LH2426 Triple 80 MHz CRT Driver General Description Features The LH2426 contains three wide bandwidth, large signal amplifiers designed for large voltage swings at high frequencies. The amplifiers work on a transimpedance principal i.e., an input current swing of g 4.38 mA results in an output voltage swing of g 25V. The device is intended for use in color CRT monitors and is a low cost solution to designs conforming to the IBMÉ 8514 graphics standard. Y Y Y Y Y Operation from 80V power supply 80 MHz bandwidth at 50 VPP swings Rise/fall time less than 4 ns Output signal can swing 70V Drives CRT directly Applications Y Y CRT driver for RGB monitors High voltage transimpedance amplifiers Schematic and Connection Diagrams (One Section) TL/H/10739 – 2 Top View Order Number LH2426S See NS Package Number HY12B TL/H/10739 – 1 IBMÉ is a registered trademark of International Business Machines Corporation. C1995 National Semiconductor Corporation TL/H/10739 RRD-B30M115/Printed in U. S. A. LH2426 Triple 80 MHz CRT Driver June 1992 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 Power Dissipation, PD 10W 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 DC Electrical Characteristics V a e 80V, R1 e 430X, C1 e 82 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 LH2426 Conditions Min Va Supply Current (per Amplifier) VINDC Input Offset Voltage VOUTDC Output Offset Voltage tR Rise Time 10% to 90% tF Fall Time BW Bandwidth Av Voltage Gain OS Overshoot LE Linearity Error VOUT from a 10V to a 70V Note 1 D Av Gain Matching Note 2 Units Typical Max 24 30 mA 1.6 1.8 V No Input or Output Load 1.4 34 40 46 V 3.5 5 ns 90% to 10% 3.5 5 b 3 dB 100 11 13 ns MHz 14 V/V 10 % 5 % 0.2 dB Note 1: Linearity error is defined as: The variation in small signal gain from a 10V to a 70V output with a 100 mVAC, 1 MHz, input signal. Note 2: Calculated value from voltage gain test on each channel. Typical Performance Characteristics Typical Test Circuit (One Section) TL/H/10739 – 3 FIGURE 1. Test Circuit (One Section) *Note: 8 pF is total load capacitance. It includes all parasitic capacitances. Figure 1 shows a typical test circuit for evaluation of the LH2426. This circuit is designed to allow testing of the LH2426 in a 50 X environment such as a pulse generator, oscillosope or network analyzer. To calibrate pulse generator, set to 2.4 VPP into 50 X. THEORY OF OPERATION The LH2426 is a two stage amplifier (see schematic on front page). Both stages are in push pull configuration. Q2 is biased with two resistors, Q1 gets its bias through the 5700X feedback resistor and the input biasing current. The bases of Q1 and Q2 are capacitively coupled and therefore Q2 is also actively driven. The LH2426 is a transimpedance amplifier: an input current is translated into an output voltage. An input current of about g 4.5 mA will provide full output swing of g 25V. A resistor in series with the input converts the LH2426 into a voltage amplifier, with 430X the voltage gain becomes b 13. Application Hints The LH2426 is designed as a triple power amplifier for delivering red, blue and green video signals to a cathode ray tube (CRT). It can provide 50V output swing and energize a 12 ns pixel. The input capacitance of a CRT grid is typically 8 pF. 2 Application Hints (Continued) The emitter resistors of Q1 and Q2 are bypassed with small capacitors. This increases the gain to the stage for high frequencies and increases the bandwidth of the amplifier. dissipation in the input transistors. If this causes a problem, it can be compensated with R-C feedback. Figure 3 illustrates the circuit and recommended component values. The power supply is internally bypassed. If low frequencies are present in the power supply line, an electrolytic capacitor is recommended. PROTECTING AMPLIFIER OUTPUT FROM TUBE ARCING During normal CRT operation, internal arcing may occasionally occur. Spark gap protectors do limit the maximum voltage, but to a value that is much higher than allowable on the LH2426. This fast, high voltage, high current pulse can damage the LH2426 output. The addition of a current limiting resistor of 50X to 100X will provide protection but will slow down response. Adding a series peaking inductor of 100 nH to 150 nH will restore the bandwidth and provide additional protection. (See Figure 3 ) The value of the inductor can be calculated from: (R0 a RD)2 C LP e 2.4 where C is the total load and R0 is the intrinsic high frequency output resistance of the amplifier, generally 160X. INPUT NETWORKS The voltage gain and the response of the amplifiers can be set by adding an R–C to the input. A 430X resistor in series, will set the voltage gain to 13. This will increase the rise and fall times of the system. (See Figure 2a ) Bypassing the resistor with a capacitor of about 50 pF will restore the rise/fall times but will result in some overshoot. (Figure 2b) Adding a resistor in series with the capacitor will reduce the overshoot but also increase the rise and fall times. (Figure 2c) Ð The addition of a second capacitor will restore the rise and fall times without significant 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. TL/H/10739 – 7 FIGURE 3. One Section of the LH2426 with Damping Resistor RD and Peaking Inductance LP in the Output SUPPLY BYPASSING Although the LH2426 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 near the supply pins. TL/H/10739 – 8 CAPACITIVE LOADS The LH2426 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. SHORT CIRCUIT PROTECTION Warning! To provide maximum output speed, the LH2426 does not have short circuit protection. Shorting the output can destroy the device. The lowest value load the LH2426 is designed to drive is 600X. If the device is used in an application where the output may be shorted, 600X should be placed in series with the output. HEAT SINKING As the LH2426 will dissipate up to 10W, an external heatsink is always required. The maximum allowed case temperature is 90§ C. To calculate maximum heatsink thermal resistance, use the following formula: Rth e (90§ C b Max Ambient)/10 TL/H/10739 – 9 FIGURE 2. Influence of Input Networks on Switching Performance Figure 2 compares different input networks and their influence on the switching waveform. DROOP COMPENSATION When low frequency square waves are amplified, some droop will occur due to the large change in the thermal 3 LH2426 Triple 80 MHz CRT Driver Physical Dimensions inches (millimeters) Note 1: Substrate: 96% alumina. Note 2: Lid seal: epoxy. Note 3: Leads: phos. bronze alloy or equiv. with 95 Sn/5 Ag solder DIP. Note 4: Four radius bumps on corners of lid are optional. Not shown on drawing. Order Number LH2426S NS Package Number HY12B 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 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. National Semiconductor Corporation 1111 West Bardin Road Arlington, TX 76017 Tel: 1(800) 272-9959 Fax: 1(800) 737-7018 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. National Semiconductor Europe Fax: (a49) 0-180-530 85 86 Email: cnjwge @ tevm2.nsc.com Deutsch Tel: (a49) 0-180-530 85 85 English Tel: (a49) 0-180-532 78 32 Fran3ais Tel: (a49) 0-180-532 93 58 Italiano Tel: (a49) 0-180-534 16 80 National Semiconductor Hong Kong Ltd. 13th Floor, Straight Block, Ocean Centre, 5 Canton Rd. Tsimshatsui, Kowloon Hong Kong Tel: (852) 2737-1600 Fax: (852) 2736-9960 National Semiconductor Japan Ltd. Tel: 81-043-299-2309 Fax: 81-043-299-2408 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.