LT3085 LINER [Linear Technology], LT3085 Datasheet - Page 14

• Manufacturer Part Number: LT3085
• Description: Adjustable 500mA Single Resistor Low Dropout Regulator
• Manufacturer: LINER [Linear Technology]

LT3085
APPLICATIONS INFORMATION
The resistor generates spot noise equal to√4kTR (k =
Boltzmann’s constant, 1.38 • 10
temperature) which is RMS summed with the reference
current noise. To lower reference noise, the voltage set-
ting resistor may be bypassed with a capacitor, though
this causes start-up time to increase as a factor of the RC
time constant.
The LT3085 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 er-
rors 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 refer-
ence 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 LT3085,
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 100nV/√Hz
(33μV
another factor that is RMS summed in to give a fi nal noise
fi gure for the regulator.
Curves in the Typical Performance Characteristics show
noise spectral density and peak-to-peak noise character-
istics for both the reference current and error amplifi er
over the 10Hz to 100kHz bandwidth.
Overload Recovery
Like many IC power regulators, the LT3085 has safe operat-
ing 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 LT3085 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.
14
RMS
over the 10Hz to 100kHz bandwidth); this is
-23
J/°K, and T is absolute
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 volt-
age to recover. Other regulators, such as the LT1085 and
LT1764A, also exhibit this phenomenon so it is not unique
to the LT3085.
The problem occurs with a heavy output load when the
input voltage is high and the output voltage is low. Com-
mon 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.
Load Regulation
Because the LT3085 is a fl oating device (there is no ground
pin on the part, all quiescent and drive current is delivered
to the load), it is not possible to provide true remote load
sensing. Load regulation will be limited by the resistance
of the connections between the regulator and the load.
The data sheet specifi cation for load regulation is Kelvin
sensed at the pins of the package. Negative side sensing
is a true Kelvin connection, with the bottom of the voltage
setting resistor returned to the negative side of the load
(see Figure 7). Connected as shown, system load regulation
will be the sum of the LT3085 load regulation and the
parasitic line resistance multiplied by the output current.
It is important to keep the positive connection between
the regulator and load as short as possible and use large
wire or PC board traces.
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