Large capacity lithium iron phosphate batteries require large power chargers.
One of the main trends in patient care is the increasing use of remote monitoring systems in patients'homes. The reason for this trend is clear: the cost of staying in a hospital is too high to bear. Therefore, many of these portable electronic surveillance systems are incorporated into RF transceivers so that data can be sent directly to the surveillance systems in hospitals for doctors to study and analyze. Obviously, such systems are usually powered by AC adapter, batteries or both at the same time by the two party. This redundancy is necessary to ensure that the system works continuously when used in places other than hospitals. In addition, many new advances have been made in the field of portable medical diagnostic devices, such as devices carried around by doctors and nurses that use batteries as primary power or as backup power to prevent AC power interruptions. Such systems require a high efficiency battery charging circuit.
In addition to medical applications, portable industrial banking terminals, rugged tablets, inventory control and bar code scanning equipment all require a single high-capacity battery to reduce shape size and weight. Batteries based on lithium materials have always been the most popular choice. However, it is not trivial to charge batteries quickly, accurately and safely. In addition, new lithium-based chemical anode/cathode combinations have been developed, and these combinations are increasingly being marketed in the mainstream. One example of this trend is that lithium iron phosphate (LiFePO4) batteries have emerged in many applications, offering higher safety and longer battery life than cobalt-based lithium ion/lithium polymer batteries. The chemical battery also has many other advantages of cobalt-based lithium-ion batteries, including lower self-discharge rates and relatively light weight. In contrast, lithium iron phosphate batteries have higher peak power ratings and less environmental impact, in addition to improving safety (because of their ability to resist "thermal runaway" and prolonging battery cycle life. Usually medical and industrial applications are willing to accept lithium iron phosphate batteries with lower energy density per unit volume in exchange for higher safety and longer life cycles. Backup applications require longer cycle life and discharge at high current.
How to get more power
The power architecture of many handheld industrial or medical devices is often similar to that of large-screen smartphones. In general, 3.7V (final charge or "floating" voltage of 4.2V) lithium-ion batteries have been used as the main power supply because of their high unit weight energy density (Wh/kg) and unit volume energy density (Wh/m3). In the past, many high-power devices used two 7.4V (8.4V) lithium-ion batteries to meet power requirements, but because the low-cost 5V power management IC was on the market, more and more handheld devices adopted lower-voltage architectures, making it possible to use a single lithium-ion battery. Typical portable medical or industrial devices have multifunctional and very large displays (in the case of portable devices). When powered by a 3.7V battery, its capacity must be measured in thousands of milliwatts. In order to charge several hours for such a large capacity battery, a few ampere charging currents are needed.
However, even if such a large charge current is required, users still want to charge their high-power devices with a USB port when no high-current AC adapter is available. To meet this requirement, the battery charger must be able to charge at high current ("2A") when an AC adapter is available, but still be able to efficiently utilize the 2.5W to 4.5W power available from the USB port. In addition, the IC product needs to protect sensitive downstream low-voltage components from overvoltage events that may result from damage, and efficiently guide large currents from USB inputs, AC adapters, or batteries to load to minimize power loss in the form of heat. At the same time, the IC must manage the battery charging algorithm safely and monitor the key system parameters.
The lower 3.6V floating voltage of lithium iron phosphate battery leads to the failure to use standard lithium ion battery chargers. If the charge is not proper, it is possible to cause unrepaired damage to the battery. Accurate floating voltage charging will extend the battery life. Compared with cobalt-based lithium-ion batteries, the advantages of LiFePO4 batteries include higher volumetric energy density (capacity per unit volume) and less prone to premature failure (if the new batteries are "deep cycling" prematurely).
The main design constraints are summarized as follows:
. Large capacity batteries require large charging current and high efficiency.
. Many portable applications, including industrial and medical equipment, require the convenience of USB compatible charging.
Lithium iron phosphate batteries have special charging requirements, i.e. lower floating voltages, which have some comforting advantages over lithium-ion batteries.
Any IC solution discussed above that meets these design limitations must be compact and monolithic, capable of fast and efficient charging of a single large-capacity battery, compatible with new chemical components such as lithium iron phosphate. Such devices will act as catalysts to increase the global adoption of portable industrial and medical products using large-capacity batteries.
Power challenges for portable devices using single cell batteries
Although the above requirements may not seem to be satisfied with monolithic IC, let's take a look at LTC4156. The LTC4156, which follows the popular lithium-based LTC4155, is a high-power, I2C-controlled, high-efficiency power Path manager, ideal diode controller, and LiFePO4 battery charger for portable applications using a single battery, such as portable medical and industrial equipment. Backup devices and high power density battery powered applications. The IC is designed to deliver up to 15W power efficiently from a variety of power sources, while minimizing power consumption and reducing heat budget constraints. The switch power path topology of LTC 4156 seamlessly manages power allocation from two input power sources, such as AC adapters and USB ports, to rechargeable lithium iron phosphate batteries on the device, giving priority to system load when the input power is limited.
TO SYSTEM LOAD: to system load
As a result of power savings, LTC4156 allows the output load current to exceed the current absorbed by the input power, thereby maximizing the use of available power to charge the battery without exceeding the input power supply specifications. For example, when powered by a 5V/2A AC adapter with an available power of 10W, the IC's switching regulator can efficiently transmit over 85% of the available power, provide up to ~2.4A charging current, and charge faster. Unlike conventional switching-on battery chargers, LTC4156 has the ability to switch on immediately to ensure that even when the battery is deeply discharged, it can be powered by a plug-in system. Because USB OTG (On-the-Go) is supported, you can, in turn, provide a 5V power supply to the USB port without any additional components.
LTC 4156's self-contained full-featured single-cell lithium iron phosphate battery charger provides up to 3.5A charging current with 15 user-selectable charging current settings. The charger includes automatic recharge, bad battery detection, programmable safety timer, temperature-qualified charging controlled by thermistors, programmable charge termination indication/termination and programmable interruption. LTC 4156 is packaged in a flat (0.75mm) 28-pin 4mm x 5mm QFN package and operates at temperatures ranging from - 40 C to 125 C.
High efficiency internal switching regulator
The LTC4156 switching regulator works like a transformer, allowing the load current at the VOUT end to exceed the current absorbed by the input power supply, and making full use of the available power to charge the battery has been greatly improved compared to a typical linear-mode charger. The above example shows how LTC 4156 can be charged efficiently with currents up to 3.5A, thus achieving faster charging speeds. Unlike ordinary switching-on battery chargers, LTC4156 has the ability to switch on immediately to ensure that even when the battery is dead or has been deeply discharged, it can be plugged in to power the system.
Switching Regulator Efficiency: switching regulator efficiency
LOAD CURRENT: load current
Safer for batteries
It is important to monitor the safety of batteries when charging batteries quickly. LTC 4156 automatically stops charging when the battery temperature drops below 0 C or rises above 60 C, as measured by an external negative temperature coefficient NTC thermistor. In addition to this Autonomous feature, LTC 4156 also provides an extended scale 7-bit analog-to-digital converter (ADC) to monitor the cell temperature at a resolution of about 1 degree C (see Figure 3). This ADC, combined with four available floating voltage settings and 15 battery charging current settings, can be used to build custom charging algorithms based on battery temperature.
The NTC ADC results can be read through a simple two-wire I2C interface to adjust the charging current and voltage settings. The communication bus allows LTC4156 to indicate additional status information, such as input power status, charger status, and fault status. Because USB On-The-Go is supported, you can, in turn, provide a 5V power supply to the USB port without any additional components.
For many portable applications, such as tablets or industrial bar code scanners, managing two inputs (such as USB and AC adapters) is enough. Portable device designers, however, have been looking for ways to recharge batteries from any available power source. LTC 4156's dual-input, priority multiplexer independently selects the most appropriate input (AC adapter or USB) according to user-defined priority (default priority is adapter input). The Overvoltage Protection (OVP) circuit protects both inputs from accidental high or reverse voltage damage. The LTC 4156 ideal diode controller guarantees sufficient power to the VOUT even if the input power is insufficient or does not exist. To minimize battery leakage when the device connects to a USB port in pause mode, an LDO is placed between VBUS and VOUT to provide the application with a permissible USB pause current. To eliminate battery leaks during manufacturing and marketing, the "shipping and storage" feature further reduces the already low battery standby current to almost zero.
Finally, LTC4156 and LTC4155 lithium ion versions are fully pin and component compatible, allowing flexible and convenient last minute replacement of batteries of different chemical compositions without requiring a large area of circuit boards to be rearranged.
Designers'work is challenging for new portable industrial and medical devices, especially when it comes to power. More and more power is required by enterprises, and the result is larger batteries. At the same time, people need convenience, hoping to use any power to charge these batteries. LiFePO4 batteries are becoming the mainstream choice due to inherent safety, low floating voltage, longer life cycle, lower self-discharge rate and relatively light weight. But like any rechargeable battery, lithium iron phosphate batteries must be treated with care. Although these trends in portable device power supply are becoming design challenges, LTC 4156 makes things much easier. In low-voltage systems, LTC 4156 efficiently provides up to 3.5A of charging current, while providing high performance and safety features.