Preface USB ports are the preferred method for fast data transfer and are rapidly becoming the preferred method for charging portable device batteries, eliminating the need for a separate AC adapter. However, there are power limitations when charging device batteries using a USB port. Furthermore, due to portability requirements, there is an increasing need for charging outside the home (e.g., in a car). However, automotive power supplies also have drawbacks, such as voltage transients or surges from the alternator. Therefore, battery charger integrated circuits need to be well protected to withstand these harsh conditions. PowerPath charging system topologies in analog integrated circuits offer numerous advantages to system designers and end-users, such as the ability to autonomously and seamlessly manage multiple input power sources, powering system loads and charging batteries. This integrated circuit topology not only reduces heat but also enables faster charging times and instant-on operation. A new trend in these integrated circuits is the integration of high-voltage capabilities and overvoltage protection to handle automotive, Firewire, or unregulated AC adapter inputs. These power path manager ICs, packaged in flat packages and requiring minimal external components, provide simple, compact, and cost-effective solutions for handheld electronics such as personal navigators, media players, digital cameras, PDAs, and smartphones. Design Challenges The ability to handle high-voltage input power sources, such as automotive power supplies, Firewire ports, or unregulated 12V/24V adapters, facilitates charging outside the home or office. For example, with adapter power, the voltage difference between the adapter and battery in a handheld device can be significant. Linear chargers may not be able to handle such high power consumption, depending on the required charging time and current. This typically necessitates an IC with a switching-mode topology to maintain fast charging while improving efficiency and reducing thermal management issues. Furthermore, ICs with high-voltage capability and/or overvoltage protection are less susceptible to input voltage transients, improving the transient resilience and reliability of the IC and system. Managing the power path in the final product is another design challenge. Today, many portable battery-powered electronics can be powered by both low-voltage sources (AC adapters, USB ports, or lithium-ion/polymer batteries, etc.) and high-voltage sources. However, autonomously managing the power paths between these power sources and batteries and powering the load presents significant technical challenges. Traditionally, designers have implemented this functionality using a small number of MOSFETs, operational amplifiers, and other components, but have faced challenges such as hot-plugging of loads, large inrush currents on loads, and large voltage transients, which can cause serious system reliability issues. Lithium-ion and lithium-polymer batteries are the preferred choice for portable consumer electronics due to their relatively high energy density, allowing for higher battery capacities than other available chemistry materials within given size and weight constraints. As portable products become increasingly complex and power-consuming, the demand for higher-capacity batteries has increased, necessitating more advanced battery chargers. Larger batteries require higher charging currents or longer charging times to fully charge. Furthermore, while USB port charging is often more convenient for users, USB compatibility imposes limitations on USB current (maximum 500mA) and power (maximum 2.5W). USB-based battery chargers must extract as much power as possible from the USB port to meet the stringent thermal constraints of today's power-intensive applications. Most consumers want shorter charging times, so increasing the charging current seems like the obvious choice. However, increasing the charging current has two major drawbacks. First, for linear chargers, increasing the current increases power consumption, which is converted into heat, thus reducing the typical actual "maximum" power to 2.1W. Second, the charger must limit the current drawn from the 5V USB bus to either 100mA (500mW) or 500mA (2.5W), depending on the mode negotiated by the host device. Any power wasted during charging directly results in longer charging times. The need for efficient charging, the high functional integration of battery charger ICs, and the need to save board space and improve product reliability all put pressure on designers of battery-powered electronics. Manufacturers are also changing how they use printed circuit boards; instead of using a single multilayer board, they are increasingly using multiple boards stacked on top of each other in space-constrained designs. Advanced packaging helps reduce height/thickness and saves printed circuit board area, enabling more efficient stacking. In summary, the main challenges faced by system designers include: ● Maximizing the current drawn from the USB port (up to 2.5W); ● Managing power paths between multiple input voltage sources and the battery while simultaneously powering the load; ● Protecting the integrated circuit from high-voltage system transients; ● Minimizing heat while fast charging; ● Maximizing charging efficiency and extending battery life; ● Minimizing the solution's footprint and height. Integrated and compact power path manager ICs with high-voltage input capability and overvoltage protection easily solve these problems. A Simple Solution An integrated circuit with power path control can autonomously and seamlessly manage power paths between different input sources such as USB, AC adapter, and battery, while prioritizing power to the load. To ensure a fully charged battery remains fully charged when connected to the USB bus, these ICs power the load through the USB bus instead of drawing power from the battery. Once the power is removed, current flows from the battery to the load through an internal low-loss ideal diode, maximizing efficiency and minimizing power consumption. The forward voltage drop of the ideal diode is much lower than that of a conventional or Schottky diode, thus maximizing energy transfer efficiency and minimizing reverse current leakage. A small forward voltage drop of 20mV, typically low, reduces power loss and self-heating, thus extending battery life. Furthermore, the three-terminal (or "intermediate bus") topology eliminates the coupling between the battery and VOUT, allowing the final product to operate immediately upon plugging in, regardless of the battery's state of charge or even if the battery is missing—a phenomenon often referred to as "instant-on" operation. The battery charger is integrated with a power path controller and ideal diode devices ("power path manager") to efficiently manage various input power sources, charge the battery, prioritize power supply to the load, and reduce power consumption. The power path control circuitry can employ linear or switching topologies, as each offers advantages depending on the specific charging requirements. Advantages of Switching Power Path Systems Compared to battery-feed systems, linear power path systems offer the advantage of high efficiency in providing power to the load/system; however, power losses exist within the linear battery charger unit, especially if the battery voltage is low (resulting in a large voltage difference between the input and battery voltages). The power path circuitry, based on a switch-mode topology, generates an intermediate bus voltage via a USB-compliant buck switching regulator, which stabilizes at a voltage 300mV higher than the battery voltage (see Figure 1). This form of adaptive output control is referred to by Linear Technology as "Bat-Track." The stabilized intermediate voltage is just high enough to allow proper charging via the internal linear charger. Tracking the battery voltage in this way minimizes power losses in the linear battery charger, improves efficiency, and maximizes the power supplied to the load. The switching architecture with average input current limiting maximizes the ability to utilize all 2.5W of power provided by the USB power supply. An optional external PFET reduces the impedance of the ideal diode between the battery and the load, further reducing heat loss. This architecture is "essential" for systems using large batteries (>1.5AHr). [img=500,206]http://cms.cn50hz.com/files/RemoteFiles/20081225/477683001.jpg[/img] Figure 1 Simplified switching power supply path circuit (4088 F01) LTC4098—Combining High-Efficiency Charging and High-Voltage Protection The LTC4098 (Figure 2) is a self-contained, high-efficiency power path manager, ideal diode controller, and battery charger for USB-powered portable devices such as media players, digital cameras, PDAs, personal navigators, and smartphones. It is housed in an ultra-thin (0.55mm) 20-pin 3mm × 4mm QFN package. For automotive, Firewire, or other high-voltage applications, the LTC4098 provides battery tracking control using Linear Technology's switching regulators, operating up to 38V (60V transient), maximizing battery charger efficiency, minimizing heat loss, and allowing seamless operation even with higher voltage supply. [img=500,289]http://cms.cn50hz.com/files/RemoteFiles/20081225/477683002.jpg[/img] Figure 2 Simplified Block Diagram of LTC4098 The LTC4098 provides overvoltage protection (OVP) up to 66V, requiring only one external NFET/resistor combination to prevent input damage from accidental high-voltage conditions. This integrated circuit automatically reduces charging current for fast turn-on, ensuring power is supplied to the system load as soon as it is plugged in, even when the battery is dead or missing. Its on-chip ideal diode guarantees sufficient power to VOUT, even if the two input pins of the LTC4098 are underpowered. The ideal diode controller of this integrated circuit can be used to drive the gate of an optional PFET, reducing the impedance to the battery to 30mΩ or lower. The LTC4098's full-featured single-cell lithium-ion/polymer battery charger allows load current to exceed the current drawn from the USB port while complying with USB load specifications. Because it conserves energy, the IC's high-efficiency switching input stage converts almost all of the 2.5W power supplied by the USB port into usable system current for fast charging, achieving up to 700mA from the 500mA limit imposed by the USB port. An additional 1.5A of usable charging current is available when powered by an AC adapter. Overvoltage Protection (OVP): The LTC4098 protects itself from damage in the event of an accidental overvoltage applied to VBUS or WALL using only two external components: an N-channel FET and a 6.04kΩ resistor. The maximum safe overvoltage is determined by the external NMOS transistor and its drain breakdown voltage. Input Current Limiting and High-Voltage Control of the Battery Tracking Switching Regulator: Power transfer from VBUS to VOUT is controlled by a 2.25MHz constant-frequency buck switching regulator. To meet the maximum load specifications of USB, this switching regulator incorporates a measurement and control system to ensure that the average input current remains below the programmed value on the CLPROG pin. This allows VOUT to drive a combination of external load and battery charger. If this combined load does not cause the switching power supply to reach its programmed input current limit, VOUT will track approximately 0.3V higher than the battery voltage. By keeping the battery charger voltage at this low value, power loss in the battery charger is minimized. If the combined external load and battery charging current are large enough to cause the switching power supply to reach its programmed input current limit, the battery charger will reduce its charging current precisely to meet the requirements of the external load. Even if the battery charging current is programmed to exceed the permissible USB current, the average input current will not fail to meet USB performance specifications. Furthermore, if the load current at VOUT causes the power to exceed the programmed power from VBUS, additional load current will be drawn from the battery through an ideal diode, even while the battery charger is operating. The WALL, /ACPR, and VC pins can be used with external high-voltage buck switching regulators such as the LT3480 to minimize heat generated when operating with higher voltage sources. The battery tracking control circuitry regulates the output voltage of the external switching regulator to a higher level (BAT + 300mV) or 3.6V. This maximizes the efficiency of the battery charger while still allowing for instantaneous turn-on even when the battery is deeply discharged. The LTC4098's advanced ultra-thin (0.55mm typical) QFN package is advantageous in space-constrained applications where printed circuit boards are stacked. This package allows for a compact "volume" solution, providing flexibility for system designers. Additionally, the device exhibits the same thermal performance as the taller (0.75mm) previous generation QFN package. Conclusion The need for small size and easy access to multiple input power sources, along with the demands for fast charging, low power consumption, and USB compatibility, presents challenges for designers of battery-powered products. Powering via automotive adapters or Firewire ports is becoming increasingly common, but the downside is the potential for high-voltage transients that can damage integrated circuits. Meanwhile, designs are becoming increasingly integrated to save board space, reduce manufacturing costs, and improve product reliability. Linear Technology's expanding family of switch-mode topology power path manager ICs makes the job much easier for product designers. These ICs can draw more power from USB ports, seamlessly manage power paths between different input power sources and batteries and prioritize power to loads, reduce heat, improve efficiency through battery tracking adaptive output control, and simplify designs by using fewer and smaller external components.