AAT3688
USB Port Lithium-Ion/Polymer Battery Charger
Where:
Capacitor Selection
Input Capacitor
TA = Ambient Temperature in Degrees C
In general, it is good design practice to place a
decoupling capacitor between the VUSB pin 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.
TJ = Maximum Device Junction Temperature
Protected by the Thermal Limit Control
PD = Total Power Dissipation by the Device
θJA = Package Thermal Resistance in °C/W
Example:
If the AAT3688 USB input is to be used in a system
with an external power supply source rather than a
USB port VBUS, such as a typical AC-to-DC wall
adapter, then a CIN capacitor 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 switch-
ing noise and input cable impedance.
For an application where the fast charge current is
set to 500mA, VUSB = 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:
VADP = 5.0V
VBAT = 3.0V
ICC = 500mA
IOP = 0.75mA
Output Capacitor
The AAT3688 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 AAT3688 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
10µF may be required to prevent the device from
cycling on and off when no battery is present.
TJ
= 140°C
θJA = 50°C/W
Using Equation 4, calculate the device power dissi-
pation for the stated condition:
Eq. 6:
PD = (5.0V - 3.0V)(500mA) + (5.0V
= 1.00375W
· 0.75mA)
Printed Circuit Board Layout
Considerations
The maximum ambient temperature before the
AAT3688 thermal limit protection will shut down
charging can now be calculated using Equation 5:
For the best results, it is recommended to physi-
cally place the battery pack as close as possible
to the AAT3688 BAT pin. To minimize voltage
drops on the PCB, keep the high current carrying
traces adequately wide. For maximum power dis-
sipation of the AAT3688 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 AAT3688 evaluation board for a good
layout example (see Figures 5 and 6).
Eq. 7:
TA = 140°C -
= 89.81°C
(
50°C/W
· 1.00375W)
Therefore, under the stated conditions for this
worst case power dissipation example, the
AAT3688 will suspend charging operations when
the ambient operating temperature rises above
89.81°C.
3688.2006.10.1.5
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AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
