AAT3688
USB Port Lithium-Ion/Polymer Battery Charger
other functions, the AAT3688 will automatically
reduce USB fast charge current to maintain port
integrity and protect the host system.
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.
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 10kΩ resistor from the CHR pin
directly to the USB input pin.
USB Charge Reduction
The following equation may be used to approximate
a USB charge reduction threshold below 4.5V:
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
R12
R12 + R11
VUSBCHR = 2.0V ÷
Eq. 1:
where R11/R12 << 1MΩ.
USB
VUSB
USBH
RSET (kΩ)
USBL
RSET (kΩ)
ICC
50
86.6
57.6
42.2
21.0
13.7
10.2
8.06
86.6
57.6
42.2
20.5
13.7
10.2
8.06
R11
1.025M
75
CHR
VCHR = 2.0V
100
200
300
400
500
R12
825k
Table 1: Recommended RSET Values.
Figure 2: Internal Equivalent
Circuit for the CHR Pin.
3688.2006.10.1.5
13
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
