APW717818QBI-TRG Просмотр технического описания (PDF) - Anpec Electronics
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APW717818QBI-TRG Datasheet PDF : 16 Pages
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APW7178
Application Information
Input Capacitor Selection
Because buck converters have a pulsating input current,
a low ESR input capacitor is required. This results in the
best input voltage filtering, minimizing the interference
with other circuits caused by high input voltage spikes.
Also, the input capacitor must be sufficiently large to sta-
bilize the input voltage during heavy load transients. For
good input voltage filtering, usually a 4.7µF input capaci-
tor is sufficient. It can be increased without any limit for
better input-voltage filtering. Ceramic capacitors show
better performance because of the low ESR value, and
they are less sensitive against voltage transients and
spikes compared to tantalum capacitors. Place the input
capacitor as close as possible to the input and GND pin of
the device for better performance.
shown in “Typical Application Circuits”. A suggestion of
maximum value of R2 is 200kΩ to keep the minimum
current that provides enough noise rejection ability through
the resistor divider. The output voltage can be calculated
as below:
VOUT
=
VREF
⋅ 1+
R1
R2
=
0.6 ⋅ 1+
R1
R2
VOUT
FB
APW7178
GND
R1≤1MΩ
R2 ≤ 200kΩ
Inductor Selection
For high efficiencies, the inductor should have a low DC
resistance to minimize conduction losses. Especially at
high-switching frequencies the core material has a higher
impact on efficiency. When using small chip inductors,
the efficiency is reduced mainly due to higher inductor
core losses. This needs to be considered when select-
ing the appropriate inductor. The inductor value deter-
mines the inductor ripple current. The larger the inductor
value, the smaller the inductor ripple current and the lower
the conduction losses of the converter. Conversely, larger
inductor values cause a slower load transient response.
A reasonable starting point for setting ripple current, ∆IL,
is 40% of maximum output current. The recommended
inductor value can be calculated as below:
L
≥
VOUT 1−
VOUT
VIN
FSW ⋅ ∆IL
I
L(MAX)
=
I
OUT(MAX)
+
1/2
x
∆IL
To avoid the saturation of the inductor, the inductor should
be rated at least for the maximum output current of the
converter plus the inductor ripple current.
Output Voltage Setting
In the adjustable version, the output voltage is set by a
resistive divider. The external resistive divider is con-
nected to the output, allowing remote voltage sensing as
Output Capacitor Selection
The current-mode control scheme of the APW7178 al-
lows the use of tiny ceramic capacitors. The higher ca-
pacitor value provides the good load transients response.
Ceramic capacitors with low ESR values have the lowest
output voltage ripple and are recommended. If required,
tantalum capacitors may be used as well. The output
ripple is the sum of the voltages across the ESR and the
ideal output capacitor.
∆VOUT
≅
VOUT
⋅ 1−
VOUT
VIN
FSW ⋅ L
⋅ ESR +
1
8 ⋅ FSW ⋅ COUT
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage char-
acteristics of all the ceramics for a given value and size.
VIN
IIN
IP-FET
IL
CIN
P-FET
SW
N-FET
IOUT VOUT
ESR
COUT
Copyright © ANPEC Electronics Corp.
10
Rev. A.2 - Jan., 2011
www.anpec.com.tw
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