AAT1143IJS-0.6-T1 AnalogicTech, AAT1143IJS-0.6-T1 Datasheet - Page 10

• Manufacturer Part Number: AAT1143IJS-0.6-T1
• Description: 1MHz 400mA Step-Down Converter
• Manufacturer: AnalogicTech

Applications Information
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 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.
This is the internal slope compensation for the adjust-
able (0.6V) version or low-voltage fixed versions. When
externally programming the 0.6V version to 2.5V, the
calculated inductance is 7.5μH.
In this case, a standard 10μH value is selected.
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.
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 4.7μH CDRH3D16 series inductor selected from
Sumida has a 105mW DCR and a 900mA DC current rat-
ing. At full load, the inductor DC loss is 17mW which gives
a 2.8% loss in efficiency for a 400mA, 1.5V output.
10
SwitchReg
L =
m =
= 3
0.75 ⋅ V
0.75 ⋅ V
μsec
TM
A
m
L
⋅ 2.5V = 7.5μH
O
O
=
=
0.24A
0.75 ⋅ 1.5V
0.75 ⋅ V
4.7μH
μsec
A
O
≈
= 0.24
3
μsec
A
μsec
A
⋅ V
w w w . a n a l o g i c t e c h . c o m
O
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 (V
for C. The calculated value varies with input voltage and
is a maximum when V
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.
The maximum input capacitor RMS current is:
The input capacitor RMS ripple current varies with the
input and output voltage and will always be less than or
equal to half of the total DC load current.
for V
Confi guration
Resistive Divider
0.6V Adjustable
With External
Fixed Output
IN
= 2 · V
1MHz 400mA Step-Down Converter
V
V
IN
V
O
V
IN
O
C
· 1 -
Table 1: Inductor Values.
O
I
· 1 -
⎛
⎝
IN(MIN)
RMS
C
⎛
⎝
Voltage Inductor
IN
Output
= I
0.6V to
0.6V to
2.5V to
V
V
=
2.0V
2.5V
2.0V
3.3V
=
IN
O
PRODUCT DATASHEET
V
V
O
IN
⎛
⎝
⎞
⎠
IN
⎛
⎝
O
·
V
V
V
is double the output voltage.
V
⎞
⎠
=
I
I
O
IN
PP
O
O
PP
=
V
V
· 1 -
IN
D · (1 - D) =
O
1
4
- ESR · 4 · F
- ESR · F
⎛
⎝
for V
4.7μH
4.7μH
4.7μH
· 1 -
10μH
AAT1143
1
⎛
⎝
V
V
⎞
⎠
⎞
⎠
IN
IN
O
V
⎞
⎠
= 2 · V
V
S
IN
O
1143.2009.02.1.11
Compensation
⎞
⎠
S
0.5
0.24A/μsec
0.24A/μsec
0.24A/μsec
0.48A/μsec
O
PP
Slope
) and solve
2
=
1
2