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APW7236 Datasheet(PDF) 10 Page - Anpec Electronics Coropration

Part No. APW7236
Description  1.5MHz Step-Up Converter for White LEDs
Download  17 Pages
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Maker  ANPEC [Anpec Electronics Coropration]
Homepage  http://www.anpec.com.tw
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APW7236 Datasheet(HTML) 10 Page - Anpec Electronics Coropration

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Copyright
© ANPEC Electronics Corp.
Rev. A.4 - Sep., 2012
APW7236
www.anpec.com.tw
10
Application Information
Input Capacitor Selection
The input capacitor (C
IN) reduces the current peaks drawn
from the input supply and reduces noise injection into the
IC. The reflected ripple voltage will be smaller with larger
CIN. For reliable operation, it is recommended to select
the capacitor voltage rating at least 1.2 times higher than
the maximum input voltage. The capacitors should be
placed close to the VIN and GND.
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,
∆I
L,
is 30% to 50% of the average inductor current. The rec-
ommended inductor value can be calculated as below:
where
V
IN = input voltage
V
OUT = output voltage
F
SW = switching frequency in MHz
I
OUT = maximum output current in amp.
η = Efficiency
∆I
L /IL(AVG) = inductor ripple current/average current
(0.3 to 0.5 typical)
To avoid saturation of the inductor, the inductor should be
rated at least for the maximum input current of the con-
verter plus the inductor ripple current. The maximum in-
put current is calculated as below:
The peak inductor current is calculated as the following
equation:
(
)
SW
OUT
IN
OUT
IN
)
MAX
(
IN
PEAK
F
L
V
V
V
V
2
1
I
I
+
=
Output Capacitor Selection
The current-mode control scheme of the APW7236 al-
lows the usage of tiny ceramic capacitors. The higher
capacitor value provides good load transient 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.
where I
PEAK is the peak inductor current.
Δ V
OUT = Δ VESR + Δ VCOUT
V
IN
V
OUT
I
L
N-FET
LX
I
OUT
I
SW
C
IN
C
OUT
I
IN
D1
ESR
I
LIM
I
L
I
PEAK
I
IN
I
OUT
I
SW
I
D
∆I
L
(
)
 ∆
η
×
×
×


AVG
L
L
)
MAX
(
OUT
SW
IN
OUT
2
OUT
IN
I
I
I
F
V
V
V
V
L
ESR
PEAK
R
I
VESR
×
η
×
×
=
IN
OUT
)
MAX
(
OUT
)
MAX
(
IN
V
V
I
I
ESR
PEAK
ESR
R
I
V
×


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