lt3080-1 Linear Technology Corporation, lt3080-1 Datasheet - Page 8

• Manufacturer Part Number: lt3080-1
• Description: Parallelable 1.1a Adjustable Single Resistor Low Dropout Regulator
• Manufacturer: Linear Technology Corporation

LT3080-1
BLOCK DIAGRAM
The LT3080-1 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 LT3080-1 is especially well suited to applications
needing multiple rails. The new architecture adjusts down
to zero with a single resistor handling modern low volt-
age 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 non-inverting input of a power operational
amplifi er. The power operational amplifi er provides a low
impedance buffered output to the voltage on the non-invert-
ing input. A single resistor from the non-inverting 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
defi ned by the input power supply.
What is not so obvious from this architecture are the ben-
efi ts 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
8
APPLICATIONS INFORMATION
V
CONTROL
IN
SET
10μA
+
–
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 LT3080-1, the loop gain is
unchanged by changing the output voltage or bypassing.
Output regulation is not fi xed at a percentage of the output
voltage but is a fi xed fraction of millivolts. Use of a true
current source allows all the gain in the buffer amplifi er
to provide regulation and none of that gain is needed to
amplify up the reference to a higher output voltage.
The LT3080-1 also incorporates an internal ballast resistor
to allow for direct paralleling of devices without the need for
PC board trace resistors or sense resistors. This internal
ballast resistor allows multiple devices to be paralleled
directly on a surface mount board for higher output current
and higher power dissipation while keeping board layout
simple and easy. It is not diffi cult to add more regulators
for higher output current; inputs of devices are all tied to-
gether, outputs of all devices are tied directly together, and
SET pins of all devices are tied directly together. Because
of the internal ballast resistor, devices automatically share
the load and the power dissipation.
The LT3080-1 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 300mV, two
supplies can be used to power the LT3080-1 to reduce
OUT
25mΩ
30801 BD
30801f