Diodes AP5726 Manuel d'utilisateur télécharger pdf (Page 9)

AP5726

WHITE LED STEP-UP CONVERTER

AP5726

Document number: DS31845 Rev. 3 - 2

9 of 17

www.diodes.com

July 2010

Applications Information

Inductor Selection

A 10μH~22μH inductor is recommended for most AP5726 applications. For high efficiency the inductor should

have low core losses at 1.2MHz and low DCR (copper wire resistance). The inductor saturation current rating

should also exceed the peak input current, especially for high load current applications (like 3S8P).

Capacitor Selection

The small size of ceramic capacitors are ideal for AP5726 applications. X5R and X7R types are recommended

because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V

or Z5U. A 1μF input capacitor and a 1μF output capacitor are sufficient for most AP5726 applications. For high

output current applications like 3S8P, larger output capacitor of 2.2uF ~ 4.7uF is recommended to minimize

output ripple.

Diode Selection

Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for AP5726

applications. The forward voltage drop of a Schottky diode represents the conduction losses in the diode, while

the diode capacitance (C

or C

) represents the switching losses. For diode selection, both forward voltage drop

and diode capacitance need to be considered. Schottky diodes with higher current ratings usually have lower

forward voltage drop and larger diode capacitance. Larger Schottky diode capacitance can cause significant

switching losses at the 1.2MHz switching frequency of the AP5726. A Schottky diode rated at 100mA to 200mA is

sufficient for most AP5726 applications.

LED Current Control

The LED current is controlled by the feedback resistor (R

SET

in Figure 1). The feedback reference is 0.31V. The

LED current is 0.31V/ R

SET

. In order to have accurate LED current, precision resistors are preferred (1% is

recommended). The formula and table for R

SET

selection are shown below.

SET

= 0.31V/I

LED

(See Table 1)

Table 1. R

SET

Resistor Value Selection

LED

(mA) R

SET

(Ω)

5 62

10 31

15 20.6

20 15.5

30 10.3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

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Diodes AP5726 Manuel d'utilisateur Page 9

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