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

AAT3680
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 highest and
the battery voltage is at the lowest during fast-
charge (this is referred to as V_MIN, nominally 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)
P_D = (5.5V - 0.1V - 3.1V) · 600mA
P_D = 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 maxi-
mum 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
R_θJA
=
(150 - 40)
1.38
R_θJA = 80°C/W
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_θϑΑ of 62.5°C/W when mounted to a PCB with ade-
quately 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 900mA rating.
5. Calculate the required current gain (β or h_FE):
β_MIN
=
I_C(MAX)
I_B(MIN)
β_MIN
=
0.60
0.02
β_MIN = 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 a factor of 3 over tem-
perature, and drops off significantly with increased
collector current. It is critical to select a transistor
with β, at full current and lowest temperature,
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:
In this design example, as shown in Figure 5, we
will use the following conditions: V_P = 5V (with 10%
supply tolerance), I_CHARGE(REG) = 750mA, 0.4V
Schottky diode, 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 highest and
the battery voltage is at the lowest during fast-
charge (this is referred to as V_MIN, nominally 3.1V
when the AAT3680-4.2 transitions from trickle-
charge to constant-current mode). In this equation
VCS is the voltage across R_SENSE, and V_D is the
voltage across the reverse-current blocking diode.
Refer to section below titled Schottky Diode for
further details. Omit the value for V_D in the equa-
tion below if the diode is not used.
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