New Circuit Topologies Conserve Board Space

May 18, 2004 12:17 PM
By John Day, Contributing Editor, Power Electronics Technology

Offering higher levels of integration, ICs simplify power conversion designs while improving efficiency.

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Linear Technology Corp. (Milpitas, Calif.) and National Semiconductor Corp. (Santa Clara, Calif.) are among the manufacturers of power management components deploying new circuit designs to conserve board space.

Linear’s LTC3441, introduced last August, is a high-efficiency (95%) single-chip buck and boost dc-dc converter designed to optimize run-time in single-cell Li-Ion, multicell alkaline or NiMH battery-powered applications. With a 2.4-V to 5.5-V input, the device can provide up to 1 A at a fixed 2.4-V to 5.25-V output. National’s LM5007, which targets next-generation communication, automotive, 48-V distributed and battery-powered systems, is a buck bias regulator IC. This chip steps down an 80-V primary-side supply to a low-voltage (10-V typ.) bias supply for secondary-side control devices.

Linear’s device employs a constant-frequency, synchronous switching topology that includes two 0.10-O N-channel switches and two 0.11-O P-channel switches. Selectable Burst Mode operation allows the four internal power MOSFETs to operate intermittently based on load demand, reducing quiescent current to 25 µA with no load.

With a single inductor, the LTC3441 can operate from a supply above, below and equal to the output voltage, providing a continuous transfer function through all the operating modes as battery voltage declines, according to Tony Armstrong, marketing manager for power products. The device’s operating frequency is set internally to 1 MHz and can be synchronized up to 2 MHz, which allows the use of surface-mount components and maintains switching noise above the pass-band of most communication systems. Another chip targeting applications with lower input voltage ranges (below 3.3 V) is the AP1605, a dual-mode PWM/PFM buck regulator from Anachip Corp. (Hsinchu, Taiwan). Featuring an integrated P-channel MOSFET and offering 92% efficiency, the device is designed for applications that require 3 A at a voltage as low as 2.5 V. Power consumption is 100 mA during normal operation and 2 mA in standby.

For applications that require a relatively low-current (< 0.5 A) bias supply powered from a high-voltage, unregulated input, National’s LM5007, introduced last September, offers better than 90% efficiency in a small package, such as a 4-mm × 4-mm LLP. The firm expects to unseat the more-costly, less-efficient transformer-coupled flyback regulators commonly used to produce bias power.

The LM5007 integrates an 80-V, 0.7-A NMOS buck switch and a simple ON-time controller implemented in a manner that improves line regulation. The chip also incorporates a level-shifting gate drive for the high-side switch, a feedback comparator with a 2.5-V threshold and current-limiting circuitry that uses a lossless current sensing technique to monitor peak current in the buck switch. The part eliminates the need for loop compensation and offers a fast transient response using an ON-time, controlled by an external resistor, that is inversely proportional to the input line voltage (Vin). Feed-forward pulse-width control compensates the duty cycle for line-voltage changes, thus producing a relatively constant, easily filtered switching frequency.

When current in the buck switch exceeds a 0.725-A threshold, the switch is held off for an interval inversely proportional to the feedback voltage. According to David Pace, director of National’s power management design center in Phoenix, the adaptive current limit OFF-time provides sufficient OFF-time to control the circuit in the buck inductor in an overload condition and to reduce the foldback of the output I-V characteristic after a current limit event.

Pace adds that the constant ON-time and variable OFF-time control in the LM5007 eliminates loop compensation and provides excellent response to load transients. In just a few nanoseconds the feedback comparator can respond to an output voltage fluctuation caused by a change in output loading.

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