AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
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.
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 AAT1147 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.
The 4.7µH CDRH3D16 series inductor selected
from Sumida has a 105mΩ DCR and a 900mA DC
current rating. 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 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 (VPP) and solve for C. The calculated
value varies with input voltage and is a maximum
when VIN is double the output voltage.
0.75 ⋅ VO 0.75 ⋅ 1.5V
= 0.24
A
m =
=
L
4.7μH
μsec
This is the internal slope compensation. When
externally programming the 0.6V version to 2.5V,
the calculated inductance is 7.5µH.
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
-
CIN =
0.75 ⋅ VO
0.75
⋅
VO
A
μsec
A
⎛
⎝
VPP
IO
⎞
- ESR
·
FS
L =
=
≈
3
⋅ VO
m
⎠
0.24A
μsec
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
1
4
-
=
for VIN = 2 · VO
μsec
A
= 3
⋅ 2.5V = 7.5μH
1
In this case, a standard 6.8µH value is selected.
Table 1 displays inductor values for the AAT1147.
CIN(MIN)
=
⎛
⎝
VPP
IO
⎞
⎠
- ESR
·
4
·
FS
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
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.
Configuration
Output Voltage
Inductor
2.2µH
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
0.6V Adjustable With
External Feedback
4.7µH
6.8µH
Table 1: Inductor Values.
10
1147.2006.05.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
