM2481. • C21, C24, and C25 on the output pins of the LM2481 (pins 6, 8, and 9) should be located close to the LM2481. The grounds of these capacitors should have a short direct return to GND (pins 3, 4, 5, 10, 11, and 12) of the LM2481. • C9 and C36 on the supply lines (pins 2 and 7) should be located close to the LM2481. The grounds of these ca- pacitors should have a short direct return to GND (pins 3, 4, 5, 10, 11, and 12) of the LM2481. • The ground of the LM2481 should have a short direct connection to the ground of the LM2421. Thermal Considerations The package the LM2481 is in uses the ground pins (3, 4, 5, 10, 11, and 12) to conduct heat from the LM2481. These pins should be connected to a ground plane that acts as a heat sink. See the example ground plane connected to these pins in the National Semiconductor PCB shown in Figure 7. Figure 4 is used to determine the size of ground plane heat sink. The lines in Figure 4 represent a junction temperature of 150˚C when the size of heat sink noted by the line is used. The area under the line is the safe operating area. Using Figure 4, the following example shows how to determine the size of the ground plane. The LM2481 uses about 0.4W of quiescent power. If V CC = +145V, and each channel of the LM2481 needs to sink an average of 1mA, the LM2481 will use: 0.4W+3x (145V x 1mA ) = 0.835W Figure 4 shows that the LM2481 dissipating 0.835W with one sq. in. of copper ground plane heat sink is in the safe operating range for any ambient temperature up to 70˚C. 20039835 FIGURE 3. V out vs Vin 20039836 FIGURE 4. Maximum Power Dissipation vs. Ambient Temperature www.national.com 3 |
Numéro de pièce similaire - LM2481 |
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Description similaire - LM2481 |
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