AAT3680
Lithium-Ion/Polymer
Linear Battery Charge Controller
Q1
RSENSE
RFD10P03L
BATT+
0.2Ω
VP
R4
C2
10µF
100k
R1
1k
DRV
CSI
VP
T2X
BATT-
VP
BAT
RT1
AAT3680
TS
TEMP
VSS
STAT
C1
4.7µF
Battery
Pack
D1
RT2
R2
1k
Figure 6: Typical Applications Schematic Using a P-Channel Power MOSFET with the AAT3680-4.2.
1. The first step is to determine the maximum power
(TJ(MAX) - TA(MAX)
)
RθJA
=
=
dissipation (PD) in the pass transistor. Worst case
is when the input voltage is the highest and the
battery voltage is the lowest during fast-charge
(this is referred to as VMIN, nominally 3.1V when
the AAT3680-4.2 transitions from trickle charge to
constant current mode). In this equation, VCS is
the voltage across RSENSE, and VD is the voltage
across the reverse current blocking diode. Refer
to section below titled Schottky Diode for further
details. Omit the value for VD in the equation
below if the diode is not used.
PD
(150 - 40)
1.4
= 79°C/W
It is recommended to choose a package with a
lower RθJA than the number calculated above.
A SOT223 package would be an acceptable
choice, as it has an RθJA of 62.5°C/W when
mounted to a PCB with an adequately sized
copper pad soldered to the heat tab.
PD = (VP(MAX) - VCS - VD - VMIN) ⋅ ICHARGE(REG)
= (5.5V - 0.1V - 0.4V - 3.1V) ⋅ 750mA
= 1.4W
3. Choose a drain-source (VDS) voltage rating
greater than the input voltage. In this example,
VP is 5.0V, so a 12V device is acceptable.
4. Choose a MOSFET with a drain current rating at
least 50% greater than the programmed
ICHARGE(REG) value. In this example, we would
select a device with at least a 1.125A rating.
2. The next step is to determine which size package
is needed to keep the junction temperature below
its rated value, TJ(MAX). Using this value, and the
maximum ambient temperature inside the system
TA(MAX), calculate the thermal resistance RθJA
required:
3680.2006.03.1.6
11
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
