Consolidating its leading linear regulator technology position, National Semiconductor Corporation today introduced a new linear regulator capable of regulating ultra-low output voltages between 0.9 V and 1.2 V. As the industry's first 150 mA linear regulator that requires an ultra low supply voltage of just 2.2 V, this new device is optimized to provide low level output voltages for high-end laptop, desktop PCs, and computing add-on cards.

National's LP2983 is a fixed voltage regulator with output voltage of 0.9 V, 1.0 V and 1.2 V. This device is designed to accommodate the high-end PC market as well as digital signal processor (DSP) applications in which supply voltages require 1.2V or less, a parameter where other conventional products on the market fall short.

Key features of National's LP2983 linear regulator include:

• Minimum peak current of 300 mA
• Ground pin current is typically 75 microA at 1 mA and 850 microA at 150 mA load current respectively
• Controlled shutdown for designers wishing to manage power dynamically in their system with less than 2 microA quiescent current in shutdown mode
• Output voltage accuracy of ý1.0% at room temperature over load and line variations and less than ý3.5% over all conditions (varying load, line, and temperature)
• Low output capacitor requirements, 2.2 microF low ESR, e.g. ceramic
• Over-temperature and over-current protection

"National Semiconductor continues to lead the linear regulator market by providing designers with the right power supply for design efficiency and fast time-to-market," said Joe Bolsenga, strategic marketing manager of Power Management Group at National Semiconductor. "With our new LP2983 linear regulator, manufacturers of computing devices can capitalize on high performance, cost effective power management solutions."

National Semiconductor, 2900 Semiconductor Dr., Santa Clara, California 95052-8090. Tel: 408-721-5000

National Semiconductor's new linear regulators are a welcomed design for the lower supply voltages and lower output voltages used by today's cutting edge laptops, cell phones, PDAs, and other devices. This is very much a niche market and National is targeting a very focused group of products. There are some impressive specs including ground pin current of typically 825 microamps @ 150 mA load, and 75 microamps @ 1 mA load, and they offer a precision output version of 1% tolerance on output voltages.

But, as is usual, the output and input need additional capacitors for filtering and load transients. This new regulator from National needs additional capacitors for regulator stability, so they provide some information on how to select capacitors. It's important if you want good performance, which is why I decided to include some helpful hints from National for selecting external capacitors.

External Capacitors

Like any low-dropout regulator, the LP2983 requires external capacitors for regulator stability. These capacitors must be correctly selected for good performance.

Input Capacitor

An input capacitor whose capacitance is ≥ 1 microfarad is required between the LP2983 input and the ground (the amount of capacitance may be increased without limit). This capacitor must be located a distance of not more than 1 cm from the input pin and returned to a clean analog ground. Any good quality ceramic, tantalum, or film capacitor may be used at the input.

Important: Tantalum capacitors can suffer catastrophic failure due to surge current when connected to a low-impedance source of power (like a battery or very large capacitor). If a Tantalum capacitor is used at the input, it must be guaranteed by the manufacturer to have a surge current rating sufficient for the application. There are no requirements for ESR on the input capacitor, but tolerance and temperature coefficient must be considered when selecting the capacitor to ensure the capacitance will be ≥ 1 microfarad over the entire operating temperature range.

Output Capacitor

The LP2983 is designed specifically to work with ceramic output capacitors, utilizing circuitry which allows the regulator to be stable across the entire range of output current with an output capacitor whose ESR is as low as zero ohms. The LP2983 requires a minimum of 2.2 microfarad on the output (output capacitor size can be increased without limit). It is important to remember that capacitor tolerance and variation with temperature must be taken into consideration when selecting an output capacitor so that the minimum required amount of output capacitance is provided over the full operating temperature range. It should be noted that ceramic capacitors could exhibit large changes in capacitance with temperature (see Capacitor Characteristics). The output capacitor must be located not more than 1 cm from the output pin and returned to a clean analog ground.

Capacitor Characteristics

The LP2983 was designed to work with ceramic capacitors on the output to take advantage of the benefits they offer: for capacitance values in the 2.2 microF to 4.7 microF ranges. Ceramics are the least expensive and also have the lowest ESR values (which makes them best for eliminating high-frequency noise). One disadvantage of ceramic capacitors is that their capacitance may vary with temperature. Most large value ceramic capacitors (≥ 2.2 microF) are manufactured with the Z5U or Y5V temperature characteristic, which results in the capacitance dropping by more than 50% as the temperature goes from 25ýC to 85ýC. This could cause problems if a 2.2 microF capacitor were used on the output, since it will drop down to approximately 1 microF at high ambient temperatures (which could cause the LP2983 to oscillate). If Z5U or Y5V capacitors are used on the output, a minimum capacitance value of 4.7 microF must be observed. A better choice for temperature coefficient in ceramic capacitors is X7R, which holds the capacitance within ý15%. Unfortunately, all manufacturers in the X7R dielectric do not offer the larger values of capacitance.

On/Off Input Operation

The LP2983 is shut off by driving the ON/OFF input low, and turned on by pulling it high. If this feature is not to be used, the ON/OFF input should be tied to VIN to keep the regulator output on at all times. To assure proper operation, the signal source used to drive the ON/OFF input must be able to swing above and below the specified turn-on/turn-off voltage thresholds listed in the Electrical Characteristics section under VON/OFF. To prevent mis-operation, the turn-on (and turn-off) voltage signals applied to the ON/OFF input must have a slew rate that is ≥ 40 mV/microsecond. Caution: the regulator output voltage can not be guaranteed if a slow-moving AC (or DC) signal is applied that is in the range between the specified turn-on and turn-off voltages listed under the electrical specification VON/OFF.

Reverse Input-Output Voltage

The PNP power transistor used as the pass element in the LP2983 has an inherent diode connected between the regulator output and input. During normal operation (where the input voltage is higher than the output) this diode is reverse-biased. However, if the output is pulled above the input, this diode will turn ON and current will flow into the regulator output. In such cases, a parasitic SCR can latch, this will allow a high current to flow into VIN (and out the ground pin), which can damage the part. In any application where the output may be pulled above the input, an external Schottky diode must be connected from VIN to VOUT (cathode on VIN, anode on VOUT), to limit the reverse voltage across the LP2983 to 0.3V.

As part of National's LP2985 linear regulator family, the LP2983 is offered in both SOT-23 package and Leadless Leadframe package (LLP). The LLP package will be available later for applications that require additional power dissipation. The LP2983 linear regulator is available in the SOT-23 packaging and priced at $0.50 each in 1,000 units.

For more information: http://www.national.com/pf/LP/LP2983.html