AAT3670 View Datasheet(PDF) - Analog Technology Inc
Part Name
Description
MFG CO.
'AAT3670' PDF : 32 Pages View PDF
Example:
For an application where the fast charge current for
the adapter mode is set to 1A, VADP = 5.0V, and the
worst-case battery voltage at 3.0V with the system
load disabled, what is the maximum ambient temper-
ature where the thermal limiting will become active?
Given:
VADP = 5.0V
VBAT = 3.0V
ICC = 1A
IOP = 0.75mA
TJ = 110°C
θJA = 37°C/W
IOUT = 0
RDS(ON) = 0.4Ω
Using Equation 7, calculate the device power dissi-
pation for the stated condition:
Eq. 9: PD = (5.0V - 3.0V)(1A) + (5.0V · 0.75mA) + (02 · 0.4Ω)
= 2.00375W
The maximum ambient temperature before the
AAT3670 thermal loop becomes active can now be
calculated using Equation 8:
Eq. 10: TA = 110°C - (37°C/W · 2.00375W)
= 35.86°C
Therefore, under the stated conditions for this
worst-case power dissipation example, the
AAT3670 will enter the thermal loop and lower the
fast charge constant current when the ambient
operating temperature rises above 35.86°C.
AAT3670
1.6A Dynamic Battery Charger
and Power Manager
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a
decoupling capacitor between the ADP and USB
pins and ground. An input capacitor in the range of
1µF to 22µF 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 AAT3670 adapter input is to be used in a sys-
tem with an external power supply source, such as
a typical AC-to-DC wall adapter, then a CIN capac-
itor in the range of 10µF 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 10µF 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 AAT3670 only requires a 1µF ceramic capaci-
tor on the BAT pin to maintain circuit stability. This
value should be increased to 10µF or more if the
battery connection is made any distance from the
charger output. If the AAT3670 is to be used in
applications where the battery can be removed
from the charger, such as with the case with desk-
top charging cradles, an output capacitor greater
than 10µF, but less than 20µF, may be required to
retard 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 physically
place the battery pack as close as possible to the
AAT3670 BAT pin as possible. To minimize voltage
drops on the PCB, keep the high current carrying
traces adequately wide. For maximum power dissi-
pation of the AAT3670 QFN package, the metal sub-
strate should be solder bonded to the board. It is
also recommended to maximize the substrate con-
tact to the PCB ground plane layer to further increase
local heat dissipation. Refer to the AAT3670 evalua-
tion board for a good layout example.
30
3670.2007.02.1.1
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