8

Date: 5/5/04

SP6682 High Efficiency Charge Pump Regulator for White LEDs

© Copyright 2004 Sipex Corporation

tor leads to larger output resistance, thus de-

creasing the output current capability and the

circuit efficiency. Place all the capacitors as

close to the SP6682 as possible for layout.

Increasing the value of the input and output

capacitors could further reduce the input and

output ripple.

Refer to Table 1 for some suggested low ESR

capacitors.

MANUFACTURERS/

PART NUMBER

CAPACITANCE/

CAPACITOR/

ESR

TELEPHONE#

VOLTAGE

SIZE/TYPE

AT 100kHz

TDK/847-803-6100

C2012X5R1A225K

2.2µF/10V

0805/X5R

0.030Ω

TDK/847-803-6100

C2012X5R0J475K

4.7µF/6.3V

0805/X5R

0.020Ω

MURATA/770-436-1300

GRM188R60J225KE01D

2.2µF/6.3V

0603/X5R

0.030Ω

MURATA/770-436-1300

GRM219R60J475KE01D

4.7µF/6.3V

0805/X5R

0.020Ω

Brightness Control Using PWM

Dimming control can be achieved by applying a

PWM control 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 the operating frequency

range of the PWM control is from 60Hz to

700Hz, the recommended maximum brightness

frequency range of the PWM signal is from

60Hz to 200Hz. A repetition 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 manufacturing tol-

erance, thus causing uneven brightness 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 LEDs.

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 resis-

tor, 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.

Rb1

8

V

FB

D1

Rb

Dn

GND

VFn

VF1

V

OUT

1

Rb2

Rb

VF2

Rb

5

D2

SP6682

I

1

I

2

I

n

Figure 14. Increasing brightness matching

Table: 1

SUGGESTED LOW ESR CAPACITORS

Power Efficiency

The efficiency of driving the white LEDs can be

calculated by:

η =

=

≈

APPLICATION INFORMATION