ADP5037ACPZ-R7 datasheet(19/28 Pages) AD

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Data Sheet

ADP5037

Rev. D | Page 19 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, referring to Figure 47, the total

combined resistance for R1 and R2 is not to exceed 400 kΩ.

Inductor

The high switching frequency of the ADP5037 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:

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 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 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:

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 +85°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 49.

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 bucks, it is imperative

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

behavior of the capacitors be evaluated for each application.

DC BIAS VOLTAGE (V)

Figure 49. Capacitance vs. Voltage Characteristic

Table 9. Suggested 1.0 μH Inductors

Vendor

Model

Dimensions (mm)

ISAT (mA)

DCR (mΩ)

Murata

LQM2MPN1R0NG0B

2.0 × 1.6 × 0.9

1400

Murata

LQM18FN1R0M00B

3.2 × 2.5 × 1.5

2300

Taiyo Yuden

CBC3225T1R0MR

3.2 × 2.5 × 2.5

2000

Coilcraft

XFL4020-102ME

4.0 × 4.0 × 2.1

5400

Coilcraft

XPL2010-102ML

1.9 × 2.0 × 1.0

3750

Toko

MDT2520-CN

2.5 × 2.0 × 1.2

1350

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