PRODUCT DATASHEET
AAT1141
SwitchRegTM
FastTransient 600mA Step-Down Converter
Applications Information
Output
Voltage (V)
Output
Capacitor (μF)
Inductor (μH)
1, 1.2
1.5, 1.8
2.5, 3.3
2.2
4.7
6.8
10
4.7
4.7
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 AAT1141 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.
Table 1: Inductor and Output Capacitor Values.
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.
0.75 · VO 0.75 · 1.5V
A
m =
=
= 0.24
L
4.7μH
μs
VO
⎛
VO ⎞
· 1 -
⎝
VIN
VIN
⎠
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.
CIN =
⎛ VPP
⎝ IO
⎞
⎠
- ESR ·FS
VO
⎛
VO ⎞
VIN ⎠
1
· 1 -
⎝
=
for VIN = 2 · VO
0.75 · VO
m
0.75 · VO
A
μs
A
VIN
4
L =
=
≈
3
· VO
1
0.24
μs
CIN(MIN)
=
⎛ VPP
⎝ IO
⎞
⎠
- ESR · 4 · FS
μs
A
= 3
· 2.5V = 7.5μH
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.
In this case, a standard 6.8μH value is selected.
For high-voltage fixed versions (≥2.5V), m = 0.48A/
μsec. Table 1 displays inductor values for the AAT1141
fixed and adjustable options.
The maximum input capacitor RMS current is:
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.
VO
⎛
VO ⎞
IRMS = IO ·
· 1 -
⎝
VIN
VIN
⎠
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.
VO
⎛
VO ⎞
VIN ⎠
1
· 1 -
⎝
=
D · (1 - D) = 0.52 =
VIN
2
for VIN = 2 · VO
The 4.7μH CDRH2D14 series inductor selected from
Sumida has a 135mΩ typical DCR and a 1A DC current
rating. At full load, the inductor DC loss is 48mW which
gives a 4.5% loss in efficiency for a 600mA, 1.8V output.
IO
IRMS(MAX)
=
2
VO
⎛
⎝
VO
⎞
⎠
·
1 -
VIN
VIN
The term
appears in both the input voltage
ripple and input capacitor RMS current equations and is
w w w . a n a l o g i c t e c h . c o m
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