LM2418

LM2418 Triple 30 MHz CRT Driver January 1995 LM2418 Triple 30 MHz CRT Driver General Description The LM2418 contains three large signal voltage amplifiers designed to directly drive CRT cathodes for VGA Color Graphics Displays Output swings greater than 50 VPP are achieved with a 90V power supply The nominal voltage gain of each amplifier is b19 with gain matching of 1 0 dB between amplifiers Packaging is the industry standard molded 11 lead TO-220 The heatsink tab is isolated and may be grounded to improve RFI shielding and simplify assembly Features Y Y Y Y Y Y Y 50 Vpp output at 30 MHz drives CRT directly Rise fall time typically 12 ns with 8 pF load 65V output swing capability Optimized output stage for low crossover distortion Gain matching of 1 dB Voltage gain of b19 Includes oscillation supression resistors Applications Y Y CRT driver for RGB monitors High voltage amplifiers Schematic and Connection Diagram (One Section) TL K 11125 – 1 TL K 11125 – 2 FIGURE 1 Top View Order Number LM2418T See NS Package Number TA11B C1995 National Semiconductor Corporation TL K 11125 RRD-B30M115 Printed in U S A Absolute Maximum Ratings If Military Aerospace specified devices are required please contact the National Semiconductor Sales Office Distributors for availability and specifications Supply Voltage V a Storage Temperature Range TSTG a 95V b 25 C to a 100 C Operating Temperature Range TCASE b20 C to a 100 C Lead Temperature (Soldering k10 sec ) 300 C ESD Tolerance tbd Electrical Characteristics V a e 90V CL e 8 pF DC input bias VIN e 3 6 VDC 50 VPP output swing VBIAS e a 12V TA e 25 C unless otherwise noted Symbol ICC VOUT tr tf BW AV OS LE D AV Parameter Supply Current (per Amplifier) Output Offset Voltage Rise Time Fall Time Bandwidth Voltage Gain Overshoot Linearity Error Gain Matching (Note 1) (Note 2) Conditions No Input or Output Load VIN e 3 6V 10% to 90% (Note 3) 10% to 90% (Note 3) b 3 dB b 17 LM2418 Min Typ 18 46 53 12 12 30 b 19 b 23 Max 26 60 20 20 Units mA VDC ns ns MHz VV % % dB 5 8 10 Note 1 Linearity Error is defined as the variation in small signal gain from a 20V to a 70V output with a 100 mV AC 1 MHz input signal Note 2 Calculated value from Voltage Gain test on each channel Note 3 Guaranteed parameter not tested AC Test Circuit Figure 2 shows a typical test circuit for evaluation of the LM2418 This circuit is designed to allow testing of the LM2418 in a 50X environment such as a pulse generator oscilloscope or network analyzer TL K 11125 – 3 8 pF is total load capacitance It includes all parasitic capacitance FIGURE 2 Test Circuit (One Section) Typical Performance Characteristics LM2418 Frequency Response LM2418 Pulse Response TL K 11125–4 TL K 11125 – 5 FIGURE 3 2 LM2418 Theory of Operation capacitance probe to match the output of the LM2418 to a 50X cable and load Typical AC performance of the circuit is shown in Figure 3 The input signal is AC coupled to the base of Q1 while a DC bias of 12V is applied to the base of Q2 (See Figure 2 ) The LM2418 is a high voltage triple CRT driver suitable for VGA display applications The LM2418 features 90V operation and low power dissipation The part is housed in the industry standard 11-lead TO-220 molded power package The heat sink is electrically isolated from the circuitry and may be grounded for ease of manufacturing and RFI shielding The circuit diagram of the LM2418 is shown in Figure 1 Q1 and R2 provide a conversion of input voltage to current while Q2 acts as a common base or cascode amplifier stage to drive the load resistor R1 Emitter followers Q3 and Q4 isolate the impedance of R1 from the capacitance of the CRT cathode and make the circuit relatively insensitive to load capacitance The gain of this circuit is bR1 R2 and is fixed at b19 The bandwidth of the circuit is set by the collector time constant formed by the load resistor R1 and associated capacitance of Q2 Q3 Q4 and stray layout capacitance Diodes D1 and D2 provide forward bias to the output stage to reduce crossover distortion at low signal levels while R3 provides a DC bias offset to match the output level characteristics of the LM1203 RGB Video Amplifier System Proprietary transistor design allows for high bandwidth with low operating power Thermal Considerations The transfer characteristics of the amplifier are shown in Figures 4 and 5 Power supply current increases as the input signal increases and consequently power dissipation also increases The LM2418 cannot be used without heat sinking Figure 5 shows the power dissipated in each channel over the operating voltage range of the device Typical ‘‘average’’ power dissipation with the device output voltage at one half the supply voltage is 1 8W per channel for a total dissipation of 5 4W package dissipation Under white screen conditions i e 20V output dissipation increases to 3 0W per channel or 9W total The LM2418 case temperature must be maintained below 100 C If the maximum expected ambient temperature is 60 C then a maximum heat sink thermal resistance can be calculated (100 C b 60 C) e44 C W Rth e 9W PRECAUTION THE LM2418 IS NOT PROTECTED AGAINST OUTPUT SHORT CIRCUITS The minimum resistance the LM2418 can drive is 800X to ground or V a Figure 2 shows a typical test circuit for evaluation of the LM2418 This circuit is designed to allow testing of the LM2418 in a 50X environment such as a pulse generator and a scope or a network analyzer In this test circuit two resistors in series totaling 4 95 kX form a wideband low TL K 11125 – 7 TL K 11125 – 9 FIGURE 4 LM2418 DC Characteristics FIGURE 5 LM2418 Output Swing and Power Characteristics 3 Typical Application A typical application of the LM2418 is shown in Figure 6 Used in conjunction with an LM1203 a complete video channel from monitor input to CRT cathode is shown Performance is satisfactory for all applications up to 640 by 480 lines Typical rise fall times of this circuit are 15 ns with better than 50 VPP drive signals available to a 10 pF load In this application feedback is local to the LM1203 an alternative scheme would feed back from the output of the LM2418 to the positive clamp inputs of the LM1203 This would provide better black level control of the system TL K 11125 – 10 FIGURE 6 Typical Application LM1203-LM2418 Application 4 5 LM2418 Triple 30 MHz CRT Driver Physical Dimensions inches (millimeters) 11 Lead TO-220 Molded Package (T) Order Number LM2418T NS Package Number TA11B 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 Fran ais 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