Energy Management For Small Portable Systems - Tutorial .

5m ago
267.38 KB
16 Pages

Maxim Design Support Technical Documents Tutorials Battery Management APP 671Maxim Design Support Technical Documents Tutorials Power-Supply Circuits APP 671Keywords: power for portable applications, power regulator, SEPIC, step-up DC-DC controller, step-up regulator, stepdown switching regulator, low-voltage logic, low-dropout DC-DC controller, linear regulator, charge-pump converter,dropout voltage, voltage converter, GaAsFET, RF power amplifier, GSM, TDMFTUTORIAL 671Energy Management for Small Portable SystemsApr 07, 2011Abstract: This application note describes power-management schemes for portable applications with four- and threecell battery systems. Optimal designs and uses for step-up/down converters, linear regulators, voltage converters,charge pumps, and inductorless regulators are shown. Several Maxim power-management devices are featured.Numerous diverse and conflicting constraints burden the designer of small handheld products. Aside from thecustomary restrictions on size and weight, these constraints include cost limitations, strict time schedules, battery-lifegoals measured in weeks instead of hours, and host computers that are (sometimes) overtaxed with the demands ofpower management.Because power requirements for handheld applications vary widely with product use, no single "best" power sourceexists for these applications. A device used intermittently is more concerned with no-load quiescent current than withfull-load efficiency, and can operate satisfactorily with alkaline batteries. Cell phones, however, must contend with highpeak loads and frequent use. This mode of operation emphasizes conversion efficiency over quiescent current, so cellphones are better served with a rechargeable battery.In handheld product design, size limitations often dictate the number of battery cells early in the process. This isfrustrating to the electrical engineer, and a substantial constraint, since the number (and type) of cells alloweddetermines the operating-voltage range. This, in turn, strongly affects the cost and complexity of the power supply.High cell counts enable the use of linear regulators and simple circuitry at the cost of extra weight and limitedefficiency. Low cell counts compel the use of a more costly switching regulator, but the low cost of the battery mayjustify this expense.Designs with Four CellsA design with four single-cell batteries often provides an attractive compromise between weight and operating life. Thatnumber is particularly popular for alkaline batteries because they are commonly sold in multiples of four. Four-cellpower for 5V circuitry presents a design challenge, however. As a battery discharges, the regulator must first stepdown and then step up. This requirement precludes use of the simpler, one-function regulator topologies that can onlystep down, step up, or invert.One effective solution to this problem is the single-ended primary inductance converter (SEPIC), in which VOUT iscapacitively coupled to the switching circuitry (Figure 1). The absence of a transformer is one of several advantagesthat this configuration has over flyback-transformer regulators and combination step-up/linear regulators.Page 1 of 16

Figure 1. This regulator topology features the MAX1771 step-up controller. It supplies 5V for inputs ranging from 3V to8V. The operation shifts smoothly between step-up and step-down conversion without steps or mode changes. Duringshutdown, the output turns off completely and sources no current.As another improvement over boost designs (in which current drains from the battery during shutdown unless you adda cutoff switch—see Figure 2), the SEPIC output fully turns off in response to a shutdown command. As VIN fallsduring normal operation, the SEPIC circuit smoothly regulates VOUT without shifting its mode of operation as VOUTapproaches VIN. Its power-conversion efficiency peaks at 86%, near 200mA (Figure 1).Figure 2. Typical DC-DC boost converters provide a current path from input to output, even when powered down. Tointerrupt this path, you must add a disconnect switch (Q2).Coils L1 and L2 (Figure 1) should be the same type and have the same value, but coupling between them is notrequired. They can be wound on the same core for convenience, but the circuit works equally well if they arecompletely separate. Each coil passes only one-half of the peak switching current (IPEAK 100mV/R1 1.22A), soeach can be rated accordingly.Page 2 of 16

Capacitor C2 couples energy to the output and requires low effective series resistance (ESR) to handle high ripplecurrents. A low-ESR SANYO OS-CON capacitor, for instance, offers 3% more efficiency than a less expensive 1µFceramic capacitor. Tantalum capacitors are not recommended because high ESR causes them to self-heat at highripple currents.Diode D2 provides a supply voltage for the IC (pin 2) by capturing switching pulses at the drain of Q1. Although thisvoltage (approximately the sum of VIN and VOUT ) limits the maximum VIN to 8V, it improves the startup capabilityunder full load and improves the low-VIN efficiency by boosting gate drive to the external MOSFET. If VIN does not fallbelow 4V, you can substitute a 3V-threshold FET for Q1 and omit D2. In that case, pin 2 connects directly to VIN,which assumes an upper limit of 16.5V.Three Cells to 3.3VFor 3-cell designs, the MAX8625A high-efficiency step-up/down regulator with integrated power MOSFETs provides3.3V and up to 0.8A output capability. The device includes a True Shutdown feature, which disconnects the outputfrom the input when the IC is disabled. Together with four internal MOSFETs (two switches and two synchronousrectifiers) and with internal compensation, the circuit of Figure 3 minimizes external components.Figure 3. Typical application circuit (fixed 3.3V output).Low-Dropout, Step-Down ConverterLow-voltage logic, such as that powered from 3.3V, now enables the use of 4-cell inputs for simple step-downconfigurations that optimize efficiency and cost. For 3.3V outputs, the key specification is dropout voltage—theminimum allowable difference between VIN and VOUT . "End-of-life" voltage for the battery varies according to cell typeand the product's pattern of use, but for all but lithium batteries it falls in the range of 0.8V to 1V per cell. As a result,it is not uncommon for 3.3V regulators to operate with input voltages as low as 3.6V.The design of Figure 4 offers an uncomplicated means for delivering intermediate current loads at 3.3V from four cells.The IC drives a low-threshold p-channel MOSFET and minimizes current-sense losses with a low current-sensevoltage of 110mV. For best performance, the MOSFET on-resistance should be specified in conjunction with thecircuit's lowest operating voltage, about 3.6V in this case.Page 3 of 16

Figure 4. A low-dropout switch-mode controller (MAX1651) and p-channel MOSFET supply 3.3V at 1.5A with inputs aslow as 3.8V. Efficiency exceeds 90% for most of the operating range.Linear RegulatorsStill the lowest-cost approach for many step-down applications (short of no regulator at all) is linear regulation,provided that its efficiency and battery-life limitations are acceptable and that its power dissipation at higher VIN ismanageable.For portable designs, even a simple linear regulator can provide some challenges. As an example, dropout voltage (thelow-VIN level at which output regulation is lost) should often be regarded as a part of normal operation rather than afault. That is, to extend operating time, it may be advisable to allow the regulator to fall out of regulation withoutshutting down. The regulator's behavior during dropout (especially its quiescent current) is important in these designs.The simple linear regulator of Figure 5 offers exceptional dropout behavior with little effect on operating current.Essentially an 8-pin surface-mount package, it delivers more than 400mA. Because the internal pass element is aMOSFET instead of a bipolar transistor, the circuit's dropout voltage is nearly zero at light loads. Moreover, itsquiescent current does not rise as VIN approaches VOUT .Page 4 of 16

Figure 5. This combination of internal MOSFET pass transistor and high-power SO-8 package provides a linearregulator (MAX604) with low dropout, an operating current of 15µA, and an output capability of over 400mA.This last characteristic is especially important for small portables whose steady-state load is no greater than 100µA. Insuch designs, the milliamp or more of quiescent-current rise (typical of a low-dropout regulator with bipolar-passtransistor) accelerates the battery discharge at a time when the battery can least afford it: near the end. Typically, theIC in Figure 5 draws 15µA of operating current whether in or out of dropout.Boosting from Low-Cell-Count BatteriesThe cell count for batteries in earlier-generation designs was high—not to provide more energy, but rather to allowgeneration of the system voltages with low-cost linear regulators (or even with no regulator at all). The latestgeneration of voltage-conversion ICs, however, lets you reduce the cell count while adding a minimum number ofexternal parts. Usually, this extra cost is more than offset by the benefits of lower cell count: smaller size, less weight,and (sometimes) longer battery life. To illustrate, the 4.5Whr of available energy in two AA cells exceeds the 3Whr ina 6-cell, 9V alkaline battery by 50%, even though the two battery topologies are comparable in size and weight.The step-up regulator of Figure 6a provides high, 88% efficiency for 2-cell and 1-cell inputs; its high, 500kHzswitching frequency enables the use of very small inductors. The IC's quiescent current is only 60µA at light or zeroloads—an attractive feature for portable products whose supply voltage must remain active when the product is turnedoff. As the product enters such an idle or suspend mode, load current falls to microamps and must not be dominatedby current into the regulator IC. For equipment that truly shuts down, the IC provides a very low-current shutdownmode in which it draws less than 1µA.One-Cell RegulatorsIt makes sense to operate from a 1-cell battery when size is of paramount importance. Reasonable efficiency and costare now possible when operating with inputs below 1V, so many handheld applications have become new candidatesfor 1-cell operation. The switching frequency for low-cost ICs now approaches 1MHz, which permits the use of smallmagnetic components available from multiple sources. It is not unusual, therefore, for the DC-DC circuitry to occupyless space than the battery it replaced.In Figure 6a, the addition of Q1 and Q2 within the dashed lines allows the regulator to start with lower input voltagesand higher load currents. Q1 also disconnects the load and battery from each other during shutdown. The on-chipcomparator does not allow Q1 to turn on again until VOUT has risen to at least 3V. Figure 6b illustrates this circuit'sloaded-start capability and its remarkably low typical startup voltage (0.8V).Page 5 of 16

Figure 6. This low-power, CMOS step-up converter (MAX856) (a) generates 3.3V from 1-cell and 2-cell inputs. Theoptional load-disconnect circuitry (dashed lines) enables the circuit to start with inputs as low as 0.8V (b).Figure 7 also shows a high-power, high-efficiency step-up regulator that operates down to 0.7V (once started) andhas a startup voltage of 0.9V. The output can be fixed at 5V or adjustable step-up (2.5V to 5.5V) and is capable ofsourcing up to 1.5A current.The MAX1703 comes in a 16-pin narrow SO package and includes an uncommitted comparator that generates apower-good or low-battery-warning output.Page 6 of 16

Figure 7. MAX1703 in high-power pulse-width modulation (PWM) mode.Inductorless Conversion Suits Tight SpacesDespite the advances made in inductor-based switching regulators, most designers would regard the ideal convertercircuit as one that has no inductor. The capacitor-based alternatives (charge-pump converters) were hampered in thepast by their lack of regulation and limited output current. Though still low compared to that of switching regulators,their output current is now adequate for many designs. And in some cases, the charge-pump advantages arecompelling: low cost, small size, and reduced electromagnetic interference (EMI). Charge pumps are particularly usefulin Personal Computer Memory Card International Association (PCMCIA) systems and other "credit-card" products inwhich the component height is limited.Figures 8, 9, and 10 illustrate three inductorless voltage converters. In Figure 8, the output of a 2-cell battery or otherlow-voltage source is converted to a regulated 5V 4%. The IC changes its operational mode with input voltage,producing a tripler at low VIN, a doubler at high VIN, and a tripler-doubler at midrange that changes modes everyswitching cycle. Efficiency ranges from 85% to 65%. Low supply current (typically 75µA for no-load operatingconditions and 1µA in shutdown) makes the circuit useful as a coin-cell-powered backup supply for DRAM orpseudostatic RAM (PSRAM).Page 7 of 16

Figure 8. With a few external capacitors, the MAX619 boosts a 2-cell or 3-cell input to 5V, and delivers 50mA (for 3Vinputs) with only 75µA of quiescent current. With an additional dual diode in a SOT23 package and two capacitors, italso produces a small negative output.The optional diode-capacitor network in Figure 8 generates an unregulated negative voltage between -1.4V and -3V.Acting as a negative supply, this output simplifies analog designs by allowing the use of inexpensive op amps. Thenegative rail assures that such op amps can swing completely to ground.Another charge-pump circuit, built in less than 0.1in² of board area, converts 5V to the 12V level required forprogramming flash memory chips (Figure 9). Common in PCMCIA cards, flash memory is popular for compact portableapplications because it