lowpowerZONE Products for the week of July 23, 2007
Linear Technology Says . . .
LT3080: Surface-Mount 1.1-A LDO Easily Paralleled for High IOUT without Hot Spots
Linear Technology Corporation announced the LT3080, a 1.1A 3-terminal LDO that may be easily paralleled for heat spreading and is adjustable with a single resistor. This new architecture regulator uses a current reference to allow sharing between multiple regulators with a small length of PC trace as ballast, enabling multi-amp linear regulation in all surface-mount systems without heat sinks.
The LT3080 achieves high performance without any compromises. Featuring wide input voltage capability from 1.2V to 40V, it has a low dropout voltage of only 300mV at full load. The output voltage is adjustable, spanning a wide range from 0V to 40V, and the on-chip trimmed reference achieves high accuracy of ±1%. The wide VIN & VOUT capability, tight line and load regulation, high ripple rejection, low external parts count and parallel capability make it ideal for modern multi-rail systems.
According to Linear Technology's VP/CTO Robert Dobkin, "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. 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."
analogZONE Says...
Most engineers might, at first blush, not think that a new three-terminal adjustable regulator is a big deal. This one is. The LT3080 represents thirty years of thinking about how to get such a regulator into production for Robert Dobkin, one of the founders of Linear Technology. What's different? If you went back to the National Semiconductor catalog of Linear ICs from 1976, you will see a lot of three-terminal regulators such as the LM78Lxx family, the LM120, 140, 320, 341 and 342. Of those I believe only the LM342 is obsolete today. The LM117, LM217 (obsolete) and LM317 were/are the most popular adjustable regulators, copied by many vendors. The key to output voltage control was a potential divider with a fixed resistor to the Adjust pin (we would title it "Feedback," now) and a potentiometer from there to ground.
The LT3080 uses a potentiometer only...although, of course, it could instead be a single fixed resistor for a fixed output voltage. The set resistance is connected from what is now called the Set pin to ground.
The older 1.5 A parts (there were/are 3 A parts in the LM150 series and 5 A in the LM138 series) guaranteed 1% output voltage tolerance with 0.01%/V line regulation and an output voltage range from 1.2 V to 37 V with a typical application of >27 V input and output up to 25 V. The LT3080 is not as highly rated, at 1.1 A (it is not in a power package) but still achieves 1% accuracy and 0.01%/V line regulation.
Additionally, and most importantly for an age where core voltage values are seemingly unstoppable in their reduction, the LT3080 offers outputs down to 0 V from an input range of 1.2 V to 30 V, or 40 V. The output voltage is determined by the set resistance value multiplied by 10 µA (the stable current reference source -- the heart of the design). Dropout voltage is just 300 mV while 100 kHz noise is a low 40 µVrms -- one of the joys of having no switching in the device!
The applications in the data sheet (very preliminary in terms of characterization curves) suggest an input capacitor of 1 µF and an output of 4.7 µF, both of which can be ceramic. Multiple parts can be paralleled to offer higher load currents, or simply to spread the dissipated heat across a larger area of the PCB.
Both over-current and over-temperature protections are provided. The data sheet suggests a large number of applications ranging from laboratory power supply circuits; to how to get a higher load current; to how to use the device as a ramp generator.
Reducing the set resistor down to a single component is an amazing jump in three-terminal adjustable regulators, but even bigger is the ability to take the output of the regulator down to 0 V. This opens up a wealth of possibilities in applications which I am having trouble getting my head around at this early stage. The variable resistance does not, of course, have to be a conventional physical component, and there are many ways this could lead to automatic loop controls, particularly for sectors where bias has to be set very exactly for optimal performance: one immediately thinks of optical and RF links of one kind or another. The ability to shut off stages of a circuit (with, perhaps, a shutdown transistor switch across the set resistance) could also be widely used. I have no trouble predicting this -- and subsequent spins -- will be wildly commercially successful. It is good to see that there is no TO-3 offering of the part! Three of the packages offered are surface-mount and allow 1 W to 2 W dissipation without heatsink. Implementation of the LT3080 in designs will be simplicity itself without the complications brought by switcher solutions.
The LT3080 is in production in thermally-enhanced DFN-8, MSOP-8, and SOT-223-8. A TO-220-5 is also offered for higher power applications (not yet defined). Prices are $1.84, $1.94, $1.81 and $2.20, respectively, in 1000-piece lots.
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