AAT2146 Datasheet, PDF (11/19 Pages) Advanced Analogic Technologies – Low Noise, Fast Transient 600mA Step-Down Converter

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PRODUCT DATASHEET
AAT2146
Low Noise, Fast Transient 600mA Step-Down Converter
Applications Information
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 adjustable and
low-voltage fixed versions of the AAT2146 is 0.24A/μs.
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
µs
This is the internal slope compensation for the adjust-
able (0.6V) version or low-voltage fixed versions. When
externally programming the 0.6V version to 2.5V, the
calculated inductance is 7.5μH.
L=
0.75 ⋅
m
V_O
=
0.75 ⋅ V_O
A
≈
3
µs
A
⋅
V_O
0.24A _µs
=
3
µs
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/μs.
Table 1 displays inductor values for the AAT2146 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 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 4.7μH CDRH3D16 series inductor selected from
Sumida has a 105mΩ DCR and a 900mA DC current rat-
ing. At full load, the inductor DC loss is 17mW which gives
a 2.8% loss in efficiency for a 400mA, 1.5V output.
Input Capacitor
Select a 2.2μF to 10μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (V_PP) and
solve for C. The calculated value varies with input volt-
age and is a maximum when V_IN is double the output
voltage.
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
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 coef-
ficient characteristics when selecting the proper value.
For example, the capacitance of a 10μF, 6.3V, X5R
ceramic capacitor with 5.0V DC applied is actually about
6μF.
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
Inductor
2.2μH
4.7μH
6.8μH
4.7μH
Table 1: Inductor Values.
The maximum input capacitor RMS current is:
I_RMS = I_O ·
V_O
V_IN
· ^⎛⎝1 -
V_O ⎞
V_IN ⎠
The input capacitor RMS ripple current varies with the
input and output voltage and will always be less than or
equal to half of the total DC load current.
V_O
V_IN
· ^⎛⎝1 -
V_O ⎞
V_IN ⎠
=
D · (1 - D) =
0.5² = ¹
2
2146.2008.04.1.1
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