LT3083MPDF datasheet(14/28 Pages) AD

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LT3083

Rev. B

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APPLICATIONS INFORMATION

resistance of 10mΩ (5mΩ for the two devices in parallel)

only adds about 30mV of output regulation drop at an out-

put of 6A. With an output voltage of 3.3V, this only adds

1% to the regulation. Of course, paralleling more than two

LT3083s yields even higher output current. Spreading the

devices on the PC board also spreads the heat. Series

input resistors can further spread the heat if the input-

to-output difference is high.

Quieting the Noise

The LT3083 offers numerous noise performance advan-

tages. Every linear regulator has its sources of noise. In

general, a linear regulator’s critical noise source is the

reference. In addition, consider the error amplifier’s noise

contribution along with the resistor divider’s noise gain.

Many traditional low noise regulators bond out the voltage

reference to an external pin (usually through a large value

resistor) to allow for bypassing and noise reduction. The

LT3083 does not use a traditional voltage reference like

other linear regulators. Instead, it uses a 50µA reference

current. The 50µA current source generates noise current

levels of 3.16pA/√Hz (1nARMS) over the 10Hz to 100kHz

bandwidth). The equivalent voltage noise equals the RMS

noise current multiplied by the resistor value.

The SET pin resistor generates spot noise equal to √4kTR

(k = Boltzmann’s constant, 1.38 • 10–23J/°K, and T is

absolute temperature) which is RMS summed with the

voltage noise. If the application requires lower noise

performance, bypass the voltage setting resistor with a

capacitor to GND. Note that this noise-reduction capaci-

tor increases start-up time as a factor of the RC time

constant.

The LT3083 uses a unity-gain follower from the SET pin

to the OUT pin. Therefore, multiple possibilities exist

(besides a SET pin resistor) to set output voltage. For

example, using a high accuracy voltage reference from

SET to GND removes the errors in output voltage due to

reference current tolerance and resistor tolerance. Active

driving of the SET pin is acceptable.

The typical noise scenario for a linear regulator is that the

output voltage setting resistor divider gains up the noise

reference, especially if VOUT is much greater than VREF.

The LT3083’s noise advantage is that the unity gain fol-

lower presents no noise gain whatsoever from the SET

pin to the output. Thus, noise figures do not increase

accordingly. Error amplifier noise is typically 126.5nV/√Hz

(40µVRMS) over the 10Hz to 100kHz bandwidth). The

error amplifier’s noise is RMS summed with the other

noise terms to give a final noise figure for the regulator.

Curves in the Typical Performance Characteristics sec-

tion show noise spectral density and peak-to-peak noise

characteristics for both the reference current and error

amplifier over the 10Hz to 100kHz bandwidth.

Load Regulation

The LT3083 is a floating device. No ground pin exists on

the packages. Thus, the IC delivers all quiescent current

and drive current to the load. Therefore, it is not pos-

sible to provide true remote load sensing. The connec-

tion resistance between the regulator and the load deter-

mines load regulation performance. The data sheet’s load

regulation specification is Kelvin sensed at the package’s

pins. Negative-side sensing is a true Kelvin connection by

returning the bottom of the voltage setting resistor to the

negative side of the load (see Figure 6).

Connected as shown, system load regulation is the sum

of the LT3083’s load regulation and the parasitic line

resistance multiplied by the output current. To minimize

load regulation, keep the positive connection between the

regulator and load as short as possible. If possible, use

large diameter wire or wide PC board traces.

–

LT3083

VCONTROL

OUT

3080 F06

SET RSET

PARASITIC

RESISTANCE

LOAD

Figure 6. Connections for Best Load Regulation

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