AAT1147 Datasheet, PDF (10/18 Pages) Advanced Analogic Technologies – High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter

DATA SHEET
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA 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 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.
m
=
0.75 ⋅
L
V_O
=
0.75 ⋅ 1.5V
4.7μH
=
0.24
A
μsec
This is the internal slope compensation. 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
⋅
V_O
0.24A _μsec
μsec
=3 _A
⋅ 2.5V = 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 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.
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 Feedback
Output Voltage
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
Inductor
2.2μH
4.7μH
6.8μH
Table 1: Inductor Values.
The maximum input capacitor RMS current is:
I_RMS = I_O ·
V_{O ·} ⎛_{1 -} V_O ⎞
V_IN ⎝ 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 ·} ⎛_{1 -} V_O ⎞ _{= D · (1 - D) =}
V_IN ⎝ V_IN ⎠
for V_IN = 2 · V_O
I = _RMS(MAX)
I_O
2
0.5² = ¹
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.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
10
201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012