AAT1143
1MHz 400mA 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
AAT1143 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 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 =
0.75 ⋅ VO
0.75
⋅
VO
A
μsec
A
⎛
⎝
VPP
IO
⎞
- ESR
·
FS
L =
=
≈
3
⋅ VO
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 AAT1143 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
Inductor
4.7μH
Slope Compensation
0.24A/μsec
0.6V Adjustable With
External Resistive Divider
10μH
0.24A/μsec
0.6V to 2.0V
2.5V to 3.3V
4.7μH
0.24A/μsec
Fixed Output
4.7μH
0.48A/μsec
Table 1: Inductor Values.
10
1143.2006.07.1.10
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
