AAT3681 AAT, AAT3681 Datasheet - Page 13

• Manufacturer Part Number: AAT3681
• Description: USB Port or AC Adapter Lithium-Ion/Polymer Battery Charger
• Manufacturer: AAT

Where:
P
V
V
I
I
By substitution, we can derive the maximum
charge current before reaching the thermal limit
condition (thermal cycling). The maximum charge
current is the key factor when designing battery
charger applications.
In general, the worst condition is the greatest volt-
age drop across the charger IC, when battery volt-
age is charged up to the preconditioning voltage
threshold. Figure 4 shows the maximum charge
current in different ambient temperatures.
3681.2007.01.1.2
CH
OP
Figure 4: Maximum Charging Current Before
D
IN
BAT
= Total Power Dissipation by the Device
= Input Voltage
= Battery Voltage as Seen at the BAT Pin
= Constant Charge Current Programmed for
= Quiescent Current Consumed by the
300
250
200
150
100
50
Thermal Cycling Becomes Active.
the Application
Charger IC for Normal Operation [0.5mA]
0
4.25
I
CH(MAX)
I
T
4.5
CH(MAX)
A
= 60°C
4.75
=
=
T
(T
5
A
= 85°C
(P
J(MAX)
5.25
D(MAX)
θ
JA
V
V
V
- T
5.5
IN
IN
IN
(V)
- V
- V
A
- V
5.75
)
- V
BAT
IN
T
BAT
A
· I
= 25°C
IN
6
OP
· I
6.25
T
)
OP
A
= 45°C
6.5
6.75
Lithium-Ion/Polymer Battery Charger
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a
decoupling capacitor between the ADP pin and
GND. An input capacitor in the range of 1µF to
22µF is recommended. If the source supply is
unregulated, it may be necessary to increase the
capacitance to keep the input voltage above the
under-voltage lockout threshold during device
enable and when battery charging is initiated. If the
AAT3681 adapter input is to be used in a system
with an external power supply source, such as a
typical AC-to-DC wall adapter, then a C
in the range of 10µF should be used. A larger input
capacitor in this application will minimize switching
or power transient effects when the power supply is
"hot plugged" in.
Output Capacitor
The AAT3681 only requires a 1µF ceramic capaci-
tor on the BAT pin to maintain circuit stability. This
value should be increased to 10µF or more if the
battery connection is made any distance from the
charger output. If the AAT3681 is to be used in
applications where the battery can be removed
from the charger, such as with desktop charging
cradles, an output capacitor greater than 10µF may
be required to prevent the device from cycling on
and off when no battery is present.
Printed Circuit Board Layout
Considerations
For the best results, it is recommended to physically
place the battery pack as close as possible to the
AAT3681 BAT pin. To minimize voltage drops on the
PCB, keep the high current carrying traces ade-
quately wide. Refer to the AAT3681 evaluation board
for a good layout example (see Figures 5 and 6).
USB Port or AC Adapter
IN
capacitor
13