lt3083idf Linear Technology Corporation, lt3083idf Datasheet - Page 15

• Manufacturer Part Number: lt3083idf
• Description: Lt3083 - Adjustable 3a Single Resistor Low Dropout Regulator
• Manufacturer: Linear Technology Corporation

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 amplifi er’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 (1nA
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
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 V
OUT
RMS
is much greater than V
) over the 10Hz to 100kHz
–23
J/°K, and T is abso-
REF
.
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 fi gures do not increase accordingly.
Error amplifi er noise is typically 126.5nV/√Hz (40μV
over the 10Hz to 100kHz bandwidth). The error amplifi er’s
noise is RMS summed with the other noise terms to give
a fi nal noise fi gure 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
amplifi er over the 10Hz to 100kHz bandwidth.
Load Regulation
The LT3083 is a fl oating 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
specifi cation 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.
V
CONTROL
Figure 6. Connections for Best Load Regulation
IN
SET R
LT3083
+
–
SET
OUT
RESISTANCE
PARASITIC
R
R
R
P
P
P
3080 F06
LT3083
LOAD
15
RMS
3083f
)