AAT2511 Datasheet, PDF (10/20 Pages) Advanced Analogic Technologies – Dual 600mA, 1MHz Step-Down DC/DC Converter

AAT2511
Dual 600mA, 1MHz Step-Down DC/DC Converter
Applications Information
Inductor Selection
The step-down converter uses peak current mode
control with slope compensation to maintain stabil-
ity for duty cycles greater than 50%. The output
inductor value must be selected so the inductor
current down slope meets the internal slope com-
pensation requirements. The internal slope com-
pensation for the adjustable and low-voltage fixed
versions of the AAT2511 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.
m
=
0.75 ⋅ V_O
L
=
0.75 ⋅ 1.5V
4.7µH
= 0.24
A
µsec
This is the internal slope compensation for the
adjustable (0.6V) version or low-voltage fixed ver-
sion. When externally programming the 0.6V ver-
sion to a 2.5V output, the calculated inductance
would be 7.5µH.
L=
0.75 ⋅ V_O
m
=
0.75V
0.24A /µsec
≈
3
µsec
A
⋅
V_O
=
3
µsec
A
⋅ 2.5V = 7.5µH
In this case, a standard 10µH value is selected.
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
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.
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
38mW for a 600mA 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 (V_PP) and solve for C. The calculated
value varies with input voltage 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
This equation provides an estimate for the input
capacitor required for a single channel.
Configuration
0.6V Adjustable With
External Resistive Divider
Output Voltage
0.6V to 2.0V
2.5V
Inductor
4.7µH
10µH
Table 1: Inductor Values.
Slope Compensation
0.24A/µsec
0.24A/µsec
10
2511.2006.02.1.2