LT3083MPDF datasheet(10/28 Pages) AD

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LT3083

Rev. B

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APPLICATIONS INFORMATION

The LT3083 regulator is easy to use and has all the pro-

tection features expected in high performance regulators.

Included are short-circuit protection and safe operat-

ing area protection, as well as thermal shutdown with

hysteresis.

The LT3083 fits well in applications needing multiple rails.

This new architecture adjusts down to zero with a single

resistor, handling modern low voltage digital IC’s as well

as allowing easy parallel operation and thermal manage-

ment without heat sinks. Adjusting to zero output allows

shutting off the powered circuitry. When the input is pre-

regulated, such as with a 5V or 3.3V input supply, external

resistors can help spread the heat.

A precision “0” TC 50μA reference current source con-

nects to the noninverting input of a power operational

amplifier. The power operational amplifier provides a low

impedance buffered output to the voltage on the nonin-

verting input. A single resistor from the noninverting input

to ground sets the output voltage. If this resistor is set

to 0Ω, zero output voltage results. Therefore, any output

voltage can be obtained between zero and the maximum

defined by the input power supply.

The benefit of using a true internal current source as the

reference, as opposed to a bootstrapped reference in older

regulators, is not so obvious in this architecture. A true

reference current source allows the regulator to have gain

and frequency response independent of the impedance on

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

ground. For the LT3083, the loop gain is unchanged with

output voltage changes or bypassing. Output regulation

is not a fixed percentage of output voltage, but is a fixed

fraction of millivolts. Use of a true current source allows

all of the gain in the buffer amplifier to provide regulation,

and none of that gain is needed to amplify up the reference

to a higher output voltage.

The LT3083 has the collector of the output transistor con-

nected to a separate pin from the control input. Since the

dropout on the collector (IN pin) is typically only 310mV,

two supplies can be used to power the LT3083 to reduce

dissipation: a higher voltage supply for the control cir-

cuitry and a lower voltage supply for the collector. This

increases efficiency and reduces dissipation. To further

spread the heat, a resistor inserted in series with the col-

lector moves some of the heat out of the IC to spread

it on the PC board (see the section

Reducing Power

Dissipation).

The LT3083 can be operated in two modes. Three termi-

nal mode has the VCONTROL pin connected to the IN pin

and gives a limitation of 1.25V dropout. Alternatively, the

VCONTROL pin is separately tied to a higher voltage and the

IN pin to a lower voltage giving 310mV dropout on the IN

pin, minimizing total power dissipation. This allows for a

3A supply regulating from 2.5VIN to 1.8VOUT or 1.8VIN to

1.2VOUT with low power dissipation.

Programming Output Voltage

The LT3083 sources a 50μA reference current that flows

out of the SET pin. Connecting a resistor from SET to

ground generates a voltage that becomes the reference

point for the error amplifier (see Figure 1). The refer-

ence voltage equals 50µA multiplied by the value of

the SET pin resistor. Any voltage can be generated and

there is no minimum output voltage for the regulator.

Figure 1. Basic Adjustable Regulator

–

LT3083

VCONTROL

OUT

3083 F01

SET

COUT

RSET

VOUT

CSET

VIN

50µA

VOUT = 50µA • RSET

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