Why do large-capacity lithium batteries require high-power chargers?

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.

The low 3.6V float voltage of LiFePO4 batteries prevents the use of standard Li-Ion battery chargers. If not charged properly, it is possible to irreparably damage this battery. Accurate float voltage charging will extend the life of the battery. Advantages of LiFePO4 batteries include higher volumetric energy density and less susceptibility to premature failure compared to cobalt-based Li-Ion batteries.

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 large-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 large-capacity batteries. Addressing the Power Challenges of Portable Devices Using 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 lithium iron phosphate 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, provide up to ~2.4A of charging current, and charge 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 curve is safer for the battery

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

For many portable applications such as tablets or industrial barcode scanners, managing two inputs (such as USB and AC adapter) is sufficient. However, portable device designers are constantly looking for ways to charge the battery from any available power source. The LTC4156's dual-input, priority multiplexer autonomously selects the most appropriate input (AC adapter or USB) based on user-defined priorities (the default priority is the adapter input). Overvoltage protection (OVP) circuitry protects both inputs from damage caused by accidentally applied high voltage or reverse voltage. The LTC4156's ideal diode controller ensures that sufficient power is always available to VOUT, even if input power is insufficient or non-existent. To minimize battery leakage when the device is connected to a USB port in suspend mode, an LDO is placed between VBUS and VOUT to provide the application with an allowable USB suspend current. To eliminate battery leakage during manufacturing and distribution, the "ship and store" feature further reduces the already low battery backup current to almost zero.

Finally, the LTC4156 is fully pin- and component-compatible with the LTC4155 Li-Ion version, allowing the flexibility to easily and last-minutely swap out battery chemistries without extensive board rescheduling.

Conclusion

Designers have a challenging job for new portable industrial and medical devices, especially when it comes to power. Businesses are demanding features that require more power, resulting in larger batteries. At the same time, people want the convenience of being able to charge these batteries from any power source . Lithium iron phosphate batteries are becoming a mainstream choice due to their inherent safety, low float voltage, longer cycle life, low self-discharge rate and relatively light weight. But like any rechargeable battery, lithium iron phosphate batteries must be treated with care. While these trends in portable device power supplies have created design challenges, the LTC4156 makes things much easier. In low voltage systems, the LTC4156 efficiently delivers up to 3.5A of charge current while providing high performance and safety features.