AAT3680 Advanced Analogic Technologies, AAT3680 Datasheet - Page 10

• Manufacturer Part Number: AAT3680
• Description: Lithium-Ion Linear Battery Charge Controller
• Manufacturer: Advanced Analogic Technologies

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.
selecting the proper device:
PNP Transistor:
In this design example, we will use the following
conditions: V
I
pack. V
I
1. The first step is to determine the maximum
power dissipation (P
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
when the AAT3680-4.2 transitions from trickle-
charge to constant-current mode). In this equation
V
2. The next step is to determine which size package
is needed to keep the junction temperature below its
rated value, T
mum ambient temperature inside the system T
calculate the thermal resistance R
It is recommended to choose a package with a lower
R
package would be an acceptable choice, as it has an
R
quately sized copper pad soldered to the heat tab.
10
CHARGE(REG)
CHARGE(REG)
CS
JA
is the voltage across R
than the number calculated above. A SOT223
of 62.5°C/W when mounted to a PCB with ade-
P
P
P
D
D
D
P
= (V
= (5.5V - 0.1V - 3.1V) · 600mA
= 1.38W
Refer to the following design guide for
is the input voltage to the AAT3680, and
= 600mA, 4.2V single cell Lithium Ion
is the desired fast-charge current.
R
R
R
P(MAX)
J(MAX)
P
=5V (with 10% supply tolerance),
JA
JA
JA
=
=
= 80°C/W
. Using this value, and the maxi-
- V
(T
(150 - 40)
D
) in the pass transistor. Worst
J(MAX)
1.38
CS
- V
SENSE
P
- T
MIN
D
A(MAX)
) · I
MIN
.
JA
, nominally 3.1V
CHARGE(REG)
required:
)
Lithium-Ion Linear Battery Charge Controller
A(MAX)
,
3. Choose a collector-emitter (V
greater than the input voltage. In this example, V
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
a device with at least 900mA rating.
5. Calculate the required current gain ( or h
where I
same as I
amount of base current drive shown in Electrical
Characteristics as I
rent gain ( or h
perature, and drops off significantly with increased
collector current. It is critical to select a transistor
with
greater than the
In summary, select a PNP transistor with ratings
V
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
supply tolerance), I
Schottky diode, 4.2V single cell Lithium Ion pack.
V
I
1. The first step is to determine the maximum
power dissipation (P
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
when the AAT3680-4.2 transitions from trickle-
charge to constant-current mode). In this equation
VCS is the voltage across R
voltage across the reverse-current blocking diode.
Refer to section below titled Schottky Diode for
further details. Omit the value for V
tion below if the diode is not used.
CHARGE(REG)
CHARGE(REG)
CE
P
is the input voltage to the AAT3680, and
15V, R
, at full current and lowest temperature,
C(MAX)
CHARGE(REG)
value. In this example we would select
is the desired fast-charge current.
JA
MIN
MIN
MIN
is the collector current (which is the
FE
=
=
= 30
80°C/W, I
) can vary a factor of 3 over tem-
MIN
SINK
I
I
0.60
0.02
D
C(MAX)
B(MIN)
CHARGE(REG)
) in the pass transistor. Worst
), and I
calculated above.
. Important Note: The cur-
C
B(MIN)
SENSE
900mA,
MIN
CE
AAT3680
= 750mA, 0.4V
P
, nominally 3.1V
is the minimum
, and V
) voltage rating
= 5V (with 10%
D
3680.2003.4.0.91
in the equa-
MIN
D
FE
is the
30 in
):
P