Datasheet LT1083, LT1084, LT1085 (Analog Devices) - 11

LT1083/LT1084/LT1085
applicaTions inForMaTion
Normally, capacitor values on the order of 100µF are used pin instantaneously shorted to ground, can damage occur. in the output of many regulators to ensure good transient A crowbar circuit at the input of the LT1083 can generate response with heavy load current changes. Output capaci- those kinds of currents, and a diode from output to input is tance can be increased without limit and larger values of then recommended. Normal power supply cycling or even output capacitor further improve stability and transient plugging and unplugging in the system will not generate response of the LT1083 regulators. current large enough to do any damage. Another possible stability problem that can occur in mono- The adjustment pin can be driven on a transient basis lithic IC regulators is current limit oscillations. These can ±25V, with respect to the output without any device deg- occur because, in current limit, the safe area protection radation. Of course, as with any IC regulator, exceeding exhibits a negative impedance. The safe area protection the maximum input to output voltage differential causes decreases the current limit as the input-to-output volt- the internal transistors to break down and none of the age increases. That is the equivalent of having a negative protection circuitry is functional. resistance since increasing voltage causes current to D1 decrease. Negative resistance during current limit is not 1N4002 (OPTIONAL) unique to the LT1083 series and has been present on all power IC regulators. The value of the negative resistance is a function of how fast the current limit is folded back V IN LT1083 OUT V IN OUT as input-to-output voltage increases. This negative resis- ADJ + R1 COUT tance can react with capacitors or inductors on the input 150µF to cause oscillation during current limiting. Depending + CADJ on the value of series resistance, the overall circuitry may R2 10µF end up unstable. Since this is a system problem, it is not 1083/4/5 ADJ F00 necessarily easy to solve; however, it does not cause any problems with the IC regulator and can usually be ignored.
Overload Recovery Protection Diodes
Like any of the IC power regulators, the LT1083 has safe area protection. The safe area protection decreases the In normal operation, the LT1083 family does not need current limit as input-to-output voltage increases and any protection diodes. Older adjustable regulators re- keeps the power transistor inside a safe operating region quired protection diodes between the adjustment pin and for all values of input-to-output voltage. The LT1083 the output and from the output to the input to prevent protection is designed to provide some output current overstressing the die. The internal current paths on the at all values of input-to-output voltage up to the device LT1083 adjustment pin are limited by internal resistors. breakdown. Therefore, even with capacitors on the adjustment pin, no protection diode is needed to ensure device safety under When power is first turned on, as the input voltage rises, short-circuit conditions. the output follows the input, allowing the regulator to start up into very heavy loads. During the start-up, as the input Diodes between input and output are usually not needed. voltage is rising, the input-to-output voltage differential The internal diode between the input and the output pins remains small, allowing the regulator to supply large of the LT1083 family can handle microsecond surge cur- output currents. With high input voltage, a problem can rents of 50A to 100A. Even with large output capacitances, occur wherein removal of an output short will not allow it is very difficult to get those values of surge currents the output voltage to recover. Older regulators, such as in normal operations. Only with a high value of output the 7800 series, also exhibited this phenomenon, so it is capacitors, such as 1000µF to 5000µF and with the input not unique to the LT1083. 108345fh For more information www.linear.com/LT1083 11 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Block Diagram Applications Information Typical Application Package Description Revision History Typical Applications Related Parts