Document
LM2427 Triple 80 MHz CRT Driver
April 1995
LM2427 Triple 80 MHz CRT Driver
General Description
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
Features
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
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 Safe Operating Power Consumption
a 85V
Storage Temperature Range TSTG Operating Temperature Range TCASE Lead Temperature (Soldering 10 sec ) ESD Tolerance
b 25 C to a 100 C b 20 C to a 90 C
14W
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 ICC VINDC VOUTDC tR tF AV LE D AV Parameter Supply Current (per Amplifier) Input Offset Voltage Output Offset Voltage Rise Time Fall Time Voltage Gain Linearity Error Gain Matching VOUT from a 10V to a 70V (Note 2) (Note 3) 10% to 90% (Note 1) 90% to 10% (Note 1)
b 11
Conditions Min No Input or Output Load 14 34
LM2427 Typical 24 16 40 35 35
b 13 b 14
Units Max 30 18 46 mA V V ns ns V V % dB
5 02
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)
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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
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
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
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) 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 val.