Trevor Barcelo Product Line Manager, Battery Management & Steve Knoth Senior Product Marketing Engineer, Power Products Group at Linear Technology Corporation explore large capacity LiFePO4 batteries and their requirement of high power chargers in portable devices such as medical equipment
A key trend in patient care is the increasing use of remote monitoring systems within the patient's own home. As a result, many of these portable electronic monitoring systems incorporate RF transmitters so that data can be sent directly back to a supervisory system within the hospital for analysis.
These types of systems are usually powered by the AC mains supply, a battery, or both. This redundancy is necessary in order to ensure continuous operation while in use at an off-site location. Many of the newest advances in portable medical diagnostic devices such as those carried around by doctors and nurses include a battery as the main power source, or used for back-up power in case the AC mains supply is interrupted. Such systems need an efficient battery charging circuit.
Lithium-based cells continue to be the most popular choice. However, charging these batteries quickly, accurately and safely is non-trivial. New Lithium-based chemical anode/cathode combinations continue to be developed and are being released into the mainstream market. Consequently, there has been an emergence of Lithium-Iron Phosphate (LiFePO4) cells in many applications, offering improved safety and longer cycle life than cobalt-based Li-Ion/polymer cells.
This chemistry offers many of the other advantages of cobalt-based Li-Ion cells as well, including a low self-discharge rate and relatively low weight.
In addition to the improved safety (due to a resistance to thermal runaway) and longer cycle life, LiFePO4 offers, by comparison, higher peak-power rating and lower environmental impact. Often, medical and industrial applications are willing to accept the lower volumetric energy density of LiFePO4 to get improved safety and cycle life. Back-up applications require the improved cycle life and frequently take advantage of the ability to discharge at high currents.
How to get more power effectively
A typical portable medical or industrial device 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 a few hours or so requires multiple amps of charge current.
To satisfy 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 a USB port. Also, the product needs to protect sensitive downstream low voltage components from potentially damage-causing overvoltage events and efficiently direct high currents to the load from a USB input, a wall adapter or the battery to minimise power lost as heat. At the same time, the IC must safely manage the battery-charging algorithm and monitor critical system parameters.
The lower 3.6V float voltage of Lithium-Iron Phosphate batteries precludes the use of a standard Li-ion battery charger. Irreparable damage to the cell may result if not properly charged.
Accurate float voltage charging will prolong the life of the cell. Advantages of LiFePO4 when compared to cobalt-based Li-Ion cells include lower volumetric energy density (capacity per unit volume) and susceptibility to fail prematurely if the new cells are ‘deep cycled’ too early.
While the above requirements might seem impossible to find in a single IC, consider the LTC4156 from Linear Technology. This device is a high power, I2C-controlled, high efficiency PowerPath manager, ideal diode controller and Lithium Iron Phosphate (LiFePO4) battery charger for single-cell portable applications such as portable medical & industrial devices, backup devices and high power density battery-powered applications.
The IC is designed to efficiently transfer up to 15W from a variety of sources, while minimising power dissipation and easing thermal budgeting constraints. This devices’ switching PowerPath topology manages power distribution from two input sources such as a wall adapter and USB port to its rechargeable Lithium Iron Phosphate battery while preferentially providing power to the system load when input power is limited (figure 1 Above Right).
Because power is conserved, this IC allows the output load current to exceed the current drawn by the input supply, maximising use of the available power for battery charging without exceeding the input supply power delivery specifications.
This IC’s switching regulator acts like a transformer, allowing the load current on VOUT to exceed the current drawn by the input supply. This IC features instant-on operation to ensure system power is available at plug-in even with a dead or deeply discharged battery (Figure 2 Above Left).
The modern product designer of portable medical and industrial devices has an extremely challenging job - particularly when it comes to power. Businesses continue to demand features that require more power and consequently, larger batteries. Meanwhile, the convenience of charging these batteries from just about any available power source is desirable.
LiFePO4 cells are becoming mainstream choices due to their inherent safety, low float voltage, longer cycle life, low self-discharge rate and relatively low weight. But like any rechargeable battery, they must be handled with care.
While these trends in portable power are design challenges, this IC makes the job considerably easier. In low voltage systems, it also provides up to 3.5A of charge current efficiently.
Linear Technology Corporation