4

Date: 07/07/04

SP6683 White LEDs Driver for Parallel Configuration

© Copyright 2004 Sipex Corporation

306mV reference voltage is compared to feed-

back output voltage to control the Vout needed

for the application. Output current is set by a

bias resistor from FB pin to GND pin chosen by

the relationship:

Configuring the SP6683 as Voltage or

Current Source

The white LED load configuration used by

customers can be discrete white LEDs or a white

LED module. Inside the white LED module,

there may or may not be resistors in series with

the white LEDs. According to the different

application requirements, the SP6683 can be

configured as either a voltage source or a current

source to provide solutions for these different

applications, as shown in figure 9~12. Figure 9

shows using the SP6683 to drive discrete white-

LEDs as a current source.

The current in one white LED current is set by

the ratio of the feedback pin voltage (306mV)

The current of the remaining white LEDs is set

according to the similarity of the white LEDs. 3-

wire white LED module with internal series

resistors as shown in figure 10 can also be driven

1

8

5

Rb

SP6683

Rb

1uF

GND

FB

V

OUT

Figure 9. Driving discrete white LEDs as current source

General Overview

The SP6683 is a current regulated charge pump

ideal for converting a Li-Ion battery input for

driving white LEDs used in backlighting color

displays in cellular phones, PDAs, digital cam-

eras and MP3 players. The SP6683 is able to

efficiently drive up to eight 20mA white LEDs

in parallel and maintain a constant brightness

over a very wide operating voltage range (2.7V

to 5.5V). The SP6683 operates with an internal

1.2MHz clock, enabling the use of small exter-

nal components. Other features of SP6683 in-

clude PWM dimming control as well as com-

plete input/out disconnect in shutdown. In shut

down mode the IC draws less than 1.5

µA cur-

rent. The output regulation is achieved by sens-

ing the voltage at the feedback pin and modulat-

ing the switch between the charge pump and

output capacitor.

Theory of Operation

The SP6683 regulated charge pump block dia-

gram consists of four main blocks (Voltage

Reference, Mode Control, Clock Manager, Start-

up and Charge-Pump Switches) and two com-

parator).

1) Voltage Reference. This block provides the

306mV and 1.25V reference voltages needed

for the two comparators.

2) Mode Control. An external voltage divider

voltage to the mode comparator which sets the

logic state of the mode selection outputs to the

1.25V bandgap voltage. For example, if one

makes a 158K/100K divider, the mode will

change at 2.58 x 1.25 V =3.23V. A comparator-

based cycle by cycle regulation ensures that no

mode change occurs during cycles.

3) Clock Manager. An internal 1.2MHz clock

is generated in this block. Depending on the

mode control, the appropriate clock phasing is

generated here and sent to the start-up and

charge-pump switches block.

OPERATION