AAT3680 Datasheet, PDF (10/18 Pages) Advanced Analogic Technologies – Lithium-Ion Linear Battery Charge Controller

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.
PNP Transistor
In this design example, we will use the following
conditions: V_P = 5V (with 10% supply tolerance),
I_CHARGE(REG) = 600mA, 4.2V single cell lithium-ion
pack. V_P is the input voltage to the AAT3680, and
I_CHARGE(REG) is the desired fast-charge current.
1. The first step is to determine the maximum
power dissipation (P_D) 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 V_MIN, nominal-
ly 3.1V when the AAT3680-4.2 transitions from
trickle charge to constant current mode). In this
equation, V_CS is the voltage across R_SENSE.
P_D = (V_P(MAX) - V_CS - V_MIN) ⋅ I_CHARGE(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, T_J(MAX). Using this value and the
maximum ambient temperature inside the system
T_A(MAX), calculate the thermal resistance R_θJA
required:
R_θJA
=
(T_J(MAX) - T_A(MAX))
P_D
= ^{(150 - 40)}
1.38
= 80°C/W
AAT3680
Lithium-Ion/Polymer
Linear Battery Charge Controller
It is recommended to choose a package with a
lower R_θJA than the number calculated above. A
SOT223 package would be an acceptable choice,
as it has an R_θJA of 62.5°C/W when mounted to a
PCB with adequately sized copper pad soldered
to the heat tab.
3. Choose a collector-emitter (V_CE) voltage rating
greater than the input voltage. In this example,
V_P 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
I_CHARGE(REG) value. In this example, we would
select a device with at least a 900mA rating.
5. Calculate the required current gain (β or h_FE);
β > 200:
β_MIN
=
I_C(MAX)
I_B(MIN)
= ^0.60
0.02
= 30
where I_C(MAX) is the collector current (which is the
same as I_CHARGE(REG)), and I_B(MIN) is the minimum
amount of base current drive shown in Electrical
Characteristics as I_SINK. Important Note: The cur-
rent gain (β or h_FE) 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 β_MIN calculated above.
In summary, select a PNP transistor with ratings
V_CE ≥ 15V, R_θJA ≤ 80°C/W, I_C ≥ 900mA, β_MIN ≥ 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: V_P = 5V (with 10%
supply tolerance), I_CHARGE(REG) = 750mA, 0.4V
Schottky diode, 4.2V single cell lithium-ion battery
pack. V_P is the input voltage to the AAT3680, and
I_CHARGE(REG) is the desired fast-charge current.
10
3680.2006.03.1.6