參數(shù)資料
型號： AAT3680ITP-4.1-T1
廠商： Advanced Analogic Technologies, Inc.
英文描述： Lithium-Ion Linear Battery Charge Controller
中文描述： 鋰離子線性電池充電控制器
文件頁數(shù)： 10/18頁
文件大?。?/td> 265K
代理商： AAT3680ITP-4.1-T1
AAT3680
Lithium-Ion Linear Battery Charge Controller
10
3680.2003.4.0.91
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
θ
JA
required:
R
θ
JA
= (T
J(MAX)
- T
A(MAX)
)
P
D
R
θ
JA
= (150 - 40)
1.38
R
θ
JA
= 80°C/W
It is recommended to choose a package with a lower
R
θ
JA
than the number calculated above. A SOT223
package would be an acceptable choice, as it has an
R
θΑ
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
θ
JA
≤
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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