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LM196 Datenblatt(PDF) 6 Page - National Semiconductor (TI) |
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LM196 Datenblatt(HTML) 6 Page - National Semiconductor (TI) |
6 / 14 page Application Hints (Continued) Input and Output Capacitors The LM196 will tolerate a wide range of input and output capacitance but long wire runs or small values of output capacitance can sometimes cause problems If an output capacitor is used it should be 1 mF or larger We suggest 10 m F solid tantalum if significant improvements in high fre- quency output impedance are needed (see output imped- ance graph) This capacitor should be as close to the regu- lator as possible with short leads to reduce the effects of lead inductance No input capacitor is needed if the regula- tor is within 6 inches of the power supply filter capacitor using 18 gauge stranded wire For longer wire runs the LM196 input should be bypassed locally with a 47 mF (or larger) solid tantalum capacitor or a 100 mF (or larger) alu- minum electrolytic capacitor Correcting for Output Wire Losses (LM196LM396) Three-terminal regulators can only provide partial Kelvin load sensing (see Load Regulation) Full remote sensing can be added by using an external op amp to cancel the effect of voltage drops in the unsensed positive output lead In Figure 7 the LM301A op amp forces the voltage loss across the unsensed output lead to appear across R3 The current through R3 then flows out the Vb pin of the op amp through R4 The voltage drop across R4 will raise the output voltage by an amount equal to the line loss just cancelling the line loss itself A small (j 40 mV) initial output voltage error is created by the quiescent current of the op amp Cancellation range is limited by the maximum output current of the op amp about 300 mV as shown This can be raised by increasing R3 or R4 at the expense of more initial output error Transformers and Diodes Proper transformer ratings are very important in a high cur- rent supply because of the conflicting requirements of effi- ciency and tolerance to low-line conditions A transformer with a high secondary voltage will waste power and cause unnecessary heating in the regulator Too low a secondary voltage will cause loss of regulation under low-line condi- tions The following formulas may be used to calculate the required secondary voltage and current ratings using a full- wave center tap Vrms e VOUTaVREGaVRECTaVRIPPLE 02 J VNOM VLOW J(11) J Irms e (IOUT) (12) (Full-wave center tap) where VOUT e DC regulated output voltage VREG e Minimum input-output voltage of regulator VRECT e Rectifier forward voltage drop at three times DC output current VRIPPLE e 12 peak-to-peak capacitor ripple voltage e (53 c 10b3)(IOUT) 2C The factor of 11 is only an approximate factor accounting for load regula- tion of the transformer VNOM e Nominal line voltage AC rms VLOW e Low line voltage AC rms IOUT e DC output current Example IOUT e 10A VOUT e 5V Assume VREG e 22V VRECT e 12V VRIPPLE e 2 Vp-p VNOM e 115V VLOW e 105V Vrms e 5a22a12a1 02 J115 105 J 11 e 801 Vrms Capacitor C e (53 c 10b3)(IOUT) 2 c VRIPPLE e (53 c 10b3)(10) 2 e 26500 mF The diodes used in a full-wave rectified capacitor input sup- ply must have a DC current rating considerably higher than the average current flowing through them In a 10A supply for instance the average current through each diode is only 5A but the diodes should have a rating of 10A – 15A There are many reasons for this both thermal and electrical The diodes conduct current in pulses about 35 ms wide with a peak value of 5 – 8 times the average value and an rms value 15 – 20 times the average value This results in long term diode heating roughly equivalent to 10A DC current The most demanding condition however may be the one cycle surge through the diode during power turn on The peak value of the surge is about 10 – 20 times the DC output current of the supply or 100A – 200A for a 10A supply The diodes must have a one cycle non-repetitive surge rating of 200A or more and this is usually not found in a diode with less than 10A average current rating Keep in mind that even though the LM196 may be used at current levels be- low 10A the diodes may still have to survive shorted output conditions where average current could rise to 12A – 15A Smaller transformers and filter capacitors used in lower cur- rent supplies will reduce surge currents but unless specific information is available on worst-case surges it is best not to economize on diodes Stud-mounted devices in a DO-4 package are recommended Cathode-to-case types may be bolted directly to the same heat sink as the LM196 because the case of the regulator is its power input Part numbers to consider are the 1N1200 series rated at 12A average cur- rent in a DO-4 stud package Additional types include com- mon cathode duals in a TO-3 package both standard and Schottky and various duals in plastic filled assemblies Schottky diodes will improve efficiency especially in low voltage applications In a 5V supply for instance Schottky diodes will decrease wasted power by up to 6W or alterna- tively provide an additional 5% ‘‘drop out’’ margin for low- line conditions Several manufacturers are producing ‘‘high efficiency’’ diodes with a forward voltage drop nearly as good as Schottkys at high current levels These devices do not have the low breakdown voltages of Schottkys so are much less prone to reverse breakdown induced failures 6 |
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