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Electronic Design

Compact ±5-V High-Current Supply Uses One 1.25-MHz Switcher

Disk drives, ADSL modems, notebook computers, and other data-acquisition circuits require high-current, ±5-V power supplies with switching frequencies above 1.1 MHz to avoid interfering with noise-sensitive circuitry. A very simple, compact, and efficient solution can be built with a single 1.25-MHz step-down switching regulator IC and only one magnetic component (Fig. 1).

This circuit can provide ±5-V supplies from a 12-V source with greater than 1-A capabilities on both rails. The IC's internal power switch saves space by eliminating the need for an external MOSFET and its traces. Typical efficiency is 84% (Fig. 2). Alternatively employing two ICs means paying a heavy toll in board space, overall cost, and complexity.

The LT1765EFE (U1) regulator IC utilizes current-mode control to regulate the positive output via its step-down converter topology. With a greater than 3-A current rating and a 1:1 winding ratio, the off-the-shelf CTX5-1A transformer (L1) induces the same voltage across the secondary winding as the primary winding and maintains a ∠5-V output. A high-current-density ceramic coupling capacitor creates a low-impedance path for current to run between the IC and negative output, with excellent cross-regulation (Fig. 3).

The 3-A minimum switch-current limit of the IC and the thermally enhanced TSSOP16 package provide high power in a more compact solution than either dual controllers or a single controller with a separate MOSFET. Dual controllers also are much more expensive. Plus, the option of a single controller and separately chosen MOSFET entails a more complex design using extra board space and taking additional design and assembly time.

The B220A Schottky diodes have a low forward-voltage rating for high efficiency and a small case size to further minimize board space. Ceramic input and output capacitors provide a tiny, low-cost solution with minimal output ripple.

Current-mode topology provides stable response to load transients on both outputs, requiring only ceramic output capacitors and a simple RC network located on the VC pin of the IC. This is a space- and cost-savings advantage over a voltage-mode controller topology. The latter would require additional compensation components to optimize load transient response.

Also, voltage-mode controllers typically need electrolytic or tantalum output capacitors to stabilize the control loop and maintain good high-frequency response. Given the same rms current-handling requirement, electrolytic and tantalum capacitors are bigger and create more output voltage ripple than the equivalent ceramic capacitor. Overall, a current-mode step-down regulator with ceramic capacitors is simpler, smaller, and less expensive than a voltage-mode solution.

A 1-kΩ preload resistor on the ∠5-V output can improve regulation if cross-regulation is an issue with a +5-V output current greater than 1.0 A and a ∠5-V output current under 5 mA. The switch current of the LT1765EFE, which has a minimum rating of 3-A total, limits the maximum outputs of the negative and positive lines. In this topology, the negative output current must be less than (and not equal to) the positive output current. So care must be taken when considering all possible load-transient conditions (Fig. 4).

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