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APW7071CI-TRG Datasheet PDF : 15 Pages
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APW7071
Application Information (Cont.)
Inductor Selection (Cont.)
converter. The switching frequency is calculated as:
( ) FSW
=
2 ⋅ IOUT
⋅
VOUT − VIN + VF
L ⋅ IP2K
where
VF is the foward voltage of the Schottky diode.
A smaller inductor gets higher switching frequency but
lower efficiency. To operate under discontinuous conduc-
tion mode, the inductor can be selected as below:
( ) L ≤ TOFF,MIN ⋅ VOUT + VF − VIN
IPK
For the white LED applications, the inductor values be-
tween 2.2 µH and 10 µH are recommended.
The inductor also affects the maximum output power.
The maximum output current is calculated as:
IOUT,MAX
=
VIN
VOUT
⋅ IIN,AVG ⋅ η =
VIN
VOUT
⋅
1
2
⋅
IPK
⋅ TON + TOFF
TS
⋅η
where
TON
=
L ⋅ IPK
VIN
TOFF
=
L ⋅ IPK
VOUT − VIN
+ VF
T
S
=
T
ON
+
T
OFF,
MIN
=
T
ON
+ 0.4µs (typical)
It can be understood by the following figure.
ILX
IPK
Recommended inductors
Part No.
LQH31CN2R2M03L
Value (µH)
2.2
LQH32CN4R7M23L
4.7
SH30184R7YSB
4.7
LQH32CN100K53L
10
SH3018100YSB
10
Vendor
Murata
Murata
ABC
Murata
ABC
Output capacitor selection
For better output voltage filtering, a low ESR output ca-
pacitor like ceramic capacitors is recommended. The se-
lection of the output capacitance directly influences the
output voltage ripple of the converter. The output voltage
ripple is calculated as:
∆VOUT
=
1
C
⋅
L ⋅ IPK
VOUT − VIN
+ VF
⋅
1
2
IPK
−
IOUT
In white LED applications, the output ripple is propor-
tional to the LED current. A proper output capacitor from
0.1µF to 1µF is recommended to limit the maximum cur-
rent ripple of the LED current.
Recommended output capacitor
Part No.
GRM188R61E105KA12
Any
Any
Value
Vender
1.0µF/X5R/0603/25V Murata
0.22µF
Any
0.1µF
Any
Input capacitor selection
For good input voltage filtering, low ESR ceramic capaci-
tors are recommended. A 4.7µF ceramic input capacitor
is sufficient for most applications. For better-input voltage
filtering the capacitor value can be increased.
Ton
Toff
Toff,min
Ts
t
Recommended input capacitor
Part No.
Value
Vender
GRM188R60J475KE19D 4.7µF/X5R/0603/6.3V Murata
GRM219R60J106KE19D 10µF/X5R/0805/6.3V Murata
Figure.5 Discontinuous Conduction Mode Operation
Waveform
The efficiency can be estimated by the section of “Typical
Operating Characteristics”.
Diode selection
To achieve high efficiency, a Schottky diode must be used.
The current rating of the diode must meet the peak cur-
rent rating of the converter.
Copyright © ANPEC Electronics Corp.
10
Rev. A.3 - Jul., 2008
www.anpec.com.tw
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