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Date: 07/07/04

SP6683 White LEDs Driver for Parallel Configuration

© Copyright 2004 Sipex Corporation

APPLICATION INFORMATION: Continued

Since the brightness of the white LED is propor-

tional to the operating current, for better bright-

ness matching, a higher output voltage could be

used. This could be done by using larger resistor,

as shown in figure 14. Rb2 is used to bias the

operating current of the white LED, Rb1 is use to

increase the output voltage. Better brightness

matching was achived at the cost of the power

wasted on the bias resistor.

Power Efficiency

The efficiency of driving the white LEDs can be

calculated by

η =

=

≈

are the forward voltage and operating current of

Rb1

8

V

FB

D1

Rb

Dn

GND

VFn

VF1

V

OUT

1

Rb2

Rb

VF2

Rb

5

D2

SP6683

I

1

I

2

I

n

Figure 14. Increasing brightness matching

Brightness Control Using PWM

Dimming control can be achieved by applying

a PWM contol signal to the EN/PWM pin. The

brightness of the white LEDs is controlled by

increasing and decreasing the duty cycle of the

PWM signal. While operating frequency range

is from 60Hz to 700Hz, the recomended

maxium brightness frequncy range is from

60Hz to 200Hz. A repition rate of at least 60Hz

is required to prevent flicker.

Brightness Matching

For white LEDs, the forward voltage drop is a

function of the operating current. However,

for a given current, the forward voltage drops

do not always match due to normal manufac-

turing tolerance, thus causing uneven bright-

ness of the white LEDs.

In figure 14, assume high-precision bias resis-

tors were used, the operating current ratio of

two different branches can be easily derived as

shown by:

=

white

white LEDs.