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LM2403 Datasheet(PDF) 5 Page - Texas Instruments |
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LM2403 Datasheet(HTML) 5 Page - Texas Instruments |
5 / 15 page OBSOLETE LM2403 www.ti.com SNOS019B – AUGUST 1999 – REVISED APRIL 2013 THEORY OF OPERATION The LM2403 is a high voltage monolithic three channel CRT driver suitable for high resolution display applications. The LM2403 operates using 80V and 12V power supplies. The part is housed in the industry standard 11-lead TO-220 molded plastic power package. The simplified circuit diagram of the LM2403 is shown in Figure 2. A PNP emitter follower, Q1, provides input buffering. The 14 k Ω feedback resistor and the 1 kΩ input resistor sets the gain of the inverting op-amp to -14. Emitter followers Q2 and Q3 isolate the output of the feedback amplifier from the capacitance of the CRT cathode, and make the circuit relatively insensitive to load capacitance. Figure 3 shows a typical test circuit for evaluation of the LM2403. This circuit is designed to allow testing of the LM2403 in a 50 Ω environment without the use of an expensive FET probe. In this test circuit, two low inductance resistors in series totaling 4.95 k Ω form a 100:1 wideband low capacitance probe when connected to a 50Ω cable and load. The input signal from the generator is ac coupled to the base of Q1. Figure 10. Figure 10 shows the large signal sine wave frequency response of the LM2403. The frequency response rolls off very rapidly above the bandwidth limit of the amplifier. There are two reasons for this fast response roll-off: 1. The LM2403 contains an input low pass filter to help remove unwanted high frequency harmonics that can cause EMI problems. This filter does not significantly affect the rise and fall times of the signal as it operates above the −3 dB bandwidth of the device. 2. The internal feedback network of the closed loop amplifier holds the gain at −14 until the loop gain drops below unity. Above this frequency, the amplifier response falls with the open loop gain of the amplifier, as the feedback ceases to have any significant effect. There is also a change in the impedance match between the op-amp and the emitter follower output stage with large signals at higher frequencies. This creates a gain boost that extends the bandwidth, then gives a sudden roll off as shown in Figure 10. The exact response of this roll off may vary slightly depending upon operating conditions, signal amplitude etc. In both cases, the fast roll of the high frequency harmonics will help to limit the creation of high frequency EMI harmonics, without limiting video rise and fall time characteristics. However, due to the very fast switching speeds of the device, good layout design for EMI is CRITICAL. Path lengths and loop areas of the video signals must be kept to a minimum. Copyright © 1999–2013, Texas Instruments Incorporated Submit Documentation Feedback 5 Product Folder Links: LM2403 |
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