AAT1142 View Datasheet(PDF) - Advanced Analogic Technologies
Part Name
Description
MFG CO.
'AAT1142' PDF : 21 Pages View PDF
SwitchRegTM
PRODUCT DATASHEET
AAT1142
800mA Voltage-Scaling Step-Down Converter
S2Cwire Timing Diagram
EN/SET
1
Data Reg
THI
TLO
T LAT
2
n-1
n ≤ 64
0
TOFF
n-1
0
Rising Clock
Edges/Data
Register
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Output
Voltage
(V)
No change
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
Rising Clock
Edges/Data
Register
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Output
Voltage
(V)
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
1.80 (default)
1.85
1.90
1.95
2.00
2.00
2.00
2.00
Table 2: AAT1142 S2Cwire Output Voltage
Programming Map.
Component Selection
Inductor Selection
The step-down converter uses peak current mode con-
trol with slope compensation to maintain stability for
duty cycles greater than 50%. The output inductor value
must be selected so the inductor current down slope
meets the internal slope compensation requirements.
The internal slope compensation for the programmable
AAT1142 is 0.61A/μs. This equates to a slope compensa-
tion that is 75% of the inductor current down slope for a
1.8V output and 2.2μH inductor.
m
=
0.75 ·
L
VO
=
0.75 · 1.8V
2.2μH
=
0.61
A
μs
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 satura-
tion 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 losses 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 2.2μH CDRH2D14 series Sumida inductor has a
94mΩ DCR and a 1.5A DC current rating. At full 800mA
load, the inductor DC loss is 60mW which gives a 4.8%
loss in efficiency for an 800mA, 1.0V output.
Input Capacitor
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (VPP) and solve
for C. The calculated value varies with input voltage and
is a maximum when VIN is double the output voltage.
VO
VIN
·
⎛⎝1 -
VO ⎞
VIN ⎠
CIN =
⎛ VPP
⎝ IO
- ESR⎞⎠ · FS
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 coeffi-
cient characteristics when selecting the proper value. For
example, the capacitance of a 10μF, 6.3V, X5R ceramic
capacitor with 5.0V DC applied is actually about 6μF.
16
www.analogictech.com
1142.2009.08.1.1
Share Link: 
