LT3080
9
3080fc
The LT3080 regulator is easy to use and has all the pro-
tection features expected in high performance regulators.
Included are short-circuit protection and safe operating
area protection, as well as thermal shutdown.
The LT3080 is especially well suited to applications needing
multiple rails. The new architecture adjusts down to zero
with a single resistor handling modern low voltage digital
IC’s as well as allowing easy parallel operation and thermal
management without heat sinks. Adjusting to “zero” output
allows shutting off the powered circuitry and when the
input is pre-regulated—such as a 5V or 3.3V input supply
—external resistors can help spread the heat.
A precision “0” TC 10µA internal current source is con-
nected to the noninverting input of a power operational
amplifier. The power operational amplifier provides a low
impedance buffered output to the voltage on the noninvert-
ing input. A single resistor from the noninverting input to
ground sets the output voltage and if this resistor is set
to zero, zero output results. As can be seen, any output
voltage can be obtained from zero up to the maximum
defined by the input power supply.
What is not so obvious from this architecture are the ben-
efits of using a true internal current source as the reference
as opposed to a bootstrapped reference in older regulators.
A true current source allows the regulator to have gain
and frequency response independent of the impedance on
the positive input. Older adjustable regulators, such as the
LT1086 have a change in loop gain with output voltage
as well as bandwidth changes when the adjustment pin
is bypassed to ground. For the LT3080, the loop gain is
unchanged by changing the output voltage or bypassing.
Output regulation is not fixed at a percentage of the output
voltage but is a fixed fraction of millivolts. Use of a true
current source allows all 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 LT3080 has the collector of the output transistor
connected to a separate pin from the control input. Since
the dropout on the collector (IN pin) is only 350mV, two
supplies can be used to power the LT3080 to reduce dis-
sipation: a higher voltage supply for the control circuitry
and a lower voltage supply for the collector. This increases
efficiency and reduces dissipation. To further spread the
heat, a resistor can be inserted in series with the collector
to move some of the heat out of the IC and spread it on
the PC board.
The LT3080 can be operated in two modes. Three-terminal
mode has the control pin connected to the power input pin
which gives a limitation of 1.35V dropout. Alternatively,
the “control” pin can be tied to a higher voltage and the
power IN pin to a lower voltage giving 350mV dropout
on the IN pin and minimizing the power dissipation. This
allows for a 1.1A supply regulating from 2.5V
IN
to 1.8V
OUT
or 1.8V
IN
to 1.2V
OUT
with low dissipation.
–
+
V
CONTROL
IN
10µA
3080 BD
OUTSET
block DiagraM
applicaTions inForMaTion
