Data Sheet

ADP5135

Rev. 0 | Page 17 of 24

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, the total combined resistance for R1

and R2 must not to exceed 400 kΩ (see Figure 37).

Inductor

The high switching frequency of the ADP5135 buck regulators

allows the selection of small chip inductors. For best performance,

use inductor values between 0.7 μH and 3 μH. Suggested induc-

tors are shown in Table 8 and Table 9.

using the following equation:

L

f

V

V

V

V

I

SW

IN

OUT

IN

OUT

RIPPLE











)

(

where:

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

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 electromagnetic interference (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 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 recom-

mended 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:

where:

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

Substituting these values in the equation yields

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.

Figure 38. Capacitance vs. DC Bias Voltage

Table 8. Suggested 1.0 μH Inductors

Vendor

Model

Dimensions (mm)

DCR (mΩ)

Murata

LQM2HPN1R0MJHL

2.5 × 2.0 × 1.2

3500

50

Wurth Electronics

74438323010

2.5 × 2.0 × 1.0

4000

75

Coilcraft®

XFL4020-102ME

4.0 × 4.0 × 2.1

5100

11.9

0

2

4

6

8

10

12

0

123

456

DC BIAS VOLTAGE (V)