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Step-down dc-dc converters

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article image Low quiescent current.

MAXIM Integrated Products, represented by Arrow Electronics , has introduced the MAX1556/MAX1557 step-down, dc-dc converters for use in digital still cameras, mobile phones, PDAs, palmtop computers, and other handheld portable equipment.

Available in a tiny 3mm x 3mm, 10-pin QFN package, the MAX1556 delivers up to 1.2A and the MAX1557 delivers up to 600mA of load current.

The devices offer a low quiescent current of 16 microamps and 27 microamps in dropout. A proprietary topology provides the benefits of high fixed-frequency operation, while still providing better than 95% efficiency at both light and full loads.

The MAX1556/MAX1557 have a unique, wideband control loop and voltage-positioning feature. By minimising the amplitude and duration of overshoot and undershoot in response to load transients, this unique capability allows less than 50mV peak-to-peak load transient response and four microseconds load step recovery time.

Other dc-dc converters with high gain-control loops use external compensation to maintain tight DC load regulation, but still allow large voltage droops of 5% or greater for several hundreds microseconds during transients.

If, for example, the load is a CPU running at 600MHz, then a dip lasting 100 microseconds corresponds to 60,000 CPU clock cycles.

The MAX1556/MAX1557 offer an internal low-on-resistance MOSFET switch and synchronous rectifier to maximize efficiency and dropout performance while minimising external component count.

Both devices also feature an adjustable soft-start to minimise battery transient loading. The input voltage range is 2.6V to 5.5V, allowing the use of 1-cell Li-Ion (Li+) or 3-cell alkaline/NiMH batteries.

With two input-select pins, designers can select an adjustable output on either device, or preset 3.3V, 2.5V or 1.8V for the MAX1556 and 1.5V, 1.3V or 1.0V for the MAX1557, without external feedback resistors.

The devices are specified for the extended temperature range (-40°C to +85°C). Pre-assembled evaluation kits with recommended external components are available to speed designs.

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