ADP1655ACBZ-R7 datasheet(20/24 Pages) AD

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ADP1655

Rev. 0 | Page 20 of 24

APPLICATIONS INFORMATION

EXTERNAL COMPONENT SELECTION

Selecting the Inductor

The ADP1655 boost converter increases the battery voltage

to allow driving of one or two LEDs, whose combined voltage

drop is higher than the battery voltage plus the current source

headroom voltage. This allows the converter to regulate the

LED current over the entire battery voltage range and with a

wide variation of LED forward voltage.

The inductor saturation current should be greater than the sum

of the dc input current and half the inductor ripple current. A

reduction in the effective inductance due to saturation increases

the inductor current ripple. Suggested inductors are shown in

Table 15.

Table 15. Suggested Inductors

Vendor

Value

(μH)

Part No.

DCR

(mΩ)

ISAT

(A)

Dimensions

L × W × H (mm)

Toko

2.2

FDSE0312

160

3.1

3 × 3 × 1.2

Toko

2.0

DE2812C

1.8

3.0 × 3.2 × 1.22

Coilcraft

2.2

LPS3010

220

1.4

3 × 3 × 1.0

Coilcraft

2.2

LPS3314

100

1.5

3 × 3 × 1.4

Selecting the Input Capacitor

The ADP1655 requires an input bypass capacitor to supply

transient currents while maintaining constant input and output

voltages. The input capacitor carries the input ripple current,

allowing the input power source to supply only the dc current.

Use an input capacitor with a sufficient ripple current rating to

handle the inductor ripple. Increased input capacitance reduces

the amplitude of the switching frequency ripple on the battery.

Because of the dc bias characteristics of ceramic capacitors, a

0603, 6.3 V X5R/X7R, 10 μF ceramic capacitor is preferable.

Higher value input capacitors help to reduce the input voltage

ripple and improve transient response. Maximum input

capacitor current is calculated using the following equation:

To minimize supply noise, place the input capacitor as close to

the VIN pin of the ADP1655 as possible. As with the output

capacitor, a low ESR capacitor is suggested. A list of suggested

input capacitors is shown in Table 16.

Table 16. Suggested Input Capacitors

Vendor

Value

Part No.

Dimensions

L × W × H (mm)

Murata

10 μF, 6.3 V

GRM188R60J106ME47

1.6 × 0.8 × 0.8

TDK

10 μF, 6.3 V

C1608JB0J106K

1.6 × 0.8 × 0.8

Tayio

Yuden

10 μF, 6.3 V

JMK107BJ106MA

1.6 × 0.8 × 0.8

Selecting the Output Capacitor

The output capacitor maintains the output voltage and supplies

the LED current during NFET power switch on period. It also

stabilizes the loop. A 10.0 μF, 16 V X5R/X7R ceramic capacitor

is suggested.

Note that dc bias characterization data is available from capa-

citor manufacturers and should be taken into account when

selecting input and output capacitors. 16 V capacitors are

recommended for most two-LED designs. Designs with 1 mm

height restrictions can also use 0603 case size, 16 V capacitors

in parallel. A list of suggested output capacitors is shown in

Table 17.

Table 17. Suggested Output Capacitors

Vendor

Value

Part No.

Dimensions

L × W × H (mm)

Murata

10.0 μF, 10 V

GRM21BR71A106KE51

2 × 1.25 × 1.25

Murata

10.0 μF, 16 V

GRM31CR61C106KA88

3.2 × 1.6 × 1.6

Tayio

Yuden

10.0 μF, 16 V

EMK212BJ106KG

2 × 1.25 × 1.25

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 different behavior over temperature and applied

voltage. Capacitors must have a dielectric that ensures the

minimum capacitance over the necessary temperature range

and dc bias conditions. X5R or X7R dielectrics with a voltage

rating of 10.0 V or 16 V are suggested for best performance.

Y5V and Z5U dielectrics are not suggested for use with any

dc-to-dc converter because of their poor temperature and dc

bias characteristics.

MAX

LOAD

CIN

)

(

≥

OUT

)

(

−

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