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SP6682 Datasheet(PDF) 8 Page - Sipex Corporation |
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SP6682 Datasheet(HTML) 8 Page - Sipex Corporation |
8 / 12 page 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 I1 = VOUT - VF1 I2 VOUT - VF2 where I1, I2 are the operating current of the white LEDs andVF1,VF2 aretheforwardvolt- 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 Figure 15. RB2 is used to bias the operating current of the white LED, RB1 is used to increase the output voltage. Better bright- ness matching was achieved at the cost of the power wasted on the bias resistor. Rb1 8 VFB D1 Rb Dn GND VFn VF1 VOUT 1 Rb2 Rb VF2 Rb 5 D2 SP6682 I1 I2 In Figure 15. Increasing brightness matching |
Similar Part No. - SP6682_06 |
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Similar Description - SP6682_06 |
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