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Charger IC touts digital edge

Posted: 21 Nov 2005 ?? ?Print Version ?Bookmark and Share

Keywords:advanced analogic technologies? aat3686? battery charger ic?

Advanced Analogic Technologies' (AnalogicTech) AAT3686 is a Li-ion/polymer battery charger IC powered by the USB port or an AC adapter that promises a number of leading edge features. According to the company these include a digital thermal loop for quicker response and more stable operation in the AC adapter mode; the highest charging current capability for a chip of this kind; circuitry that cuts back on the charge current depending on the condition of the alternative (USB) power source; and a serial data output to an external processor to provide system feedback on a wide range of battery charge and battery status states.

The AAT3686, working from a 4V to 5.5V input, can deliver up to a programmable 1.5A (AC adapter mode) or 100mA to 500mA when powered from a USB port, with the chip charging from the highest voltage source. "If youre charging a portable device at 500mA using a USB port, and you want to actually use the device while it's charging, the voltage can easily fall out of regulation if worst-case current budgets havent been set by the designer," said David Brown, senior applications engineering manager. "The AAT3686 simplifies design, decreases charging time and increases system reliability by integrating a charge reduction function that automatically compensates for voltage sags and throttles back the charge current depending upon how much current other loads on the device are consuming. "

Charging proceeds in the traditional phases: preconditioning, constant-current, constant-voltage and end-of-charge cycle. The charger's digital thermal regulation loop (when adapter supply is in use) engages when the chip's die temperature exceeds 900C. Once the thermal loop control becomes active, the fast charge current is initially reduced to about 44 percent of the chips nominal preset value. Assume for example, said the company, a charging current of 440mA at 1100C (nominal charging current would be 1A). As the chips temperature (checked every 3s) drops, the output current increases in staircase fashion. At 900C, the average output current would be about 650mA. The charger resumes normal operation to deliver its full output current as the temperature drops to below 850C.

The aforementioned chip circuitry senses the source voltage from the USB port and appropriately sets the charge current. A watchdog timer for the adapter mode shuts down the chip if the trickle charge exceeds 25mins and the combined time for trickle charge plus fast charge exceeds three hours. Charging levels are set with resistors placed between the chips ADPSET, USBH and USBL pins and ground.

Under extreme (failure) conditions, the chips overvoltage and over-temperature protection circuits automatically shut down the charger to protect the overall system and the battery under charge. The chip also provides a temperature sense feedback function from the battery to shut down the device if the battery exceeds its own thermal limit during charging.

The chip has two outputs that drive two external LEDs to provide visual feedback of battery charging status. In addition, its serial interface output reports up to 22 charging status states to a system microcontroller. Under these conditions, the microcontroller asserts a data request pulse and the chip replies back with the data word. The feedback to the system includes information on the state of the constant-current, constant-voltage, and end-of-charge modes, watchdog timer status and battery temperature.

The AAT3686, available in a Pb-free, 4-by-4 mm TDFN package (16 pins), is priced at $1.64 in 1,000-unit pieces.

- Vince Biancomano


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