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ADVERTORIAL: Battery Charging in Portable Products – The Way Forward

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By Trevor Barcelo
Product Line Manager – Battery Management Products, Linear Technology Corporation

Applications in portable power are extensive and diverse. Products range from wireless sensor nodes that consume average power measured in microwatts to cart-based medical or data acquisition systems with multi-hundred watt-hour battery packs.  However, despite this variety, a few trends emerge – designers continue to demand more power in their products to support increased functionality and look to charge the battery from any available power source. The first trend requires increased battery capacities. Unfortunately, users are often impatient and these increased capacities must be charged in a reasonable time, which leads to increased charge currents. The second trend requires tremendous flexibility from the battery charging solution.  Each of these issues will be examined in greater detail.

Wanting More Power
Consider modern handheld devices – both consumer-oriented devices and industrial devices may include a cellular phone modem, a WiFi module, a Bluetooth module, a large, back-lit display … the list continues.  The power architecture of many handheld devices mirrors that of a cell phone. Typically, a 3.7V Li-ion battery is used as the primary power source due to its high gravimetric (Wh/kg) and volumetric (Wh/m3) energy density.  In the past, many high powered devices used a 7.4V
Li-ion battery to reduce current requirements, but the availability of inexpensive 5V power management ICs has pushed more and more handhelds to the lower voltage architecture. The tablet computer illustrates this point well – a typical tablet computer incorporates significant functionality along with a very large (for a portable device) screen.  When powered from a 3.7V battery, the capacity must be measured in thousands of milliamp-hours.  In order to charge such a battery in hours, thousands of milliamps of charge current are required.
However, this high charge current does not prevent consumers from also wanting to charge their high powered devices from a USB port if a high current wall adapter is not available.  To satisfy these requirements, a battery charger must be able to charge at a high current (>2A) when a wall adapter is available, but still efficiently make use of the 2.5W to 4.5W available from USB. Furthermore, the product needs to protect sensitive downstream low voltage components from potentially damage-causing overvoltage events and seamlessly direct high currents to the load from a USB input, a wall adapter or the battery while minimising power loss.  At the same time, the IC must safely manage the battery-charging algorithm and monitor critical system parameters.

Charging Single-Cell Portables
While the above requirements might seem impossible to find in a single IC, consider the LTC4155, a high power, I2C controlled high efficiency PowerPath manager, ideal diode controller and lithium-ion battery charger. It is designed to efficiently transfer up to 3A from a variety of 5V sources, resulting in over 3.5A available for battery charging and system use (see Figure 1). Even at these high current levels, the LTC4155’s 88 – 94% efficiency eases thermal budgeting constraints (see Figure 2). The LTC4155’s switching PowerPath topology seamlessly manages power distribution from two input sources such as a wall adapter and USB port to the device’s rechargeable lithium-ion battery while preferentially providing power to the system load when input power is limited.
The LTC4155’s switching regulator acts like a transformer, allowing the load current on VOUT to exceed the current drawn by the input supply and making greatly improved use of the available power for battery charging, compared to typical linear-mode chargers. The previous example illustrated how the LTC4155 can efficiently charge at up to 3.5A, resulting in faster charge times. Unlike ordinary switching battery chargers, the LTC4155 features instant-on operation to ensure system power is available at plug-in even with a dead or deeply discharged battery.
While charging a battery at a high rate, it is important to monitor battery safety.  The LTC4155 will automatically stop charging when the battery temperature falls below 0°C or rises above 40°C (as measured by an external negative temperature coefficient [NTC] thermistor).  In addition to this autonomous feature, the LTC4155 provides a 7-bit expanded scale analogue-to-digital converter (ADC) to monitor the battery temperature with approximately 1°C resolution. Combined with the four available float voltage settings and fifteen battery charge current settings, this ADC can be used to create custom charge algorithms based on battery temperature.
A simple 2-wire I2C port provides access to the NTC ADC results, enabling adjustment of the charge current and voltage settings. The I2C port also provides USB compliance by controlling 16 input current limit settings (including USB 2.0 and 3.0 compatible settings). The communication bus allows the LTC4155 to indicate additional status information such as input supply status, charger status and fault status. USB On-The-Go support provides a 5V supply back to the USB port without any additional components.
The LTC4155’s dual input, priority multiplexer autonomously selects the most appropriate input, wall adapter or USB, based on a user-defined priority (default priority goes to the adapter input). An overvoltage protection (OVP) circuit simultaneously protects both inputs from damage caused by accidental application of high voltage or reverse voltage. The LTC4155’s ideal diode controller guarantees that ample power is always available to VOUT even if input power is insufficient or absent.
Managing two inputs (e.g., USB and wall adapter) is sufficient for many portable applications such as tablet computers or industrial bar code scanners.  However, designers of portable devices continue to seek ways to charge the battery from any available power source.

Multiple Input Sources
There are several reasons for users to charge batteries from multiple input sources.  Some applications may need to be untethered from the grid and look to solar panels to provide power.  Other applications demand the convenience of being able to charge from a wall adapter or an automotive battery or a high voltage industrial or telecom supply. Whatever the reason, the requirement places a significant burden on the battery charging system. Most battery chargers make use of a step-down (either switching or linear) architecture to charge a battery from a voltage supply higher than the maximum battery voltage.  Prior charger products have typically been limited to input voltages of about 30V.  These limitations prevent a designer from considering a telecom supply as a viable input supply or a solar panel with a 42V open-circuit voltage.  In some cases the voltage range of the desired input supplies varies both above and below the battery voltage. Designing a solution for these challenges typically involves a compilation of high precision current sense amplifiers, ADCs, a microprocessor to control charging, a high performance DC/DC converter and ideal diode or multiplexing circuitry.  Linear Technology proposes an alternative and improved solution.

Powerful Charging Solution
The LTC4000 converts any externally compensated DC/DC power supply into a full- featured battery charger with PowerPath control. Typical DC/DC converter topologies that can be driven by the LTC4000 include, but are not limited to, buck, boost, buck-boost, Sepic and flyback. The device offers precision input and charge current regulation and operates across a wide input and output voltage range of 3V to 60V, allowing compatibility with a variety of different input voltage sources, battery stack sizes and chemistries. Typical applications are widespread due to the device’s general purpose configuration and include high power battery charger systems, high performance portable instruments, battery backup systems, industrial battery equipped devices and notebook/subnotebook computers.
The high voltage capability of the LTC4000, in addition to the fact that it can be combined with many different DC/DC topologies, allows it to create a powerful battery charging solution with virtually any input supply (see Figure 3).  To ensure that power from these inputs flows to the appropriate load, the LTC4000 features an intelligent PowerPath topology that preferentially provides power to the system load when input power is limited. The LTC4000 controls external PFETs to provide low loss reverse current protection, low loss charging and discharging of the battery and instant-on operation to ensure system power is available at plug-in even with a dead or deeply discharged battery.  External sense resistors provide input current and battery charge current information, allowing the LTC4000 to work with converters that span the power range from milliwatts to kilowatts.
The LTC4000’s full-featured battery charge controller charges a variety of battery chemistries including lithium-ion/polymer/phosphate, sealed lead acid (SLA), and nickel. The battery charger also includes precision current sensing that allows lower sense voltages for high current applications.

Conclusion
The modern portable product designer has an extremely challenging job – particularly when it comes to power.  Customers continue to demand features that require more power and consequently, larger batteries.  Meanwhile, customers want the convenience of charging these batteries from just about any available power supply.  While these trends in portable power are design challenges, the LTC4155 and LTC4000 make the job considerably easier. In low voltage systems, the LTC4155 provides up to 3.5A of charge current efficiently and with a host of high performance features.  The LTC4000 creates a powerful charging solution from virtually any input with unparalleled performance and flexibility.
Linear Technology (UK) ltd
Tel 01628 477066 • Email: uksales@linear.com
www.linear.com

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