AAT2550_08 Datasheet, PDF (15/34 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

SystemPower^TM
PRODUCT DATASHEET
AAT2550178
Total Power Solution for Portable Applications
Fast Charge/Constant Current Charging
Battery preconditioning continues until the voltage on
the BAT pin exceeds the preconditioning voltage thresh-
old (V_MIN). At this point, the charger begins the constant
current fast charging phase. The fast charge constant
current (I_CH) amplitude is programmed by the user via
the R_SET resistor. The charger remains in the constant
current charge mode until the battery reaches the volt-
age regulation threshold, V_{BAT_EOC}.
Constant Voltage Charging
The system transitions to a constant voltage charging
mode when the battery voltage reaches the output
charge regulation threshold (V_{BAT_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.
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 volt-
age mode, the device terminates charging and goes into
a sleep state. The charger will remain in a sleep state
until the battery voltage decreases to a level below the
battery recharge voltage threshold (V_RCH). When the
input supply is disconnected, the charger will automati-
cally 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 particularly useful in applications where
the input supply level may fall below the battery charge
or under-voltage lockout level. In such cases where the
input voltage 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 converters minimize
external component size and optimize efficiency over the
entire load range. Both converters can be 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 volt-
age tracks the input voltage minus the R_DS(ON) drop of the
P-channel MOSFET.
Input voltage range is 2.7V to 5.5V and each converter’s
efficiency has been optimized for all load conditions,
ranging from no load to 600mA. The internal error
amplifier and compensation provides excellent transient
response, load regulation, and line regulation. Soft start
eliminates output voltage overshoot when the enable or
the input voltage is applied.
Soft Start / Enable
The internal soft start limits the inrush current during
start-up. This prevents possible sagging of the input
voltage and eliminates output voltage overshoot. Typical
start-up time for a 4.7μF output capacitor and load cur-
rent 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 converter into a
low-power, non-switching, shutdown state. The total
input current during shutdown is less than 1μA for each
channel.
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current is limit-
ed. To minimize power dissipation and stresses under
current limit and short-circuit conditions, switching is
terminated after entering current limit for a series of
pulses. Switching is terminated for seven consecutive
clock cycles after a current limit has been sensed for a
series of four consecutive clock cycles.
Thermal protection completely disables switching when
internal dissipation becomes excessive. The junction
over-temperature threshold is 140°C with 15°C of hys-
teresis. Once an over-temperature or over-current fault
conditions is removed, the output voltage automatically
recovers.
Under-Voltage Lockout
The under-voltage lockout circuit prevents the device from
improper operation at low input voltages. Internal bias of
all circuits is controlled via the VIN input. Under-voltage
lockout (UVLO) guarantees sufficient V_IN bias and proper
operation of all internal circuitry prior to activation.
2550.2008.02.1.3
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