Choosing the correct power supply management topology for your application - Embedded.com

Choosing the correct power supply management topology for your application

The issue of efficiently generating a 3.3V rail from a Li-ion batteryis not new, andsolutions for this problem already exist. This article discusses somepopular solutions, including cascaded buck-and-boost, buck-boost, buck and low-dropoutregulator (LDO) power supply topologies, and the trade-offsof each design. System runtime is also measured and compared.

The discharge profile of a typical Li-ion battery startsat 4.2Vwhen fully charged. The x-axis inFigure 1 below starts at -5 mins to show the battery's fullycharged open-circuit voltage. At 0 min, the battery is loaded, and thevoltage drops due to its internal impedance and protection circuitry.

The battery voltage gradually drops until about 3.4V, where itstarts to do so rapidly because its discharge cycle is almost done. Tomaximize the battery's stored energy, a 3.3V rail requires a stepdownconverter for most of the discharge cycle and a boost converter for therest of the cycle.

Figure1: A 3.3V rail requires a step-down converter for most of the dischargecycle and a boost converter for the rest of the cycle.

A cascaded buck-and-boost converter consists of two separate,discrete converters: a buck converter and a boost converter.

A buck converter regulates the battery voltage to an intermediatevoltage such as 1.8V. A boost converter then increases the intermediatevoltage up to 3.3V.

This architecture is useful if the system already requires the lowervoltage rail. It uses 100 percent of the battery's capacity. From theefficiency point of view, however, this solution is not optimal becauseof the two conversion stages.

The effective power-conversion efficiency is the product of both thebuck regulator's and boost converter's efficiencies.

Typical efficiency numbers for buck-and-boost converters operatingat these voltages are 90 percent each. These efficiencies provide aneffective 3.3V converter efficiency of 90% * 90% = 81%. The twoseparate converters increase the number of parts and solution size ofthis architecture, making it prohibitive in small portables. Anadditional drawback is the additional cost associated with two separateconverters.

An often overlooked solution for generating 3.3V from a Li-ionbattery is the buck converter, which has not gained widespread use.However, it has clear benefits that should not be overlooked. Designerstypically dismiss this solution after examining the battery's dischargecurve in Figure 1, above.

Figure 1 shows that a buckregulator cannot generate a 3.3V rail over the battery's full dischargecurve. When a buck converter's input voltage drops near its outputvoltage, many buck converters enter a 100 percent duty cycle mode. Inthis condition, the converter stops switching and passes the inputvoltage directly through to the output. In 100 percent duty cycle mode,the output voltage equals the input voltage minus a voltage drop acrossthe converter.

This voltage drop is a function of the power MOSFET's on-resistance,the output inductor's DC resistance and the load current. It sets theminimum battery voltage where the output is still considered to be inregulation.

Assuming that a system allows the 3.3V rail to drop five percent andstill be in regulation, this equation calculates the minimum batteryvoltage for system operation:

V battery_min = V out_nom *0.95 = (R dson + R L ) * I out

where Vout_nom is the nominal 3.3V setpoint; Rdson is the power MOSFET's on-resistance; RL is the outputinductor's DC resistance; and Iou t is the converter's 3.3Voutput current.

When the battery voltage drops to Vbattery_min , thesystem must shut down to ensure that data is not corrupted by runningwith the 3.3V rail below its minimum tolerance. The system may shutdown even though the battery still contains anywhere from 5-15 percentof its rated capacity.

The actual unused capacity depends on many factors, includingcomponent resistances, load currents, battery age and ambienttemperature.

Most designers dismiss the buck-only topology for this reason alone,but careful examination of actual system runtime reveals that thisdecision may have been made in haste.

The efficiencies of traditional buckboosts and the cascadedbuck-and-boost topologies are much lower than the standalone buckconverter. Although these other topologies use the full batterycapacity, they do it at a much lower efficiency than the buckconverter.

In many cases, the standalone buck converter's runtime exceeds thatof the other two topologies. Until about 2005, the fully-integratedbuck converter was often the best choice for generating the 3.3V rail.

Wider acceptance
Another solution that doesn't get much widespread use is the LDO. Likethe buck-only solution, the LDO cannot fully use the entire batterycapacity.

This is because it only maintains regulation when its input voltageis greater than the output voltage plus the LDO's dropout voltage. Ifthe LDO has a dropout voltage of 0.15V, the 3.3V output voltage startsto drop when the battery voltage falls below 3.3V + 0.15V = 3.45V.

Depending on the dropout voltage of the LDO, this solution hasgreater potential to leave more unused energy in the battery than thebuck-only solution. Despite this drawback, the LDO has benefits thatmake it an attractive solution in the right situation.

An LDO typically provides the smallest solution size, making it thesolution of choice when space constraint is the main systemrequirement. Usually the cheapest solution, the LDO is attractive invery cost-sensitive applications. Many designers dismiss the LDO due toits low ef- ficiency, but close examination of the efficiency in thisapplication shows a respectable solution:

Since the fully charged Li-ion battery voltage starts at 4.2V, theLDO's efficiency starts at 78 percent and increases as the batteryvoltage drops.

The buck-boost topology is becoming widely accepted. This topologycombines the best features of all the other solutions discussedearlier. As the name implies, it provides both buck and boostfunctionalities, using 100 percent of the battery capacity. Dependingon how the buckboost converter is implemented, it can have a very highefficiency.

For example, the TPS63000 fully-integrated buck-boost converter hasan efficiency that hovers around 95 percent for a 3.6V to 3.3Vconversion ratio. Using the entire battery capacity at a highefficiency provides the longest runtime of all solutions.

A fully-integrated buck-boost converter that integrates the powerswitches, compensation components and feedback circuitry has a verysmall solution size. The only external parts required are the input andoutput capacitors, and inductor, which are comparable to the buck forparts count and solution size. The single, highly-integrated ICsolution helps minimize overall cost.

Figure2: The buck-boost topology consists of a buck power stage with its twopower switches connected through the power inductor to a boost powerstage with its two power switches.

Figure 2, above, shows thebuckboost power stage. This topology consists of a buck power stagewith its two power switches connected through the power inductor to aboost power stage with its two power switches. These switches can becontrolled in three distinct modes of operation: buck-boost, buck andboost modes. A specific IC's mode of operation is a function of theinput- to-output voltage ratio, and the IC's control topology.

Buck-boost variety
The need for buck-boost converters in portable applications has beenthere for a long time. Often, these buck-boost converters have strictsize and efficiency requirements.

Silicon and packaging technology only recently has advanced to thepoint where integrating fourMOSFETswitches into a small package with a suitable control loop is feasible.Several integrated buck-boost converters are available, but often, theyhave very different operating characteristics. Although the variousbuckboost solutions have the same power stage topology, they havevastly different control circuitry.

Three types of standard buckboosts are available. The first typeoperates all four MOSFETs during each switching cycle. This type ofoperation generates the classical buck-boost waveforms. Carefulanalysis of these waveforms shows that the rms current through theinductor and MOSFETs is significantly higher than that of a standardbuck or boost converter.

This increases both the conduction and switching losses in theclassical buck-boost. Operating all four switches simultaneously alsoincreases gate-drive losses, which can significantly lower efficiencyat lower output currents. The second buck-boost control scheme is newerand reduces losses by only operating two MOSFETs per switch cycle.

Referring to Figure 2 above, thiscontrol scheme operates in three distinct modes. When Vin is greaterthan Vout , theconverter opens Q4 and closes Q3. It then controls Q1and Q2 as a classical buck converter. When Vin is below Vout , thecontrol circuitry opens Q2 and closes Q1. It then controls Q3 and Q4 asa classical boost converter.

This control mode has several operational and control problemsaround the transition region between the buck and boost modes. Thesolution is to operate as a classical buckboost mode during thetransition region.

In this operating mode, all four switches are operational. Thebuck-boost mode eliminates the control issues. However, it introduces aregion with significant efficiency drop in the transition region due tothe increased switching losses and increased rms currents.

Unfortunately, the transition region falls near the battery voltagewhere most of the energy is available. Thus, the converter operates inthe inefficient buck-boost mode for much of the battery's dischargecurve.

The third buck-boost control scheme provides a significantimprovement in performance and efficiency by eliminating the transitionregion between buck and boost modes. The TPS63000 buck-boost convertercontains an advanced control topology that eliminates the traditionalbuckboost issues.

Regardless of operating conditions, it operates only two switchesper switching cycle. This results in reduced power losses and highefficiency across the full battery discharge curve.

Figure3: Each setup uses the same battery to eliminate variations in data dueto differing battery capacities.

Figure 3 above shows aside-by-side comparison of the battery discharge curves and runtimesfor four Li-ion to 3.3V solutions. These solutions are the cascadedbuck-and-boost, the buck only, the LDO and the buckboost converter.

The setup uses a fully-charged 18650 Li-ion battery with 1650mA-Hrcapacity. The load current is set at 500mA, and system shutdown isdefined as the point where the 3.3V rail drops five percent below theinitial set point.

Each setup uses the same battery to eliminate variations in data dueto differing battery capacities. As anticipated, the LDO achieved thelowest runtime with only 190 mins, and the buck-boost achieved thesecond highest runtime with 203 mins. As expected, the cascadedbuck-and-boostsolution achieved the shortest runtime with only 175 mins. Table 1 below compares several keyareas of concern for an actual system.

Table1. Design engineers have many choices for generating 3.3V from a Li-ionbattery, and the optimal solution depends on the specific systemrequirements.

Other considerations
The data in Figure 3 are takenwith a constant DC load. This is typical of bench testing, but nottypical in real applications. To maximize runtime in portableapplications, loads are switched on only as long as required, thenswitched off when not needed.

Displays, processors and power amplifiers are examples of loads thatproduce significant transients on the system battery. Their load stepsresult in voltage drops on the battery bus due to the battery'sinternal source resistance, protection circuitry and distribution busimpedance.

When these load steps occur near the end of the discharge cycle,they can pull the battery voltage below 3.3V. With the buck and LDOsolutions, this results in early system shutdown.

The buck-boost solution continues to operate through thesetransients, thereby extending the system's operating time. Loadtransients that appear insignificant in lab testing get muchworse under real-world conditions. This is because a Li-ion battery'sinternal resistance doubles with 150 charge/discharge cycles. Internalresistance also doubles when operated at 0°C vs. 25°C.

Figure4: The buck and buck-boost converters have a constant 250mA load, whilethe battery bus is loaded with a 500mA current transient.

Figure 4 above shows aLi-ion battery's bus voltage when operated with a load transient. Thebuck and buck-boost converters have a constant 250mA load, while thebattery bus is loaded with a 500mA current transient. Thebuck-converter output drops out of regulation, which could cause asystem shutdown. The buck-boost converter operates through thetransients with no change in output voltage.

Design engineers have many choices for generating 3.3V from a Li-ionbattery. The optimal solution really depends on the specific systemrequirements. Most systems will benefit from the advantages provided bya buck-boost converter. With the longest runtimes, small size andrelatively low cost, this is the best overall solution for mostportable applications.

When choosing a buck-boost converter, take note that not allbuck-boost converters are created equally. Pay close attention to theoperating modes, the efficiency over the full battery operating rangeand the total solution size.

MichaelDay is Power Management Application Supervisor and Bill Johns is anapplication engineer in the Portable Power Group at Texas Instruments Inc.

For a PDF version of this story,go to “ Choosethe best buck-boost converter.” 

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