AAT3683 Datasheet, PDF (17/24 Pages) Advanced Analogic Technologies – 1A Linear Li-Ion Battery Charger

Next, the power dissipation can be calculated by
the following equation:
P_D = [(V_IN - V_BAT) · I_CH + (V_IN · I_OP)]
Where:
P_D = Total Power Dissipation by the Device
V_IN = Input Voltage
V_BAT = Battery Voltage as Seen at the BAT Pin
I_CH = Constant Charge Current Programmed for
the Application
I_OP = Quiescent Current Consumed by the
Charger IC for Normal Operation [0.3mA]
By substitution, we can derive the maximum
charge current 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 volt-
age drop across the charger IC, when battery volt-
age is charged up to the preconditioning voltage
threshold and before entering thermal loop regula-
tion. Figure 5 shows the maximum charge current
in different ambient temperatures.
1000
800
T_A = 45°C
T_A = 25°C
600
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
V_IN (V)
Figure 5: Maximum Charging Current Before
the Digital Thermal Loop Becomes Active.
3683.2007.06.1.0
AAT3683
1A Linear Li-Ion Battery Charger
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a
decoupling 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 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 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 physi-
cally 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.
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