powerZONE Products for the week of June 28, 2004
Linear Technology Says . . .
LT1941: Triple Switching Regulator Delivers Big Power
from Tiny Footprint
Linear Technology Corporation announced the LT1941, a triple output monolithic switching regulator offered in a 28-lead TSSOP package with an exposed copper bottom. Two of the regulators are step-down converters with 3A and 2A switches. The third regulator can be configured as a boost, inverter or SEPIC converter and has a 1.5A switch. All three converters are synchronized to a 1.1MHz oscillator. The two step-down converters run on opposite phases, reducing input ripple current. It is ideal for xDSL, hard disk drives and general-purpose applications that require dual high current step-down converters for microprocessor cores and I/Os, plus a high power boost/inverter for biasing requirements.
With an input voltage range of 3.5V to 25V, the LT1941 regulates a broad
array of power sources from 4-cell batteries and 5V logic rails to unregulated
wall transformers, lead acid batteries and distributed power supplies. Each
regulator generates a power good signal when its output is in regulation,
easing power supply sequencing and interfacing with microcontrollers or
DSPs. The high switching frequency provides several advantages by permitting
the use of small inductors and ceramic capacitors, ensuring a very compact
triple output solution. The constant switching frequency, combined with
low impedance ceramic capacitors, results in low and predictable input ripple.
analogZONE Says...
This is the first triple regulator I have seen where there are actually three switches on-chip. Triple parts up to now have had two switches and then either a charge pump or a linear regulator to provide the third voltage. The great advantage with the third switch approach is the simplicity of inverting the supply and the ability to boost if that is what you need -- with a decent current rating, to boot. As is, the LT1941 looks like a shoe-in for products that need a core voltage, an I/O voltage and a bias such as cable and DSL modems. And the combination of the supplies in a single IC, with the protections that can be offered, means that the part thoroughly deserves a price premium
The input voltage range is from 3.5 V to 25 V, so that it can be used over the gamut of supply sources from batteries to wall transformers. The closer the supply voltage to the two step-down voltages the more efficient the part will be, of course. Producing 1.8 V from a 5-V supply, for example, on the first switch gives efficiencies above 70%, peaking at about 76% with a 1 A load. On the second switch, producing 3.3 V the efficiencies are above 80%, peaking at about 87% at 0.75 A load. The third switch providing -12 V in such a scenario gives about 80% effciency above a 150-mA load.
The non-switching quiescent current is a typical 2 mA and when the RUN/SS (soft-start) pins are tied to ground the part shuts down with a quiescent of 50 µA. The switches share a common oscillator (running at a nominal 1.1 MHz) and voltage reference but are otherwise independent of each other, each with its own feedback pin and power-good outputs. The oscillator frequency is high enough to allow for inductors that are small, but not small enough that they can't be sourced and the capacitors can also be small, low-impedance ceramics. The data sheet is comprehensive in helping select capacitors and, just as importantly, in Schottky diode selection.
Sequencing can be set up with external capacitor choices on the RUN/SS pins or by using a power-good indication as a trigger to a RUN/SS pin. Sequencing only operates on power-up and all supplies shut down simultaneously. The first switch is rated at 3 A step-down, the second at 2 A step-down, and the third switch is rated at 1.5 A in either inverter and/or boost modes. The switching is in antiphase between the first two switches so that the ripple is not only minimized but also predictable.
The LT1941 smells of success story and we should expect the part will spawn further products for the specific requirements of the larger users.
The LT1941 is in production in a thermally-enhanced TSSOP-28 and is priced at $5.25 in 1000-piece lots.