Adjustable three-terminal voltage
regulators made their debut in
Electronic Design in an April 12, 1977 article called "Break Loose From
Fixed IC Regulators" by Robert Dobkin,
then an IC designer at National Semiconductor. Dobkin had adapted National's
bandgap-based fixed regulator to make it
adjustable via a voltage divider on the output. The divider's center tap is applied to
the anode of the regulator's voltage reference (see the figure, a).
In 1981, Dobkin and Bob Swanson left
National to found Linear Technology,
where Dobkin continues as chief technology officer. Remarkably, after more than 30
years, Dobkin is still perfecting the
adjustable three-terminal regulator. To be
fair, there's been a lot of other work in the
meantime. His patent portfolio is prodigious. But he insists that Linear's new
LT3080 has been his pet project since
the program's beginning.
SO WHAT'S NEW?
To power
today's low-voltage logic, this adjustable
low-dropout (LDO) regulator can deliver
from 0 V up to VIN – 300 mV from sources
up to 40 V. To deliver high power without
the need for heatsinks, it's designed to be
easily paralleled, spreading regulation's
thermal footprint so the circuit board
alone can handle heat dissipation.
"The LT3080 regulator allows designers to have an all-surface-mount solution
in high-current, noise-sensitive applications such as high-frequency serial data
links," Dobkin said. "Also, with the ability
to provide zero output, it can control powering down parts of the system. Having
the collector of the pass transistor available further enhances the options of
spreading the heat."
Enabling the parallel use of regulators
for greater current capacity was crucial
for Dobkin's design team. Generally, 1 or 2 W is about as much dissipation that
can be accommodated by conductive-heat transfer between an active device
and the circuit board it's mounted on. To
allow the use of multiple LT3080s to power big ICs, Dobkin and his design
team needed as much gain in the regulation feedback loop as possible to minimize the size of the external ballast
resistors used for current sharing among
multiple LDOs.
"My solution was to substitute a current
reference for the voltage reference. That
allows use of a very small ballast resistance, on the order of 10 mΩ, which can be
achieved simply with a few inches of pcboard trace," Dobkin said. "Using a current
source means there is no attenuation of
amplifier gain, which helps keep output
regulation constant."
UNDER THE HOOD
The LT3080
has four terminals (none of them ground)
(see the figure, b). The IN terminal connects to the collector to the NPN power
device inside the regulator. Most output
load current is supplied through this pin.
About 1% comes through VCONTROL.
This terminal and OUT define the regulator's LDO characteristics. That is, with the
regulator's internal NPN pass transistor
driven into saturation, the voltage difference from IN to OUT can range from as low
as 100 mV at light loads (down to 1 mA) to
300 mV at 1.1 A. The terminal labeled OUT
is the power output of the regulator. Load
current must be greater than 1 mA.
VCONTROL is the supply pin for the regulating circuitry. This comprises a precision 10µA current source, that unity-gain current follower that's key to the chip's design, and
current- and thermal-limiting circuitry. The
voltage requirements for this terminal are
higher than for the IN terminal. It must be
greater than the OUT-terminal voltage by1.2 to 1.35 V.
The pin labeled SET is the input to the current-follower error amplifier. The regulator's internal current generator drives
10 µA out of this terminal and through an
external resistor to ground to program the
output voltage. The programmed output
voltage is the voltage drop across that
resistor times 100. Note that when you
parallel these regulators to increase maximum output current, you also parallel all
the SET current sources, so you'll have to
reduce the value of the voltage-setting
resistor proportionally.
New MEMS packaging also sets the
LT3080 apart. Intended primarily for surface mounting to maximize heat conduction to the pc board, the LT3080 comes in
low-profile (0.75 mm), eight-lead DFNs (3
by 3 mm); eight-lead, thermally enhanced
MSOPs; and three-lead SOT-223s. These
packages can dissipate 1 to 2 W in surface-mount applications without a
heatsink. Also, there's a TO-220 version for
mounting to heatsinks for higher power
dissipation. Pricing starts at $1.88, $1.94,
$1.81, and $2.20 each in lots of 1000,
respectively.
Linear Technology
www.linear.com