Datasheet

Data Sheet ADP2138/ADP2139
Rev. C | Page 13 of 20
APPLICATIONS INFORMATION
ADIsimPower DESIGN TOOL
The ADP2138/ADP2139 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.
EXTERNAL COMPONENT SELECTION
Trade-offs between performance parameters such as efficiency
and transient response can be made by varying the choice of
external components in the applications circuit, as shown in
Figure 1.
Inductor
The high switching frequency of the ADP2138/ADP2139 allows
for the selection of small chip inductors. For best performance,
use inductor values between 0.7 μH and 3 μH. Recommended
inductors are shown in Table 6.
The peak-to-peak inductor current ripple is calculated using
the following equation:
LfV
VVV
I
SW
IN
OUT
IN
OUT
RIPPLE
××
−×
=
)(
where:
f
SW
is the switching frequency.
L is the inductor value.
The minimum dc current rating of the inductor must be greater
than the inductor peak current. The inductor peak current is
calculated using the following equation:
2
)(
RIPPLE
MAXLOAD
PEAK
I
II +=
Inductor conduction losses are caused by the flow of current
through the inductor, which has an associated internal DCR.
Larger sized inductors have smaller DCR, which may decrease
inductor conduction losses. Inductor core losses are related to
the magnetic permeability of the core material. Because the
ADP2138/ADP2139 are high switching frequency dc-to-dc
converters, shielded ferrite core material is recommended for its
low core losses and low electromagnetic interference (EMI).
Table 6. Suggested 1.0 μH Inductors
Vendor Model
Dimensions
(mm)
I
SAT
(mA)
DCR
(mΩ)
Murata LQM2MPN1R0NG0B 2.0 × 1.6 × 0.9 1400 85
LQM18PN1R0 1.6 × 0.8 × 0.33 700 52
Taiyo Yuden CBMF1608T1R0M 1.6 × 0.8 × 0.8 290 90
EPL2014-102ML 2.0 × 2.0 × 1.4 900 59
Coilcraft TDK GLFR1608T1R0M-LR 1.6 × 0.8 × 0.8 360 80
0603LS-102
1.8 × 1.27 × 1.1
400
81
Coilcraft
Toko
MDT2520-CN 2.5 × 2.0 × 1.2 1800 100
Output Capacitor
Higher output capacitor values reduce the output voltage ripple
and improve load transient response. When choosing this value,
it is also important to account for the loss of capacitance due to
output voltage dc bias.
Ceramic capacitors are manufactured with a variety of dielectrics,
each with different behavior over temperature and applied voltage.
Capacitors must have a dielectric adequate to ensure the
minimum capacitance over the necessary temperature range
and dc bias conditions. X5R or X7R dielectrics with a voltage
rating of 6.3 V or 10 V are recommended for best performance.
Y5V and Z5U dielectrics are not recommended for use with any
dc-to-dc converter because of their poor temperature and dc bias
characteristics.
The worst-case capacitance accounting for capacitor variation
over temperature, component tolerance, and voltage is calcu-
lated using the following equation:
C
EFF
= C
OUT
× (1 − TEMPCO) × (1 − TOL)
where:
C
EFF
is the effective capacitance at the operating voltage.
TEMPCO is the worst-case capacitor temperature coefficient.
TOL is the worst-case component tolerance.
In this example, the worst-case temperature coefficient (TEMPCO)
over −40°C to +85°C is assumed to be 15% for an X5R dielectric.
The tolerance of the capacitor (TOL) is assumed to be 10%, and
C
OUT
is 4.0466 μF at 1.8 V, as shown in Figure 35.
Substituting these values in the equation yields
C
EFF
= 4.0466 μF × (1 − 0.15) × (1 − 0.1) = 3.0956 μF
To guarantee the performance of the ADP2138/ADP2139, it is
imperative that the effects of dc bias, temperature, and tolerances
on the behavior of the capacitors be evaluated for each application.