AAT3685 AAT, AAT3685 Datasheet - Page 13

• Manufacturer Part Number: AAT3685
• Description: Lithium-Ion/Polymer Battery Charger
• Manufacturer: AAT

Application Information
USB System Power Charging
The USB charge mode provides two programma-
ble fast charge levels up to 1A for each, SETH and
SETL. The SETH or SETL modes may be exter-
nally selected by the select pin (PWRSEL). When
the PWRSEL pin is connected to a logic high level,
the SETH level will be active. Conversely, when
PWRSEL is pulled to a logic low level (ground), the
SETL level will be used for fast charging. These
two charge levels may be user programmed to any
level between 50mA and 1A by selecting the appro-
priate resistor values for R
Table 1 for recommended R
for the desired input constant current charge levels.
Charge Reduction
In many instances, product system designers do
not know the real properties of a potential 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 port being used to supply
the charger is unable to provide the programmed
fast charge current, or if the system under charge
must also share supply current with other func-
tions, the AAT3685 will automatically reduce USB
3685.2006.10.1.3
100
200
300
400
I
50
75
CC
R
SET
SETH
86.6
57.6
42.2
21.0
13.7
10.2
Table 1: Recommended R
(kΩ Ω ) R
SET
SETL
86.6
57.6
42.2
20.5
13.7
10.2
SETH
(kΩ Ω )
SETH
and R
and R
1000
500
600
700
800
900
I
CC
SETL
SETL
. Refer to
SET
R
values
SET
SETH
8.06
6.65
5.62
4.87
4.32
3.83
Values.
(kΩ Ω )
Lithium-Ion/Polymer Battery Charger
R
SET
SETL
8.06
6.65
5.62
4.87
4.32
3.83
fast charge current to maintain port integrity and
protect the host system.
The charge reduction system becomes active when
the voltage on the input falls below the charge
reduction threshold (V
ly 4.5V. Regardless of which charge function is
selected (SETH or SETL), the charge reduction
system will reduce the fast charge current level in a
linear fashion until the voltage sensed on the input
recovers above the charge reduction threshold volt-
age. The charge reduction threshold (V
may be externally set to a value lower than 4.5V by
placing a resistor divider network between V
and ground with the center connected to the CHR
pin. The charge reduction feature may be disabled
by connecting a 10kΩ resistor from the CHR pin
directly to the ADP/USB input pin.
The following equation may be used to approximate
a USB charge reduction threshold below 4.5V:
where R11/R12 << 1MΩ.
(kΩ Ω )
Eq. 1:
V
ADP/USB_CHR
Figure 2: Internal Equivalent
Circuit for the CHR Pin.
R11
R12
ADP/USB_CHR
V
ADP/USB
= 2.0V ÷
CHR
ADP/USB
), which is typical-
AAT3685
R12 + R11
825k
1.025M
R12
V
ADP/USB_CHR
CHR
= 2.0V
ADP/USB
13
)