AAT2556
Battery Charger and Step-Down
Converter for Portable Applications
low temperature operation applications, a 10µF
(X5R, X7R) ceramic capacitor is recommended to
stabilize extreme pulsed load conditions.
current down slope meets the internal slope com-
pensation requirements. The internal slope com-
pensation for the AAT2556 is 0.45A/µsec. This
equates to a slope compensation that is 75% of the
inductor current down slope for a 1.8V output and
3.0µH inductor.
The output voltage droop due to a load transient is
dominated by the capacitance of the ceramic out-
put capacitor. During a step increase in load cur-
rent, the ceramic output capacitor alone supplies
the load current until the loop responds. Within two
or three switching cycles, the loop responds and
the inductor current increases to match the load
current demand. The relationship of the output volt-
age droop during the three switching cycles to the
output capacitance can be estimated by:
0.75 ⋅ VO 0.75 ⋅ 1.8V
= 0.45
A
µsec
m =
=
L
3.0µH
0.75 ⋅ VO
0.75
⋅
VO
A
µsec
A
L =
=
≈
1.67
⋅ VO
m
0.45A
µsec
3
·
VDROOP FS
ΔILOAD
COUT
=
For most designs, the step-down converter operates
with an inductor value of 1µH to 4.7µH. Table 3 dis-
plays inductor values for the AAT2556 with different
output voltage options.
·
Once the average inductor current increases to the
DC load level, the output voltage recovers. The
above equation establishes a limit on the minimum
value for the output capacitor with respect to load
transients.
Manufacturer's specifications list both the inductor
DC current rating, which is a thermal limitation, and
the peak current rating, which is determined by the
saturation characteristics. The inductor should not
show any appreciable saturation under normal load
conditions. Some inductors may meet the peak and
average current ratings yet result in excessive loss-
es due to a high DCR. Always consider the losses
associated with the DCR and its effect on the total
converter efficiency when selecting an inductor.
The internal voltage loop compensation also limits
the minimum output capacitor value to 4.7µF. This
is due to its effect on the loop crossover frequency
(bandwidth), phase margin, and gain margin.
Increased output capacitance will reduce the
crossover frequency with greater phase margin.
The maximum output capacitor RMS ripple current
is given by:
The 3.0µH CDRH2D09 series inductor selected
from Sumida has a 150mΩ DCR and a 470mA DC
current rating. At full load, the inductor DC loss is
9.375mW which gives a 2.08% loss in efficiency for
a 250mA, 1.8V output.
1
VOUT · (VIN(MAX) - VOUT)
IRMS(MAX)
=
·
L · FS · VIN(MAX)
2 · 3
Output Voltage (V)
L1 (µH)
Dissipation due to the RMS current in the ceram-
ic output capacitor ESR is typically minimal,
resulting in less than a few degrees rise in hot-
spot temperature.
1.0
1.2
1.5
1.8
2.5
3.0
3.3
1.5
2.2
2.7
3.0/3.3
3.9/4.2
4.7
Inductor Selection
The step-down converter uses peak current mode
control with slope compensation to maintain stabil-
ity for duty cycles greater than 50%. The output
inductor value must be selected so the inductor
5.6
Table 3: Inductor Values.
20
2556.2006.05.1.0
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