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LT3088

3088fb

For more information www.linear.com/LT3088

APPLICATIONS INFORMATION

Quieting the Noise

The LT3088 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

LT3088 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 18pA/√Hz (5.7nARMS over a 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,

bypassthevoltagesettingresistorwithacapacitortoGND.

Notethatthisnoise-reductioncapacitorincreasesstart-up

time as a factor of the RC time constant.

The LT3088 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

outputvoltagesettingresistordividergainsupthereference

noise, especially if VOUT is much greater than VREF. The

LT3088’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 typical 85nV/√Hz(27µVRMS over

a 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.

Paralleling of regulators adds the benefit that output noise

is reduced. For n regulators in parallel, the output noise

drops by a factor of √n.

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 a 10Hz to 100kHz bandwidth.

Load Voltage Regulation

The LT3088 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 re-

sistance 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. 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 13).

Connected as shown, system load regulation is the sum

of the LT3088’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.

Figure 13. Connections for Best Load Regulation

SET

–

LT3088

50µA

3088 F13

OUT

RSET

PARASITIC

RESISTANCE

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