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Glitch-free portable charging

Posted: 14 Jan 2008 ?? ?Print Version ?Bookmark and Share

Keywords:USB-based battery charger? PowerPath controllers? portable charging?

By Steve Knoth
Linear Technology

Users are fast taking to the USB port as the preferred way for portable device battery charging. But while the port eliminates a separate wall adapter, it has its power limitations: generally 2.5W maximum at no more than 500mA. More applications also call for charging portable devices away from the office or home, such as from the car. Here again, there's also a drawback to charging the device from the auto's cigarette lighter because of occasional line-voltage transients and surges that come from the alternator. These conditions require a well-protected battery charger IC.

The controller/manager topologies in some of the newer analog ICs offer a viable solution to many of these problems. Integrating both high-voltage capability and over-voltage protection features to handle automotive, Firewire, and unregulated wall adapter inputs, they autonomously and seamlessly manage multiple input power sources to the system load and charge the battery while providing higher efficiency, faster charging times, and instant-ON operation. These systems come in lower-profile packages and require few external components for handheld device applications such as personal navigators, media players, digital cameras, PDAs and smart phones. Here's what this class of chips, in this case Linear Technology's PowerPath manager ICs, do for you.

Design challenges
High-input voltage sources from automobile-based systems, Firewire ports, and unregulated 12V and 24V adapters provide convenient charging in locations outside the home or office. When the power source is an adapter, the voltage difference between the adapter's output and the battery voltage in the handheld device can be very large. Thus, depending on the required charge-time and current, a linear charger may not be suitable to handle the power dissipation. In these applications, the designer seeks an IC with a switchmode topology to maintain fast charging while at the same time improving efficiency and reducing thermal management issues. And if the IC has high-voltage capability and/or overvoltage protection to protect circuitry from voltage input transients, so much the better.

Managing power flow within the end product is another challenge. Many of today's portable battery-powered electronics can be powered from low-voltage sources such as the USB port or directly from a battery. However, autonomous management of the power flow between these various power sources and the battery and load presents a significant technical obstacle. Designers can attack the problem with a discrete solution, i.e., a handful of MOSFETs, op-amps, and other components. If you do it this way, however, you'll likely face problems with hot plugging, large inrush currents and large voltage transients to the load.

What's needed
A USB-based battery charger must extract as much power from the USB port as efficiently as possible to meet the stringent thermal constraints of today's power-intensive applications. But quick charge times and high charge current come with two major penalties. First, with a linear charger, increased current creates additional power dissipation as heat, reducing the typical practical power "maximum" to 2.1W. 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 that the host device has negotiated. Any power wasted in the charging process directly results in longer charge cycle times.

Manufacturers are also changing the way they populate printed circuit boards. Instead of using a single, multilayer board, they're stacking multiple boards in space-constrained designs. Advanced packaging helps save height/thickness and area on PCBs and enables more efficient stacking. So designers need high efficiency charging, and high functionality in a densely integrated IC to save board space and increase product reliability.

Simplifying the solution
IC-based controllers such as Linear's PowerPath control/manager devices autonomously and seamlessly manage power flow between various input sources such as USB, wall adapters and the battery. These ICs direct power to the load rather than extracting power from the battery to ensure that a fully charged battery remains fresh when the USB is connected. Once the power source is removed, current flows from the battery to the load through an internal low-loss "ideal diode," maximizing efficiency and minimizing power dissipation. The forward voltage drop of an "ideal diode" is far less than that of a conventional or Schottky diode, and the reverse leakage is smaller. The smaller voltage drop across an optimized diode will be typically 20mV. Further, a three-terminal (or "intermediate bus") topology decouples the battery from Vout, allowing the end product to operate immediately when plugged in regardless of the battery's state of charge or even if it's missing. This feature is commonly referred to as "instant-ON" operation.

Figure 1: Simplified switching PowerPath circuit.
Click to view image.

Battery chargers integrated with such PowerPath controllers containing the so-called ideal diode device ("PowerPath managers") efficiently manage a wide variety of input power sources, charge the battery, and preferentially power the load and reduce power dissipation. PowerPath control circuits are available in both linear and switching topologies.

Switching-system advantages
While a linear PowerPath system offers significant power delivery efficiency advantages to the load/system over a battery-fed system, power is lost in the linear battery charger element especially if the battery voltage is low (resulting in a large differential between the input voltage and the battery voltage). Alternatively, a switchmode-based topology PowerPath device produces an intermediate-bus voltage via a USB-compliant step-down (buck) switching regulator that is regulated to 300mV above the battery voltage (Figure 1). This form of adaptive output control is referred to as "Bat-Track" (by Linear Technology). The regulated intermediate voltage is just high enough to allow proper charging through the internal linear charger.

Figure 2: Simplified block diagram, LTC4098.
Click to view image.

The switching architecture with average-input-current limiting maximizes the ability to use all of the 2.5W available from a USB supply. An optional external PFET reduces "ideal diode" impedance between the battery and the load, further reducing heat dissipation. This architecture is particularly advantageous for systems with large (>1.5A-h) batteries.

A manager with an "ideal-diode" system and battery charger for portable USB powered devices (such as LTC4098) will suit the vast majority of media player, digital camera, PDA, personal navigator, and smart phone applications. It's housed in a 20-pin 3mm x 4mm QFN package just 0.55mm high. The chip is suited for control of a compatible switching regulator up to 38V (60V transient) to maximize battery charger efficiency.

The LTC4098 provides overvoltage protection (OVP) up to 66Vrequiring only an external NFET/resistor combinationto prevent damage to its inputs caused by accidental application of high voltage. The IC's automatic charge current reduction circuitry enables fast instant-ON operation, ensuring system load power at plug-in even with a dead or missing battery. Its onboard "ideal diode" guarantees that ample power is always available to Vout even if there is insufficient power at the LTC4098's two input pins. The IC's ideal diode controller can be used to drive the gate of an optional PFET, thus reducing impedance to the battery to 30m or less (Figure 2).

Figure 3: Available current from the USB.
Click to view image.

The LTC4098's single-cell Li-ion/polymer charger allows the load current to exceed the current drawn from the USB port while conforming to USB load specifications. The IC's switching input stage converts nearly all of the 2.5W available from the USB port to available system current, thus enabling up to 700mA from a 500mA limited USB port (Figure 3). Up to 1.5A of charge current is available if the system is wall powered.

Overvoltage protection
The LTC4098 can protect itself from inadvertent application of excessive voltage to Vbus or WALL with just two external components: an n-channel FET and a 6.04K resistor. The maximum safe overvoltage value is determined by the designer's choice of the external NMOS transistor and its associated drain breakdown voltage.

Figure 4 shows the response of the OVP circuitry. The first trace at the bottom of the scope photo is the input voltage, which ramps up to 10V (2V/div). The second is the Vbus pin. It follows the input voltage ramp up to 6V, and then falls away as the OVP protection activates. The top trace shows the OVGATE output (5V/div), which ramps up to approximately 10V before going to ground (protection mode).

The LTC4098's power delivery from Vbus to Vout is controlled by a 2.25MHz constant frequency step-down switching regulator. To meet the USB maximum load specification, the switching regulator contains a measurement and control system that ensures that the average input current remains below the level programmed at CLPROG. Thus, Vout can drive the combination of the external load and the battery charger.

Figure 4: OVP capabilities.
Click to view image.

If the combined load does not cause the switching power supply to reach the programmed input current limit, the IC's Vout pin will track approximately 0.3V above the battery voltage. Thus the power lost to the battery charger will be minimal.

If the combined external load plus battery charge current is large enough to cause the switching power supply to reach the programmed input current limit, the battery charger will reduce its charge current by precisely the amount necessary to enable the external load to be satisfied. Even if the battery charge current is programmed to exceed the allowable USB current, the USB specification for average input current will not be violated. Moreover, if the load current at Vout exceeds the programmed power from Vbus, additional load current will be drawn from the battery via the "ideal diodes" even when the battery charger is enabled.

The WALL, /ACPR and VC pins can be used in conjunction with an external high voltage step-down switching regulator, such as the LT3480, to minimize heat production when operating from higher voltage sources. Bat-Track control circuitry regulates the external switching regulator's output voltage to (BAT + 300mV) or 3.6V, whichever is greater. Thus the chip maximizes the efficiency of the battery charger efficiency while still allowing instant-ON operation when the battery is deeply discharged.

About the author
Steve Knoth
is a product marketing engineer in Linear Technology's Power Products group. He earned his bachelor's degree in Electrical Engineering in 1988 and a masters degree in Physics in 1995, both from San Jose State University. Steve also received an MBA in Technology Management from the University of Phoenix in 2000.

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