New USB Power Solution

To simplify the task of drawing power from a battery or USB cable, Linear Technology offers several devices. These devices seamlessly manage the power flow between the AC adapter, USB cable and Li-Ion battery and maintain compliance with USB power specifications. As battery capacity increases, the battery charger must keep pace by steadily improving efficiency to minimize thermal issues and charging time. USB-based battery chargers must extract as much power as possible from the USB, and this power drawing process must be done efficiently to meet the stringent space and thermal constraints of today's power-intensive applications.

The LTC3555 combines a USB switching power manager and battery charger with three synchronous buck regulators and an LDO to provide a complete power solution in a small form factor (4mm x 5mm) package (Figure 1). This constant-current, constant-voltage Li-Ion/Polymer battery charger uses Bat-Track to maximize the efficiency of the battery charger by generating an input voltage that automatically tracks the battery voltage (see below). An I2C serial interface gives the system designer full control over the charger and DC/DC step-down regulator, designed to maximize adaptability to changing operating modes in a wide range of applications.

Figure 1: An all-in-one USB power solution integrates a switching power manager, battery charger, three synchronous buck regulators, and an LDO

Switching Power Path Controllers Maximize

Available Power to System LoadThe LTC®3555 improves upon earlier generations of USB battery chargers with new features. It uses a proprietary switching power manager to draw power from a current-limited USB port with the highest possible efficiency while maintaining compliance with average input current specifications. It minimizes power losses in the linear charger with its Bat-Track feature.

The first generation of USB applications implemented a current limited battery charger directly between the USB port and the battery, where the battery voltage powered the system. This is called a battery-fed system. In a battery-fed system, the available system power is IUSB • VBAT, since VBAT is the only voltage available to the system load. When the battery voltage is low, almost half of the available power will be dissipated as heat inside the linear battery charger components.

Second generation USB chargers generate an intermediate voltage between the USB port and the battery. This intermediate bus voltage topology is known as the PowerPath™ system. In the PowerPath IC, a current limited switch is placed between the USB port and the intermediate voltage. The intermediate voltage VOUT is responsible for powering the linear battery charger and the system load. By using an intermediate bus voltage topology, the battery is isolated from the system load while charging can be performed efficiently. Since the intermediate voltage is available to the system load as soon as power is applied to the circuit (regardless of the state of the battery), the PowerPath system has the added benefit of being “instant on”. In the PowerPath system, more of the 2.5W available from the USB port can be delivered to the system load as long as the input current limit is not exceeded. Although the PowerPath system is an improvement over the battery-fed system, if the battery voltage is low, a significant amount of power can still be lost in the linear battery charger components.

The LTC3555 is the first third generation USB PowerPath charger IC to hit the market. These PowerPath devices
generate an intermediate bus voltage from a USB compliant step-down regulator that is regulated to a fixed value (Bat-Track feature) over the battery voltage range. The regulated intermediate voltage is just high enough to allow proper charging operation by the linear charger. By tracking the battery voltage in this way, power losses in the linear battery charger are minimized, efficiency is improved, and available power to the load enables efficiency and power savings for the system charger. When charging large batteries, the amount of power savings can make the difference between running a device in thermal limits or operating at low temperatures.

Figure 2 compares switching and linear PowerPath

Figure 2: Efficiency and power savings of a switching PowerPath battery charger compared to a linear charger. (VBUS = 5V, 5x mode, RCLPROG = 2.94k, RPROG = 1k, IBAT = 0.7A at VBAT = 2.8V)

Complete power solution in a single IC

The LTC3555 also includes three user-configurable step-down DC/DC converters capable of delivering 0.4A, 0.4A and 1A currents, respectively. Regulator 1 has a fixed reference voltage of 0.8V, while regulators 2 and 3 can have their reference voltages varied from 0.8V to 0.425V via the I2C interface. All converters operate at a 2.25MHz switching frequency, allowing the use of small passive components and maintaining efficiencies up to 92% at output voltages above 1.8V (see Figure 3). All three regulators can be set to operate in pulse-skipping mode, Burst Mode® operation or LDO mode via the I2C port or I/O pins. In Burst Mode operation, the output ripple amplitude is slightly increased and the switching frequency varies with load current to improve efficiency at light loads. If noise is a concern, all regulators can be set to operate in LDO mode or pulse-skipping mode. The device also provides an always-on 3.3V output capable of sourcing 25mA to meet system requirements such as real-time clocks or push-button monitors.

Figure 3: Efficiency of Switching Regulator 1 and Regulator 2 using Burst Mode Operation

in conclusion

The LTC3555 is an advanced and complete single-chip power solution. The third-generation PowerPath power management technology, coupled with reduced heat and battery charging time, makes it ideal for future high-density, multi-function battery-powered products. By integrating three I2C-controlled, high-efficiency step-down DC/DC converters, the LTC3555 provides system designers with full flexibility to adapt to changing needs and operating modes.