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LM2476 monolithic triple channel 6.5 ns high gain crt driver and bias clamp general description the LM2476 is a monolithic triple channel crt driver and triple bias clamp for low-cost color monitor applications. the highly integrated ic contains three wide-band amplifiers for driving the rgb cathodes of a crt through external cou- pling capacitors and three low-band clamp amplifiers for dc restoration and cutoff adjustment of the video outputs. the crt drivers have a high, fixed gain of -27 and can drive crt capacitive loads as well as resistive loads present in other applications, limited only by the package?s power dissipation. the ic is packaged in a 19-lead to-247 molded plastic package and must be operated with a properly chosen heat sink. see the package mounting and thermal consider- ations sections for more information. features n well-matched to the lm123x/4x family of preamplifiers n operates with v cc = 60v to 90v n convenient to-247 staggered lead package style crt drivers n high gain of ?27 for up to 60v p-p output swing n stable with capacitive loads and inductive peaking networks bias clamps n gain of ?17 for up to 60v dc output range applications n 1024 x 768 displays up to 85 hz refresh rate n pixel clock frequencies up to 95 mhz n monitors using video blanking pinout diagram and pin descriptions pin name pin description ind driver input pins (1, 3, 4) inc clamp input pins (6, 7, 8) v bb bias voltage pin (2) v cc supply voltage pin (14) gnd* ground pins (5, 10, 12, 16, 18) outc clamp output pins (9, 11, 13) outd driver output pins (15, 17, 19) * note: all gnd pins should be connected together via low hf impedance traces on the pcb. 20121901 note: tab is at gnd figure 1. pinout and connection diagram january 2004 LM2476 monolithic triple channel 6.5 ns high gain crt driver and bias clamp ? 2005 national semiconductor corporation ds201219 www.national.com
schematic diagrams 20121902 figure 2. crt driver simplified schematic (one channel) 20121915 figure 3. bias clamp simplified schematic (one channel) LM2476 www.national.com 2
absolute maximum ratings (notes 1, 3) if military/aerospace specified devices are required, please contact the national semiconductor sales office/ distributors for availability and specifications. supply voltage (v cc ) 96v bias voltage (v bb ) 10v driver input voltage (v ind ) 0v to 4.5v clamp input voltage (v inc ) 0v to 5.0v storage temperature range (t stg ) ?65?c to +150?c lead temperature (soldering, < 10 sec.) 300?c esd tolerance human body model 2 kv machine model 200v operating ranges (note 2) v cc 60v to 85v v bb 7v to 9v v ind 0v to 3.5v v inc 0v to 4.0v v outd (driver output voltage) 12v to v cc v outc (clamp output voltage) 12v to v cc case temperature (device tab) ?20?c to +100?c do not operate the part without a heat sink. electrical characteristics (see figure 4 for test circuit) unless otherwise noted: v cc = 85v, v bb = 8v, c l = 8 pf, t c = 40?c dc tests: v ind = 2.30v, v inc = 2.35v ac tests: v outd = 40v p-p (35v ? 75v) at 1 mhz, v inc = 2.35v symbol parameter conditions LM2476 units min typical max i cc supply current all three channels, no ac input signal, no output load 34 45 ma i bb bias current all three channels, no ac input signal, no output load 21 30 ma v outd, 1 driver dc output voltage no ac input signal, v ind = 2.30v 41 46 51 v v outd, 2 driver dc output voltage no ac input signal, v ind = 1.15v 72 77 82 v a v-outd driver dc voltage gain no ac input signal ?24 ?27 ?30 ? a v-outd driver gain matching (note 4), no ac input signal 1.0 db le outd driver linearity error (notes 4, 5), no ac input signal 5 % t r rise time (note 6), 10% to 90% 6.0 ns t f fall time (note 6), 90% to 10% 6.7 ns os overshoot (note 6) 3 % v outc clamp dc output voltage v inc = 2.35v 46 51 56 v v outc-range clamp dc output voltage range v inc-range = 0.5v to 4.0v 58 vdc a v-outc clamp dc voltage gain no ac input signal ?14.5 ?16.5 ?18.5 le outc clamp linearity error (notes 4, 5), no ac input signal 5 % note 1: absolute maximum ratings indicate limits beyond which damage to the device may occur. note 2: operating ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. for guaranteed specifi cations and test conditions, see the electrical characteristics. datasheet min/max specification limits are guaranteed by design, test, or statistical analy sis. the guaranteed specifications apply only for the test conditions listed. some performance characteristics may change when the device is not operated under the list ed test conditions. note 3: all voltages are measured with respect to gnd, unless otherwise specified. note 4: calculated value from voltage gain test on each channel. note 5: driver linearity error is the variation in dc gain from v ind = 1.1v to v ind = 3.6v. note 6: input from signal generator: t r ,t f < 1 ns. note 7: clamp linearity error is the variation in dc gain from v inc = 1.0v to v inc = 4.0v. LM2476 www.national.com 3
LM2476 test circuits figure 4 shows a typical test circuit to evaluate the LM2476 crt driver and bias clamp electrical characteristics. the driver test circuit is designed to allow for testing the transient response in a 50 ? environment without the use of an expensive fet probe. an input from a 50 ? pulse generator output can be ac coupled and biased with an external supply via the v bias input. the two 2.49 k ? resistors form a 200:1 divider with the 50 ? resistor and the oscilloscope. the clamp test circuit is designed to allow for testing the clamp outputs. a clamp input can be biased with an external supply via the v dc input and a high impedance voltmeter ( > 100m ? ) can be used to measure the dc voltage at the clamp outputs. test points can included to accommodate voltmeter or oscilloscope probes. 20121903 note: 8 pf load includes parasitic capacitance. figure 4. crt driver and bias clamp test circuits (one channel) LM2476 www.national.com 4
typical performance characteristics (v cc = 85v, v bb = 8v, c l = 8 pf, v outd = 40v p-p (35v ? 75v), v inc = 2.35v, test circuit - figure 4 unless otherwise specified) 20121904 figure 5. v outd vs v ind 20121917 figure 6. v outc vs v inc 20121906 figure 7. LM2476 pulse response 20121908 figure 8. speed vs offset 20121909 figure 9. speed vs load capacitance 20121905 figure 10. speed vs temperature LM2476 www.national.com 5
typical performance characteristics (v cc = 85v, v bb = 8v, c l =8 pf, v outd = 40v p-p (35v ? 75v), v inc = 2.35v, test circuit - figure 4 unless otherwise specified) (continued) theory of operation the LM2476 is a high voltage monolithic three channel crt driver and triple bias clamp suitable for low-cost color moni- tor applications. the LM2476 operates with 85v and 8v power supplies. the part is housed in the 19-lead to-247 molded plastic power package. the pinout and internal con- nection diagram is shown in figure 1 . the crt driver circuit diagram is shown in figure 1 . the pnp emitter follower, q5, provides input buffering. q1 and q2 form a fixed gain cascode amplifier with resistors r1 and r2 setting the gain at ?27. emitter followers q3 and q4 isolate the high output impedance of the cascode stage from the capacitance of the crt cathode which decreases the sensitivity of the device to load capacitance. q6 provides biasing to the output emitter follower stage to reduce cross- over distortion at low signal levels. the typical driver dc transfer function is shown in figure 5 . the bias clamp circuit diagram is shown in figure 3 . the clamp circuit amplifies the dc inputs, v inc , by the internally fixed gain of ?16.5. each clamp output, v outc , will require a pull-up resistor to v cc . the typical clamp dc transfer func- tion is shown in figure 6 . 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. when working with high bandwidth circuits, good layout practices are also critical to achieving maximum per- formance. important information the LM2476 performance is targeted for the vga (640 x 480) to xga (1024 x 768, 85hz) resolution market. the application circuits shown in this document to optimize per- formance and to protect against damage from crt arcover are designed specifically for the LM2476. if another member of the nsc crt driver or bias clamp family is used, please refer to its data sheet. power supply bypass since the LM2476 contains wide bandwidth video amplifiers, proper power supply bypassing is critical for optimum per- formance. improper power supply bypassing can result in large overshoot, ringing or oscillation. 0.1 f capacitors should be connected from the supply pins, v cc and v bb ,to ground, as close to the LM2476 as is practical. additionally, a 47 f or larger electrolytic capacitor should be connected from both supply pins to ground reasonably close to the LM2476. for optimal supply bypassing, the bypass capaci- tors should have the shortest connections between the sup- ply and ground pins of the LM2476. arc protection during normal crt operation, internal arcing may occasion- ally occur. a spark gap sg1 ? in the range of 200v ? connected from each of the crt cathodes to crt ground will limit the maximum voltage, but to a value that is much higher than allowable on the LM2476. this fast, high voltage, high energy pulse can damage the LM2476 driver and/or clamp output stages. the application circuit shown in figure 12 is designed to help clamp the voltage at the outputs of the LM2476 to a safe level. the arc protection clamp diodes, d1 and d2, should have a fast transient response, high peak current rating, low series impedance and low shunt capaci- tance. fdh400 or equivalent diodes are recommended. do not use 1n4148 diodes for the clamp diodes. d1 and d2 should have short, low impedance connections to v cc and ground respectively. the cathode of d1 should be located very close to a separately decoupled bypass capacitor (c1 in figure 12 ). the ground connection of d2 and the decoupling capacitor should be closest to the crt ground. this will significantly reduce the high frequency voltage transients that the LM2476 would be subjected to during an arcover condition. resistor r2 limits the arcover current that is seen by the diodes while r1 limits the current into the LM2476 as well as the voltage stress at the outputs of the device. r2 should be a 1 M 2 w solid carbon type resistor. r1 can be a 1 M 4 w metal or carbon film type resistor. having large value resis- tors 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 LM2476 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. current-limiting resistor r3 and bypass capacitor c2 should be placed very close to the clamp output pins to protect the LM2476 against damage during an arcover condition. the ground connection of c2 should have a short return path to crt ground to shunt arcover currents away from the LM2476. for proper arc protection, it is important to not omit any of the arc protection components shown in figure 12 . 20121907 figure 11. power dissipation vs frequency LM2476 www.national.com 6
application hints (continued) optimizing transient response referring to figure 12 , there are three components (r1, r2 and l1) that can be adjusted to optimize the transient re- sponse of the application circuit. increasing the values of r1 and r2 will slow the circuit down while decreasing over- shoot. increasing the value of l1 will speed up the circuit as well as increase overshoot. it is very important to use induc- tors 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 can be used as a good starting point for the evaluation of the LM2476. using a variable resistor for r1 will simplify finding the value needed for optimum performance in a given application. once the opti- mum value is determined, the variable resistor can be re- placed with a fixed value. effect of offset figure 8 shows the variation in rise and fall times when the output offset of the device is varied from 50 to 60vdc. the rise time shows a maximum variation of 13% relative to the center data point (55 v dc ). the fall time shows a maximum variation of less than 3% relative to the center data point. effect of load capacitance figure 9 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. thermal considerations figure 10 shows the performance of the LM2476 in the test circuit shown in figure 4 as a function of case temperature (on the device tab). the figure shows that the rise and fall times of the LM2476 increase by approximately 10% and 5%, respectively, as the case temperature increases from 50?c to 100?c. this corresponds to a speed degradation of 2% and 1%, respectively, for every 10?c rise in case tem- perature. figure 11 shows the maximum power dissipation of the LM2476 vs. frequency when all three channels of the device are driving an 8 pf load with a 40 v p-p 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 monitor application. the other 28% of the time the device is assumed to be sitting at the black level (75v in this case). this graph gives the designer the information needed to determine the heat sink requirement for his appli- cation. the designer should note that if the load capacitance is increased the ac component of the total power dissipation will also increase. the LM2476 case temperature must be maintained below 100?c. if the maximum expected ambient temperature is 70?c and the maximum power dissipation is about 6w (from figure 11 , 95 mhz bandwidth), then a maximum heat sink thermal resistance can be calculated: package mounting considerations mounting of the to-247 package to a heat sink must be done such that there is sufficient pressure from the mounting screws to insure good contact with the heat sink for efficient heat flow. the surface of the heat sink should be free of contaminants before mounting to insure good contact. incor- rect mounting may lead to both thermal and mechanical problems. over tightening the mounting screws will cause the package to warp, reducing contact area with the heat sink. less contact with the heat sink will increase the thermal resistance from the package case to the heat sink ( cs ) resulting in higher operating die temperatures. extreme over tightening of the mounting screws will cause severe physical stress resulting in a cracked or chipped package and pos- sible catastrophic ic failure. the recommended mounting screw size is m3 with a maxi- mum torque of 50 n-cm. it is best to use fiber washers under the screws to distribute the force over a wider area. addi- tionally, if the mounting screws are used to force the pack- age into correct alignment with the heat sink, package stress 20121910 figure 12. one channel of the LM2476 with the recommended application circuit LM2476 www.national.com 7
application hints (continued) will be increased. this increase in package stress will result in reduced contact area with the heat sink increasing die operating temperature and possible catastrophic ic failure. typical application a typical application of the LM2476 is shown in figure 14 . used in conjunction with an lm123x/4x preamplfier, a com- plete video channel from monitor input to crt cathode can be achieved. performance is ideal for 1024 x 768 resolution displays with pixel clock frequencies up to 95 mhz. please see the next two sections below for hints on how to properly evaluate the lm123x/4x and LM2476 combination in a monitor. figure 13 shows the typical cathode response for this application. the peaking component values used are shown in figure 12 and figure 14 . pc board layout considerations for optimum performance, use single-point grounds sys- tems with adequate ground planes, isolate between chan- nels, apply good supply bypassing, and minimize unwanted feedback and parasitic capacitance. also, the length of the video signal traces from the preamplifier to the LM2476 and from the LM2476 to the crt cathodes should be as short as possible. the following references are recommended: ott, henry w., ?noise reduction techniques in electronic systems?, john wiley & sons, new york, 1976. ?video amplifier design for computer monitors?, national semiconductor application note 1013. pease, robert a., ?troubleshooting analog circuits?, butterworth-heinemann, 1991. because of its high small signal bandwidth, the part may oscillate in a monitor if feedback occurs around the video channel through the chassis wiring. to prevent this, leads to the video amplifier input circuit should be shielded, and input circuit wiring should be spaced as far as possible from output circuit wiring. nsc demonstration board figure 14 is the schematic for the nsc lm1276/3x/4x_2476 demonstration pcb that can be used to evaluate the lm1276, lm123x, or lm124x preamp with the LM2476 in a monitor. figure 15 shows the routing and component place- ment on the nsc demonstration pcb. this board provides a good example of a layout that can be used as a guide for future layouts. note the location of the following compo- nents: c26?v cc bypass capacitor, located very close to pin 14 and gnd pins c27?v bb bypass capacitors, located close to pin 2 and gnd pins c28, c30, c33 ? v cc bypass capacitors, near LM2476 and v cc clamp diodes. these are very important for arc protection. the routing of the LM2476 driver outputs to the crt is very critical to achieving optimum performance. figure 16 shows the routing and component placement from pin 17 of the LM2476 to the red cathode. note that the components are placed so that they almost line up from the output pin of the LM2476 to the red cathode pin of the crt connector. this is done to minimize the length of the video path between these two components for optimal performance and to mini- mize emi. note also that l4, d9, d13 and r23, r19, d10 are placed to minimize the size of the video nodes that they are attached to. this minimizes parasitic capacitance in the video path and also enhances the effectiveness of the pro- tection diodes. the anode of protection diode d13 is con- nected directly to a section of the ground trace that has a short and direct path to crt ground to shunt arcover current away from the LM2476. the cathode of d9 is connected to v cc very close to decoupling capacitor c30 (see figure 16 ) which is connected to the same section of the ground trace as d13. the diode placement and routing is very important for minimizing the voltage stress on the LM2476 during an arcover event. r37 and c29 are placed very close to the clamp output on pin 11. these components will help limit the current and voltage stress to the clamp output. lastly, notice that s2 is placed very close to the red cathode and is connected directly to crt ground. 20121916 40v p-p (35v ? 75v) figure 13. typical cathode response LM2476 www.national.com 8
application hints (continued) 20121911 figure 14. nsc demonstration pcb schematic LM2476 www.national.com 9
application hints (continued) 20121913 figure 15. nsc demonstration pcb layout LM2476 www.national.com 10
application hints (continued) 20121914 figure 16. trace routing and component placement for red channel output LM2476 www.national.com 11
physical dimensions inches (millimeters) unless otherwise noted controlling dimension is inch values in [ ] are millimeters ns package number tb19a order number LM2476 national does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and national reserves the right at any time without notice to change said circuitry and specifications. for the most current product information visit us at www.national.com. life support policy national?s products are not authorized for use as critical components in life support devices or systems without the express written approval of the president and general counsel of national semiconductor corporation. as used herein: 1. life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. a critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. banned substance compliance national semiconductor certifies that the products and packing materials meet the provisions of the customer products stewardship specification (csp-9-111c2) and the banned substances and materials of interest specification (csp-9-111s2) and contain no ??banned substances?? as defined in csp-9-111s2. national semiconductor americas customer support center email: new.feedback@nsc.com tel: 1-800-272-9959 national semiconductor europe customer support center fax: +49 (0) 180-530 85 86 email: europe.support@nsc.com deutsch tel: +49 (0) 69 9508 6208 english tel: +44 (0) 870 24 0 2171 fran?ais tel: +33 (0) 1 41 91 8790 national semiconductor asia pacific customer support center email: ap.support@nsc.com national semiconductor japan customer support center fax: 81-3-5639-7507 email: jpn.feedback@nsc.com tel: 81-3-5639-7560 www.national.com LM2476 monolithic triple channel 6.5 ns high gain crt driver and bias clamp

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