AAT3684 Datasheet, PDF (13/17 Pages) Advanced Analogic Technologies – 500mA USB Port or AC Adapter Lithium-Ion/Polymer Battery Charger

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PRODUCT DATASHEET
AAT3684
500mA USB Port or AC Adapter Lithium-Ion/Polymer Battery Charger
Next, the power dissipation can be calculated by the fol-
lowing equation:
P_D = [(V_IN - V_BAT) · I_CC + (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_CC = Constant Charge Current Programmed for the
Application
I_OP = Quiescent Current Consumed by the Charger IC
for Normal Operation [0.5mA]
By substitution, we can derive the maximum charge cur-
rent before reaching the thermal limit condition (thermal
cycling). The maximum charge current is the key factor
when designing battery charger applications.
I_CC(MAX) =
(P_D(MAX) - V_IN · I_OP)
V_IN - V_BAT
I_CC(MAX) =
(T_J(MAX) -
θ_JA
T_A)
-
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.
Figure 4 shows the maximum charge current in different
ambient temperatures.
500
450
400
T_A = 60°C
350
T_A = 45°C
300
250
200
T_A = 85°C
150
100
50
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 4: Maximum Charging Current Before
Thermal Cycling Becomes Active.
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a decou-
pling 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
AAT3684 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 AAT3684 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
AAT3684 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
AAT3684 BAT pin. To minimize voltage drops on the PCB,
keep the high current carrying traces adequately wide.
Refer to the AAT3684 evaluation board for a good layout
example (see Figures 5 and 6).
For better thermal enhancement, solder the exposed
paddle of the IC onto the thermal landing of the PCB,
where the thermal landing is connected to the ground
plane. If heat is still an issue, multi-layer boards with
dedicated ground planes are recommended. Adding
more thermal vias on the thermal landing will also help
transfer heat to the PCB effectively. Figure 7 illustrates
the exploded view of the layout at the bottom of the IC.
3684.2007.12.1.2
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