Datasheet LT3080 (Analog Devices) - 9

LT3080
BLOCK DIAGRAM
IN VCONTROL 10µA + – 3080 BD SET OUT
APPLICATIONS INFORMATION
The LT3080 regulator is easy to use and has all the pro- regulators, such as the LT1086 have a change in loop tection features expected in high performance regulators. gain with output voltage as well as bandwidth changes Included are short-circuit protection and safe operating when the adjustment pin is bypassed to ground. For the area protection, as well as thermal shutdown. LT3080, the loop gain is unchanged by changing the out- The LT3080 is especially well suited to applications need- put voltage or bypassing. Output regulation is not fixed at ing multiple rails. The new architecture adjusts down to a percentage of the output voltage but is a fixed fraction of zero with a single resistor handling modern low voltage millivolts. Use of a true current source allows all the gain digital IC’s as well as allowing easy parallel operation in the buffer amplifier to provide regulation and none of and thermal management without heat sinks. Adjusting that gain is needed to amplify up the reference to a higher to “zero” output allows shutting off the powered circuitry output voltage. and when the input is pre-regulated—such as a 5V or The LT3080 has the collector of the output transistor 3.3V input supply —external resistors can help spread connected to a separate pin from the control input. Since the heat. the dropout on the collector (IN pin) is only 350mV, two A precision “0” TC 10µA internal current source is con- supplies can be used to power the LT3080 to reduce dis- nected to the noninverting input of a power operational sipation: a higher voltage supply for the control circuitry amplifier. The power operational amplifier provides a low and a lower voltage supply for the collector. This increases impedance buffered output to the voltage on the nonin- efficiency and reduces dissipation. To further spread the verting input. A single resistor from the noninverting input heat, a resistor can be inserted in series with the collector to ground sets the output voltage and if this resistor is set to move some of the heat out of the IC and spread it on to zero, zero output results. As can be seen, any output the PC board. voltage can be obtained from zero up to the maximum The LT3080 can be operated in two modes. Three-terminal defined by the input power supply. mode has the control pin connected to the power input pin What is not so obvious from this architecture are the which gives a limitation of 1.35V dropout. Alternatively, benefits of using a true internal current source as the the “control” pin can be tied to a higher voltage and the reference as opposed to a bootstrapped reference in power IN pin to a lower voltage giving 350mV dropout older regulators. A true current source allows the regu- on the IN pin and minimizing the power dissipation. This lator to have gain and frequency response independent allows for a 1.1A supply regulating from 2.5VIN to 1.8VOUT of the impedance on the positive input. Older adjustable or 1.8VIN to 1.2VOUT with low dissipation. Rev. D For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Output Voltage Stability and Output Capacitance Paralleling Devices Thermal Performance Quieting the Noise Overload Recovery Load Regulation Thermal Considerations T Package, 5-Lead TO-220 Calculating Junction Temperature Reducing Power Dissipation Typical Applications Package Description Revision History Typical Application Related Parts