Chip Takes Charge In Handhelds

Taking its power from either an AC adapter or USB port, we look at an IC that performs power-source selection, power delivery, battery charging and other functions for popular single-cell Li-ion devices.

Offering the longest run times among the popular re-chargeable chemistries, single-cell lithium-ion (Li-ion) batteries can be found in many handheld devices. But the factors of multiple charging sources, multiple supply voltages within the product, demands for optimum efficiency and very limited space complicate power management 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. Marrying 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 4mm, 24pin QFN package. It has a low-battery indicator.

POWER DELIVERY
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 is because power cannot 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 run time. 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 (ie battery charging) is eliminated.

TECHNICAL CHALLENGE
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 (Figure 2). It also operates autonomously as a stand alone controller.

Other on-chip features include accurate USB current limiting. Its pin-selectable options include a 500mA limit, a 100mA limit, or a suspend mode. In addition, charging current is reduced automatically as USB current increases to ensure that the USB current limit is not 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 1.5MHz switching frequency, enabling the use of very small external inductors.

The devices 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.

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