AAT1123
1MHz Step-Down Converter
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.
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
AAT1123 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
µsec
m =
=
L
4.7µH
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.
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
-
CIN =
⎛
⎝
VPP
IO
⎞
0.75 ⋅ VO
0.75
⋅
VO
A
µsec
⋅ VO
A
- ESR
·
FS
L =
=
≈
3
⎠
m
0.24A
µsec
VO
VIN
⎛
VO
VIN
⎞
⎠
1
4
· 1
-
=
for VIN = 2 × VO
µsec
A
⎝
= 3
⋅ 2.5V = 7.5µH
1
In this case, a standard 10µH value is selected.
CIN(MIN)
=
⎛
⎝
VPP
IO
⎞
⎠
- ESR
·
4
·
FS
For high-voltage fixed versions (2.5V and above),
m = 0.48A/µsec. Table 1 displays inductor values
for the AAT1123 fixed and adjustable options.
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.
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
Configuration
Output Voltage
0.6V to 2.0V
2.5V to 3.3V
0.6V to 2.0V
2.5V to 3.3V
Inductor
4.7µH
Slope Compensation
0.24A/µsec
0.6V Adjustable With
External Resistive Divider
10µH
0.24A/µsec
4.7µH
0.24A/µsec
Fixed Output
4.7µH
0.48A/µsec
Table 1: Inductor Values.
10
1123.2007.02.1.6
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
