Datasheet LT3082 (Analog Devices) - 8

LT3082
APPLICATIONS INFORMATION Introduction
reference current source allows the regulator to have gain The LT3082 regulator is easy to use and has all the pro- and frequency response independent of the impedance on tection features expected in high performance regulators. the positive input. On older adjustable regulators, such as Included are reverse-input, reverse-output and reverse the LT1086, loop gain changes with output voltage and input-to-output protection for sensitive circuitry and loads. bandwidth changes if the adjustment pin is bypassed Additional protection includes short-circuit protection and to ground. For the LT3082, loop gain is unchanged with thermal shutdown with hysteresis. output voltage changes or bypassing. Output regulation is not a fi xed percentage of output voltage, but is a fi xed The LT3082 fi ts well in applications needing multiple rails. fraction of millivolts. Use of a true current source allows This new architecture adjusts down to zero with a single all of the gain in the buffer amplifi er to provide regulation, resistor, handling modern low voltage digital IC’s as well and none of that gain is needed to amplify up the reference as allowing easy parallel operation and thermal manage- to a higher output voltage. ment without heat sinks. Adjusting to zero output allows shutting off the powered circuitry. When the input is pre-
Programming Output Voltage
regulated—such as a 5V or 3.3V input supply—external The LT3082 generates a 10μA reference current that resistors can help spread the heat. fl ows out of the SET pin. Connecting a resistor from SET A precision “0” TC 10μA reference current source connects to GND generates a voltage that becomes the reference to the noninverting input of a power operational amplifi er. point for the error amplifi er (see Figure 1). The reference The power operational amplifi er provides a low impedance voltage equals 10μA multiplied by the value of the SET buffered output to the voltage on the noninverting input. pin resistor. Any voltage may be generated and there A single resistor from the noninverting input to ground is no minimum output voltage for the regulator. Table sets the output voltage. If this resistor is set to 0Ω, zero 1 lists many common output voltages and the closest output voltage results. Therefore, any output voltage be- standard 1% resistor values used to generate that output tween zero and the maximum defi ned by the input power voltage. supply voltage is obtainable. Regulation of the output voltage requires a minimum load The benefi t of using a true internal current source as the current of 0.5mA. For a true 0V output operation, return reference, as opposed to a bootstrapped reference in older this minimum 0.5mA load current to a negative supply regulators, is not so obvious in this architecture. A true voltage. IN LT3082 CIN 10μA + – SET OUT VOUT = 10μA • RSET CSET RSET COUT RLOAD 3082 F01
Figure 1. Basic Adjustable Regulator
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