Datasheet

EVAL-ADP2118
Rev. 0 | Page 5 of 16
ADP2118 DESIGN PROCESS
EXTERNAL COMPONENT SELECTION
This section describes how to select the external components
for the ADP2118 application.
Input Capacitor Selection
The input decoupling capacitor is used to attenuate high fre-
quency noise on the input. This capacitor should be a ceramic
capacitor in the range of 22 μF to 100 μF. This capacitor must
be placed close to the PVIN pin. The loop, composed of C
IN
,
PFET, and NFET, must be kept as small as possible.
VIN RC Filter
An RC filter is needed at the VIN pin to attenuate switching
noise. Generally, a 10 Ω resistor and 0.1 μF, 6.3 V capacitor is
recommended.
Output Filter Selection
The ADP2118 has internal compensation, so there are some
limitations in choosing the output filter. Table 2 to Table 7 show
the stability with different output filter components.
For the inductor selection, check the rms current and saturation
current rating.
The selected inductor rms current must be higher than the
value calculated by Equation 1, and the saturation current must
be higher than the value calculated by Equation 2.

2
2
_
12
1











LfV
VVV
II
S
IN
OUT
IN
OUT
ORMSL
(1)

LfV
VVV
II
S
IN
OUT
IN
OUT
O
PeakL



2
1
_
(2)
where:
I
O
is the output current.
V
OUT
is the output voltage.
V
IN
is the input voltage.
f
S
is the switching frequency.
L is the selected inductance.
For the ADP2118 design, X5R or X7R ceramic capacitors are
recommended. When selecting the capacitor, the dc voltage
rating and the rms current rating must be considered.
Make sure that the capacitor dc voltage rating is greater than the
output voltage.
The capacitor’s rms current rating must be larger than the value
calculated by Equation 3.

LfV
VVV
I
S
IN
OUT
IN
OUT
RMSC
OUT



12
1
_
(3)
Output Voltage Setting
If using the adjustable output version of the ADP2118, the
output voltage is set by the resistor divider (see Figure 3).
Equation 4 is used to set the output voltage:
R
BOT
R
TOP
V
OUT
FB
08742-003
Figure 3. Resistor Divider for Adjustable Version
BOT
BOTTOP
OUT
R
RR
VV

 6.0
(4)
To limit the output voltage accuracy degradation due to FB bias
current (0.1 μA maximum) to less than 0.5%, ensure that R
BOT
is
less than 30 kΩ. It is recommended to use a 10 kΩ, 1% accuracy
resistor for R
BOT
.
If using the fixed output version, connect V
OUT
directly to
the FB pin.