Technical secrets: Large-capacity lithium iron phosphate batteries require high-power chargers

　　One of the major trends in patient care is the increasing use of remote monitoring systems in the patient's home. The reason for this trend is obvious, as the cost of keeping patients in the hospital is too high to bear. Therefore, many of these portable electronic monitoring systems incorporate RF transceivers so that data can be sent directly to the monitoring system in the hospital for doctors to study and analyze. Obviously, these systems are usually powered by AC power, batteries, or both. This redundancy is necessary to ensure that the system can continue to operate when used in locations other than the hospital. In addition, many new developments in portable medical diagnostic equipment, such as equipment that doctors and nurses carry around, use batteries as the main power source or as a backup power source in case the AC power is interrupted. These systems require efficient battery charging circuits.

　　In addition to medical applications, portable industrial banking terminals, rugged tablets, inventory control and barcode scanning devices require single-cell high-capacity batteries to reduce size and weight. Lithium-based batteries have been the most popular choice. However, charging these batteries quickly, accurately and safely is not a trivial matter. In addition, new lithium-based chemical anode/cathode combinations have been developed and are being introduced to the mainstream market. An example of this trend is that lithium iron phosphate (LiFePO4) batteries have emerged in many applications, providing higher safety and longer battery life compared to cobalt-based lithium-ion/lithium polymer batteries. And this battery chemistry also combines many other advantages of cobalt-based lithium-ion batteries, including lower self-discharge rate and relatively light weight. In contrast, in addition to improved safety (due to resistance to "thermal runaway") and extended battery cycle life, lithium iron phosphate batteries have higher peak power ratings and lower environmental impact. Medical and industrial applications are generally willing to accept the lower energy density per unit volume of lithium iron phosphate batteries in exchange for higher safety and longer cycle life. Backup applications require longer cycle life and the ability to discharge at high currents.

　　How to get more power

　　The power architecture of many handheld industrial or medical devices is often similar to that of large-display smartphones. Typically, 3.7V (final charge or "float" voltage is 4.2V) lithium-ion batteries have been used as the primary power source because of their high energy density per unit weight (Wh/kg) and per unit volume (Wh/m3). In the past, many high-power devices used two 7.4V (8.4V float voltage) lithium-ion batteries to meet the power requirements, but the availability of inexpensive 5V power management ICs has led to a growing number of handheld devices adopting lower voltage architectures that allow the use of a single lithium-ion battery. A typical portable medical or industrial device has many features and a very large (for a portable device) display. When powered by a 3.7V battery, its capacity must be measured in thousands of milliwatt hours. In order to charge such a large battery over a few hours, a charging current of several amps is required.

　　However, even with these high charging currents, users still want to use the USB port to charge their high-power devices when a high-current AC adapter is not 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 efficiently utilize the 2.5W to 4.5W of power available from the USB port. In addition, the IC needs to protect sensitive downstream low-voltage components from overvoltage events that could cause damage, and efficiently direct high current from the USB input, AC adapter, or battery to the load to minimize power lost as heat. At the same time, the IC must safely manage the battery charging algorithm and monitor key system parameters.

　　The low 3.6V float voltage of LiFePO4 batteries precludes the use of standard Li-Ion battery chargers. If improperly charged, it is possible to irreparably damage this battery. Accurate float voltage charging will extend the life of the battery. Advantages of LiFePO4 batteries over cobalt-based Li-Ion batteries include higher volumetric energy density (capacity per unit volume) and less susceptibility to premature failure if new batteries are “deep cycled” too early.

　　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, namely lower float voltage, and some comforting advantages over lithium-ion batteries

　　Any IC solution that meets these design constraints discussed above must be compact and monolithic, address the problem of fast, efficient charging of single-cell high-capacity batteries, and be compatible with new chemistries such as lithium iron phosphate. Such a device would be a catalyst for increasing the global adoption of portable industrial and medical products using high-capacity batteries.

　　Meeting the Power Challenges of Portable Devices Running on Single-Cell Batteries

　　While the above requirements may seem impossible to meet with a monolithic IC, consider the LTC4156. The successor to the popular lithium-based LTC4155, the LTC4156 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 and industrial equipment, backup devices and high power density battery-powered applications. The IC is designed to efficiently deliver up to 15W of power from a variety of sources while minimizing power dissipation and alleviating thermal budget constraints. The LTC4156'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 LiFePO4 battery, while giving priority to system loads when input power is limited. See Figure 1.

Figure 1: Typical application circuit for LTC4156

　　Because of power conservation, the LTC4156 allows the output load current to exceed the current drawn by the input power supply, 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 10W of available power, the IC's switching regulator can efficiently deliver more than 85% of the available power, providing up to ~2.4A of charging current and charging faster. Unlike ordinary switching battery chargers, the LTC4156 has instant-on operation to ensure that the system can be powered as soon as the plug is plugged in, even when the battery is deeply discharged. Because it supports USB OTG (On-the-Go), it can provide a 5V power supply to the USB port in turn without any additional components.

　　The LTC4156's autonomous, fully-featured single-cell lithium iron phosphate charger provides up to 3.5A of charge current with 15 user-selectable charge current settings. The charger includes automatic recharging, bad battery detection, programmable safety timer, thermistor-controlled temperature-qualified charging, programmable end-of-charge indication/termination, and programmable interrupts. The LTC4156 is available in a low-profile (0.75mm) 28-pin 4mm x 5mm QFN package and is guaranteed to operate over the -40°C to 125°C temperature range.

　　High Efficiency Internal Switching Regulator

　　The LTC4156's switching regulator works like a transformer, allowing the load current at VOUT to exceed the current drawn by the input supply, and the ability to fully utilize the available power to charge the battery is greatly improved compared to typical linear mode chargers. The previous example shows how the LTC4156 can charge efficiently at currents up to 3.5A, resulting in faster charging speeds. Unlike ordinary switching battery chargers, the LTC4156 has instant-on operation to ensure that power is supplied to the system as soon as the power source is plugged in, even when the battery is dead or deeply discharged.

Figure 2: LTC4156 VOUT efficiency vs. load current

　　Safer for batteries

　　When fast charging a battery, it is important to monitor the safety of the battery. The LTC4156 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 function, the LTC4156 also provides an extended-scale 7-bit analog-to-digital converter (ADC) to monitor the battery temperature with a resolution of approximately 1°C (see Figure 3). This ADC, combined with the 4 available float voltage settings and 15 battery charge current settings, can be used to create a custom charging algorithm based on battery temperature.

Figure 3: 7-bit thermistor ADC showing preset LTC4156 temperature trip points

　　The results of the NTC ADC can be read through a simple two-wire I2C interface, allowing the charging current and voltage settings to be adjusted. The communication bus allows the LTC4156 to indicate additional status information, such as input power status, charger status, and fault status. Because it supports USB On-The-Go, it can in turn provide a 5V power supply to the USB port without any additional components.

　　High Efficiency Internal Switching Regulator

　　The LTC4156's switching regulator works like a transformer, allowing the load current at VOUT to exceed the current drawn by the input supply, and the ability to fully utilize the available power to charge the battery is greatly improved compared to typical linear mode chargers. The previous example shows how the LTC4156 can charge efficiently at currents up to 3.5A, resulting in faster charging speeds. Unlike ordinary switching battery chargers, the LTC4156 has instant-on operation to ensure that power is supplied to the system as soon as the power source is plugged in, even when the battery is dead or deeply discharged.

Figure 2: LTC4156 VOUT efficiency vs. load current

　　Safer for batteries

　　When fast charging a battery, it is important to monitor the safety of the battery. The LTC4156 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 function, the LTC4156 also provides an extended-scale 7-bit analog-to-digital converter (ADC) to monitor the battery temperature with a resolution of approximately 1°C (see Figure 3). This ADC, combined with the 4 available float voltage settings and 15 battery charge current settings, can be used to create a custom charging algorithm based on battery temperature.

Figure 3: 7-bit thermistor ADC showing preset LTC4156 temperature trip points

　　The results of the NTC ADC can be read through a simple two-wire I2C interface, allowing the charging current and voltage settings to be adjusted. The communication bus allows the LTC4156 to indicate additional status information, such as input power status, charger status, and fault status. Because it supports USB On-The-Go, it can in turn provide a 5V power supply to the USB port without any additional components.