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LM2427 Datasheet(PDF) 4 Page - National Semiconductor (TI) |
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LM2427 Datasheet(HTML) 4 Page - National Semiconductor (TI) |
4 / 6 page Application Hints (Continued) 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 TLH11967 – 7 FIGURE 3 Tilt on a Low Frequency Signal and Its Effects The tilt can be compensated by adding an external RC feed- back network as shown in Figure 4 The RC feedback helps by reducing the gain of the amplifier during the edge tran- sition 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) TLH11967 – 8 FIGURE 4 RC Feedback Network for Tilt Compensation To find the value of resistor R the following formula can be used R e (100 b x%) x% RF where x% is the percentage value of DV to the peak-to- peak output swing (VPP) RF is internally fixed to 5700X The value of capacitor C is determined by C e u R 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 PROTECTlNG AMPLIFIER OUTPUT FROM TUBE ARCING During normal CRT operation internal arcing may occasion- ally occur Spark gap protectors will limit the maximum volt- age but to a value that is much higher than allowable on the LM2427 This fast high voltage high energy pulse can dam- age 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 re- sponse 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 Lp e (RO a R1 a R2)2 24 CL where CL is the total load and RO is the intrinsic high fre- quency output resistance of the amplifier generally 160X TLH11967 – 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 de- stroy 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 (Va) pins of the LM2427 CAPACITlVE LOADS The LM2427 is designed to drive capacitive loads however the very high output slew rate of about 13700 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 HEAT SlNKlNG Power consumption by the LM2427 will depend on the sup- ply voltage used the output loading the peak-to-peak out- put swing and the operating frequency Since the LM2427 will dissipate up to 14W an external heatsink is always re- quired The maximum allowed case temperature is 90 C To calculate maximum heatsink thermal resistance use the fol- lowing formula Rth e (90 C b Max Ambient) 14 4 |
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