AAT3663 Datasheet, PDF (18/22 Pages) Advanced Analogic Technologies – 1A Linear Li-Ion Battery Charger for Single and Dual Cell Applications

AAT3663
1A Linear Li-Ion Battery Charger
for Single and Dual Cell Applications
First, the maximum power dissipation for a given sit-
uation should be calculated:
Where:
P_D(MAX) =
(T_J - T_A)
θ_JA
P_D(MAX) = Maximum Power Dissipation (W)
θ_JA = Package Thermal Resistance (°C/W)
T_J = Thermal Loop Entering Threshold (ºC)
[115ºC]
T_A = Ambient Temperature (°C)
Figure 5 shows the relationship between maximum
power dissipation and ambient temperature of
AAT3663
2.50
2.00
1.50
1.00
0.50
0.00
0
25
50
75
100
T_A (°C)
Figure 5: Maximum Power Dissipation Before
Entering Thermal Loop.
Next, the power dissipation can be calculated by
the following equation:
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
·
V_IN - V_BAT
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 [0.5mA]
By substitution, we can derive the maximum
charge current before reaching the thermal limit
condition which will activate digital thermal loop
operation. The maximum charge current is the key
factor when designing battery charger applications.
In general, the worst case condition is when the great-
est input to output voltage drop occurs across the
charger IC. Specifically when battery voltage is
charged up just above the preconditioning voltage
threshold and the charger enters into the constant
current fast charging mode. Under this condition, the
device will suffer the maximum possible power dissi-
pation since both the voltage difference across the
device and the charge current will be at their respec-
tive maximums. Figure 6 shows the safe fast charge
current operating region for different ambient temper-
atures. Exceeding these limits will drive the charge
control into digital thermal loop operation. When
under digital thermal loop operation, the device will
remain active and continue to charge the battery at a
reduced current level for the given ambient condition.
1000
800
T_A = 85°C T_A = 60°C
600
T_A = 45°C
T_A = 25°C
400
200
0
4
5
6
7
8
9 10 11 12 13
V_IN (V)
Figure 6: Maximum Charging Current Before
the Digital Thermal Loop Becomes Active.
Capacitor Selection
Input Capacitor
In general, it is a good design practice to place a
decoupling capacitor between the IN pin and ground.
An input capacitor in the range of 1μF to 22μF is rec-
ommended. If the source supply is unregulated, it
may be necessary to increase the capacitance to
keep the input voltage above the under-voltage lock-
out threshold during device enable and when battery
charging is initiated. If the AAT3663’s input is to be
used in a system with an external power supply
source, such as a typical AC-to-DC wall adapter,
18
3663.2007.10.1.0