AAT2550
Total Power Solution for Portable Applications
ing or un-terminated, as this will cause errors in the
internal timing control circuit.
10000
1000
100
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-
ADP
10
1
10
100
racy of the internal timer is dominated by the
RSET (kΩ)
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.
Figure 2: IFASTCHARGE vs. RSET
.
Over-Voltage Protection
An over-voltage event is defined as a condition
where the voltage on the BAT pin exceeds the max-
imum battery charge voltage and is set by the over-
voltage protection threshold (VOVP). If an over-volt-
age condition occurs, the AAT2550 charge control
will shut down the device until voltage on the BAT
pin drops below the over-voltage protection thresh-
old (VOVP). The AAT2550 will resume normal charg-
ing 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.
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 transitions to the con-
stant voltage mode, the timing counter is reset and
will time out after three hours and shut down the
charger (see Table 2).
Over-Temperature Shutdown
The AAT2550 has a thermal protection control cir-
cuit which will shut down charging functions should
the internal die temperature exceed the preset
thermal limit threshold.
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
Battery Temperature Fault Monitoring
In the event of a battery over-temperature condi-
tion, 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.
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 capac-
itor were to be doubled from the nominal 0.1µF
value, the time-out durations would be doubled.
The AAT2550 checks battery temperature before
starting the charge cycle, as well as during all
stages of charging. This is accomplished by moni-
toring 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
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-
18
2550.2006.07.1.0
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