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National DC/DC converters target RF power amplifiers

Posted: 24 Nov 2004 ?? ?Print Version ?Bookmark and Share

Keywords:national semiconductor? DC/DC converter? power design? lm3200?

National Semiconductor Corp. launched a family of DC/DC converters that is designed for powering RF power amplifiers from a single Li-ion battery.

According to the company, these new chips will enable cellphone makers to reduce power consumption by up to 80 percent, while increasing talk time as much as 90 minutes.

Compared with previous step-down switchers, the new LM3200 has a high switching frequency that allows it to use small external components such as a 2.2?H inductor. The converter also has an analog voltage-control input that dynamically controls the output voltage of the device depending on the power requirements of the power amplifier. Its bypass operation allows it to support the power amplifier's power requirement during peak usage periods to keep the phone running longer. The converter steps down an input voltage from 2.7V to 5.5V to a variable output voltage from 0.8V to 3.6V, while output voltage is set using a voltage-control analog input for adjusting power levels and efficiency of the RF power amplifier.

"With these new products, National is targeting the power amplifiera previously unregulated portion of the cellphoneto enable dramatic improvements in battery life and talk time," said Peter Henry, the company's VP for its Portable Power Products Group. "The LM3200 features a small-size chip-scale package, high switching frequencies and compatibility with a wide variety of industry-standard platform reference designs that will enable our customers to offer phones and other portable devices with outstanding performance."

Designed on National's CMOS7 process, the LM3200 offers four modes for mobile phones and similar RF PA applications. Fixed-frequency pulse-width modulator (PWM) mode minimizes RF interference and provides efficient performance under typical operating conditions; forced bypass mode turns on an internal bypass switch to power the PA directly from the battery; automatic bypass mode minimizes dropout by turning on the bypass switch when the battery decays close to the output voltage; and shutdown mode turns the device off and reduces battery consumption to 0.1?A (typ.).

The LM3200 is available in a 10-pin lead-free micro surface mount package, measuring 2.2-by-1.8mm. A high switching frequency (2MHz) allows use of tiny surface-mount components. Only three external components, a 2.2?H inductor and two 10?F ceramic capacitors, are needed to complete the application circuit. The device is priced at $1.60 each in quantities of 1,000 pieces.

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