AAT2556 Datasheet, PDF (19/27 Pages) Advanced Analogic Technologies – Battery Charger and Step-Down Converter for Portable Applications

DATA SHEET
AAT2556
Battery Charger and Step-Down Converter for Portable Applications
Inductor Selection
The step-down converter uses peak current mode con-
trol with slope compensation to maintain stability for
duty cycles greater than 50%. The output inductor value
must be selected so the inductor current down slope
meets the internal slope compensation requirements.
The internal slope compensation 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.
m=
0.75 ⋅
L
V_O
=
0.75 ⋅ 1.8V
3.0µH
=
0.45
A
µsec
L=
0.75 ⋅ V_O
m
=
0.75 ⋅ V_O
A
≈
1.67
µsec
A
⋅
V_O
0.45A _µsec
µsec
= 1.67 _A
⋅ 3.0V =
5.0µH
For most designs, the step-down converter operates
with an inductor value of 1μH to 4.7μH. Table 3 displays
inductor values for the AAT2556 with different output
voltage options.
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 satura-
tion 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 losses 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 150m DCR and a 470mA DC current rat-
ing. At full load, the inductor DC loss is 9.375mW which
gives a 2.08% loss in efficiency for a 250mA, 1.8V out-
put.
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: Inductor Values.
Adjustable Output Resistor Selection
Resistors R3 and R4 of Figure 5 program the output to
regulate at a voltage higher than 0.6V. To limit the bias
current required for the external feedback resistor string
while maintaining good noise immunity, the suggested
value for R4 is 59k. Decreased resistor values are nec-
essary to maintain noise immunity on the FB pin, result-
ing in increased quiescent current. Table 4 summarizes
the resistor values for various output voltages.
R3
=
⎛ ^VOUT
⎝ V_REF
-1^⎞⎠
·
R4
=
⎛ ^3.3V
⎝ 0.6V
-
1 ^⎞⎠
·
59kΩ
=
267kΩ
With enhanced transient response for extreme pulsed
load application, an external feed-forward capacitor (C5
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
R4 = 59k
R3 (k)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R4 = 221k
R3 (k)
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 4: Adjustable Resistor Values For
Step-Down Converter.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
202177B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 19, 2013
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