AAT1141_0810 Datasheet, PDF (11/20 Pages) Advanced Analogic Technologies – Fast Transient 600mA Step-Down Converter

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PRODUCT DATASHEET
AAT1141
Fast Transient 600mA Step-Down Converter
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 require-
ments. The internal slope compensation for the adjust-
able 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.
m
=
0.75 ·
L
V_O
=
0.75 · 1.5V
4.7μH
=
0.24
A
μs
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.
L=
0.75 ·
m
V_O
=
0.75 · V_O
A
≈3
μs
A
· V_O
0.24 _μs
=
3
μs
A
· 2.5V = 7.5μH
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.
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.
Output
Voltage (V)
1, 1.2
1.5, 1.8
2.5, 3.3
Inductor (μH)
2.2
4.7
6.8
Output
Capacitor (μF)
10
4.7
4.7
Table 1: Inductor and Output Capacitor Values.
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 out-
put.
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_PP) and
solve for C. The calculated value varies with input volt-
age and is a maximum when V_IN is double the output
voltage.
V_O
V_IN
· ^⎛⎝1 -
V_O ⎞
V_IN ⎠
C_IN =
⎛ V_PP
⎝ I_O
- ESR^⎞⎠ · F_S
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
=
1
4
for
V_IN
=
2
·
V_O
1
C_IN(MIN) = _{⎛ VPP}
⎝ I_O
- ESR^⎞⎠ · 4 · F_S
Always examine the ceramic capacitor DC voltage coef-
ficient 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:
I_RMS = I_O ·
V_O
V_IN
· ^⎛⎝1 -
V_O ⎞
V_IN ⎠
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.
V_O
V_IN
· ^⎛⎝1 -
V_O ⎞
V_IN ⎠
=
D · (1 - D) =
0.5² = ¹
2
for V_IN = 2 · V_O
I = _RMS(MAX)
I_O
2
The term
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
appears in both the input voltage
ripple and input capacitor RMS current equations and is
a maximum when V_O is twice V_IN. This is why the input
voltage ripple and the input capacitor RMS current ripple
are a maximum at 50% duty cycle.
1141.2008.10.1.6
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