ADP5138

Data Sheet

Rev. A | Page 20 of 23

APPLICATIONS INFORMATION

INPUT CAPACITOR SELECTION

The input capacitor reduces the input voltage ripple caused by

the switch current on PVINx. Place the input capacitor as close

as possible to the PVINx pin. A ceramic capacitor in the 10 μF

to 47 μF range is recommended. The loop composed of the input

capacitor, the high-side MOSFET, and the low-side MOSFET

must be kept as small as possible.

The voltage rating of the input capacitor must be greater than

the maximum input voltage. Ensure that the rms current rating

of the input capacitor is larger than the value calculated from

the following equation:

)

1

(

_

D

D

I

I

OUT

RMS

CIN

−

×

×

=

where:

OUTPUT VOLTAGE SETTING

programmed by an external resistor divider.

If the output voltage is factory set, connect the FBx pin to the

output voltage directly.

If the output voltage is programmable, use the following

equation to set the output voltage:













+

×

=

BOT

TOP

OUT

R

R

V

1

8

.

0

where:

Table 8. Resistor Divider Values for Various Output Voltages

1.0

4.99

20

1.2

10

20

1.5

10

11.5

1.8

18.7

15

2.5

24.3

11.5

3.3

35.7

11.5

INDUCTOR SELECTION

The inductor value is determined by the operating frequency,

input voltage, output voltage, and inductor ripple current. Using

a small inductor value leads to a faster transient response but

degrades efficiency due to a larger inductor ripple current. Using

a large inductor value leads to a smaller ripple current and

better efficiency but results in a slower transient response.

As a guideline, an inductor with its value in the range from

0.68 µH to 2.2 µH is recommended for the best balance between

transient and efficiency performance. The inductor ripple

current.

Use the following equation to calculate the inductor value:

SW

L

OUT

IN

f

I

D

V

V

L

×

∆

×

−

=

)

(

where:

Use the following equation to calculate the peak inductor current:

2

L

OUT

PEAK

I

I

I

∆

+

=

The saturation current of the inductor must be larger than the

peak inductor current. For ferrite core inductors with a quick

saturation characteristic, the saturation current rating of the

inductor must be higher than the current-limit threshold of the

switch to prevent the inductor from reaching saturation.

Use the following equation to calculate the rms current of the

inductor:

12

2

2

L

OUT

I

I

∆

+

Shielded ferrite core materials are recommended for low core

loss and low EMI.

OUTPUT CAPACITOR SELECTION

The output capacitor selection affects both the output voltage

ripple and the loop dynamics of the regulator. The ADP5138

operates with small ceramic capacitors that have low equivalent

series resistance (ESR) and low equivalent series inductance

(ESL) and can, therefore, easily meet the output voltage ripple

specifications.

When the regulator operates in continuous conduction mode, the

overall output voltage ripple is the sum of the voltage spike caused

by the output capacitor ESR plus the voltage ripple caused by the

charging and discharging of the output capacitor.

















+

×

×

×

∆

=

∆

OUT

C

OUT

SW

L

RIPPLE

ESR

C

f

I

V

8

1

where:

Capacitors with lower ESR are preferable to guarantee low

output voltage ripple, as shown in the following equation:

L

RIPPLE

C

I

V

ESR

OUT

∆

∆

≤

Ceramic capacitors are manufactured with a variety of dielectrics,

each with different behavior over temperature and applied voltage.

X5R or X7R dielectrics are recommended for best performance

due to the low ESR and small temperature coefficients.