Embedded Systems Design - July 2008 - (Page 28)

feature Commercial converter characteristics. Manufacturer Model TRACO POWER XP Power RECOM C&D Technologies BOURNS RECOM Table 1 Vi (V) 12 12 12 12 12 12 Vo (V) 3.3 3.3 3.3 5.0 3.3 3.3 Io (A) 1.2 1.2 0.7 0.4 3.0 1.0 Ii (mA) Io=0. 20 38 21 40 11 7 η (%) 77 83 65 60 93 81 Isolation Yes Yes Yes Yes No No TEN 5-1210 JCA0412S03 RW-123.3S HL02R12S05 MX3A-12SA R-78A3.3-1 third depends directly on power conany modules are commercially available for this purpose, due to the neces- sumption in the DC-DC converter itself. You must take extra care, theresary trade-offs between isolation and fore, to minimize the current draw low power consumption. under no-load conditions. As a result, To address such issues, described we stress this third point in recomhere is an isolated, switched-mode power supply intended for wireless de- mending design techniques for optimizing the system. vices. It accepts a nominal 12-V input and delivers an isolated 3.6-V output, Discontinuous transmission and and its quiescent current draw is reception among the lowest available. Designed Because transmitters and receivers as a power supply for EGSM (Extended GSM), WiFi, and ZigBee communi- consume the most power in a wireless device, many such devices implement cation modules, it also provides rediscontinuous transmission/reception mote control for electromechanical actuators and electronic sensors in harsh Discontinuous operation also helps environments. Filtering or regulation of the powersupply voltage The supply voltage can be filtered with a high-valued capacitor or other techniques described in a paper by Jose Ignacio Garate and others.5 The voltage is regulated with a DC-DC converter operating in linear or switched mode. Regulation is not only necessary to reduce voltage ripple, but also to reduce EMC problems and maintain performance of the radio. Topology of the power supply for high efficiency Power-supply effito reduce ciency is important, so a switching topolpower consumption, . . . but it ogy offers the best INCREASING introduces voltage ripple and current peaks power-supply option. AUTONOMY However, the DC-DC in the power supply. Because the main characconverter modules teristic of a power supply listed in Table 1, to optimize the resources of the air infor wireless devices is battery life, the which are typical of those commercialterface and the efficiency of the commain design goal was to reduce power ly available, don’t meet the requiremunications link. Discontinuous oper- ment we’re looking for: ultra-low powconsumption while maintaining peration also helps to reduce power formance in the radio-frequency syser consumption under no-load consumption, because the radio’s actem. Thus, care was taken that the auconditions. tonomy of the wireless-communication tive elements are not continuously on. Even the nonisolated converters On the other hand, discontinuous device increased when these conditions draw a relatively high current with no transmission introduces voltage ripple are present: load. Taking this data into account, we and current peaks in the power therefore set a goal of 12 mA maxisupply.4 The level of bias voltage afmum for the DC-DC converter’s noDiscontinuous transmission and load current. To achieve that goal, we reception. fects transceiver performance directly, distinguish between standby current and a drop in supply voltage degrades Filtering or regulation of the supradio performance, mainly at extremes and quiescent current as follows: ply voltage. of the voltage range. This degradation A high-efficiency power-supply can make it difficult to meet the applitopology. Quiescent current is the supply curcable specifications for certifying a rent necessary to maintain a reguwireless device. Battery life cycle and The first of these characteristics lated supply voltage under no-load discharge characteristics are also sensidepends on the transmission system. conditions. tive to current peaks in a load, if the The second can be obtained with a Standby current is the supply system is powered by a secondary cell. switched-mode power supply, and the current drawn when the system is • • • • • 28 JULY 2008 | embedded systems design | www.embedded.com http://www.embedded.com

Table of Contents Feed for the Digital Edition of Embedded Systems Design - July 2008

Embedded Systems Design - July 2008 Contents #Include Parity Bit Programming Pointers Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design Building a Power Supply for Discontinuous Transmission Wireless Networks An Exception Primer Advertising Index Break Points Marketplace

Embedded Systems Design - July 2008

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