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- LT1086 have a change in loop gain with output voltage tection features expected in high performance regulators. as well as bandwidth changes when the adjustment pin Included are short-circuit protection and safe operating is bypassed to ground. For the LT3080, the loop gain is area protection, as well as thermal shutdown. unchanged by changing the output voltage or bypassing. The LT3080 is especially well suited to applications needing Output regulation is not fixed at a percentage of the output multiple rails. The new architecture adjusts down to zero voltage but is a fixed fraction of millivolts. Use of a true with a single resistor handling modern low voltage digital current source allows all the gain in the buffer amplifier IC’s as well as allowing easy parallel operation and thermal to provide regulation and none of that gain is needed to management without heat sinks. Adjusting to “zero” output amplify up the reference to a higher output voltage. allows shutting off the powered circuitry and when the The LT3080 has the collector of the output transistor input is pre-regulated—such as a 5V or 3.3V input supply connected to a separate pin from the control input. Since —external resistors can help spread 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 noninvert- efficiency and reduces dissipation. To further spread the ing input. A single resistor from the noninverting input to heat, a resistor can be inserted in series with the collector 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 ben- which gives a limitation of 1.35V dropout. Alternatively, efits of using a true internal current source as the reference the “control” pin can be tied to a higher voltage and the as opposed to a bootstrapped reference in older regulators. power IN pin to a lower voltage giving 350mV dropout A true current source allows the regulator to have gain on the IN pin and minimizing the power dissipation. This and frequency response independent of the impedance on allows for a 1.1A supply regulating from 2.5VIN to 1.8VOUT the positive input. Older adjustable regulators, such as the or 1.8VIN to 1.2VOUT with low dissipation. 3080fc 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts