AAT2550
Total Power Solution for Portable Applications
used NTC thermistors in battery packs are approx-
imately 10kΩ at room temperature (25°C).
be accomplished by using the STAT1 pin and a sin-
gle LED. Using two LEDs and both STAT pins simply
gives the user more information to the charging
states. Refer to Table 3 for LED display definitions.
The TS pin has been specifically designed to
source 80µA of current to the thermistor. The volt-
age on the TS pin that results from the resistive
load should stay within a window from 330mV to
2.3V. If the battery becomes too hot during charg-
ing due to an internal fault, the thermistor will heat
up and reduce in value, pulling the TS pin voltage
lower than the TS1 threshold, and the AAT2550 will
signal the fault condition.
The LED anodes should be connected to ADP. The
LEDs should be biased with as little current as nec-
essary to create reasonable illumination; therefore,
a ballast resistor should be placed between the
LED cathodes and the STAT1/2 pins. LED current
consumption will add to the overall thermal power
budget for the device package, so it is wise to keep
the LED drive current to a minimum. 2mA should
be sufficient to drive most low-cost green or red
LEDs. It is not recommended to exceed 8mA for
driving an individual status LED.
If the use of the TS pin function is not required by
the system, it should be terminated to ground with
a 10kΩ resistor.
The required ballast resistor value can be estimat-
ed using the following formulas:
Battery Charge Status Indication
The AAT2550 indicates the status of the battery
under charge with two different systems. First, the
device has two status LED driver outputs. These
two LEDs can indicate simple functions such as no
battery charge activity, battery charging, charge
complete, and charge fault. The AAT2550 also pro-
vides a bi-directional data reporting function so that
a system microcontroller can interrogate the DATA
pin and read any one of 13 system states.
For connection to the adapter supply:
VADP - VF(LED)
ILED(STAT1/2)
RB(STAT1/2)
=
Example:
RB(STAT1)
5.5V - 2.0V
2mA
=
= 1.75kΩ
Status Indicator Display
Simple system charging status states can be dis-
played using one or two LEDs in conjunction with the
STAT1 and STAT2 pins on the AAT2550. These two
pins are simple switches to connect the LED cath-
odes to ground. It is not necessary to use both dis-
play LEDs if a user simply wants to have a single
lamp to show "charging" or "not charging." This can
Note: Red LED forward voltage (VF) is typically
2.0V @ 2mA. Green LED forward voltage (VF) is
typically 3.2V @ 2mA.
The four status LED display conditions are
described in Table 3.
Event Description
STAT1
STAT2
Charge Disabled or Low Supply
Charge Enabled Without Battery
Battery Charging
Off
Flash1
On
Off
Flash1
Off
Charge Completed
Off
On
Fault
On
On
Table 3: Status LED Display Conditions.
1. Flashing rate depends on output capacitance.
2550.2006.07.1.0
19
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
