AAT3683_08 Datasheet, PDF (16/23 Pages) Advanced Analogic Technologies – 1A Linear Li-Ion Battery Charger

BatteryManager^TM
Most of the commonly used NTC thermistors in battery
packs are approximately 10kΩ at room temperature
(25°C). The TS pin (3683-2 only) has been specifically
designed to source 75μA of current to the thermistor.
The voltage on the TS pin resulting from the resistive
load should stay within a window of 331mV to 2.39V. If
the battery becomes too hot during charging due to an
internal fault or excessive constant charge current, the
thermistor will heat up and reduce in value, pulling the
TS pin voltage lower than the TS1 threshold, and the
AAT3683-2 will stop charging until the condition is
removed, when charging will be resumed.
If the use of the TS pin function is not required by the
system, it should be terminated to ground using a 10kΩ
resistor. Alternatively, on the AAT3683-2, the TS pin may
be left open.
For AAT3683-3 and -4, the internal battery temperature
sensing system is comprised of two comparators which
establish a voltage window for safe operation. The
thresholds for the TS operating window are bounded by
the TS1 and TS2 specifications. Referring to the electrical
characteristics table in this datasheet, the TS1 threshold
= 0.30 · V_IN and the TS2 threshold = 0.60 · V_IN.
VIN
IN
AAT3683-3 and -4
0.60x VIN
TS
Battery Cold Fault
Battery
Pack
Battery Hot Fault
x VIN
Figure 3: AAT3683-3 and -4 Battery Temperature
Sense Circuit.
Thermal Considerations
The AAT3683 is offered in two packages (STDFN2.2x2.2-10
and QFN33-16) both of which can provide up to 2W of
power dissipation when properly bonded to a printed
circuit board and have a maximum thermal resistance of
50°C/W. Many considerations should be taken into
account when designing the printed circuit board layout,
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AAT3683
1A Linear Li-Ion Battery Charger
as well as the placement of the charger IC package in
proximity to other heat generating devices in a given
application design. The ambient temperature around the
charger IC will also have an effect on the thermal limits
of a battery charging application. The maximum limits
that can be expected for a given ambient condition can
be estimated by the following discussion.
First, the maximum power dissipation for a given situa-
tion should be calculated:
P_D(MAX) =
(T_J - T_A)
θ_JA
Where:
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 4 shows the relationship of maximum power dis-
sipation and ambient temperature of AAT3683.
2.5
2
1.5
1
0.5
0
0
25
50
75
100
T_A (°C)
Figure 4: Maximum Power Dissipation Before
Entering Thermal Loop.
Next, the power dissipation can be calculated by the fol-
lowing 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]
16
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3683.2007.11.1.2