AAT2554 Datasheet, PDF (25/33 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

AAT2554
Total Power Solution for Portable Applications
For most designs, the step-down converter operates
with inductor values from 1µH to 4.7µH. Table 3 dis-
plays inductor values for the AAT2554 for various
output voltages.
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 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.
Output Voltage (V)
1.0
1.2
1.5
1.8
2.5
3.0
3.3
L1 (µH)
1.5
2.2
2.7
3.0/3.3
3.9/4.2
4.7
5.6
Table 3: Step-Down Converter
Inductor Values.
Adjustable Output Resistor Selection
Resistors R2 and R3 of Figure 5 program the out-
put to regulate at a voltage higher than 0.6V. To
limit the bias current required for the external feed-
back resistor string while maintaining good noise
immunity, the suggested value for R3 is 59kΩ.
Decreased resistor values are necessary to main-
tain noise immunity on the FB pin, resulting in
increased quiescent current. Table 4 summarizes
the resistor values for various output voltages.
R2
=
⎛ ^VOUT
⎝ V_REF
-1^⎞⎠
·
R3
=
⎛ ^3.3V
⎝ 0.6V
-
1 ^⎞⎠
·
59kΩ
=
267kΩ
With enhanced transient response for extreme
pulsed load application, an external feed-forward
capacitor (C8 in Figure 5) can be added.
V_OUT (V)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
R3 = 59kΩ
R2 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R3 = 221kΩ
R2 (kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 4: Adjustable Resistor Values For
Step-Down Converter.
2554.2007.01.1.2
25