AAT2550_08 Datasheet, PDF (21/34 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

SystemPower^TM
PRODUCT DATASHEET
AAT2550178
Total Power Solution for Portable Applications
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a decou-
pling capacitor between the ADP pin and ground. An
input capacitor in the range of 1μF to 22μF is recom-
mended. If the source supply is unregulated, it may be
necessary to increase the capacitance to keep the input
voltage above the under-voltage lockout threshold during
device enable and when battery charging is initiated.
If the AAT2550 adapter input is to be used in a system
with an external power supply source, such as a typical
AC-to-DC wall adapter, then a C_IN capacitor in the range
of 10μF should be used. A larger input capacitor in this
application will minimize switching or power bounce
effects when the power supply is “hot plugged.”
Output Capacitor
The AAT2550 only requires a 1μF ceramic capacitor on
the BAT pin to maintain circuit stability. This value should
be increased to 10μF or more if the battery connection is
made any distance from the charger output. If the
AAT2550 is to be used in applications where the battery
can be removed from the charger, such as in the case of
desktop charging cradles, an output capacitor greater
than 10μF may be required to prevent the device from
cycling on and off when no battery is present.
Step-Down Converter
Functional Description
The AAT2550 has two step-down converters and both
are designed with the goal of minimizing external com-
ponent size and optimizing efficiency over the complete
load range (600mA). Apart from the small bypass input
capacitor, only a small L-C filter is required at the output.
Typically, a 4.7μH inductor and a 4.7μF ceramic capacitor
are recommended (see Table 5).
Configuration
0.6V Adjustable With
External Feedback
Output Voltage
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
Inductor
2.2μH
4.7μH
6.8μH
Table 5: Inductor Values.
The two step-down converters can be programmed with
external feedback to any voltage, ranging from 0.6V to
the input voltage. An additional feed-forward capacitor
can also be added to the external feedback with a 10μF
output capacitor for improved transient response (see
C10 and C11 in Figure 4).
At dropout, the converter duty cycle increases to 100%
and the output voltage tracks the input voltage minus
the R_DS(ON) drop of the P-channel high-side MOSFET.
The input voltage range is 2.7V to 5.5V. The converter
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, and line regulation.
Soft start eliminates any output voltage overshoot when
the enable or the input voltage is applied.
Control Loop
Both step-down converters are peak current mode control
converters. The current through the P-channel MOSFET
(high side) is sensed for current loop control, as well as
short-circuit and overload protection. A fixed slope com-
pensation signal is added to the sensed current to main-
tain stability for duty cycles greater than 50%. The peak
current mode loop appears as a voltage-programmed cur-
rent source in parallel with the output capacitor.
The output of the voltage error amplifier programs the
current mode loop for the necessary peak switch current
to force a constant output voltage for all load and line
conditions. Internal loop compensation terminates the
transconductance voltage error amplifier output. The
error amplifier reference is fixed at 0.6V.
Soft Start / Enable
Soft start limits the current surge seen at the input and
eliminates output voltage overshoot. When pulled low,
the enable input forces the AAT2550 into a low-power,
non-switching state. The total input current during shut-
down is less than 1μA.
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.
2550.2008.02.1.3
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