AAT1112
1.5A, 1.4MHz Step-Down Converter
VIN
U1
3.3V
C1
10µF
AAT1112
TDFN33-12
L1
3.3µH
11
7
12
4
VP
LX
C3
(optional)
C2
VCC
EN
N/C
FB
R2
22µF
3
2
1
8
6
9
3
SYNC PGND
Enable
R3
10
5
2
N/C
PGND
59K
1
3
2
1
GND PGND
SYNC
Figure 1: AAT1112 Schematic.
Input Capacitor
Component Selection
Select a 10µF to 22µF 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.
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 inductor should be
set equal to the output voltage numeric value in µH.
This guarantees that there is sufficient internal
slope compensation.
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
-
CIN =
⎛
⎝
VPP
IO
⎞
- ESR
·
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
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.
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
1
4
-
=
for VIN = 2 · VO
1
CIN(MIN)
=
⎛
⎝
VPP
IO
⎞
⎠
- ESR
·
4
·
FS
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.
The 3.3µH CDRH4D28 series Sumida inductor has
a 49.2mΩ worst case DCR and a 1.57A DC current
rating. At full 1.5A load, the inductor DC loss is
97mW which gives less than 1.5% loss in efficien-
cy for a 1.5A, 3.3V output.
12
1112.2007.01.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
