LM2427 Triple 80 MHz CRT Driver General Description Features The LM2427 is a high performance triple CRT driver for simplifying color monitor designs. The device contains three large signal transimpedance amplifiers, and provides direct cathode drive capability. A plastic power package and pinto-pin compatibility make the LM2427 ideal for new designs or as a low cost replacement for designs using the LH2426 or CR5527. Y Y Y Y Low-cost plastic power package Typical rise/fall times of 3.5 ns 80 MHz video bandwidth at 50 VPP with 8 pF load Operation from 80V power supply Applications Y Y Y CRT driver for color monitors Drives CRT cathode directly Pin-to-pin compatible with the LH2426 and CR5527 CRT drivers Schematic and Connection Diagrams (One Section) TL/H/11967 – 2 Top View Order Number LM2427T See NS Package Number MKT-TA12A TL/H/11967 – 1 C1995 National Semiconductor Corporation TL/H/11967 RRD-B30M115/Printed in U. S. A. LM2427 Triple 80 MHz CRT Driver April 1995 Absolute Maximum Ratings Storage Temperature Range, TSTG If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/Distributors for availability and specifications. Supply Voltage, V a a 85V Safe Operating Power Consumption 14W b 25§ C to a 100§ C Operating Temperature Range, TCASE b 20§ C to a 90§ C Lead Temperature (Soldering, 10 sec.) ESD Tolerance 300§ C TBD Electrical Characteristics V a e 80V, RG e 430X, C1 e 47 pF, CL e 8 pF, 50 VPP output swing with 40V DC offset. See Figure 1. TCASE e 25§ C unless otherwise noted. Symbol Parameter LM2427 Conditions Min No Input or Output Load Units Typical Max 24 30 mA V ICC Supply Current (per Amplifier) VINDC Input Offset Voltage 1.4 1.6 1.8 VOUTDC Output Offset Voltage 34 40 46 tR Rise Time 10% to 90% (Note 1) tF Fall Time 90% to 10% (Note 1) V 3.5 ns 3.5 b 11 AV Voltage Gain LE Linearity Error VOUT from a 10V to a 70V (Note 2) DAV Gain Matching (Note 3) b 13 ns b 14 V/V 5 % 0.2 dB Note 1: Input signal: tr, tf k 1.5 ns, fin e 1 MHz. Note 2: Linearity error is defined as: The variation in small signal gain from a 20V to a 70V output with a 100 mVAC, 1 MHz, input signal. Note 3: Calculated value from voltage gain test on each channel. Typical Performance Characteristics Typical Test Circuit (One Section) TL/H/11967 – 3 Note: CL, total load capacltance, includes all parasitic capacitances. FIGURE 1. Test Circuit (One Section) This test circuit is used for both characteristic plots. Typical Rise Time vs Capacitive Loading Typical Fall Time vs Capacitive Loading TL/H/11967–10 TL/H/11967 – 5 2 Bypassing the resistor with a capacitor of about 47 pF will restore the rise and fall times but will result in some overshoot. (Figure 2b) Test Circuit Figure 1 shows a typical test circuit for evaluation of the LM2427. This circuit is designed to allow testing of the LM2427 in a 50X environment, such as a pulse generator, oscilloscope or network analyzer. The 4950X resistor in series with the output of the LM2427 forms a 100:1 voltage divider when connected to a 50X-input oscilloscope or network analyzer. To calibrate pulse generator, set to 2.4 VPP into 50X. Adding a resistor in series with the 47 pF capacitor will reduce the overshoot but also increases the rise and fall times. (Figure 2c) The addition of a second capacitor offers a compromise between the above networks by improving the rise and fall times at the expense of some overshoot. (Figure 2d) Suggested values for the resistors and capacitors are shown, however, optimum values may differ depending upon the stray inductances and capacitances present in different board layouts. THEORY OF OPERATlON The LM2427 is a triple channel transimpedance amplifier for CRT’s, suitable for SVGA, XGA, IBM and Macintosh display resolution monitors. The LM2427 is pin-to-pin compatible with the LH2426 and CR5527 CRT drivers. The device is packaged in the industry standard 12-lead SIP TO-220 molded plastic power package. The heat sink is electrically isolated and may be grounded for ease of manufacturing and RFI/EMI shielding. Applying an input current to the LM2427 will result in an output voltage. An input current of about g 4.5 mA will provide a full output swing of g 25V. A resistor in series with the input converts the device into a voltage amplifier; with a resistor value of 430X the voltage gain becomes b13. The LM2427 is a two stage amplifier configured in a pushpull configuration (see schematic on front page). Q2 is biased by resistors R4 and R5, Q1 gets its bias through a 5700X feedback resistor and the input biasing current. The bases of Q1 and Q2 are capacitively coupled and, therefore, Q2 will be actively driven. The emitter resistors of Q1 and Q2 are bypassed with small capacitors. This increases the gain of the stage for high frequencies and increases the bandwidth of the amplifier. Emitter followers Q3 and Q4 isolate the input stage from the output capacitance load, and minimizes the circuit sensitivity to load capacitance. The power supply pin is intemally bypassed. If low frequencies are present in the power supply line, an electrolytic capacitor is recommended. Application Hints The LM2427 is designed as a triple power amplifier for delivering red, green, and blue video signals to a cathode ray tube (CRT). It can provide a 50V output swing and energize a 12 ns pixel at a CRT cathode with 8 pF of capacitance. As with any CRT driver, when designing a video amplifier board with the LM2427, careful attention should be paid at reducing stray capacitance along the entire video signal path. This is especially important in the path between the output of the CRT driver and the cathodes, because any additional capacitance load will increase rise and fall times and will result in reduced picture quality. INPUT NETWORKS The voltage gain and the response of the amplifiers can be set by adding an R-C network to the input. A 430X resistor in series with the input will set the voltage gain to b13, but this will increase the rise and fall times of the system (see Figure 2a ). TL/H/11967 – 6 FIGURE 2. Influence of Input Networks on Switching Performance 3 Application Hints (Continued) PROTECTlNG AMPLIFIER OUTPUT FROM TUBE ARCING TILT AND OVERSHOOT COMPENSATlON When a low frequency square is displayed on a monitor screen, some tilt may appear on the video signal due to the large power and thermal dissipation changes in the input transistors. This problem is illustrated in Figure 3. During normal CRT operation, internal arcing may occasionally occur. Spark gap protectors will limit the maximum voltage, but to a value that is much higher than allowable on the LM2427. This fast, high voltage, high energy pulse can damage the LM2427 output stage. The addition of two current limiting resistors of 50X to 100X total, and clamping diodes D1 and D2, will provide protection but will slow down the response. The diodes should have a fast transient response, high peak current rating, low series impedance and low shunt capacitance. FDH400 or equivalent diodes are recommended. Adding a series peaking inductor of 100 nH to 150 nH will restore the bandwidth and provide additional protection. (See Figure 5 ) The value of the inductor can be calculated from: (RO a R1 a R2)2 CL Lp e 2.4 where CL is the total load and RO is the intrinsic high frequency output resistance of the amplifier, generally 160X. TL/H/11967 – 7 FIGURE 3. Tilt on a Low Frequency Signal and Its Effects The tilt can be compensated by adding an external RC feedback network as shown in Figure 4. The RC feedback helps by reducing the gain of the amplifier during the edge transition for a duration corresponding to u. The values of R and C should be selected so that the gain is reduced (DV e 0) for the duration of the tilt (u). TL/H/11967 – 9 FIGURE 5. One Section of the LM2427 with Tilt Compensation, Arc Protection and Peaking Inductance LP in the Output SHORT CIRCUIT PROTECTION WARNING! To provide maximum output speed, the LM2427 does not have short circuit protection. Shorting the output can destroy the device. SUPPLY BYPASSlNG Although the LM2427 has internal supply bypassing, some values of supply line inductance can cause ringing in the supply lines. If this occurs, an additional bypass capacitor or a low-pass filter should be placed as close as possible to the supply (V a ) pins of the LM2427. TL/H/11967 – 8 FIGURE 4. RC Feedback Network for Tilt Compensation CAPACITlVE LOADS The LM2427 is designed to drive capacitive loads, however, the very high output slew rate of about 13,700 V/ms can result in charging currents of over 200 mA into a 20 pF load. These very high currents can damage the output transistors. To find the value of resistor R, the following formula can be used: (100 b x%) RF x% where x% is the percentage value of DV to the peak-topeak output swing (VPP). RF is internally fixed to 5700X. The value of capacitor C is determined by: C e u/R Re HEAT SlNKlNG Power consumption by the LM2427 will depend on the supply voltage used, the output loading, the peak-to-peak output swing and the operating frequency. Since the LM2427 will dissipate up to 14W, an external heatsink is always required. The maximum allowed case temperature is 90§ C. To calculate maximum heatsink thermal resistance, use the following formula: where u is the duration of the tilt. For optimum results in a specific application, the values for R and C may need to be tested and adjusted in the given application board. Rth e 4 (90§ C b Max Ambient) . 14 PC BOARD LAYOUT CONSIDERATIONS Input pins 2, 6 and 10 are amplifier summing junctions. All connections to these points should be as short as possible and should be separated from other signals. The components connected to these pins should be located close to the LM2427, and the total conductor length connected to these points should be no more than one inch. For optimum performance, an adequate ground plane, isolation between channels, good supply bypassing and minimizing unwanted feedback are necessary. Also, the length of the signal traces from the preamplifier to the LM2427 and from the LM2427 to the CRT cathode 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. ‘‘Guide to CRT Video Design’’, National Semiconductor Application Note 861. 5 LM2427 Triple 80 MHz CRT Driver Physical Dimensions inches [millimeters] Note 1: Unless otherwise specified. Standard Lead Finish: 200 microinches/5.08 micrometers minimum. Lead/Tin (Solder) on Olin 194 or equivalent. Note 2: No JEDEC Registration as of 09-01-93. 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