ADP1653

Rev. A | Page 18 of 24

EXTERNAL COMPONENT SELECTION

Selecting the Inductor

The ADP1653 step-up converter increases the battery voltage to

allow driving one, two, or three LEDs, whose combined voltage

drop is higher than the battery voltage plus the 0.32 V (typical)

current source headroom voltage. This allows the converter to

regulate the HPLED current over the entire battery voltage range

and with a wide variation of LED forward voltage.

Users should choose an inductor value such that the inductor

ripple current is approximately 2/5th of the maximum dc input

load current. In general, lower inductance values have higher

saturation current and lower series resistance for a given physical

size. For most applications, an inductor in the range of 1.5 μH

to 3.3 μH works well.

To determine the inductor ripple current, users should first

calculate the switch duty cycle for the step-up converter, which

drop plus 320 mV (typical) overhead for the HPLED current

regulator.

D

V

V

V

F

OUT

IN

−

=

+

1

(14)

Solving for D

F

OUT

IN

F

OUT

F

OUT

IN

V

V

V

V

V

V

V

V

D

+

−

+

=

+

−

= 1

The HPLED (output) current is regulated as low as 50 mA

(torch mode) and as high as 500 mA (flash mode). The

maximum dc input current is related to the maximum dc

output current by the following equation:

η

V

V

I

I

IN

OUT

MAX

OUT

MAX

IN

1

)

(

)

(

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

=

(15)

where η is efficiency (assume η ≈ 0.80 in the two-LED case).

Choose the initial inductor value by using the equation

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

−

+

×

Δ

=

F

OUT

IN

F

OUT

SW

L

IN

V

V

V

V

V

f

I

V

L

(16)

where:

L is the inductor value (reduce L to reduce solution size).

maximum dc input current.

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 but improves loop stability, reducing

the amount of output capacitance required. Ensure that the peak

inductor current (dc + 1/2 of inductor ripple) is less than the

LX minimum current limit (1.5 A).

Selecting the Input Capacitor

The ADP1653 requires an input bypass capacitor to supply

transient currents while maintaining constant input and output

voltage. The input capacitor carries the input ripple current,

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

Use an input capacitor with sufficient ripple current rating to

handle the inductor ripple. A 4.7 μF X5R/X7R ceramic capacitor

rated for 6.3 V is the minimum recommended input capacitor.

Increased input capacitance reduces the amplitude of the switching

frequency ripple on the battery. Because of the dc bias charac-

teristics of ceramic capacitors, a 0603, 6.3 V X5R/X7R, 10 μF

ceramic capacitor is preferable.

Selecting the Diode

The ADP1653 is a nonsynchronous boost and, as such, requires

an external Schottky rectifier to conduct the inductor current to

the output capacitor and HPLEDs when the LX switch is off.

Ensure that the Schottky peak current rating is greater than the

maximum inductor current. Choose a diode with an average

current rating that is significantly larger than the maximum

LED current. To prevent thermal runaway, derate the Schottky

rectifier to ensure reliable operation at high junction temperatures.

Selecting the Output Capacitor

The output capacitor maintains the output voltage and supplies

the HPLED current when the LX switch is on. It also stabilizes

the loop. A 4.7 μF, 16 V X5R/X7R ceramic capacitor is generally

recommended. The minimum required capacitance for loop

stability for the two-LED and one-LED cases is shown in Figure 31

and Figure 32, respectively. Choose a capacitor with a capacitance

greater than the minimum shown in Figure 31 and Figure 32

for the worst case dc bias voltage and temperature condition.

Note that dc bias characterization data is available from

capacitor manufacturers and should be taken into account

when selecting input and output capacitors.