Datasheet

ADP2118 Data Sheet
Rev. C | Page 16 of 24
APPLICATIONS INFORMATION
This section describes the external components selection for the
ADP2118. The typical application circuit is shown in Figure 41.
08301-004
R
TOP
10kΩ
R
BOT
10kΩ
R2
10kΩ
R1
10Ω
SYNC/MODE
FREQ
TRK
FB
PVIN
ADP2118
SW
SW
SW
1
2
3
4
12
11
10
9
C
IN
100µF
X5R,
6.3V
C
OUT
100µF
X5R,
6.3V
C1
0.1µF
V
IN
5V
V
OUT
1.2V
3A
L
1µH
16 15 14 13
5 6
7 8
PGND GND PGND PGND
PGOOD
EN
VIN
PVIN
Figure 41. Application Circuit
ADISIMPOWER DESIGN TOOL
The ADP2118 is supported by ADIsimPower design tool set.
ADIsimPower is a collection of tools that produce complete power
designs optimized for a specific design goal. The tools enable
the user to generate a full schematic, bill of materials, and calculate
performance in minutes. ADIsimPower can optimize designs for
cost, area, efficiency, and parts count while taking into consideration
the operating conditions and limitations of the IC and all real
external components. For more information about ADIsimPower
design tools, refer to www.analog.com/ADIsimPower. The tool
set is available from this website, and users can also request an
unpopulated board through the tool.
OUTPUT VOLTAGE SELECTION
The output voltage of the adjustable version of the ADP2118
can be set by an external resistive voltage divider by using the
following equation to set the voltage:








+×=
BOT
TOP
OUT
R
R
V 16.0
To limit output voltage accuracy degradation due to FB bias
current (0.1 µA maximum) to less than 0.5% (maximum),
ensure that R
BOT
is less than 30 kΩ.
INDUCTOR SELECTION
The inductor value is determined by the operating frequency,
input voltage, output voltage, and ripple current. Using a small
inductor leads to larger inductor current ripple and provides
fast transient response but degrades efficiency, whereas a large
inductor value leads to small current ripple and good efficiency
but slow transient response. As a guideline, the inductor current
ripple, ΔI
L
, is typically set to 1/3 of the maximum load current
trade-off between the transient response and efficiency. The
inductor can be calculated using the following equation:
( )
S
L
OUT
IN
fI
DVV
L
×∆
×−
=
where:
V
IN
is the input voltage.
V
OUT
is the output voltage.
ΔI
L
is the inductor current ripple.
D is the duty cyle.
IN
OUT
V
V
D =
The ADP2118 uses slope compensation in the current loop to
prevent subharmonic oscillations when the duty cycle is larger
than 50%. The internal slope compensation limits the minimum
inductor value.
The negative current limit (−0.9 A) also limits the minimum
inductor value. The inductor current ripple (ΔI
L
) calculated by
the selected inductor should not exceed 1.8 A.
The peak inductor current should be kept below the peak
current limit threshold value and can be calculated as
2
L
O
PEAK
I
II
∆
+
=
Ensure that the rms current of the selected inductor is greater
than the maximum load current and that its saturation current
is greater than the peak current limit of the regulator.
OUTPUT CAPACITOR SELECTION
The output voltage ripple, load step transient, and loop stability
determine the output capacitor selection.
The output ripple is determined by the ESR and the
capacitance.








××
+×∆=∆
S
OUT
L
OUT
fC
ESRIV
8
1
The load transient response depends on the inductor, output
capacitor, and the control loop.