LT3083MPDF datasheet(15/28 Pages) LINER

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LT3083

3083f

APPLICATIONS INFORMATION

resistance of 10mΩ (5mΩ for the two devices in paral-

lel) only adds about 30mV of output regulation drop at

an output 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 abso-

lute 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 capacitor 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 follower

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 possible

to provide true remote load sensing. The connection resis-

tance between the regulator and the load determines load

regulation performance. The data sheet’s load regulation

speciﬁcation is Kelvin sensed at the package’s pins. Nega-

tive-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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