AAT2550
Total Power Solution for Portable Applications
goes into a sleep state. The charger will remain in a
sleep state until the battery voltage decreases to a
Battery Preconditioning
Before the start of charging, the charger checks sev-
eral conditions in order to assure a safe charging
environment. The input supply must be above the
minimum operating voltage, or under-voltage lock-
out threshold (VUVLO), for the charging sequence to
begin. Also, the battery temperature, as reported by
a thermistor connected to the TS pin from the bat-
tery, must be within the proper window for safe
charging. When these conditions have been met
and a battery is connected to the BAT pin, the charg-
er checks the state of the battery. If the battery volt-
age is below the preconditioning voltage threshold
(VMIN), then the charge control begins precondition-
ing the battery. The preconditioning trickle charge
current is equal to the fast charge constant current
divided by 10. For example, if the programmed fast
charge current is 1A, then the preconditioning mode
(trickle charge) current will be 100mA. Battery pre-
conditioning is a safety precaution for deeply dis-
charged batteries and also helps to limit power dis-
sipation in the pass transistor when the voltage
across the device is at the greatest potential.
level below the battery recharge voltage threshold
(VRCH). When the input supply is disconnected, the
charger will automatically transition into a power-
saving sleep mode. Consuming only an ultra-low
0.3µA in sleep mode, the charger minimizes battery
drain when it is not charging. This feature is particu-
larly useful in applications where the input supply
level may fall below the battery charge or under-volt-
age lockout level. In such cases where the input volt-
age drops, the device will enter sleep mode and
resume charging automatically once the input sup-
ply has recovered from the fault condition.
Step-Down Converters
The AAT2550 offers two high-performance,
600mA, 1.4MHz step-down converters. Both con-
verters minimize external component size and opti-
mize efficiency over the entire load range. The
fixed output version requires only three external
power components (CIN, COUT, and L) for each con-
verter. The adjustable version is programmed with
external feedback resistors to any voltage ranging
from 0.6V to the input voltage. At dropout, the con-
verter duty cycle increases to 100% and the output
voltage tracks the input voltage minus the RDS(ON)
drop of the P-channel MOSFET.
Fast Charge/Constant Current Charging
Battery preconditioning continues until the voltage on
the BAT pin exceeds the preconditioning voltage
threshold (VMIN). At this point, the charger begins the
constant current fast charging phase. The fast charge
constant current (ICH) amplitude is programmed by
the user via the RSET resistor. The charger remains in
the constant current charge mode until the battery
Input voltage range is 2.7V to 5.5V and each convert-
er's efficiency has been optimized for all load condi-
tions, ranging from no load to 600mA. The internal
error amplifier and compensation provides excellent
transient response, load regulation, and line regula-
tion. Soft start eliminates output voltage overshoot
when the enable or the input voltage is applied.
reaches the voltage regulation threshold, VBAT_EOC
.
Constant Voltage Charging
The system transitions to a constant voltage charging
mode when the battery voltage reaches the output
charge regulation threshold (VBAT_EOC) during the con-
stant current fast charge phase. The regulation voltage
level is factory programmed to 4.2V (±1%). The charge
current in the constant voltage mode drops as the bat-
tery under charge reaches its maximum capacity.
Soft Start / Enable
The internal soft start limits the inrush current dur-
ing start-up. This prevents possible sagging of the
input voltage and eliminates output voltage over-
shoot. Typical start-up time for a 4.7µF output
capacitor and load current of 600mA is 100µs.
The AAT2550 offers independent enable pins for
each converter. When connected to logic low, the
enable input forces the respective step-down con-
verter into a low-power, non-switching, shutdown
state. The total input current during shutdown is
less than 1µA for each channel.
End of Charge Cycle Termination and
Recharge Sequence
When the charge current drops to 7.5% of the pro-
grammed fast charge current level in the constant
voltage mode, the device terminates charging and
2550.2006.07.1.0
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