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LT1086IT-5 Datasheet(PDF) 8 Page - Linear Technology |
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LT1086IT-5 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LT1086 Series The LT1086 family of 3-terminal regulators is easy to use and has all the protection features that are expected in high performance voltage regulators. They are short-circuit protected, have safe area protection as well as thermal shutdown to turn off the regulator should the temperature exceed about 165 °C at the sense point. These regulators are pin compatible with older 3-terminal adjustable devices, offer lower dropout voltage and more precise reference tolerance. Further, the reference stabil- ity with temperature is improved over older types of regulators. The only circuit difference between using the LT1086 family and older regulators is that they require an output capacitor for stability. Stability The circuit design used in the LT1086 family requires the use of an output capacitor as part of the device frequency compensation. For all operating conditions, the addition of 150 µF aluminum electrolytic or a 22µF solid tantalum on the output will ensure stability. Normally capacitors much smaller than this can be used with the LT1086. Many different types of capacitors with widely varying charac- teristics are available. These capacitors differ in capacitor tolerance (sometimes ranging up to ±100%), equivalent series resistance, and capacitance temperature coeffi- cient. The 150 µF or 22µF values given will ensure stability. When using the LT1086 the adjustment terminal can be bypassed to improve ripple rejection. When the adjust- ment terminal is bypassed the requirement for an output capacitor increases. The values of 22 µF tantalum or 150µF aluminum cover all cases of bypassing the adjustment terminal. For fixed voltage devices or adjustable devices without an adjust pin bypass capacitor, smaller output capacitors can be used with equally good results and the table below shows approximately what size capacitors are needed to ensure stability. Recommended Capacitor Values INPUT OUTPUT ADJUSTMENT 10 µF10µF Tantalum, 50µF Aluminum None 10 µF22µF Tantalum, 150µF Aluminum 20 µF Normally, capacitor values on the order of 100 µF are used in the output of many regulators to ensure good transient APPLICATIONS INFORMATION response with heavy load current changes. Output capaci- tance can be increased without limit and larger values of output capacitor further improve stability and transient response of the LT1086 regulators. Another possible stability problem that can occur in mono- lithic IC regulators is current limit oscillations. These can occur because in current limit, the safe area protection exhibits a negative impedance. The safe area protection decreases the current limit as the input-to-output voltage increases.That is the equivalent of having a negitive resis- tance since increasing voltage causes current to decrease. Negative resistance during current limit is not unique to the LT1086 series and has been present on all power IC regulators. The value of negative resistance is a function of how fast the current limit is folded back as input-to-output voltage increases. This negative resistance can react with capacitors or inductors on the input to cause oscillation during current limiting. Depending on the value of series resistance, the overall circuitry may end up unstable. Since this is a system problem, it is not necessarily easy to solve; however it does not cause any problems with the IC regulator and can usually be ignored. Protection Diodes In normal operation the LT1086 family does not need any protection diodes. Older adjustable regulators required protection diodes between the adjustment pin and the output and from the output to the input to prevent over- stressing the die. The internal current paths on the LT1086 adjustment pin are limited by internal resistors. Therefore, even with capacitors on the adjustment pin, no protection diode is needed to ensure device safety under short-circuit conditions. Diodes between input and output are usually not needed. The internal diode between the input and the output pins of the LT1086 family can handle microsecond surge currents of 10A to 20A. Even with large output capaci- tances, it is very difficult to get those values of surge currents in normal operation. Only with high value output capacitors such as 1000 µF to 5000µF, and with the input pin instantaneously shorted to ground, can damage occur. A crowbar circuit at the input of the LT1086 can generate those kinds of currents and a diode from output to input is then recommended. Normal power supply cycling or even |
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