ADP2109 Analog Devices, ADP2109 Datasheet - Page 12

• Manufacturer Part Number: ADP2109
• Description: Compact 600 mA, 3 MHz, Step-Down Converter with Output Discharge
• Manufacturer: Analog Devices

ADP2109
APPLICATIONS INFORMATION
EXTERNAL COMPONENT SELECTION
Parameters like efficiency and transient response can be
affected by varying the choice of external components in
the applications circuit, as shown in Figure 1.
Inductor
The high switching frequency of the ADP2109 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:
where:
f
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:
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
ADP2109 is a high switching frequency dc-to-dc converter,
shielded ferrite core material is recommended for its low core
losses and low EMI.
Table 6. Suggested 1.0 μH Inductors
Vendor
Murata
Coilcraft
Toko
TDK
SW
is the switching frequency.
I
I
RIPPLE
PEAK
Model
LQM2HPN1R0M
LPS3010-102
MDT2520-CN
CPL2512T
=
=
I
V
LOAD
OUT
V
(
MAX
IN
×
(
×
V
)
f
+
IN
SW
I
−
RIPPLE
Dimensions
2.5 × 2.0 × 1.1
3.0 × 3.0 × 0.9
2.5 × 2.0 × 1.2
2.5 × 1.5 × 1.2
×
V
2
L
OUT
)
I
1500
1700
1800
1500
SAT
(mA)
DCR (mΩ)
90
85
100
100
Rev. A | Page 12 of 16
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 that is adequate to
ensure the minimum capacitance over the necessary temper-
ature 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 recom-
mended 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:
where:
C
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
Substituting these values in the equation yields
To guarantee the performance of the ADP2109, it is imperative
that the effects of dc bias, temperature, and tolerances on the
behavior of the capacitors be evaluated for each application.
EFF
OUT
12
10
is the effective capacitance at the operating voltage.
8
6
4
2
0
C
C
is 9.2481 μF at 1.8 V from the graph in Figure 28.
0
EFF
EFF
= C
= 9.2481 μF × (1 – 0.15) × (1 – 0.1) = 7.0747 μF
OUT
1
Figure 28. Typical Capacitor Performance
× (1 – TEMPCO) × 1(1 – TOL)
2
DC BIAS VOLTAGE (V)
3
4
5
6