LT3080EDD-1 LINER [Linear Technology], LT3080EDD-1 Datasheet - Page 12

• Manufacturer Part Number: LT3080EDD-1
• Description: Parallelable 1.1A Adjustable Single Resistor Low Dropout Regulator
• Manufacturer: LINER [Linear Technology]

LT3080-1
Quieting the Noise
The LT3080-1 offers numerous advantages when it comes
to dealing with noise. There are several sources of noise
in a linear regulator. The most critical noise source for any
LDO is the reference; from there, the noise contribution
from the error amplifi er must be considered, and the gain
created by using a resistor divider cannot be forgotten.
Traditional low noise regulators bring the voltage reference
out to an external pin (usually through a large value resistor)
to allow for bypassing and noise reduction of reference
noise. The LT3080-1 does not use a traditional voltage
reference like other linear regulators, but instead uses a
reference current. That current operates with typical noise
current levels of 3.2pA/√Hz (1nA
100kHz bandwidth). The voltage noise of this is equal to the
noise current multiplied by the resistor value. The resistor
generates spot noise equal to √4kTR (k = Boltzmann’s
constant, 1.38 • 10
which is RMS summed with the reference current noise.
To lower reference noise, the voltage setting resistor may
be bypassed with a capacitor, though this causes start-up
time to increase as a factor of the RC time constant.
The LT3080-1 uses a unity-gain follower from the SET pin
to drive the output, and there is no requirement to use
a resistor to set the output voltage. Use a high accuracy
voltage reference placed at the SET pin to remove the
errors in output voltage due to reference current tolerance
and resistor tolerance. Active driving of the SET pin is
acceptable; the limitations are the creativity and ingenuity
of the circuit designer.
One problem that a normal linear regulator sees with
reference voltage noise is that noise is gained up along
with the output when using a resistor divider to operate
at levels higher than the normal reference voltage. With
the LT3080-1, the unity-gain follower presents no gain
whatsoever from the SET pin to the output, so noise
fi gures do not increase accordingly. Error amplifi er noise
is typically 125nV/√Hz (40μV
bandwidth); this is another factor that is RMS summed in
to give a fi nal noise fi gure for the regulator.
APPLICATIONS INFORMATION
12
-23
J/°K, and T is absolute temperature)
RMS
over the 10Hz to 100kHz
RMS
over the 10Hz to
Curves in the Typical Performance Characteristics
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.
Overload Recovery
Like many IC power regulators, the LT3080-1 has safe
operating area (SOA) protection. The SOA protection
decreases current limit as the input-to-output voltage
increases and keeps the power dissipation at safe levels for
all values of input-to-output voltage. The LT3080-1 provides
some output current at all values of input-to-output voltage
up to the device breakdown. See the Current Limit curve
in the Typical Performance Characteristics section.
When power is fi rst turned on, the input voltage rises
and the output follows the input, allowing the regulator to
start into very heavy loads. During start-up, as the input
voltage is rising, the input-to-output voltage differential
is small, allowing the regulator to supply large output
currents. With a high input voltage, a problem can occur
wherein removal of an output short will not allow the
output voltage to recover. Other regulators, such as the
LT1085 and LT1764A, also exhibit this phenomenon so it
is not unique to the LT3080-1.
The problem occurs with a heavy output load when the
input voltage is high and the output voltage is low. Common
situations are immediately after the removal of a short
circuit. The load line for such a load may intersect the
output current curve at two points. If this happens, there
are two stable operating points for the regulator. With this
double intersection, the input power supply may need to
be cycled down to zero and brought up again to make the
output recover.
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