PRODUCT DATASHEET
AAT1146
SwitchRegTM
FastTransient 400mA Step-Down Converter
Input Capacitor
Applications Information
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (VPP) and
solve for C. The calculated value varies with input volt-
age and is a maximum when VIN is double the output
voltage.
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 AAT1146 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.
VO
VIN
⎛
VO ⎞
VIN ⎠
· 1 -
⎝
CIN =
⎛ VPP
⎝ IO
⎞
- ESR ·FS
⎠
VO
VIN
⎛
VO ⎞
VIN ⎠
1
· 1 -
=
for VIN = 2 × VO
⎝
4
0.75 ⋅ VO 0.75 ⋅ 1.5V
= 0.24
A
m =
=
L
4.7μH
μsec
1
CIN(MIN)
=
⎛ VPP
⎝ IO
⎞
⎠
- ESR · 4 · FS
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.
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.
0.75 ⋅ VO
0.75
⋅
VO
A
μsec
A
L =
=
≈
3
⋅ VO
m
0.24A
μsec
μsec
Configuration
Output Voltage
Inductor
= 3
⋅ 2.5V = 7.5μH
A
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
0.6V to 3.3V
2.2μH
4.7μH
6.8μH
4.7μH
0.6V Adjustable With
External Feedback
In this case, a standard 6.8μH value is selected.
Fixed Output
For high-voltage fixed versions (≥2.5V), m = 0.48A/
μsec. Table 1 displays inductor values for the AAT1146
fixed and adjustable options.
Table 1: Inductor Values.
The maximum input capacitor RMS current is:
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.
VO
VIN
⎛
VO ⎞
VIN ⎠
IRMS = IO ·
· 1 -
⎝
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.
VO
VIN
⎛
VO ⎞
VIN ⎠
1
2
D · (1 - D) = 0.52 =
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.
· 1 -
⎝
=
w w w . a n a l o g i c t e c h . c o m
1146.2008.06.1.6
11
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