AAT3688
USB Port Lithium-Ion/Polymer Battery Charger
The LED anodes should be connected to VUSB
.
regarding the state of the charging system. The
DATA pin is a bi-directional port which will read
back a series of data pulses when the system
microcontroller asserts a request pulse. This sin-
gle strobe request protocol will invoke one of 14
possible return pulse counts in which the micro-
controller can look up based on the serial report
shown in Table 3.
The LEDs should be biased with as little current as
necessary to create reasonable illumination; there-
fore, a ballast resistor should be placed between
each of the LED cathodes and the STAT1/2 pins.
LED current consumption will add to the over-ther-
mal power budget for the device package, hence it
is recommended to keep the LED drive current to a
minimum. 2mA should be sufficient to drive most
low-cost green, red, or multi-color LEDs. It is not
recommended to exceed 8mA for driving an indi-
vidual status LED.
The DATA pin function is active low and should nor-
mally be pulled high to VUSB. This data line may
also be pulled high to the same level as the high
state for the logic I/O port on the system microcon-
troller. In order for the DATA pin control circuit to
generate clean sharp edges for the data output and
to maintain the integrity of the data timing for the
system, the pull-up resistor on the data line should
be low enough in value so that the DATA signal
returns to the high state without delay. If the value
of the pull-up resistor used is too high, the strobe
pulse from the system microcontroller may exceed
the maximum pulse time and the DATA output con-
trol could issue false status reports. A 1.5kΩ resis-
tor is recommended when pulling the DATA pin
high to 5.0V at the VUSB input. If the data line is
pulled high to a voltage level less than 5.0V, the
pull-up resistor may be calculated based on a rec-
ommended minimum pull-up current of 3mA. Use
the following formula:
The required ballast resistor value can be estimat-
ed using the following formulas:
(VUSB
-
ILED(STAT1/2)
VF(LED)
)
RB(STAT1/2)
=
Eq. 2:
Example:
(5.0V - 2.0V)
2mA
RB(STAT1)
=
= 1.5kΩ
Note: Red LED forward voltage (VF) is typically
2.0V @ 2mA.
Table 2 shows the four status LED display conditions.
VPULL-UP
3mA
RPULL-UP
≤
Eq. 3:
Digital Charge Status Reporting
The AAT3688 has a comprehensive digital data
reporting system by use of the DATA pin feature.
This function can provide detailed information
16
3688.2006.10.1.5
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
