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LM2405 Fiches technique(PDF) 4 Page - National Semiconductor (TI) |
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LM2405 Fiches technique(HTML) 4 Page - National Semiconductor (TI) |
4 / 7 page Application Hints (Continued) the supply pin as is practical (preferably less than 1⁄4" from the supply pin). Additionally, a 10 µF to 100 µF electrolytic capacitor should be connected from the supply pin to ground. The electrolytic capacitor should also be placed rea- sonably close to the LM2405’s supply pin. A 0.1 µF capacitor should be connected from the bias pin, V BB, to ground, as close as is practical to the part. ARC PROTECTION During normal CRT operation, internal arcing may occasion- ally occur. Spark gaps of 200V to 300V at the cathodes will limit the maximum voltage, but to a value that is much higher than allowable on the LM2405. This fast, high voltage, high energy pulse can damage the LM2405 output stage. The ad- dition of clamp diodes D1 and D2 (as shown in Figure 7) will help clamp the voltage at the output of the LM2405 to a safe level. The clamp diodes should have a fast transient re- sponse, high peak current rating, low series impedance and low shunt capacitance. FDH400 or equivalent diodes are recommended. Resistor R2 in Figure 7 limits the arcover current while R1 limits the current into the LM2405 and re- duces the power dissipation of the output transistors when the output is stressed beyond the supply voltage. (Peaking inductor Lp also helps protect the CRT driver from arc over.) Having large value resistors for R1 and R2 would be desir- able, but this has the effect of increasing rise and fall times. For proper arc protection, it is important to not omit any of the arc protection components shown in Figure 7. There are also ESD protection diodes built into the part. To avoid damaging these diodes, do not apply an input voltage from a low impedance source when the V BB and VCC pins are held at ground potential. IMPROVING RISE AND FALL TIMES Because of an emitter follower output stage, the rise and fall times of the LM2405 are relatively insensitive to capactive loading. However, the series resistors R1 and R2 (see Fig- ure 7) will increase the rise and fall times when driving the CRT’s cathode which appears as a capacitive load. The ca- pacitance at the cathode typically ranges from 8 pF to 12 pF. To improve the rise and fall times at the cathode, a small in- ductor is often used in series with the output of the amplifier. The inductor L P in Figure 7 peaks the amplifier’s frequency response at the cathode, thus improving rise and fall times. It also acts with the output load capacitance to form a low pass filter, which reduces the amplitudes of high frequency harmonics of the video signal, to lower radiated electromag- netic interference. The inductor value is empirically deter- mined and is dependent on the load. An inductor value of 0.22 µH is a good starting value. Note that excessive peak- ing of the amplifier’s frequency response will increase the overshoot. (Increasing the value of resistor R1 or R2 will re- duce ringing and overshoot.) EFFECT OF LOAD CAPACITANCE The output rise and fall times will be slower than specified if the load capacitance at the output is more than 8 pF, as shown in Figure 8. The monitor designer should ensure that stray capacitance applied to the LM2405 is as low as possible. THERMAL CONSIDERATIONS Power supply current increases as the input signal increases and consequently power dissipation also increases. The LM2405 cannot be used without heat sinking. Typical “average” power dissipation with the device output voltage at one half the supply voltage is 2.4W per channel for a total dissipation of 7.2W package dissipation. Under white screen conditions, i.e., 25V output, dissipation increases to 3.5W per channel or 10.5W total. The LM2405 case temperature must be maintained below 100˚C. If the maximum expected ambient temperature is 50˚C, then a maximum heat sink thermal resistance can be calculated: This example assumes a typical CRT capacitive load and is without a resistive load. Note that this thermal resistance must be achieved when the heat sink is operating in the monitor. INPUT RESISTANCE The LM2405 has a fixed resistor of 3000 Ω connected from each signal input pin to ground. In the Figure 2 Test Circuit, the input DC voltage level, Vtest, must be adjusted, (to about +3.5V) to allow for the voltage drop across the 1000 Ω resis- tor, to set the actual voltage at the input pins to +2.6V. In ac- tual use in a monitor, the 1000 Ω resistor is not used and the video preamp supplies the 2.6V offset. PC BOARD LAYOUT CONSIDERATIONS For optimum performance, an adequate ground plane, isola- tion between channels, good supply bypassing and minimiz- ing unwanted feedback are necessary. Also, the length of the DS012682-8 FIGURE 7. One Section of the LM2405 with Arc Protection and Peaking Inductor L P DS012682-9 FIGURE 8. Effect of Load Capacitance on Rise/Fall Time www.national.com 4 |
Numéro de pièce similaire - LM2405 |
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Description similaire - LM2405 |
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