Datasheet

Data Sheet ADP5024
Rev. E | Page 21 of 28
APPLICATIONS INFORMATION
BUCK 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.
Feedback Resistors
For the adjustable model, shown in Figure 47, the total
combined resistance for R1 and R2 is not to exceed 400 kΩ.
Inductor
The high switching frequency of the ADP5024 bucks allows for
the selection of small chip inductors. For best performance, use
inductor values between 0.7 μH and 3 μH. Suggested inductors
are shown in Table 9.
The peak-to-peak inductor current ripple is calculated using
the following equation:
L
f
V
V
VV
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
MAX
LOAD
PEAK
I
I
I +
=
Inductor conduction losses are caused by the flow of current
through the inductor, which has an associated internal dc
resistance (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 bucks are high switching frequency dc-to-dc
converters, shielded ferrite core material is recommended for
its low core losses and low EMI.
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 dielec-
trics, each with a different behavior over temperature and applied
voltage. Capacitors must have a dielectric that is 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 per-
formance. 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 9.2 μF at 1.8 V, as shown in Figure 49.
Substituting these values in the equation yields
C
EFF
= 9.2 μF × (1 − 0.15) × (1 − 0.1) ≈ 7.0 μF
To guarantee the performance of the bucks, it is imperative
that the effects of dc bias, temperature, and tolerances on the
behavior of the capacitors be evaluated for each application.
0
2
4
6
8
10
12
0 1 2 3 4 5 6
DC BIAS VOLTAGE (V)
CAPACITANCE (µF)
09888-049
Figure 49. Capacitance vs. Voltage Characteristic
Table 9. Suggested 1.0 μH Inductors
Vendor Model Dimensions (mm) I
SAT
(mA) DCR (mΩ)
Murata LQM2MPN1R0NG0B 2.0 × 1.6 × 0.9 1400 85
Murata LQH32PN1R0NN0 3.2 × 2.5 × 1.6 2300 45
Taiyo Yuden CBC3225T1R0MR 3.2 × 2.5 × 2.5 2000 71
Coilcraft XFL4020-102ME 4.0 × 4.0 × 2.1 5400 11
Coilcraft XPL2010-102ML 1.9 × 2.0 × 1.0 1800 89
Toko MDT2520-CN 2.5 × 2.0 × 1.2 1350 85