AAT2550_08 Datasheet, PDF (18/34 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

SystemPower^TM
PRODUCT DATASHEET
AAT2550178
Total Power Solution for Portable Applications
10000
1000
100
ADP
10
1
10
100
R_SET (kΩ)
Figure 2: Constant Charging Current vs. R_SET.
Protection Circuitry
Programmable Watchdog Timer
The AAT2550 contains a watchdog timing circuit for the
adapter input charging mode. Typically, a 0.1μF ceramic
capacitor is connected between the CT 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 tran-
sitions 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).
Mode
Trickle Charge (TC) Time Out
Trickle Charge (TC) + Constant Current (CC)
Mode Time Out
Constant Voltage (VC) Mode Time Out
Time
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
durations would be doubled.
If the programmable watchdog timer function is not need-
ed, it can be disabled by connecting the CT pin to ground.
The CT pin should not be left floating or un-terminated, as
this will cause errors in the internal timing control circuit.
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 physically located on the
printed circuit board layout as closely as possible to the
CT pin. Since the accuracy of the internal timer is domi-
nated by the capacitance value, 10% tolerance or better
ceramic capacitors are recommended. Ceramic capacitor
materials, such as X7R and X5R type, are a good choice
for this application.
Over-Voltage Protection
An over-voltage event is defined as a condition where
the voltage on the BAT pin exceeds the maximum bat-
tery charge voltage and is set by the over-voltage pro-
tection threshold (V_OVP). If an over-voltage condition
occurs, the AAT2550 charge control will shut down the
device until voltage on the BAT pin drops below the over-
voltage protection threshold (V_OVP). The AAT2550 will
resume normal charging operation after the over-voltage
condition is removed. During an over-voltage event, the
STAT LEDs will report a system fault, and the actual fault
condition may be read via the DATA pin signal.
Over-Temperature Shutdown
The AAT2550 has a thermal protection control circuit
which will shut down charging functions should the inter-
nal die temperature exceed the preset thermal limit
threshold.
Battery Temperature Fault Monitoring
In the event of a battery over-temperature condition,
the charge control will turn off the internal pass device
and report a battery temperature fault on the DATA pin
function. The STAT LEDs will also display a system fault.
After the system recovers from a temperature fault, the
device will resume charging operation.
The AAT2550 checks battery temperature before start-
ing the charge cycle, as well as during all stages of
charging. This is accomplished by monitoring the voltage
at the TS pin. This system is intended to use negative
temperature coefficient thermistors (NTC), which are
typically integrated into the battery package. Most of the
commonly used NTC thermistors in battery packs are
approximately 10kΩ at room temperature (25°C).
The TS pin has been specifically designed to source 80μA
of current to the thermistor. The voltage 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 charging due to an internal fault, the thermis-
tor 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.
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2550.2008.02.1.3