Datasheet

Data Sheet ADD5205
Rev. 0 | Page 11 of 16
EXTERNAL COMPONENT SELECTION GUIDE
Inductor Selection
The inductor is an integral part of the step-up converter. It stores
energy during the switch-on time and transfers that energy to
the output through the output diode during the switch-off time.
An inductor in the 3.3 μH to 6.8 μH range is recommended. In
general, lower inductance values result in higher saturation
current and lower series resistance for a given physical size.
The input (V
IN
) and output (V
OUT
) voltages determine the switch
duty cycle (D), which in turn can be used to determine the
inductor ripple current.
OUT
IN
OUT
V
VV
D
−
=
Use the duty cycle and switching frequency (f
SW
) to determine
the on time (t
ON
).
SW
ON
f
D
t =
The inductor ripple current (ΔI
L
) in a steady state is
L
tV
I
ON
IN
L
×
=Δ
Solve for the inductance value (L).
L
ON
IN
I
tV
L
Δ
×
=
Ensure that the peak inductor current (that is, the maximum
input current plus half of the inductor ripple current) is less
than the rated saturation current of the inductor. In addition,
ensure that the maximum rated rms current of the inductor is
greater than the maximum dc input current to the regulator.
L
LAVG
LPK
III Δ
2
1
+= ,
IN
OUTOUT
LAVG
Vη
V
I
I
×
×
=
OUT
SW
IN
OUT
INL
Vf
VV
V
L
I
×
−
=Δ
1
where:
I
LPK
is the peak inductor current.
I
LAVG
is the input average current.
Tabl e 6 shows a list of recommend inductors.
Table 6. Recommended Inductors
Coilcraft Part No. L (μH) I
SAT
(A) Size (mm)
XFL4020-332ML 3.3 2.7 4 × 4 × 2
LPS4012-472ML 4.7 1.6 3.9 × 3.9 × 1.1
LPS4018-472ML 4.7 1.8
3.9 × 3.9 × 1.7
LPS4018-682ML 6.8 1.2 3.9 × 3.9 × 1.7
Input and Output Capacitors Selection
The ADD5205 requires input and output bypass capacitors to
supply transient currents while maintaining a constant input
and output voltage. Use a low effective series resistance (ESR)
4.7 μF or greater capacitor for the input capacitor to prevent noise
at the ADD5205 input. Place the input between VIN and GND,
as close as possible to the ADD5205.
The output capacitor maintains the output voltage and supplies
current to the load while the ADD5205 switch is on. The value
and characteristics of the output capacitor greatly affect the
output voltage ripple and stability of the regulator. Use a ceramic
X5R or X7R dielectric capacitor, and for the output capacitor, a
4.7 μF or greater capacitor is preferred.
Place a 100 nF or greater capacitor as close as possible to the
OVP pin of ADD5205.
Diode Selection
The output diode conducts the inductor current to the output
capacitor and loads while the switch is off. For high efficiency,
minimize the forward voltage drop of the diode. Schottky diodes
are recommended.
The output diode for a boost regulator must be chosen depending
on the output voltage and the output current. The diode must
be rated for a reverse voltage greater than the output voltage used.
The average current rating must be greater than the maximum
load current expected, and the peak current rating must be greater
than the peak inductor current.