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參數(shù)資料
| 型號： | AAT1147 |
| 廠商： | Advanced Analog Technology,lnc. |
| 英文描述： | High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter |
| 中文描述： | 高效率，低噪聲，快速瞬態(tài)400mA的降壓轉(zhuǎn)換器 |
| 文件頁數(shù)： | 10/19頁 |
| 文件大?。?/td> | 620K |
| 代理商： | AAT1147 |
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 AAT1147 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. When
externally programming the 0.6V version to 2.5V,
the calculated inductance is 7.5μH.
In this case, a standard 6.8μH value is selected.
Table 1 displays inductor values for the AAT1147.
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 105m
Ω
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 (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.
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.
C
IN(MIN)
=
1
- ESR
·
4
·
F
S
I
O
V
PP
· 1
-
= 1
IN
= 2 · V
O
V
IN
V
IN
O
V
O
4
· 1
-
V
O
V
IN
C
IN
=
V
O
V
IN
- ESR
·
F
S
V
PP
I
O
0.75
V
L = = 0.75
≈
3
V
O
m
0.24A
= 3 A
2.5V = 7.5
μ
H
V
O
sec
A
μ
sec
A
μ
sec
0.75
V
L
m = = 0.75
1.5V
4.7
μ
H
A
μ
sec
AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
10
1147.2006.05.1.0
Table 1: Inductor Values.
Configuration
Output Voltage
Inductor
0.6V Adjustable With
External Feedback
1V, 1.2V
2.2μH
1.5V, 1.8V
4.7μH
2.5V, 3.3V
6.8μH
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