The power architecture of many industrial or medical devices is often similar to that of a large-display smartphone. Typically, 3.7V (final charge or "float" voltage of 4.2V) Li-Ion has been used as the main power source because of its high energy density per unit weight (Wh/kg) and energy density per unit volume (Wh/m3) . In the past, many high-power devices used two 7.4V (8.4V float voltage) Li-Ion batteries to meet power requirements, but due to the availability of inexpensive 5V power management, more and more handheld devices use lower voltage architecture, which makes it possible to use a single-cell Li-Ion battery. A typical portable medical or industrial device has many functions and a very large (for portable devices) display screen. 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 in 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. Improper charging can cause irreparable damage to this battery. Accurate float voltage charging will extend battery life. The advantages of LiFePO4 batteries compared to cobalt-based lithium-ion batteries include higher volumetric energy density and less prone to premature failure.
The main design constraints are summarized as follows:
High-capacity batteries require high charging current and high efficiency
Many portable applications, including industrial and medical equipment, require the convenience of USB-compatible charging
LiFePO4 batteries have special charging requirements, namely lower float voltage, and have some comforting advantages over Li-Ion batteries
Any IC solution discussed above that meets these design constraints must be compact and monolithic, capable of fast, efficient charging of single-cell high-capacity batteries, and compatible with new chemistries such as lithium iron phosphate. Such a device could be a catalyst for increasing the global adoption of portable industrial and medical products with high-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, take a look at the LTC4156. The LTC4156 follows the popular lithium-based LTC4155 and is a high-power, I2C-controlled, high-efficiency PowerPath™ manager, ideal controller and lithium iron phosphate (LiFePO4) battery charger for Portable applications using single-cell batteries, such as portable medical and industrial equipment, backup equipment, and high power density battery-powered applications. The IC is designed to efficiently deliver up to 15W of power from a variety of power supplies, while minimizing power consumption and alleviating thermal budget constraints. The LTC4156's pass-through topology seamlessly manages power distribution from two input sources, such as the AC adapter and USB port, to the device's rechargeable lithium iron phosphate battery, while prioritizing power to the system load when input power is limited. See Figure 1.
Εικόνα 1: Τυπικό κύκλωμα εφαρμογής για το LTC4156
Due to the power savings, the LTC4156 allows the output load current to exceed the current drawn by the input supply, thereby maximizing the use of available power to charge the battery without exceeding the input supply supply specification. For example, when powered by a 5V/2A AC adapter with 10W of available power, the IC's switching regulator can efficiently deliver over 85 percent of the available power, provide up to 2.4A of charging current, and charge faster . Unlike common switching battery chargers, the LTC4156 features instant-on operation to ensure power is available as soon as the system is plugged in, even when the battery is deeply discharged. Since it supports USBOTG (On-the-Go), it can in turn provide a 5V power supply to the USB port without any additional components.
The LTC4156's autonomous full-featured single-cell LiFePO4 battery charger can deliver 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, controlled temperature qualified charging, programmable end-of-charge indication/termination, and programmable interrupts. The LTC4156 is packaged in a low profile (0.75mm) 28-pin 4mmx5mmQFN package and is guaranteed to operate over the -40C to 125C temperature range.
High Efficiency Internal Switching Regulator
The LTC4156's switching regulator operates like a transformer, allowing the load current at VOUT to exceed the current drawn by the input supply, and the ability to fully charge the battery with the available power is greatly improved compared to typical linear mode chargers. The preceding examples illustrate how the LTC4156 can efficiently charge up to 3.5A, enabling faster charging speeds. Unlike ordinary switching battery chargers, the LTC4156 features instant-on operation to ensure that power is available to the system as soon as it is plugged in, even when the battery is dead or deeply discharged.
Εικόνα 2: Αποδοτικότητα LTC4156VOUT έναντι ρεύματος φορτίου
Πιο ασφαλές για μπαταρίες
It is important to monitor the safety of the battery when rapidly charging the battery. When the battery temperature drops below 0C or rises above 60C (as measured by an external negative temperature coefficient NTC thermistor), the LTC4156 automatically stops charging. In addition to this autonomous feature, the LTC4156 provides an extended-scale 7-bit analog-to-digital converter (ADC) to monitor battery temperature with 1C resolution (see Figure 3). This ADC, combined with the 4 available float voltage settings and 15 battery charge current settings, can be used to build custom charge algorithms based on battery temperature.
Εικόνα 3: 7-bit θερμίστορ ADC που δείχνει προκαθορισμένα σημεία διακοπής θερμοκρασίας LTC4156
For many portable applications such as tablet PCs or industrial barcode scanners, managing two inputs (eg USB and AC adapter) is sufficient. However, portable device designers are constantly looking for ways to charge batteries from any available power source. The LTC4156's dual-input, priority multiplexer autonomously selects the most appropriate input (AC adapter or USB) based on a user-defined priority (default priority is adapter input). An overvoltage protection (OVP) circuit protects both inputs simultaneously from damage caused by accidentally applied high or reverse voltages. The ideal diode controller of the LTC4156 guarantees that sufficient power is always available to VOUT even if the 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 allowable USB suspend current. To eliminate battery leakage during manufacturing and sales, 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 for the flexibility and convenience of last-minute battery chemistries without the need for large-area rearrangements.
in conclusion
For new portable industrial and medical equipment, the designer's job is extremely challenging, especially when power is involved. Enterprise-requested functions increasingly require more power and, as a result, larger batteries. At the same time, people need convenience and want to charge these batteries with any power source. Lithium iron phosphate batteries are becoming the mainstream choice due to their inherent safety, low float voltage, longer cycle life, lower 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 power supplies have created design challenges, the LTC4156 makes things a lot easier. In low voltage systems, the LTC4156 efficiently delivers up to 3.5A of charge current while providing high performance and safety features.
