Datasheet

ADP5033 Data Sheet
Rev. F | Page 18 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.
Inductor
The high switching frequency of the ADP5033 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
fV
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
MAXLOAD
PEAK
I
II +=
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.
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 LQM18FN1R0M00B 1.6 × 0.8 × 0.8 150 26
Taiyo Yuden BRC1608T1R0M 1.6 × 0.8 × 0.8 520 180
Coilcraft EPL2014-102ML 2.0 × 2.0 × 1.4 900 59
TDK GLFR1608T1R0M-LR 1.6 × 0.8 × 0.8 230 80
Coilcraft
0603LS-102
1.8 × 1.69 × 1.1
400
81
Toko MDT2520-CN 2.5 × 2.0 × 1.2 1350 85
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 a 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 calculated
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 45.
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)
09788-004
Figure 45. Typical Capacitor Performance