Datasheet LT3085 (Analog Devices) - 8

LT3085
BLOCK DIAGRAM
IN VCONTROL 10μA + – 3085 BD SET OUT
APPLICATIONS INFORMATION
The LT3085 regulator is easy to use and has all the pro- What is not so obvious from this architecture are the ben- tection features expected in high performance regulators. efi ts of using a true internal current source as the reference Included are short-circuit protection and safe operating as opposed to a bootstrapped reference in older regulators. area protection, as well as thermal shutdown. A true current source allows the regulator to have gain and frequency response independent of the impedance on The LT3085 is especially well suited to applications needing the positive input. Older adjustable regulators, such as the multiple rails. The new architecture adjusts down to zero LT1086, have a change in loop gain with output voltage with a single resistor, handling modern low voltage digital as well as bandwidth changes when the adjustment pin IC’s as well as allowing easy parallel operation and thermal is bypassed to ground. For the LT3085, the loop gain is management without heat sinks. Adjusting to “zero” output unchanged by changing the output voltage or bypassing. allows shutting off the powered circuitry and when the Output regulation is not fi xed at a percentage of the output input is pre-regulated – such as a 5V or 3.3V input supply voltage but is a fi xed fraction of millivolts. Use of a true – external resistors can help spread the heat. current source allows all the gain in the buffer amplifi er A precision “0” TC 10μA internal current source is connected to provide regulation and none of that gain is needed to to the non-inverting input of a power operational amplifi er. amplify up the reference to a higher output voltage. The power operational amplifi er provides a low impedance The LT3085 has the collector of the output transistor buffered output to the voltage on the non-inverting input. connected to a separate pin from the control input. Since the A single resistor from the non-inverting input to ground dropout on the collector (IN pin) is only 275mV, two supplies sets the output voltage and if this resistor is set to zero, can be used to power the LT3085 to reduce dissipation: a zero output results. As can be seen, any output voltage higher voltage supply for the control circuitry and a lower can be obtained from zero up to the maximum defi ned by voltage supply for the collector. This increases effi ciency and the input power supply. reduces dissipation. To further spread the heat, a resistor can be inserted in series with the collector to move some of the heat out of the IC and spread it on the PC board. 3085fb 8 Document Outline FEATURES DESCRIPTION APPLICATIONS 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