AAT3680
Lithium-Ion/Polymer
Linear Battery Charge Controller
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 adequately sized copper pad soldered
to the heat tab.
Applications Information
Choosing an External Pass Device
(PNP or PMOS)
The AAT3680 is designed to work with either a
PNP transistor or P-channel power MOSFET.
Selecting one or the other requires looking at the
design tradeoffs, including performance versus
cost issues. Refer to the following design guide for
selecting the proper device.
3. Choose a collector-emitter (VCE) voltage rating
greater than the input voltage. In this example,
VP is 5.0V, so a 15V device is acceptable.
4. Choose a transistor with a collector current rating
at least 50% greater than the programmed
ICHARGE(REG) value. In this example, we would
select a device with at least a 900mA rating.
PNP Transistor
In this design example, we will use the following
conditions: VP = 5V (with 10% supply tolerance),
ICHARGE(REG) = 600mA, 4.2V single cell lithium-ion
pack. VP is the input voltage to the AAT3680, and
ICHARGE(REG) is the desired fast-charge current.
5. Calculate the required current gain (β or hFE);
β > 200:
IC(MAX)
1. The first step is to determine the maximum
power dissipation (PD) in the pass transistor.
Worst case is when the input voltage is the high-
est and the battery voltage is the lowest during
fast-charge (this is referred to as VMIN, nominal-
ly 3.1V when the AAT3680-4.2 transitions from
trickle charge to constant current mode). In this
βMIN
=
=
IB(MIN)
0.60
0.02
= 30
equation, VCS is the voltage across RSENSE
.
where IC(MAX) is the collector current (which is the
same as ICHARGE(REG)), and IB(MIN) is the minimum
amount of base current drive shown in Electrical
Characteristics as ISINK. Important Note: The cur-
rent gain (β or hFE) can vary by a factor of three
over temperature and drops off significantly with
increased collector current. It is critical to select a
transistor with β, at full current and lowest temper-
ature, greater than the βMIN calculated above.
PD = (VP(MAX) - VCS - VMIN) ⋅ ICHARGE(REG)
= (5.5V - 0.1V - 3.1V) ⋅ 600mA
= 1.38W
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:
In summary, select a PNP transistor with ratings
VCE ≥ 15V, RθJA ≤ 80°C/W, IC ≥ 900mA, βMIN ≥ 30 in
a SOT223 (or better thermal) package.
P-Channel Power MOSFET
The following conditions apply to Figure 6, for use
with the AAT3680-4.2V version: VP = 5V (with 10%
supply tolerance), ICHARGE(REG) = 750mA, 0.4V
Schottky diode, 4.2V single cell lithium-ion battery
pack. VP is the input voltage to the AAT3680, and
ICHARGE(REG) is the desired fast-charge current.
(TJ(MAX) - TA(MAX)
)
RθJA
=
=
PD
(150 - 40)
1.38
= 80°C/W
10
3680.2006.03.1.6
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
