APW7137
Application Information
The peak inductor current is calculated as the following
equation:
Input Capacitor Selection
The input capacitor (CIN) reduces the ripple of the input
current drawn from the input supply and reduces noise
injection into the IC. The reflected ripple voltage will be
smaller when an input capacitor with larger capacitance
is used. For reliable operation, it is recommended to
select the capacitor with maximum voltage rating at least
1.2 times of the maximum input voltage. The capacitors
should be placed close to the VIN and the GND.
1 V ×
VOUT - V
IN
IN
IPEAK = I
+
×
IN(MAX)
2
VOUT ×L×FSW
IL
IOUT
LX
D1
VOUT
VIN IIN
ESR
COUT
N-FET
ISW
CIN
Inductor Selection
Selecting an inductor with low dc resistance reduces con-
duction losses and achieves high efficiency. The efficiency
is moderated whilst using small chip inductor which op-
erates with higher inductor core losses. Therefore, it is
necessary to take further consideration while choosing
an adequate inductor. Mainly, the inductor value deter-
mines the inductor ripple current: larger inductor value
results in smaller inductor ripple current and lower con-
duction losses of the converter. However, larger inductor
value generates slower load transient response. A rea-
sonable design rule is to set the ripple current, DIL, to be
30% to 50% of the maximum average inductor current,
IL(AVG). The inductor value can be obtained as below,
IL
ILIM
IPEAK
DIL
IIN
ISW
ID
æ
ç
è
ö2
÷
÷
ø
V
VOUT - V
h
IOUT
IN
IN
L ³
´
´
ç
VOUT
FSW ×IOUT(MAX)
æ
ö
÷
÷
ø
DIL
ç
ç
IL
(
AVG)
è
Output Capacitor Selection
where
The current-mode control scheme of the APW7137 al-
lows the usage of tiny ceramic capacitors. The higher
capacitor value provides 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.
VIN = input voltage
VOUT = output voltage
FSW = switching frequency in MHz
IOUT = maximum output current in amp.
η = Efficiency
DIL /IL(AVG) = inductor ripple current/average current
(0.3 to 0.5 typical)
To avoid the saturation of the inductor, the inductor should
be rated at least for the maximum input current of the
converter plus the inductor ripple current. The maximum
input current is calculated as below:
ΔVOUT = ΔVESR + ΔVCOUT
æ
IN ö
IOUT
VOUT - V
ç
×
÷
÷
ø
DVCOUT
»
ç
COUT
VOUT ×FSW
è
DVESR » IPEAK ×RESR
where IPEAK is the peak inductor current.
I
OUT(MAX) × VOUT
I
=
IN(MAX)
VIN ×h
Copyright ã ANPEC Electronics Corp.
13
www.anpec.com.tw
Rev. A.4 - Oct., 2008
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
