7

Date: 5/5/04

SP6682 High Efficiency Charge Pump Regulator for White LEDs

© Copyright 2004 Sipex Corporation

In this application, the bias resistor can be se-

lected by:

of all the white LEDs.

To use SP6682 as a voltage source for fixed

voltage applications, a voltage divider is need to

program the ouput voltage, as shown in figure 12.

The output voltage is set by the ratio of the two

Anode

5

8

SP6682

Cathode

V

OUT

R6

1

R5

2.2uF

V

FB

2-wire W-LED

module

GND

Figure 12. Driving 2-wire white LED module as voltage

source

resistors and the feedback control voltage as

shown by:

Programming the Operating Mode

SP6682 can automatically change from X1.5

mode to X2 mode for highest efficiency. To use

this feature, divider resistors should be chosen

according to the specific application, as shown

in figure 13.

The guideline for divider resistor selections is as

follows. For high input voltage, the SP6682 will

work in X1.5 mode. When the input voltage

drops to Vth threshold voltage, it will switch to

X2 mode automatically. The Vth threshold volt-

age for mode change can be calculated by:

number of the white LEDs, Rout is the output

resistance of the SP6682.

which can be expressed as R1:

For the typical SP6682 application, Using

Capacitor Selection

Ceramic capacitors are recommended for their

inherently low ESR, which will help produce

low peak to peak output ripple, and reduce high

frequency spikes.

The fly capacitor controls the strength of the

charge pump. Selection of the fly capacitor is a

trade-off between the output voltage ripple and

the output current capability. Decreasing the fly

capacitor will reduce the output voltage ripple

because less charge will be delivered to the

output capacitor. However, smaller fly capaci-

V

IN

V

IN

C2

GND

4

V

MODE

R1

SP6682

8

R2

3

Figure 13. Programming the Vmode Resistors

APPLICATION INFORMATION: Continued