Datasheet LT3083 (Analog Devices) - 10

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APPLICATIONS INFORMATION
The LT3083 regulator is easy to use and has all the pro- The LT3083 has the collector of the output transistor con- tection features expected in high performance regulators. nected to a separate pin from the control input. Since the Included are short-circuit protection and safe operat- dropout on the collector (IN pin) is typically only 310mV, ing area protection, as wel as thermal shutdown with two supplies can be used to power the LT3083 to reduce hysteresis. dissipation: a higher voltage supply for the control cir- The LT3083 fits well in applications needing multiple rails. cuitry and a lower voltage supply for the collector. This This new architecture adjusts down to zero with a single increases efficiency and reduces dissipation. To further resistor, handling modern low voltage digital IC’s as well spread the heat, a resistor inserted in series with the col- as allowing easy parallel operation and thermal manage- lector moves some of the heat out of the IC to spread ment without heat sinks. Adjusting to zero output allows it on the PC board (see the section Reducing Power shutting off the powered circuitry. When the input is pre- Dissipation). regulated, such as with a 5V or 3.3V input supply, external The LT3083 can be operated in two modes. Three termi- resistors can help spread the heat. nal mode has the VCONTROL pin connected to the IN pin A precision “0” TC 50μA reference current source con- and gives a limitation of 1.25V dropout. Alternatively, the nects to the noninverting input of a power operational VCONTROL pin is separately tied to a higher voltage and the amplifier. The power operational amplifier provides a low IN pin to a lower voltage giving 310mV dropout on the IN impedance buffered output to the voltage on the nonin- pin, minimizing total power dissipation. This allows for a verting input. A single resistor from the noninverting input 3A supply regulating from 2.5VIN to 1.8VOUT or 1.8VIN to to ground sets the output voltage. If this resistor is set 1.2VOUT with low power dissipation. to 0Ω, zero output voltage results. Therefore, any output
Programming Output Voltage
voltage can be obtained between zero and the maximum defined by the input power supply. The LT3083 sources a 50μA reference current that flows out of the SET pin. Connecting a resistor from SET to The benefit of using a true internal current source as the ground generates a voltage that becomes the reference reference, as opposed to a bootstrapped reference in older point for the error amplifier (see Figure 1). The refer- regulators, is not so obvious in this architecture. A true ence voltage equals 50µA multiplied by the value of reference current source allows the regulator to have gain the SET pin resistor. Any voltage can be generated and and frequency response independent of the impedance on there is no minimum output voltage for the regulator. the positive input. On older adjustable regulators, such as the LT1086, loop gain changes with output voltage and bandwidth changes if the adjustment pin is bypassed to IN LT3083 ground. For the LT3083, the loop gain is unchanged with VCONTROL output voltage changes or bypassing. Output regulation 50µA is not a fixed percentage of output voltage, but is a fixed + + + fraction of millivolts. Use of a true current source allows VIN VCONTROL – OUT all of the gain in the buffer amplifier to provide regulation, VOUT and none of that gain is needed to amplify up the reference SET C to a higher output voltage. OUT RSET CSET 3083 F01 VOUT = 50µA • RSET
Figure 1. Basic Adjustable Regulator
Rev. B 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts