AAT3688 datasheet(13/23 Pages) AAT

13 / 23 page

AAT3688

USB Port Lithium-Ion/Polymer Battery Charger

3688.2006.10.1.5

Application Information

USB System Power Charging

The USB charge mode provides two programma-

ble fast charge levels up to 500mA for each, USB

high and USB low, USBH and USBL, respectively.

The USBH or USBL modes may be externally

selected by the USB select pin (USBSEL). When

the USBSEL pin is connected to a logic high level,

the USBH level will be active. Conversely, when

USBSEL is pulled to a logic low level (ground), the

USBL level will be used for fast charging. Typically

USBH is set for 500mA and USBL is set for 100mA.

However, these two USB charge levels may be

user programmed to any level between 50mA and

500mA by selecting the appropriate resistor values

for RSETH and RSETL. Refer to Table 1 for recom-

mended RSETH and RSETL values for the desired

USB input constant current charge levels.

USB Charge Reduction

In many instances, product system designers do

not know the real properties of a potential USB port

to be used to supply power to the battery charger.

Typical powered USB ports commonly found on

desktop and notebook PCs should supply up to

500mA. In the event a USB port being used to sup-

ply the charger is unable to provide the pro-

grammed fast charge current, or if the system

under charge must also share supply current with

other functions, the AAT3688 will automatically

reduce USB fast charge current to maintain port

integrity and protect the host system.

The USB charge reduction system becomes active

when the voltage on the USB input falls below the

USB charge reduction threshold (VUSBCHR), which

is typically 4.5V. Regardless of which USB charge

function is selected (USBH or USBL), the charge

reduction system will reduce the fast charge cur-

rent level in a linear fashion until the voltage

sensed on the USB input recovers above the

charge reduction threshold voltage.

The USB

charge reduction threshold (VUSBCHR) may be

externally set to a value lower than 4.5V by placing

a resistor divider network between VUSB and

ground with the center connected to the CHR pin.

The USB charge reduction feature may be disabled

by connecting a 10k

Ω resistor from the CHR pin

directly to the USB input pin.

The following equation may be used to approximate

a USB charge reduction threshold below 4.5V:

Eq. 1:

where R11/R12 << 1M

Ω.

USBCHR = 2.0V

R12

R12 + R11

Table 1: Recommended RSET Values.

Figure 2: Internal Equivalent

Circuit for the CHR Pin.

1.025M

825k

R11

R12

VCHR = 2.0V

VUSB

USB

CHR

USBH

USBL

ICC

RSET (kΩ

Ω)RSET (kΩ

Ω)

86.6

57.6

100

42.2

200

21.0

20.5

300

13.7

400

10.2

500

8.06

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