LT3081
11
3081fc
For more information
applicaTions inForMaTion
Introduction
The LT3081 regulator is easy to use and has all the pro-
tection features expected in high performance regulators.
Included are short-circuit protection, reverse-input protec-
tion and safe operating area protection, as well as thermal
shutdown with hysteresis. Safe operating area (SOA) for
the LT3081 is extended, allowing for use in harsh indus-
trial and automotive environments where sudden spikes
in input voltage lead to high power dissipation.
The LT3081 fits well in applications needing multiple rails.
This new architecture adjusts down to zero with a single
resistor, handling modern low voltage digital ICs as well
as allowing easy parallel operation and thermal manage-
ment without heat sinks. Adjusting to zero output allows
shutting off the powered circuitry.
A precision “0” TC 50μA reference current source connects
to the noninverting input of a power operational amplifier.
The power operational amplifier provides a low impedance
buffered output to the voltage on the noninverting input.
A single resistor from the noninverting input to ground
sets the output voltage. If this resistor is set to 0Ω, zero
output voltage results. Therefore, any output voltage can
be obtained between zero and the maximum defined by
the input power supply is obtainable.
The benefit of using a true internal current source as the
reference, as opposed to a bootstrapped reference in older
regulators, is not so obvious in this architecture. A true
reference current source allows the regulator to have gain
and frequency response independent of the impedance on
the positive input. On older adjustable regulators, such as
the LT1086 loop gain changes with output voltage and
bandwidth changes if the adjustment pin is bypassed to
ground. For the LT3081, the loop gain is unchanged with
output voltage changes or bypassing. Output regulation
is not a fixed percentage of output voltage, but is a fixed
fraction of millivolts. Use of a true current source allows
all of the gain in the buffer amplifier to provide regulation,
and none of that gain is needed to amplify up the reference
to a higher output voltage.
The LT3081 has many additional features that facilitate
monitoring and control. Current limit is externally pro-
grammable via a single resistor between the I
LIM
pin and
OUT. Shorting this resistor out disables all output current
to the load, only bias currents remain.
The I
MON
pin produces a current output proportional to
load current. For every 1A of load current, the I
MON
pin
sources 200µA of current. This can be sensed using an
external resistor to monitor load requirements and detect
potential faults. The I
MON
pin can operate at voltages above
OUT, so it operates even during a short-circuit condition.
One additional monitoring function is the TEMP pin, a cur-
rent source that is proportional to average die temperature.
For die temperatures above 0°C, the TEMP pin sources a
current equal to 1µA/°C. This pin operates normally during
output short-circuit conditions.
Programming Linear Regulator Output Voltage
The LT3081 generates a 50μA reference current that flows
out of the SET pin. Connecting a resistor from SET to
ground generates a voltage that becomes the reference
point for the error amplifier (see Figure 1). The reference
voltage equals 50µA multiplied by the value of the SET
pin resistor. Any voltage can be generated and there is
no minimum output voltage for the regulator.
3081 F01
IN
SET
OUT
+
–
LT3081
50µA
R
LOAD
C
SET
R
SET
C
IN
V
OUT
= 50µA • R
SET
C
OUT
Figure 1. Basic Adjustable Regulator
