ADP1660ACBZ-R7 datasheet(24/28 Pages) AD

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ADP1660

Data Sheet

Rev. 0 | Page 24 of 28

APPLICATIONS INFORMATION

EXTERNAL COMPONENT SELECTION

Selecting the Inductor

The ADP1660 boost converter increases the battery voltage to

allow driving of two LEDs when the forward voltage of the LEDs

is higher than the battery voltage minus 2× 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 voltages.

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 ripple current. Table 28 provides a list of suggested

inductors.

Table 28. Suggested Inductors

Vendor

Value

(µH)

Part No.

DCR

(mΩ)

ISAT

(A)

Dimensions

L × W × H (mm)

Toko

1.0

FDSD0312

4.5

3.0 × 3.0 × 1.2

Toko

1.0

DFE2520

3.4

2.5 × 2.0 × 1.0

Coilcraft

1.0

XFL3010

2.4

3.0 × 3.0 × 1.0

Murata

1.0

LQM32P_G0

3.2 × 2.5 × 1.0

FDK

1.0

MIP3226D

3.2 × 2.6 × 1.0

Selecting the Input Capacitor

The ADP1660 requires an input bypass capacitor to supply tran-

sient 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.

Increased input capacitance reduces the amplitude of the switching

frequency ripple on the battery. Due to the dc bias characteristics

of ceramic capacitors, the recommended capacitor is a 10.0 µF,

6.3 V, X5R/X7R ceramic capacitor.

Higher input capacitor values help to reduce the input voltage

ripple and improve transient response.

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

the VIN pin of the ADP1660 as possible. A low ESR capacitor is

required. Table 29 provides a list of suggested input and output

capacitors.

Table 29. Suggested Input and Output 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

Taiyo

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 the on period of the N-FET power

switch. It also stabilizes the loop. The recommended capacitor

is a 10.0 µF, 6.3 V, X5R/X7R ceramic capacitor (see Table 29).

Note that dc bias characterization data is available from capacitor

manufacturers and should be taken into account when selecting

input and output capacitors. Capacitors of 6.3 V or 10 V are best

for most designs.

Higher output capacitor values reduce the output voltage ripple

and improve load transient response. When selecting an output

capacitor value, it is also important to account for the loss of

capacitance due to output voltage dc bias.

Ceramic capacitors have a variety of dielectrics, 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 6.3 V or 10 V are recommended

for best performance. Y5V and Z5U dielectrics are not recom-

mended 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.

For example, a 10 μF, X5R capacitor has the following

characteristics:

TEMPCO from −40°C to +85°C is 15%.

TOL is 10%.

COUT at VOUT (MAX) = 5 V is 3 μF (see Figure 31).

–10

–20

–30

–40

–50

–60

–70

–80

–90

6.30

5.04

3.78

2.52

DC BIAS VOLTAGE (V)

1.26

Figure 31. DC Bias Characteristic of a 10 μF, 6.3 V Ceramic Capacitor

Substituting these values in the equation yields

CEFF = 3 μF × (1 − 0.15) × (1 − 0.1) = 2.3 μF

The effective capacitance needed for stability, which includes

temperature and dc bias effects, is 3.0 μF.

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