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New P-Channel MOSFET Low Drop-Out (LDO) Controller from National Semiconductor Drives High-Current and/or Ultra-Low Drop-Out Regulators

Dec 9, 1997

December 9, 1997 - National Semiconductor Corporation® today announced availability of a new LDO controller for regulator applications requiring the extremes of ultra-low drop-out (<0.10V) and high current (>1A). The LP2975 driver/controller is designed for use with any external P-MOSFET pass device, greatly simplifying the design of a high-current LDO regulator. The LP2975 and a P-MOSFET will also deliver ultra-low dropout regulation with extremely low quiescent current (180 microamps typical at Vin = 5V).

“We’ve seen a growing demand for LDO regulation from 3.3V to 2.5V to supply the I/O systems of today’s PCs and information appliances,” said Venkatesh Shan, National's power management marketing director. “This trend dictated the development of the LP2975.”

For battery-powered applications from mobile telephones to portable bar code scanners, the LP2975 can operate as an ultra-low drop out controller regulating within tens of millivolts of the battery voltage. “Ultra-low dropout is especially important in battery applications where the cell voltage can drop very low before system operation is lost. It’s also critical in heat-sensitive applications where the power dissipated by the regulator must be minimized,” Shan said.

At high current, the LP2975 allows designers to select a P-MOSFET that is rated for the required current, yet can also accommodate power dissipation requirements. “The need for low-dropout at high current is increasing. Medical instrumentation and PC servers are just two examples of these types of applications,” added Shan.

The advantage of using a controller with an external MOSFET is that it gives the designer the flexibility to select the external FET with the appropriate Rds-on (resistance from drain to source when on) for the application. Using the LP2975 controller with a P-MOSFET has several advantages over pairing an LDO regulator with an N-Channel device. Since N-MOSFETs require a gate voltage exceeding the supply voltage to turn on in LDO applications, an additional DC supply of about 10V above Vout is needed. Also, some N-FET designs use charge pump technology, which creates system noise, necessitating additional filtering capacitors.

Three fixed output voltage versions of 12.0V, 5.0V and 3.3V are available with guaranteed accuracy of 1.5% (A-grade) and 2.5% (B-grade) at 25 degrees C. Output voltage may be programmed with two external resistors. High open loop gain assures excellent regulation and ripple rejection performance. To facilitate extremely small circuit designs, the LP2975 is housed in a mini-small outline (MSOP-8) package that is only one-half the size of a standard small outline IC (SOIC)

Price and Availability
The LP2975 is currently available in production quantities and is priced at $1.23 each in 1000s for the A-grade device (LP2975AIM) and $.98 for the B-grade device (LP2975IM).

National Semiconductor Corporation, a Fortune 500 company, produces system-on-a-chip silicon solutions for the information highway, based on its leadership in analog and mixed signal technologies. National is headquartered in Santa Clara, California, and focuses on the communications, personal systems and consumer markets. National has annual sales of approximately $2.5 billion and 13,000 employees worldwide. Additional company and product information is available on the World Wide Web at www.national.com.