Milpitas, Calif. - Adding a welcome twist for multiphase applications, Linear Technology Corp.'s synchronous LTC3425 dc/dc converter chip incorporates a switching four-phase boost topology for 3.3- to 5-volt power conversion for handheld computers, ADSL and interface electronics.
Operating at up to 8 MHz (up to 2 MHz per phase), it's the first such monolithic multiphase solution for step-up applications, according to the company. The device delivers a low-ripple output at up to 5.25 V and 3 amps (nominal output of 10 watts) from a 0.5 to 4.5-V input. It's thus suited to lithium-ion and nickel metal hydride battery applications and general-purpose fixed-rail systems.
In operation, the LTC3425's multiple phases are spaced equally at 90 degrees apart. Although this architecture requires four inductors rather than one, the peak inductor current for each is lower, allowing the use of smaller, low-cost components. Output ripple current is greatly reduced, minimizing output capacitance requirements. Input ripple current is also reduced, the company said.
The device can operate as a two- or three-phase converter by simply eliminating the corresponding inductor from its unused phase output. On the other hand, two LTC3425s can be connected together to achieve eight-phase, 10-MHz operation.
Four-phase operation minimizes the size of external inductors (four are required) and capacitors, providing a total solution profile of less than 2 mm. Internal switches consisting of four N-channel MOSFETs and four p-channel synchronous rectifiers have an RDS(on) of about 50 milliohms, which contributes to power conversion efficiencies of up to 95 percent, according to LTC. The low on-resistance and low gate charge of the synchronous switches thus eliminate the need for external Shottkky diodes in most applications.
The chip has what the company calls a true output disconnect for high inrush currents, along with inrush current limiting. In noise-sensitive apps, the IC can also be configured to provide forced continuous operation to reduce noise and RF interference.
Other features with an accent on power saving include burst-mode operation (programmable using a single external resistor), quiescent current of 12 microamps, shutdown current of 1 microamps, programmable oscillator with sync output, programmable soft-start, anti-ringing control, thermal shutdown, adjustable current limit and power-good output comparator.
The LTC3425 is available from stock in a 32-pin, thermally enhanced QFN package measuring 5 x 5 mm. Pricing starts at $4.38 each in quantities of 1,000. Evaluation boards are available.
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