AAT3686
USB Port/AC Adapter
Lithium-Ion/Polymer Battery Charger
Programming Charge Current
Protection Circuitry
The fast charge constant current charge level for
both adapter and USB input modes are pro-
grammed with set resistors placed between the
ADPSET, USBH, and USBL pins and ground. The
accuracy of the fast charge, as well as the precon-
ditioning trickle charge current, is dominated by the
tolerance of the set resistor used. For this reason,
1% tolerance metal film resistors are recommend-
ed for the set resistor function.
Programmable Watchdog Timer
The AAT3686 contains a watchdog timing circuit for
the adapter input charging mode. No watchdog tim-
ing functions are active for the USB input mode.
Typically, a 0.1µF ceramic capacitor is connected
between the TC pin and ground. When a 0.1µF
ceramic capacitor is used, the device will time a
shutdown condition if the trickle charge mode
exceeds 25 minutes and a combined trickle charge
plus fast charge mode of three hours. When the
device transitions to the constant voltage mode, the
timing counter is reset and will time out after three
hours and shut down the charger (see Table 2).
Fast charge constant current levels from 50mA to
1.5A can be set by selecting the appropriate resistor
value from Table 1. The RSET resistor should be con-
nected between the ADPSET pin and ground.
The USB input fast charge constant current charge
control provides for two current set levels, USBH
and USBL. The USBSEL pin is used to select the
high or low charge current levels in the USB charge
mode. When the USBSEL pin is pulled to a voltage
level above the VUSBSEL(H) threshold, the USBH cur-
rent level will be selected. Conversely, this pin
should be pulled below VUSBSEL(L) to enable the
USBL charge level. Typically, the two RSETH and
RSETL resistors for the USBH and USBL functions
are fixed for 500mA and 100mA USB fast charge
levels. However, these two charge levels can be set
to any level between 50mA and 500mA depending
upon the system design requirements for a given
USB charge application. Refer to Table 1 and
Figure 3 for recommended RSETH and RSETL values.
Mode
Time
Trickle Charge (TC) Time Out
Trickle Charge (TC) +
Fast Charge (CC) Time Out
Constant Voltage (VC) Mode
Time Out
25 minutes
3 hours
3 hours
Table 2: Summary for a 0.1µF Used for the
Timing Capacitor.
The CT pin is driven by a constant current source
and will provide a linear response to increases in
the timing capacitor value. Thus, if the timing
capacitor were to be doubled from the nominal
0.1µF value, the time-out times would be doubled.
If the programmable watchdog timer function is not
needed, it can be disabled by connecting the CT
pin to ground. The CT pin should not be left float-
ing or un-terminated, as this will cause errors in the
internal timing control circuit.
10000
1000
The constant current provided to charge the timing
capacitor is very small, and this pin is susceptible
to noise and changes in capacitance value.
Therefore, the timing capacitor should be physical-
ly located on the printed circuit board layout as
closely as possible to the CT pin. Since the accu-
racy of the internal timer is dominated by the
capacitance value, 10% tolerance or better ceram-
ic capacitors are recommended. Ceramic capacitor
materials, such as X7R and X5R type, are a good
choice for this application.
ADP
USBL
100
USBH
10
1
10
100
RSET (kΩ)
Figure 3: IFASTCHARGE vs. RSET
.
3686.2006.10.1.9
15
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
