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ACT4527 Folha de dados(PDF) 7 Page - Active-Semi, Inc

Nome de Peças ACT4527
Descrição Electrónicos  40V/3.0A CV/CC Buck Converter Featuring USB-PD
PDF  14 Pages
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Fabricante Electrônico  ACTIVE-SEMI [Active-Semi, Inc]
Página de início  http://www.active-semi.com
Logo ACTIVE-SEMI - Active-Semi, Inc

ACT4527 Folha de dados(HTML) 7 Page - Active-Semi, Inc

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ACT4527
Rev 2, 06-Jan-2017
Innovative Power
TM
- 7 -
www.active-semi.com
Copyright
©
2
0
1
5
-2017 Active-Semi, Inc.
APPLICATIONS INFORMATION
Inductor Selection
The inductor maintains a continuous current to the
output load. This inductor current has a ripple that is
dependent on the inductance value.
Higher inductance reduces the peak-to-peak ripple
current. The trade off for high inductance value is
the increase in inductor core size and series
resistance, and the reduction in current handling
capability. In general, select an inductance value L
based on ripple current requirement:
Where VIN is the input voltage, VOUT is the output
voltage, fSW is the switching frequency, ILOADMAX is
the maximum load current, and KRIPPLE is the ripple
factor.
Typically,
choose
KRIPPLE
=
30%
to
correspond to the peak-to-peak ripple current being
30% of the maximum load current.
With a selected inductor value the peak-to-peak
inductor current is estimated as:
The peak inductor current is estimated as:
The selected inductor should not saturate at ILPK.
The maximum output current is calculated as:
LLIM is the internal current limit.
Input Capacitor
The input capacitor needs to be carefully selected
to maintain sufficiently low ripple at the supply input
of the converter. A low ESR capacitor is highly
recommended. Since large current flows in and out
of this capacitor during switching, its ESR also
affects efficiency.
The input capacitance needs to be higher than
10
µ
F
.
T
h
e
b
e
s
t
c
h
o
i
c
e
i
s
t
h
e
c
e
r
a
m
i
c
t
y
p
e
.
However, low ESR tantalum or electrolytic types
may also be used provided that the RMS ripple
current rating is higher than 50% of the output
current. The input capacitor should be placed close
to the IN and GND pins of the IC, with the shortest
traces
possible.
In
the
case
of
tantalum
or
electrolytic
types,
a
ceramic
capacitor
is
recommended
to
parallel
with
tantalum
or
electrolytic capacitor, which should be placed right
next to the IC.
Output Capacitor
The output capacitor also needs to have low ESR to
keep low output voltage ripple. The output ripple
voltage is:
Where IOUTMAX is the maximum output current,
KRIPPLE is the ripple factor, RESR is the ESR of the
output capacitor, fSW is the switching frequency, L is
the
inductor
value,
and
COUT
is
the
output
capacitance.
In
the
case
of
ceramic
output
capacitors, RESR is very small and does not
contribute
to
the
ripple.
Therefore,
a
lower
capacitance value can be used for ceramic type. In
the case of tantalum or electrolytic capacitors, the
ripple is dominated by RESR multiplied by the ripple
current. In that case, the output capacitor is chosen
to have sufficiently low ESR.
For ceramic output capacitor, typically choose a
capacitance
of
about
22
µ
F
.
F
o
r
t
a
n
t
a
l
u
m
o
r
electrolytic capacitors, choose a capacitor with less
than 50m
Ù
ESR. If an 330
µ
F
o
r
4
7
0
µ
F
e
l
e
c
t
r
o
l
y
t
i
c
capacitor is used, where ripple is dominantly
caused
by
ESR,
an
2.2uF
ceramic
is
recommended.
Rectifier Schottky Diode
Use a Schottky diode as the rectifier to conduct
current when the High-Side Power Switch is off.
The Schottky diode must have current rating higher
than the maximum output current and a reverse
voltage rating higher than the maximum input
voltage. Further more, the low forward voltage
Schottky is preferable for high efficiency and
smoothly operation.
.
(1)
(
)
RIPPLE
LOADMAX
SW
IN
OUT
IN
OUT
K
I
f
V
V
V
V
L
_
×
=
(2)
(
)
SW
IN
OUT
IN
OUT
PK
LPK
f
V
L
V
V
V
I
×
×
×
=
_
_
PK
LPK
LOADMAX
LPK
_
I
2
1
I
I
+
=
(3)
(4)
PK
LPK
LIM
OUTMAX
I
2
1
I
I
_
_
=
(5)
IN
OUT
SW
OUT
OUT
IN
ESR
RIPPLE
OUTMAX
RIPPLE
V
LC
f
V
V
V
R
K
I
V
2
8
)
(



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