8

Date: 7/1

9/06 Rev D

SP6682 High Efficiency Charge Pump Regulator for White LEDs

© 2006 Sipex Corporation

APPLICATION INFORMATION: Continued

BRIGHTNESS CONTROL USING PWM

Dimmingcontrolcanbeachievedbyapplying

a PWM control signal to the EN/PWM pin.

The brightness of the white LEDs is con-

trolledbyincreasinganddecreasingtheduty

cycleofthePWMsignal.Whiletheoperating

frequency range of the PWM control is from

60Hz to 700Hz, the recommended maxim-

um brightness frequency range of the PWM

signalisfrom60Hzto200Hz.Arepetitionrate

of at least 60Hz is required to prevent

flicker.

BRIGHTNESS MATCHING

For white LEDs, the forward voltage drop is

afunctionoftheoperatingcurrent.However,

foragivencurrent,theforwardvoltagedrops

do not always match due to normal man-

ufacturing tolerances, thus causing uneven

brightness of the white LEDs.

In Figure 15, assume high-precision bias

resistors were used, the operating current

ratio of two different branches can be easily

derived as shown by:

put current capability and the circuit effi-

ciency. Place all the capacitors as close to

the SP6682 as possible for layout. Increas-

ing the value of the input and output capaci-

tors could further reduce the input and out-

put ripple.

Refer to Table 1 for some suggested low

ESR capacitors.

MANUFACTURER PART NUMBER

CAPACITANCE/

CAPACITOR/ ESR

VOLTAGE

SIZE/TYPE

AT 100kHz

TDK

C2012X5R1A225K

2.2µF/10V

0805/X5R

0.030�

TDK

C2012X5R0J475K

4.7µF/6.3V

0805/X5R

0.020�

MURATA

GRM188R60J225KE01D

2.2µF/6.3V

0603/X5R

0.030�

MURATA

GRM219R60J475KE01D

4.7µF/6.3V

0805/X5R

0.020�

Table 1:

Suggested Low ESR Capacitors

age of the white LEDs.

Since the brightness of the white LED is

proportional to the operating current, for

better brightness matching, a higher output

voltage could be used. This could be done

by using a larger resistor, as shown in

increase the output voltage. Better bright-

ness matching was achieved at the cost of

the power wasted on the bias resistor.

Rb1

8

D1

Rb

Dn

GND

VFn

VF1

Rb2

Rb

VF2

Rb

5

D2

SP6682

Figure 15. Increasing brightness matching