Data Sheet ADP3050
Rev. C | Page 11 of 20
APPLICATIONS INFORMATION
ADIsimPower DESIGN TOOL
The ADP3050 is supported by the 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 considera-
tion 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 request an
unpopulated board through the tool.
The complete process for designing a step-down switching
regulator using the ADP3050 is provided in the following
sections. Each section includes a list of recommended devices.
These lists do not include every available device or manufacturer.
They contain only surface-mount devices. Equivalent through-
hole devices can be substituted if needed. In choosing components,
keep in mind what is most important to the design, for example,
efficiency, cost, and size. These ultimately determine which compo-
nents are used. It is also important to ensure that the design
specifications are clearly defined and reflect the worst-case
conditions. Key specifications include the minimum and
maximum input voltage, the output voltage and ripple, and the
minimum and maximum load current.
INDUCTOR SELECTION
The inductor value determines the mode of operation for the
regulator: continuous mode, where the inductor current flows
continuously; or discontinuous mode, where the inductor current
reduces to zero during every switch cycle. Continuous mode is
the best choice for many applications. It provides higher output
power, lower peak currents in the switch, inductor, and diode,
and a lower inductor ripple current, which means lower output
ripple voltage. Discontinuous mode allows the use of smaller
magnetics, but at a price: lower available load current and
higher peak and ripple currents. Designs with a high input
voltage or a low load current often operate in discontinuous
mode to minimize inductor value and size. The ADP3050 is
designed to work well in both modes of operation.
Continuous Mode
The inductor current in a continuous mode system is a triangular
waveform (equal to the ripple current) centered around a dc
value (equal to the load current). The amount of ripple current
is determined by the inductor value, and is usually between 20%
and 40% of the maximum load current. To reduce the inductor
size, ripple currents between 40% and 80% are often used in
continuous mode designs with a high input voltage or a low
output current.
The inductor value is calculated using the following equation:
)(
)(
1
MAXIN
OUT
SW
RIPPLE
OUT
MAXIN
V
V
fI
L ××
=
(2)
Where V
IN(MAX)
is the maximum input voltage, V
OUT
is the
regulated output voltage, and f
SW
is the switching frequency
(200 kHz). The initial choice for the amount of ripple current
may seem arbitrary, but it serves as a good starting point for
finding a standard off-the-shelf inductor value, such as
10 μH, 15 μH, 22 μH, 33 μH, and 47 μH. If a specific inductance
value is to be used, simply rearrange Equation 2 to find the
ripple current. For an 800 mA, 12 V to 5 V system, and a ripple
current of 320 mA (40% of 800 mA) is chosen, the inductance is
μH45.5
12
5
10200
1
0.32
512
3
=×
×
×
−
=L
A 47 μH inductor is the closest standard value that gives a ripple
current of about 310 mA. The peak switch current is equal to
the load current plus one-half the ripple current (this is also the
peak current for the inductor and the catch diode).
A95.0155.08.0
2
1
)()(
=+=+=
RIPPLE
MAXOUTPKSW
III
(3)
Pick an inductor with a dc (or saturation) current rating about 20%
larger than I
SW(PK)
to ensure that the inductor is not running near
the edge of saturation. For this example, 1.20 × 0.95 A = 1.14 A, use
an inductor with a dc current rating of at least 1.2 A. The maxi-
mum switch current is internally limited to 1.5 A, and this limit,
along with the ripple current, determines the maximum load
current the system can provide.
If the load current decreases to below one-half the ripple
current, the regulator operates in discontinuous mode.
Discontinuous Mode
For load currents less than approximately 0.5 A, discontinuous
mode operation can be used. This allows the use of a smaller
inductor, but the ripple current is much higher (which means a
higher output ripple voltage). If a larger output capacitor must
be used to reduce the output ripple voltage, the overall system
may take up more board area than if a larger inductor is used.
The operation and equations for the two modes are quite different,
but the boundary between these two modes occurs when the ripple
current is equal to twice the load current (when I
RIPPLE
= 2 × I
OUT
).
From this, Equation 2 is used to find the minimum inductor
value needed to keep the system in continuous mode operation
(solve for the inductor value with I
RIPPLE
= 2 × I
OUT
).
)(
)(
1
2
MAXIN
OUT
SW
OUT
OUT
MAXIN
DIS
V
fI
L ××
×
=
(4)
Using an inductor below this value causes the system to operate
in discontinuous mode.
