AAT1123 Datasheet, PDF (10/18 Pages) Advanced Analogic Technologies – 1MHz Step-Down Converter

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 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 AAT1123 is 0.24A/μsec. This equates
to a slope compensation that is 75% of the inductor cur-
rent 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
µsec
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 ⋅ V_O
m
=
0.75 ⋅ V_O
A
≈
3
µsec
A
⋅
0.24A _µsec
µsec
= 3 _A ⋅ 2.5V = 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
AAT1123 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 105m 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.
DATA SHEET
AAT1123
1MHz Step-Down Converter
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 voltage 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 ceram-
ic capacitor with 5.0V DC applied is actually about 6μF.
Configuration
0.6V Adjustable
With External
Resistive Divider
Fixed Output
Output
Voltage
0.6V to
2.0V
2.5V to
3.3V
0.6V to
2.0V
2.5V to
3.3V
Inductor
4.7μH
10μH
4.7μH
4.7μH
Slope
Compensation
0.24A/μsec
0.24A/μsec
0.24A/μsec
0.48A/μsec
Table 1: Inductor Values.
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
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