Synchronous Controllers

Oct 6, 2004 9:43 AM
Edited by PETech Staff

National Semiconductor Corp.’s LM274x new family of high-speed, synchronous switching regulator controllers work from a 1-V to 16-V input power rail and provide output voltages down to 0.6 V. Applications include set-top boxes, cable modems, rack-mounted equipment, networking products and any other system that employs a DSP, FPGA, ASIC or processor that requires supply voltages of 1.8 V or lower.

The LM2742, a high-speed, synchronous switching regulator controller, is capable of controlling currents of 0.7 A to 20 A with up to 95% conversion efficiency. Power up and down sequencing is achieved with the power-good flag, adjustable soft-start and output enable features. The LM2742 operates from a low-current 5-V bias rail and converts from a 1-V to 16-V power rail. It uses a fixed-frequency, voltage-mode pulse width modulation (PWM) control architecture, and its switching frequency can be adjusted from 50 kHz to 2 MHz by setting an external resistor.

Current limit is achieved by monitoring the voltage drop across the on-resistance of the conducting low-side MOSFET, with best-in-class 40 ns minimum on time. The wide range of operating frequencies gives the power supply designer flexibility to fine-tune component size, EMI and efficiency. The adaptive, non-overlapping MOSFET gate drivers and high-side bootstrap driver help maximize efficiency without compromising reliability. This part keeps the low-side MOSFET switched off during shutdown, preventing the discharge of the output filter capacitor.

The LM2743 is a high-speed, N-Channel synchronous buck regulator controller with a 2% 0.6-V feedback reference voltage. It uses a fixed-frequency, voltage-mode PWM control architecture that is adjustable from 50 kHz to 1 MHz through an external resistor. This wide range of PWM frequencies gives the power supply designer the flexibility to make tradeoffs among component size, EMI and efficiency.

The power MOSFETs can run on a separate 1 V to 16 V input voltage rail, while the regulator is biased from a 3 V to 6 V, 2 mA rail. A power-good flag, precision shutdown threshold and soft-start features make power supply tracking and sequencing easy. The LM2743 also employs current limit, which is achieved by monitoring the voltage drop across the on-resistance of the conducting low-side MOSFET, as well as an output undervoltage and overvoltage flag. The adaptive, non-overlapping MOSFET gate drivers help avoid shoot-through currents while maintaining high efficiency. The high-side and low-side MOSFETs are the N-Channel type, and the IC bootstrap structure enhances the high-side MOSFET for the highest efficiency. The LM2744 has the same feature set as the LM2743 and includes an external reference that the user can connect to an error amplifier for tighter dc output accuracy.

For more information, visit www.national.com.

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