Electronic Design

Inventor Updates A Classic 30 Years Later

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

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