AAT3680
Lithium-Ion/Polymer
Linear Battery Charge Controller
RT1 and RT2 for use with PTC Thermistor
5 ⋅ RTH ⋅ RTC
impedance load and assure device stability over all
operating conditions.
RT1
=
Operation Under No-Load Conditions
3 ⋅ (RTC - RTH)
Under no-load conditions, that is when the
AAT3680 is powered with no battery connected
between the BAT pin and VSS, the output capacitor
is charged up very quickly by the trickle charge
control circuit to the BAT pin until the output reach-
es the recharge threshold (VRCH). At this point, the
AAT3680 will drop into sleep mode. The output
capacitor will discharge slowly by the capacitor's
own internal leakage until the voltage seen at the
BAT pin drops below the VRCH threshold. This
100mV cycle will continue at approximately 3Hz
with a 0.1µF capacitor connected. A larger capaci-
tor value will produce a slower voltage cycle. This
operation mode can be observed by viewing the
STAT LED blinking on and off at the rate estab-
lished by the COUT value.
5 ⋅ RTH ⋅ RTC
(2 ⋅ RTH) - (7 ⋅ RTC)
RT2
=
Where RTC is the thermistor's cold temperature
resistance and RTH is the thermistor's hot tempera-
ture resistance. See thermistor specifications for
information. To ensure there is no dependence on
the input supply changes, connect the divider
between VP and VSS. Disabling the temperature-
monitoring function is achieved by applying a volt-
age between VTS1 and VTS2 on the TS pin.
Capacitor Selection
Input Capacitor
For desktop charger applications, where it might
not be desirable to have a "charger ready" blinking
LED, a large COUT capacitor in the range of 100µF
or more would prevent the operation of this mode.
In general, it is good design practice to place a
decoupling capacitor between the VP and VSS pins.
An input capacitor in the range of 1µF to 10µF is rec-
ommended. If the source supply is unregulated, it
may be necessary to increase the capacitance to
keep the input voltage above the under-voltage lock-
out threshold.
Reverse Current Blocking Diode
Bipolar Circuit Application
If the AAT3680 is to be used in a system with an
external power supply source, 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.
When using the AAT3680 with a PNP transistor, a
reverse blocking diode is not required because
there is no current path from BAT to VP. However,
it is advisable to still place a blocking diode
between the bipolar transistor collector and the
BAT pin connection to the circuit output. In the
event where the input supply is interrupted or
removed during the constant current or constant
voltage phases of the charging cycle, the battery
under charge will discharge through the circuit
pass transistor, rendering it impossible to turn off.
If the circuit is unable to turn off, the reverse leak-
age will eventually discharge the battery. A block-
ing diode will prevent this undesirable effect.
Output Capacitor
The AAT3680 does not need an output capacitor
for stability of the device itself. However, a capaci-
tor connected between BAT and VSS will control the
output voltage when the AAT3680 is powered up
when no battery is connected. The AAT3680 can
become unstable if a high impedance load is
placed across the BAT pin to VSS. Such a case is
possible with aging lithium-ion/polymer battery
cells. As cells age through repeated charge and
discharge cycles, the internal impedance can rise
over time. A 10µF or larger output capacitor will
compensate for the adverse effects of a high-
MOSFET Circuit Application
A reverse blocking diode is generally required for
the circuit shown in Figure 6. For this application,
the blocking diode gives the system protection
from a shorted input, when the AAT3680 is used
3680.2006.03.1.6
13
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