參數(shù)資料
型號: AAT3685
廠商: Advanced Analog Technology,lnc.
英文描述: Lithium-Ion/Polymer Battery Charger
中文描述: 鋰離子電池充電器
文件頁數(shù): 19/23頁
文件大?。?/td> 557K
代理商: AAT3685
Where:
T
A
T
J
= Ambient Temperature in Degrees C
= Maximum Device Junction Temperature
Protected by the Thermal Limit Control
P
D
θ
JA
= Total Power Dissipation by the Device
= Package Thermal Resistance in °C/W
Example:
For an application where the fast charge current is
set to 500mA, V
USB
= 5.0V and the worst case bat-
tery voltage at 3.0V, what is the maximum ambient
temperature at which the thermal limiting will
become active
Given:
V
USB
= 5.0V
V
BAT
= 3.0V
I
CC
= 500mA
I
OP
= 0.75mA
T
J
= 140°C
θ
JA
= 50°C/W
Using Equation 4, calculate the device power dissi-
pation for the stated condition:
Eq. 6:
The maximum ambient temperature before the
AAT3685 thermal limit protection will shut down
charging can now be calculated using Equation 5:
Eq. 7:
Therefore, under the stated conditions for this
worst case power dissipation example, the
AAT3685 will suspend charging operations when
the ambient operating temperature rises above
89.81°C.
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a
decoupling capacitor between the ADP/USB pin
and ground. An input capacitor in the range of 1
μ
F
to 22
μ
F is recommended. If the source supply is
unregulated, it may be necessary to increase the
capacitance to keep the input voltage above the
under-voltage lockout threshold during device
enable and when battery charging is initiated.
If the AAT3685 input is to be used in a system with
an external power supply source rather than a USB
port V
BUS
, such as a typical AC-to-DC wall adapter,
then a C
IN
capacitor in the range of 10
μ
F should be
used. A larger input capacitor in this application will
minimize switching or power bounce effects when
the power supply is "hot plugged" in. Likewise, a
10
μ
F or greater input capacitor is recommended
for the USB input to help buffer the effects of USB
source power switching noise and input cable
impedance.
Output Capacitor
The AAT3685 only requires a 1
μ
F ceramic capaci-
tor on the BAT pin to maintain circuit stability. This
value should be increased to 10
μ
F or more if the
battery connection is made any distance from the
charger output. If the AAT3685 is to be used in
applications where the battery can be removed
from the charger, such as in the case of desktop
charging cradles, an output capacitor greater than
10
μ
F may be required to prevent the device from
cycling on and off when no battery is present.
Printed Circuit Board Layout
Considerations
For the best results, it is recommended to physi-
cally place the battery pack as close as possible
to the AAT3685 BAT pin. To minimize voltage
drops on the PCB, keep the high current carrying
traces adequately wide. For maximum power dis-
sipation of the AAT3685 3x3mm TDFN package,
the metal substrate should be solder bonded to
the board. It is also recommended to maximize
the substrate contact to the PCB ground plane
layer to further increase local heat dissipation.
Refer to the AAT3685 evaluation board for a good
layout example (see Figures 5 and 6).
T
A
= 140
°
C -
(
50
°
C/W
·
1.00375W)
= 89.81
°
C
P
D
= (5.0V - 3.0V)
(500mA)
+ (5.0V
·
0.75mA)
= 1.00375W
AAT3685
Lithium-Ion/Polymer Battery Charger
3685.2006.10.1.3
19
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