Choosing a Power Management IC for Energy

8/13/2019 Choosing a Power Management IC for Energy

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Choosing a Power Management IC for Energy-Harvesting ApplicationsBy Bill Schweber, Electronic Products5/22/2013

The power-management IC (PMIC) supports and manages the transducer and

energy-collection channel, the energy-storage element (battery, conventionalcapacitor or supercapacitor), and the processor/wireless link. This critical block ofany energy-harvesting design implements several major functions:

Captures and extracts the random, miniscule energy from the sourcetransducer

Transforms that extracted power into energy for the storage element, usuallyvia a DC/DC converter

Manages the outflow of power from the storage element, while ensuring thatpower is not drawn when the stored energy is below a threshold value andwould be wasted

Perhaps most challenging, it has to manage its own start-up sequence in thetransition from when there is insufficient available stored energy for the PMICitself

For designers of energy-harvesting systems, there is good news: a wider choice ofPMICs is now available from vendors, each with differing attributes andcharacteristics. Some are optimized for a specific harvesting source, while others aremore general; some fulfill primarily one part of the PMIC role, while others integratemore of the essential functions.

Engineers should always keep in mind that all energy-harvesting designs are basedon one principleto accumulate over time the trickle of minuscule, sporadic energyfrom the source, store it, and then release it as a power pulse when needed by therest of the system for data collection, analysis and transmission. Therefore, there arelong quiescent periods when the PMIC's only activity is to allow the energy to flow tothe storage element and little else, as well as periods when there is no incomingenergy.

You must also be aware that a conventional DC/DC switching converter core, nomatter how efficient, is generally not a good choice for energy harvesting. This isbecause these are designed to work with a consistent source such as battery, which

is a low-impedance, somewhat regulated source. Further, the DC/DC switchingconverter of a PMIC must have extremely low quiescent current requirements, asthere may be long periods without available source energy and thus no power-converter activity.

MaximsMAX17710

The MaximMAX17710accepts energy from a variety of transducers, manages thecharging of the energy-storage element, and provides a regulated DC output to theload (Figure 1). It is optimized to work with the THINERGY ultra-thin, postage-stamp-sized micro-energy cell (MEC) rechargeable battery from Infinite Power Solutions.

When no harvesting source is present, the quiescent battery drain of this IC is under1 nA, thus increasing the likelihood that enough battery power will remain for basic
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operation even after a long period of harvest inactivity.

The part includes protection for the battery against undervoltage and overvoltageconditions, which many batteriesespecially the very small ones used forharvestingcannot tolerate. For the first problem, the MAX17710 locks out any load

drain to prevent overdischarge of the MEC (which may damage the MEC), andunlocks it only when the harvesting source has delivered enough energy to the MECand raised its voltage. For overvoltage conditions, which may be due to excessivecharging or component failure, it regulates the input voltage and shunts excesspower away from the MEC.

Figure 1: The Maxim MAX17710 is designed to primarily work with one batteryvendor's component, but the harvested energy can come from a variety of sources.

Linear TechnologysLTC3588 series

TheLTC3588is an energy-harvesting power supply, which is designed for use withvibration-sensitive piezoelectric transducers with their high output impedance andrelatively high output voltage (up to about 20 V). By combining a full-wave bridgerectifier and a buck converter, both in low-loss implementations and undervoltagelockout functions with wide hysteresis, it directs charge from the transducer toaccumulate on an input capacitor. Then, when there is enough charge for the buckconverter to operate efficiently, it directs the stored charge to the output (Figure 2).
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Figure 2: The start-up profile of the Linear Technology LTC3588, designed forpiezoelectric source harvesting, shows how it manages the difficult transition as itaccumulates enough energy for its own operation.

Texas Instrumentsbq25504

In contrast to the piezoelectric transducer, photovoltaic (PV, or solar cell) orthermoelectric generators (TEG) are low-voltage, high-impedance sources.Thebq25504(Figure 3) is centered on a boost converter/charger which starts whenthe input voltage is as low as 330 mV, and then will continue to function with VIN aslow as 80 mV.

Since it is designed for sources such as solar cells, this device implements analgorithm to operate at the optimal energy-extraction and transfer point, called

maximum peak power or power point tracking (MPP or MPPT), which matches thesource impedance with the load impedance. For a PV source, the MPP is typically at80 percent of the open-circuit voltage. For enhanced efficiency, the bq25504 lets thesystem microcontroller externally establish the MPP level; this also allows use of thebq25504 with other sources, such as the ambient electric field, which have differentMPPs.
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Figure 3: Optimized for solar (photovoltaic) and thermal sources, the TexasInstruments bq25504 implements an MPP algorithm but also lets the user set the

MPP point.

CymbetsCBC915

In contrast to PMICs which are designed for one or just a few types of energysources, theCBC915EnerChip EP Energy Processor is designed to be used withalmost any energy source, including vibration, light, thermal, RF induction, or EMfields. It incorporates a sophisticated MPPT approach, which dynamically matchesthe output impedance of the transducer to the input impedance of the power-conversion stage (Figure 4); in contrast, some PMIC collection topologies use asimpler but less efficient "charge accumulation" approach. When you are only

collecting nanojoules of harvested energy, even a few percentage points ofimprovement can be critical.
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Figure 4: A sophisticated harvesting PMIC such as the Cymbet CBC915 is wellsuited to sources such as PV cells, where the power curve is highly nonlinear, thusconnecting the cell to a load that is not matched to its impedance results in inefficientpower transfer.

Why would a designer use a more general-purpose PMIC such as this one? Thereare several reasons: the specific harvesting transducer may not be known inadvance; the end-product is intended to be used with a variety of sources and use ofthis type of PMIC simplifies the bill of materials (BOM); or the user wants access tomore advanced MPP control for that last extra bit of performance.

The CBC915 also includes a serial UART for communication to/from the systemmicrocontroller. Using this interface, it can accept basic operational commands andalso report back on key status indicators.

Factors to consider

When choosing a PMIC for harvesting applications, it is vital to look at the detailedspecifications, of course. But along with that, there are broader questions toconsider:

Do you want a transducer-specific PMIC or one which can be tailored todifferent harvesting sources?

What level of power management and algorithm sophistication is needed? What amount of processor control/status with the PMIC is preferred?

What source duty cycle do you expect: as little as a few seconds per hour, or


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much more?

Summary

The power-management IC (PMIC) is one of the major blocks of an energy-

harvesting design, along with the transducer and energy-collection channel, theprocessor/wireless link, and the energy-storage element (battery or supercapacitor).All four of these blocks are now seeing new product introductions from multiplevendors, as new harvesting opportunities stimulate innovation in the others. Thisarticle has reviewed recent developments in PMICs from various vendors, along withtheir key attributes and capabilities. For more information on the parts discussed,use the provided links to access product pages on the Digi-Key website.

Further Reading

1. TechZone article, Power Management ICs Simplify Integration of Supercapacitors in EnergyMicroharvesting Designs

2. TechZone article, Managing the Energy and Lifetimes of Thin-Film Batteries3. TechZone article, A Hands-on Look at the Maxim MAX17710 Energy Harvesting PMIC4. TechZone article, Optimal Power Management Techniques in Energy Harvesting Designs
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Choosing a Power Management IC for Energy

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