AAT3683_08 Datasheet, PDF (17/23 Pages) Advanced Analogic Technologies – 1A Linear Li-Ion Battery Charger

BatteryManager^TM
By substitution, we can derive the maximum charge cur-
rent before reaching the thermal limit condition (thermal
loop). The maximum charge current is the key factor
when designing battery charger applications.
I_CH(MAX) =
(P_D(MAX) - V_IN · I_OP)
V_IN - V_BAT
I_CH(MAX) =
(T_J - T_A)
θ_JA
- V_IN · I_OP
V_IN - V_BAT
In general, the worst condition is the greatest voltage
drop across the charger IC, when battery voltage is
charged up to the preconditioning voltage threshold and
before entering thermal loop regulation. Figure 5 shows
the maximum charge current in different ambient tem-
peratures.
1000
800
T_A = 45°C
600
T_A = 25°C
400
200
T_A = 85°C _{TA = 60°C}
0
4.25 4.5 4.75 5 5.25 5.5 5.75 6 6.25 6.5 6.75 7
V_IN (V)
Figure 5: Maximum Charging Current Before the
Digital Thermal Loop Becomes Active.
PRODUCT DATASHEET
AAT3683
1A Linear Li-Ion Battery Charger
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a decou-
pling capacitor between the IN 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
AAT3683 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 capacitor 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 AAT3683 only requires a 1μF ceramic capacitor 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
AAT3683 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
AAT3683 BAT pin. To minimize voltage drops on the PCB,
keep the high current carrying traces adequately wide.
When designing with >500mA charging current system,
a multilayer PCB design is highly recommended. Putting
thermal vias on the thermal pad design will effectively
transfer heat from the top metal layer of the PCB to the
inner or bottom layers. The number of thermal vias will
depend on the application and power dissipation. Refer
to the AAT3683 evaluation board for a good layout
example.
3683.2007.11.1.2
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