Power Electronics

Power Management ICs Improving Rechargeable Battery Lifetimes

As rechargeable batteries surge in usage, power management ICs become a key element in many portable and vehicle power circuits.

Getting the most power and lifetimes out of the rechargeable lithium-ion (Li-Ion), lithium-polymer (Li-Poly), nickel-cadmium (NiCd) and nickel metal-hydride (NiMH) batteries found in a plethora of mobile and consumer electronics items is a leading user requirement. Most of these use Li-Ion batteries.

This in turn is propelling the growth of power management ICs. Also helping this trend is the move toward alternate energy products like electric vehicles, solar power, wind power, and a smart electric grid.

So says market research firm iSuppli, part of IHS Inc. It predicts global semiconductor revenues will climb up to $36.2 billion this year, up 13.9% from last year's banner revenues of $31.8 billion (Fig. 1). A good deal of these revenues will come from the enormous growth of portable electronic devices everywhere, including digital cameras, laptop and notebook computers, portable digital assistants (PDAs), MP3 players, mobile phones, tablets, portable navigation devices, and a host of other consumer electronic products.

Powering such batteries is calling for newer design strategies in the power management circuits that monitor, charge, and maintain battery performance. These circuits are becoming more highly integrated, more efficient and smaller. Many portable consumer electronics products provide fuel gauges that allow users to know when the batteries need charging and how much life is left in that charge.

We're witnessing the introduction of highly integrated pulse-width-modulation (PWM) controllers, high-efficiency dc-dc converters, sophisticated microcontroller units (MCUs) and more potent power devices going into the power supplies that run the portable and consumer electronics products as well as charge their batteries. Another trend is a move to making USB ports available for charging.

An advanced single- or dual-cell Li-Ion/Li-Poly) charge management controller can be found in the MCP73841/2/3/4 family of charge-management MCUs from Microchip Technology. Designed for use in space-limited and cost-sensitive applications, they offer high accuracy (±0.5% maximum) with four pre-set voltage options: 4.1 V, 4.2 V, 8.2 V and 8.4 V. They feature constant-current and constant voltage regulation cell-temperature monitoring and pre-conditioning, advanced safety timers, automatic charge termination and charge-status indication, all within a 10-pin MCSOP package.

There are even devices like the Linear Technology LTC4425 designed for charging super-capacitors that work with batteries (Fig. 2). This constant-current/constant-voltage linear charger can charge a 2-cell super-capacitor stack from either a Li-Ion/Li-Poly battery, from a USB port, or a 2.7-V to 5.5-V current source. It operates as a 50-mΩ ideal diode making it suitable for high peak-power/low average-power applications.

CELL BALANCING

Balancing a rechargeable battery pack's cells is a critical requirement for longer battery life and greater levels of safety. Cell balancing must also be coupled with the manner in how the cells are used and the environment in which they operate.

Thermal mismatch of a cell's battery pack is just one of many factors involved. Self-discharge rates for a Li-Ion battery just about double for every rise in temperature by 10°C. This in turn affects a cell's state of charge. In general, care must be exercised on where a battery pack is placed, for minimal temperature effects. The closer the pack is to the equipment it powers, the greater the risk of overheating those cells in contact with the equipment.

Li-Ion batteries can store more energy than conventional NiMH batteries and are about 30% smaller and 50% lighter. But they tend to overheat when overcharged or during deep discharging, necessitating protection and safety features for high-cell-count Li-Ion battery packs.

Atmel offers a solution with its ATA6870, the first to support active cell balancing in high-cell-count Li-Ion battery packs. It supports voltages up to several hundred volts, voltages found in electric/hybrid vehicles, uninterruptable power supplies (UPSs) and e-bikes. Active cell balancing avoids the energy loss that occurs with passive cell balancing methods where overcharged cells are discharged through a resistor. The charge from active cell balancing is transferred from a shuffle capacitor or inductor, from one cell to another, with nearly no losses, leading to higher efficiency and longer battery life.

Linear Technology uses passive cell balancing with its LT6802-1 cell-monitoring IC. It is simpler than the active balancing method. The technique works by shunting a resistor across each cell, in a multi-cell series stack, to dissipate imbalance currents, when the stack is fully charged.

There are also other balancing factors to consider. For example, the quality of the battery cell involved, the impedance mismatch between cells, the number of cycles and the rate a battery pack operates at, and the size of the battery pack (how many cells are involved) can all contribute to cell imbalance.

A MATTER OF GAUGING

Most portable battery powered consumer electronics items contain a fuel gauge for modern battery power management. They're designed to gauge single-cell and multiple-cell battery packs, in various battery pack cell configurations, denoted by the letters S for series and P for parallel. For example, 1S1P means one serial-one parallel, 1S2P means one-serial 2-parallel, and so on. These try to provide an accurate estimate of battery capacity remaining. Although the displayed data - either a bar graph, amount of charge and time left or both - is not always very accurate, it is reasonably accurate depending on whose fuel gauge IC is being used and the algorithm it is based on. Most offer accuracies to within 1% or better, provided that certain operating conditions are observed.

Some of these gauges can also protect the primary battery, implement cell-balancing of the battery pack, and can also maintain records of battery use history. Most of these gauges support the two-wire I2C bus or the one-wire HDQ (high-speed DQ) for communication with the host device, which can be a portable device like a laptop computer or a charging module like those used for cell phones. One company, Maxim Integrated Products, has its own proprietary 1-Wire bus.

Maxim uses the Model Gauge algorithm for its MAX17040/041 single-cell (1S)/dual-cell (2S) ultra-compact host-side fuel gauge for Li-Ion batteries. The Model Gauge algorithm monitors the battery's relative state of charge continuously over a widely varying charge/discharge profile. The algorithm eliminates the need for battery re-learn cycles and an external current-sense resistor used by fuel gauge ICs from other companies. Temperature compensation is performed with minimal interaction between a microcontroller and the device.

The ICs interface via the I2C bus and provide a quick-start mode that gives an initial estimate of the battery's state of charge, enabling the placement of the gauge ICs on the system side, reducing cost and supply chain constraints. They're available in either 0.4-mm-pitch or 9-bump Maxim UCSP packages or 2-by-3-mm 8-pin thin dual flat no-lead (TDFN) packages.

HIGH INTEGRATION

Recently, Maxim introduced the MAX17435/535 850-kHz/500-kHz multi-chemistry SMBus-programmable battery chargers that permit the charging of current, charge voltage, input current limits, relearn voltage, and digital IINP voltage readback (Fig. 3). Other competitive products require the use of external components to program charge settings.

Maxim is working on the MAX17830, the next-generation higher-accuracy electric-vehicle battery management solution with safety enabling features and a hardened SMBus ladder. It is said to be the industry's only 80-V tolerant 12-cell device and provides a cell-error reading of 0.1%. It offers 48-cell hot-plug performance for fast-pack assembly.

One of the first companies to introduce a sophisticated algorithm that provides high-accuracy fuel gauging is Texas Instruments (TI). Its Impedance Track algorithm uses a model of cell-impedance change to update the cell's chemical capacity during operation. This algorithm has been continually improved and works with many battery types and can provide 99% accuracies and higher. For this algorithm to work properly, however, a battery should not be used close to the maximum charge and discharge levels. That is, the battery should not be allowed to reach the maximum discharge rate before being re-charged, nor should it be overcharged and get too hot.

O2Micro international has a patented technique for fuel gauging of high-S-count battery packs used in electric vehicles and high-energy drive electric motors. The method involves a controlled trickle-charging of the battery pack to protect it from large current faults conditions like a short circuit. It enables continuous monitoring and graceful recovery of the fault without using conventional methods that may damage the battery pack or cause it to permanently open.

Intersil makes a low-voltage fuel gauge IC, the ISL6295, that measures, stores and reports all of the critical parameters required for rechargeable battery monitoring with a minimum of external components. The ISL6295 chip comprises a 16-bit integrating analog-to-digital converter (ADC) that performs current measurements to within ±0.5%.

ENTER USB

One noticeable trend in battery power management is the move to a USB-based technology platform. According to Pericom, this move provides a greener approach to changing portable devices, by eliminating the arbitrary hardware and form-factor differences between chargers for all types of portable battery powered equipment.

Pericom makes available a USB solution for notebook and mobile device chargers via its PowerNap technology that is being extended to automotive applications using a console-based approach. PowerNap's PI5USB56 Sleep-and-Charge controller technology allows the user to quickly connect and charge portable devices without waking up the PC (Fig. 4). It is said to be the first to offer auto-detection and auto-switching support for mobile device charging.

Until recently, the USB 2.0 specification was ill-suited for portable consumer electronic devices that needed to communicate and exchange data with one another. This has now been simplified with the release of the USB on-the-go (OTG) addendum, which defines a new class of devices with increased functionality and limited host-computer capabilities. USB-OTG is a host-centric point-to-point bus which allows a user to plug in two devices. It specifies that USB-OTG's host must supply at least 8 mA between 4.4 and 5.25 V.

USB-OTG has given rise to a number of USB -compliant ICs for power management. Devices like the Texas Instruments bq24150/51 fully integrated switch-mode single-cell Li-Ion charger ICs fully comply with the USB-OTG specification for Li-Ion and Li-Poly batteries. Aimed at mobile and smart phones, MP3 players and hand-held devices, the IC provides faster charging than linear chargers and provides high-accuracy voltage and current regulation. Input current regulation is accurate to within ±5% (100 to 500 mA) and charging voltage regulation is accurate to within ±0.5% at 25°C. Charging current is regulated within ±5%.

The IC can be programmed through an I2C interface. It integrates a synchronous PWM controller, power MOSFETs, input current sensing, high-accuracy current and voltage regulation, and charge termination, all in a tiny 1.976-by-1.946-mm, 20-pin wafer-level chip-scale package (CSP). The IC's boost mode of operation simplifies the support needed for USB-OTG, by eliminating the need for a separate boost converter to supply 5 V.

TI also makes a number of other USB battery-power management ICs for single-cell Li-Ion and Li-Poly batteries. These include the TPS6507X family, the bq2408x family of 3-by-3-mm micro-lead packages (MLPs) for space-limited applications, and the bq24314 and bq24316, highly integrated ICs designed to provide protection to Li-ion batteries from failures of the charging circuit.

An interesting device is TI's bq24085. It provides charge front-end control between the USB port and a battery charger like the bq2408x. It offers protection for input overvoltage, with rapid response in less than 1 µs, user-programmable over-current protection with current limiting, and 30-V maximum battery over-voltage input protection.

MORE POWERFUL CONVERTERS

More powerful dc-dc converters are emerging that can be used for limited-space battery-power management tasks in mobile devices. One is the Fujitsu MKB39C326 power-management IC (Fig. 5). This buck-boost converter is designed to power the RF amplifiers in mobile phone handsets and other mobile products that use a single-cell Li-Ion battery. It provides up to 800 mA of output current over an input voltage range of 3.1 V to 4.6 V.

Fairchild Semiconductor's USB-OTG FAN5400 ICs for Li-Ion batteries features fast switch-mode charging times with up to 94% efficiency (Fig. 6). These 5-V 300-mA boost regulators overcome the thermal issues associated with linear charging. They're suited for single-cell or parallel-cell battery packs.

National Semiconductor has just unveiled the industry's first full bridge PWM controllers with integrated MOSFET gate drivers. The LM5045 and LM5046 primary side controllers drive higher-power modules into smaller form factors using current-mode or voltage-mode control. They're available in either 28-pin 5.0-by-5.0-by-0.8-mm thermally-enhanced leadless leadframe packages (LLPs), or 4.4-by-9.7-by-0.9-mm thin shrink small outline packages (TSSOPs) (Fig. 7).

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