Datasheet LT1086 (Analog Devices) - 8

LT1086 Series
U U W U APPLICATIONS INFORMATION
The LT1086 family of 3-terminal regulators is easy to use response with heavy load current changes. Output capaci- and has all the protection features that are expected in high tance can be increased without limit and larger values of performance voltage regulators. They are short-circuit output capacitor further improve stability and transient protected and have safe area protection as well as thermal response of the LT1086 regulators. shutdown to turn off the regulator should the temperature Another possible stability problem that can occur in mono- exceed about 165°C at the sense point. lithic IC regulators is current limit oscillations. These can These regulators are pin compatible with older 3-terminal occur because in current limit, the safe area protection adjustable devices, offer lower dropout voltage and more exhibits a negative impedance. The safe area protection precise reference tolerance. Further, the reference stabil- decreases the current limit as the input-to-output voltage ity with temperature is improved over older types of increases.That is the equivalent of having a negitive resis- regulators. The only circuit difference between using the tance since increasing voltage causes current to decrease. LT1086 family and older regulators is that they require an Negative resistance during current limit is not unique to output capacitor for stability. the LT1086 series and has been present on all power IC regulators. The value of negative resistance is a function of
Stability
how fast the current limit is folded back as input-to-output The circuit design used in the LT1086 family requires the voltage increases. This negative resistance can react with use of an output capacitor as part of the device frequency capacitors or inductors on the input to cause oscillation compensation. For all operating conditions, the addition of during current limiting. Depending on the value of series 150µF aluminum electrolytic or a 22µF solid tantalum on resistance, the overall circuitry may end up unstable. Since the output will ensure stability. Normally capacitors much this is a system problem, it is not necessarily easy to solve; smaller than this can be used with the LT1086. Many however, it does not cause any problems with the IC different types of capacitors with widely varying charac- regulator and can usually be ignored. teristics are available. These capacitors differ in capacitor
Protection Diodes
tolerance (sometimes ranging up to ±100%), equivalent series resistance, and capacitance temperature coeffi- In normal operation the LT1086 family does not need any cient. The 150µF or 22µF values given will ensure stability. protection diodes. Older adjustable regulators required protection diodes between the adjustment pin and the When using the LT1086 the adjustment terminal can be output and from the output to the input to prevent over- bypassed to improve ripple rejection. When the adjust- stressing the die. The internal current paths on the LT1086 ment terminal is bypassed the requirement for an output adjustment pin are limited by internal resistors. Therefore, capacitor increases. The values of 22µF tantalum or 150µF even with capacitors on the adjustment pin, no protection aluminum cover all cases of bypassing the adjustment diode is needed to ensure device safety under short-circuit terminal. For fixed voltage devices or adjustable devices conditions. without an adjust pin bypass capacitor, smaller output capacitors can be used with equally good results. The table Diodes between input and output are usually not needed. below shows approximately what size capacitors are needed The internal diode between the input and the output pins to ensure stability. of the LT1086 family can handle microsecond surge currents of 10A to 20A. Even with large output capaci-
Recommended Capacitor Values
tances, it is very difficult to get those values of surge
INPUT OUTPUT ADJUSTMENT
currents in normal operation. Only with high value output 10µF 10µF Tantalum, 50µF Aluminum None capacitors such as 1000µF to 5000µF, and with the input 10µF 22µF Tantalum, 150µF Aluminum 20µF pin instantaneously shorted to ground, can damage occur. A crowbar circuit at the input of the LT1086 can generate Normally, capacitor values on the order of 100µF are used those kinds of currents and a diode from output to input is in the output of many regulators to ensure good transient then recommended. Normal power supply cycling or even sn1086 1086ffs 8