Electronic Design

Power-Management Chip Takes Charge In Handhelds

Taking its power from either an ac adapter or USB port, this unique IC performs power-source selection, power delivery, battery charging, and other functions for popular single-cell Li-ion devices.

Offering the longest runtimes among the popular rechargeable chemistries, single-cell lithium-ion (Li-ion) batteries can be found in many handheld devices. But multiple charging sources, multiple supply voltages within the product, demands for optimum efficiency, and very limited space complicate power management.

These factors have driven the development of highly integrated power-management ICs for cell phones. Yet digital still cameras, PDAs, MP3 players, GPS receivers, and other single-cell Li-ion applications also need integrated power-management designs.

Linear Technology's LTC3455 dual dc-dc converter with USB power manager and Li-ion battery charger addresses this need. Wedding several functions that previously required five or more chips, this device performs power-source selection, battery charging, two stages of dc-dc conversion, and hot-swap control in a 4- by 4-mm, 24-pin QFN package. It additionally features a low-battery indicator.

Unlike other converters, this IC can select and take power from the appropriate power source. If the ac adapter is present, the chip derives power from the adapter. Otherwise, it takes power from the USB port. And if neither source is connected, the IC will grab power from the battery.

The chip isn't merely selecting the ac or USB power source when either is available, though. It's actually using that source to directly power the device in which it's used. The selected input source is routed to the output, where it provides one supply voltage, and to the chips' two dc-dc converters and hot-swap controller, which generate three other supplies (Fig. 1).

The LTC3455's power delivery differs from existing charger-fed systems implemented in battery and power-management ICs. In such systems, the external power source doesn't power the loads directly. Instead, the adapter or USB port would be used to charge the battery, which then powers the loads. If the battery is deeply discharged, there will be a delay in getting power to the loads. That's because power can't be taken from the battery until it obtains the required minimum amount of charge.

The LTC3455 eliminates this delay so the handheld device can be powered up as soon as the ac or USB power source is connected. In addition, the chip will take any available power not being used by the loads and use it to charge the battery.

These two benefits, elimination of charging delays and simultaneous battery charging and powering of loads, extend the battery's effective runtime. This power-management technique also increases efficiency whenever the ac or USB power source is available. In these instances, an unnecessary stage of power conversion (that is, battery charging) is eliminated.

The key technical challenge in developing the LTC3455 was power-path control. Previously, system designers had to implement this function discretely with FETs, op amps, and other components. But hot-plugging problems such as large inrush currents, which cause big system problems, plague these designs. The LTC3455 provides hot-plug protection for both the USB and wall adapter while always using the appropriate input source for power (Fig. 2). It also operates autonomously as a standalone controller.

Other on-chip features include accurate USB current limiting. Its pin-selectable options include a 500-mA limit, a 100-mA limit, or a suspend mode. In addition, charging current is reduced automatically as USB current increases to ensure that the USB current limit isn't exceeded.

The chip's pair of buck converters operate under current-mode control and achieve efficiencies as high as 96% with pin-selectable burst-mode operation. These dc-dc converters operate at a fixed 1.5-MHz switching frequency, enabling the use of very small external inductors.

The chip's battery charger uses a thermal regulation loop to ensure that the battery is charged at the maximum rate without fear of overheating the IC. The charger also features a pin-programmable maximum charge current and end-of-charge timer. The LTC3455 costs $3.95 each in quantities of 1000.

Linear Technology Corp.
www.linear.com
(408) 432-1900

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