PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Applications Information
Configuration
Output Voltage
Inductor
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
0.6V to 3.3V
2.2μH
4.7μH
6.8μH
4.7μH
0.6V Adjustable With
External Feedback
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 AAT1140 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.
Fixed Output
Table 1: Inductor 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
VO
VIN
VO
VIN
L
4.7μH
μsec
· 1 -
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
VIN
VO
VIN
1
· 1 -
=
for VIN = 2 · VO
4
0.75 VO
m
0.75 VO
μsec
A
L =
=
≈
3
VO
A
1
0.24A
CIN(MIN)
=
μsec
VPP
IO
- ESR · 4 · FS
μsec
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 AAT1140
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
VIN
VO
VIN
IRMS = IO ·
· 1 -
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
VIN
VO
VIN
1
2
· 1 -
=
D · (1 - D) = 0.52 =
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
VIN
VO
VIN
·
1 -
The term
appears in both the input voltage
ripple and input capacitor RMS current equations and is
a maximum when VO is twice VIN. This is why the input
w w w . a n a l o g i c t e c h . c o m
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