AAT2550 Datasheet, PDF (23/35 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

AAT2550
Total Power Solution for Portable Applications
Configuration
0.6V Adjustable With
External Feedback
Fixed Output
Output Voltage
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
0.6V to 3.3V
Table 5: Inductor Values.
Inductor
2.2µH
4.7µH
6.8µH
4.7µH
Step-Down Converter
Applications Information
Inductor Selection
The step-down converter uses peak current mode
control with slope compensation to maintain stability
for duty cycles greater than 50%. The output induc-
tor value must be selected so the inductor current
down slope meets the internal slope compensation
requirements. The internal slope compensation for
the adjustable and low-voltage fixed versions of the
AAT2550 is 0.24A/µsec. This equates to a slope
compensation that is 75% of the inductor current
down slope for a 1.5V output and 4.7µH inductor.
m
=
0.75 ⋅
L
V_O
=
0.75 ⋅ 1.5V
4.7μH
=
0.24
A
μsec
This is the internal slope compensation for the
adjustable (0.6V) version or low-voltage fixed ver-
sions. When externally programming the 0.6V ver-
sion to 2.5V, the calculated inductance is 7.5µH.
L=
0.75 ⋅ V_O
m
=
0.75 ⋅ V_O
A
≈
3
μsec
A
⋅
V_O
0.24A _μsec
μsec
=3 _A
⋅ 2.5V = 7.5μH
In this case, a standard 6.8µH value is selected.
For high-voltage fixed versions (≥2.5V), m = 0.48A/
µsec. Table 5 displays inductor values for the
AAT2550 fixed and adjustable 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
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 Sumida 4.7µH CDRH3D16 series inductor has
a 105mΩ DCR and a 900mA DC current rating. At
full load, the inductor DC loss is 38mW, which gives
a 4% loss in efficiency for a 600mA, 1.5V output.
Input Capacitor
Select a 4.7µF to 10µF X7R or X5R ceramic capac-
itor for the input. To estimate the required input
capacitor size, determine the acceptable input rip-
ple level (V_PP) and solve for C. The calculated
value varies with input voltage and is a maximum
when V_IN is double the output voltage.
V_{O ·} ⎛_{1 -} V_O ⎞
V_IN ⎝ V_IN ⎠
C_IN =
⎛ V_PP
⎝ I_O
- ESR^⎞⎠ · F_S
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
=
1
4
for
V_IN
=
2
·
V_O
1
C_IN(MIN) = _{⎛ VPP}
⎝ I_O
- ESR^⎞⎠ · 4 · F_S
Always examine the ceramic capacitor DC voltage
coefficient characteristics when selecting the prop-
er value. For example, the capacitance of a 10µF,
6.3V, X5R ceramic capacitor with 5.0V DC applied
is actually about 6µF.
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