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LM2427T Datasheet(PDF) 3 Page - National Semiconductor (TI) |
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LM2427T Datasheet(HTML) 3 Page - National Semiconductor (TI) |
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3 / 6 page  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 se- ries with the output of the LM2427 forms a 1001 voltage divider when connected to a 50X-input oscilloscope or net- work analyzer To calibrate pulse generator set to 24 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 RFIEMI shielding Applying an input current to the LM2427 will result in an output voltage An input current of about g45 mA will pro- vide a full output swing of g25V 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 push- pull configuration (see schematic on front page) Q2 is bi- ased 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 frequen- cies are present in the power supply line an electrolytic capacitor is recommended Application Hints The LM2427 is designed as a triple power amplifier for deliv- ering 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 ) Bypassing the resistor with a capacitor of about 47 pF will restore the rise and fall times but will result in some over- shoot (Figure 2b) Adding a resistor in series with the 47 pF capacitor will re- duce 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 dif- ferent board layouts TLH11967 – 6 FIGURE 2 Influence of Input Networks on Switching Performance 3 |
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