Next-generation PMIC with switching management

Next-generation PMIC with switching management

New technologies solves power limitations when USB is used for charging batteries in devices

BY STEVE KNOTH
Linear Technology
Milpitas, CA
http:\www.linear.com

USB technology has increased portability of electronic devices, while larger battery capacities are needed to accommodate increased feature content. Take the personal media player, for example. The explosion of downloadable media content has resulted in the desire for PC data transfer to the portable handheld device; USB makes this transfer occur faster. It is also convenient to charge the device from this same USB port. However, there are some power limitations when the USB is used for charging the device’s batteries.

For example PowerPath-topology ICs solve these problems, offering advantages to the end user such as the ability to autonomously and seamlessly manage various input power sources, the battery, and provide power to the load, charge quickly with minimal heat, and provide instant-on operation. Linear Technology, for example, offers the LTC3555, a PMIC with onboard switching-based PowerPath manager, lithium battery charger, triple buck regulator, and LDO. It contains many high-performance features to benefit the end product, and its miniature low-profile QFN packaging and few external components comprise a simple, compact cost-effective solution for handheld electronic devices.

Key design challenges

USB-capable battery charging in many cases means more convenience to the user. However, USB-compatibility poses the constraint of USB current limits. A USB-based battery charger must extract as much power from the USB as efficiently as possible, to meet the stringent space and thermal constraints of today’s power-intensive applications.

Managing power flow within the product is another issue. Many of today’s portable battery-powered electronics can be powered from a wall adapter, automotive adapter, a USB port, or a Li-ion/polymer battery for example. However, autonomously managing the power flow between these various power sources, the load and the battery presents a significant technical challenge. Traditionally, designers have tried to perform this function discretely by using a handful of MOSFETs, op amps, and other discrete components, but have faced tremendous problems with hot plugging and large inrush currents, which may cause big system reliability problems. More recently, even discrete IC solutions require several chips to implement a practical solution.

Li-ion and Li-polymer batteries are becoming commonplace in portable consumer products because of their relatively high energy densitythey provide more capacity than other available chemistries within given size and weight constraints. As portable handheld products become more complex, they consume more power, so the need for higher-capacity batteries increases, with a corresponding need for more advanced battery chargers. Larger batteries require either higher charging current or additional time to charge to their full capacity.

Most consumers look for quick charge times, so increasing the charge current seems obvious, but this increase comes with two major penalties. First, with a linear charger, increased current creates additional power dissipation as heat, reducing the typical practical power “maximum” to 2.1 W. Second, the charger must limit the current drawn from the 5-V USB bus to either 100 mA (500 mW) or 500 mA (2.5 W), depending on the mode that the host controller has negotiated. This need for high-efficiency charging, combined with the high level of feature integration required of the battery charger IC, need to save board space and increase product reliability, exerts pressure on the designers of battery-powered electronic devices.

Summarizing, the main challenges for the system designer include:

• Maximizing current delivered from the USB port (2.5 W available)

• Managing power flow between multiple input voltage sources, the battery and load

• Minimizing heat

• Maximizing charging efficiency

• Minimizing solution footprint and profile

A simple solution

PowerPath control offers the ability to autonomously and seamlessly manage power flow between various input sources such as USB ports, wall adapters and other types of ac adapters, the battery, and also to provide sufficient power to the load. PowerPath systems also offer “instant–on” operation because the intermediate voltage is available for system loads as soon as power is applied to the circuit–this allows the end product to operate immediately when plugged in, regardless of the battery’s state of charge. A device with PowerPath control provides power to the device load and charges its single-cell Li-ion/polymer battery from the power source.

To ensure that a fully charged battery remains fresh when the USB bus is connected, the IC directs power to the load through the USB bus rather than extracting power from the battery. Once the power source is removed, current flows from the battery to the load through an internal low loss ideal diode, minimizing voltage drop and power dissipation. Refer to Fig. 1 for details, which illustrate a simplified switching PowerPath block diagram. The forward voltage drop of an ideal diode is far less than that of a conventional or Schottky diode and so maximizes the efficiency of energy transfer, and the reverse current leakage is also smaller. The tiny forward voltage drop reduces power losses and self-heating, resulting in extended battery life.

Fig. 1. In the above simplified switching PowerPath diagram the forward-voltage drop of an ideal diode is far less than that of a conventional or Schottky diode and so maximizes the efficiency of energy transfer, and the reverse-current leakage is also smaller.

A switching PowerPath system

First-generation USB charging system applications implemented a current-limited battery charger directly between the USB port and the battery, where the battery directly powered the system. Second-generation linear-based USB charging systems develop an intermediate voltage between the USB port and the battery (a PowerPath system). New third-generation USB charging systems feature a switching-based topology.

This type of PowerPath device produces an intermediate bus voltage from a USB-compliant step-down (buck) switching regulator that is regulated to a fixed voltage above the battery voltage (see Fig. 1) . Linear Technology calls this form of adaptive output control “Bat-Track.” The regulated intermediate voltage is just high enough to allow proper charging through the linear charger. However, by tracking the battery voltage in this manner, power lost in the linear battery charger is minimized, efficiency increases and power available to the load is maximized. Additionally, the switching average input current limit maximizes the ability to use all 2.5 W available from a USB supply. An optional external PFET reduces ideal diode impedance for less heat dissipation. This architecture is a “must” for systems with large (>1.5-AHr) batteries.

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