ADP2108AUJZ-3.3-R7 datasheet(13/20 Pages) AD

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Data Sheet

ADP2108

Rev. G | Page 13 of 20

APPLICATIONS INFORMATION

ADIsimPower DESIGN TOOL

The ADP2108 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 ADP2108 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:

OUT

RIPPLE

−

)

(

where:

fSW 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:

)

(

RIPPLE

MAX

LOAD

PEAK

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

ADP2108 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

Model

Dimensions

ISAT (mA)

DCR (mΩ)

Murata

LQM21PN1R0M

2.0 × 1.25 × 0.5

800

190

Murata

LQM31PN1R0M

3.2 × 1.6 × 0.85

1200

120

Murata

LQM2HPN1R0M

2.5 × 2.0 × 1.1

1500

Coilcraft

LPS3010-102

3.0 × 3.0 × 0.9

1700

Toko

MDT2520-CN

2.5 × 2.0 × 1.2

1800

100

TDK

CPL2512T

2.5 × 1.5 × 1.2

1500

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:

CEFF = COUT × (1 − TEMPCO) × (1 − TOL)

where:

CEFF 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 +125°C is assumed to be 15% for an X5R dielectric.

The tolerance of the capacitor (TOL) is assumed to be 10%, and

COUT is 9.2 μF at 1.8 V, as shown in Figure 30.

Substituting these values in the equation yields

CEFF = 9.2 μF × (1 − 0.15) × (1 − 0.1) = 7.0 μF

To guarantee the performance of the ADP2108, it is imperative

that the effects of dc bias, temperature, and tolerances on the

behavior of the capacitors be evaluated for each application.

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