AN268| Application Note
Maxim > App Notes > POWER-SUPPLY CIRCUITS Keywords: RF PA power, PDA, CDPD, IC, integrated circuit, rf, power amplifiers, hand-held devices
Aug 01, 2000
APPLICATION NOTE 268
Wireless-Modem Power for Hand-Held Devices
Abstract: Thanks to lackluster efficiency in currently available RF power amplifiers (PAs), the addition of wireless communications to hand-held computing devices entails unusual power-management techniques. Some communications protocols allow burst-transmission techniques and shutdown when not transmitting (duty-cycle control), but during a transmission the typical PA efficiency is no greater than 40% to 60%. In contrast, efficiency for the main power supply in a hand-held device is typically 90% to 95%. Many hand-held devices operate from one to four nonrechargeable alkaline batteries. Others that demand the higher levels of instantaneous power associated with backlit displays achieve longer life (between charges or between battery replacements) with NiMH or even Li+ batteries. Regardless of battery type and configuration, however, the modem, PA, and radio circuitry necessary for wireless communications call for extra battery capacity if the system is to maintain a reasonable operating life. Typical of suc
h systems is a PCMCIA wireless modem for transferring cellular digital packet data (CDPD). It might plug into a hand-held personal digital assistant (PDA) or a hand-held computer running Windows CE, drawing several hundred milliamps from a 3.3V supply. The PCMCIA card usually includes a secondary battery to avoid excessive drain on the host battery. To supply the power surge demanded during transmission, secondary batteries usually exhibit the low equivalent series resistance (ESR) found in today's popular rechargeable chemistries. Actual power for the wireless link depends primarily on the PA's transmitting power and efficiency. As an example, the power supply (including the backup battery) for a wireless data-communications link must interface with a host hand-held system operating at 3.3V (Figure 1 - note the tiny packages appropriate to a hand-held system: IC1 in a 16-pin QSOP, and IC2 in an 8-pin uMAX ). The secondary battery is a single lithiumion cell that exhibits a full-charge voltage of 4.1V to 4.
2V and has little residual energy below 2.9V. IC1 converts this voltage to 3.3V, and IC2 forces the resulting backup voltage to track the host supply within 12mV (0.36%).
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