Datasheet LT1083, LT1084, LT1085 (Analog Devices) - 10

LT1083/LT1084/LT1085 Fixed
applicaTions inForMaTion
resistance since increasing voltage causes current to
Overload Recovery
decrease. Negative resistance during current limit is not Like any of the IC power regulators, the LT1083 has safe unique to the LT1083 series and has been present on all area protection. The safe area protection decreases the power IC regulators. The value of negative resistance is current limit as input-to-output voltage increases and keeps a function of how fast the current limit is folded back as the power transistor inside a safe operating region for all input-to-output voltage increases. This negative resistance values of input-to-output voltage. The LT1083 protection can react with capacitors or inductors on the input to is designed to provide some output current at all values cause oscillations during current limiting. Depending on of input-to-output voltage up to the device breakdown. the value of series resistance, the overall circuitry may end up unstable. Since this is a system problem, it is not When power is first turned on, as the input voltage rises, necessarily easy to solve; however it does not cause any the output follows the input, allowing the regulator to start problems with the IC regulator and can usually be ignored. up into very heavy loads. During the start-up, as the input voltage is rising, the input-to-output voltage differential
Protection Diodes
remains small allowing the regulator to supply large In normal operation the LT1083 family does not need any output currents. With high input voltage a problem can protection diodes, The internal diode between the input occur wherein removal of an output short will not allow and the output pins of the LT1083 family can handle the output voltage to recover. Older regulators such as microsecond surge currents of 50A to 100A. Even with the 7800 series, also exhibited this phenomenon so it is large output capacitances it is very difficult to get those not unique to the LT1083. values of surge current in normal operation. Only with The problem occurs with a heavy output load when the high value output capacitors, such as 1000µF to 5000µF input voltage is high and the output voltage is low, such and with the input pin instantaneously shorted to ground, as immediately after a removal of a short. The load line for can damage occur. A crowbar circuit at the input of the such a load may intersect the output current curve at two LT1083 can generate those kinds of currents and a diode points. If this happens there are two stable output operat- from output-to-input is then recommended. Normal ing points for the regulator. With this double intersection power supply cycling or even plugging and unplugging the power supply may need to be cycled down to zero and in the system will not generate currents large enough to brought up again to make the output recover. do any damage.
Ripple Rejection
D1 1N4002 In applications that require improved ripple rejection the (OPTIONAL) LT1083 series adjustable regulators should be used. With LT1083 series adjustable regulators the addition of a by- pass capacitor from the adjust pin to ground will reduce V IN LT1083 OUT V IN OUT output ripple by the ratio of V ADJ + OUT/1.25V. See LT1083 series COUT adjustable regulator data sheet. 150µF 1083/4/5 AI01 1083ffe 10 For more information www.linear.com/LT1083 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Typical Performance Characteristics Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts