Analog Angle Article

RF PA 6-MHz DC/DC converter implements dynamic voltage scaling for improved efficiency of 3G handsets

San Jose, Calif—It's not surprising that a large part of the overall operating time in a 3G mobile handset is related to the operation and efficiency of its RF power amplifier (PA), rather than its processor and associated digital/multimedia ICs, and this, in turn, is a function of its power-supply and bias-current management. The Fairchild Semiconductor FAN5902 works to maximum RF PA performance by two techniques: it operates at 6 MHz for smallest size and high basic efficiency; and it implements dynamic voltage scaling (DVS) to adjust the RF PA supply voltage to the actual transmitted power requirements (which are often less than maximum) rather than use the supply rail directly at full value. The result, the vendor maintains, is savings of 20 to 30% of battery power and about 40 minutes of extended talk time in today's 3G (CDMA, W-CDMA) handsets.

Fairchild FAN5902 6-MHz DC/DC converter dynamic voltage scaling (DVS) for maximum RF PA efficiency

(Click on image to enlarge)

While the DVS concept is not new, implanting it is a challenge, due to discontinuities in the supply voltage which make calibration of the 3G PA stage difficult and adversely affects distortion and other performance parameters. The FAN5902, says the vendor, overcomes these problems with its discontinuity-free dynamic voltage scaling operation, resulting in more efficient operation, and is compatible with any single-level RF PA. In addition to improved battery life, thermal dissipation is reduced, a further benefit.

External components required for synchronous 6-MHz operation include a 0.5 μH inductor and 0603 and 0402-size capacitors. The 6-MHz frequency also moves switching noise out of the signal baseband channel, minimizing supply-induced, in-band noise-degradation issues. The load transient response includes full Vout transitions in under 20 μsec.

Output current capability for this dual-mode (PFM/PWM) IC is 800 mA rms, with 0.4 to 3.4 V adjustable output, controlled by an analog voltage; the IC includes thermal and current limits as well as a slew-rate controller. Overall efficiency ranges between 92% (for high RF-power output) and 67% (low RF-power output), and the device includes a 50 m&#937 on-resistance bypass FET for low-battery operation down to 2.7 V.—Bill Schweber

Price, package, and availability : The FAN5902 is housed in a 12-bump, 0.5 mm chip-scale package and is priced at $0.75 in 1000-piece orders; it is available now.

For more information : contact Fairchild Semiconductor Corp. ,; for the data sheet, go to

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