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SP6682 Datasheet(PDF) 8 Page - Exar Corporation |
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SP6682 Datasheet(HTML) 8 Page - Exar Corporation |
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8 / 13 page  S SP P6 66 68 82 2 H Hiig gh h E Effffiic ciie en nc cy y W Wh hiitte e L LE ED D C Ch ha arrg ge e P Pu um mp p R Re eg gu ulla atto orr © 2009 Exar Corporation 8/13 Rev. 2.0.0 use this feature, divider resistors should be chosen according to the specific applica-tion, as shown in Figure 16. Fig. 16: Programming the VMODE Resistors The guideline for divider resistor selections is as follows: For high input voltage, the SP6682 will work in X1.5 mode. When the input voltage drops to the VTH threshold voltage, it will switch to X2 mode automati-cally. The VTH threshold voltage for mode change can be calculated by: () 5 . 1 306 . 0 OUT LED F TH R I M V V × × + + = Where VF and M are the forward voltage and number of the white LEDs, ROUT is the output resistance of the SP6682. The equation for the voltage divider R1 and R2 with VMODE = 1.25V is: ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + × = 2 1 1 25 . 1 R R V TH which can be expressed as R1: 2 1 25 . 1 1 R V R TH × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = For the typical SP6682 application, using VF=3.6V, M=4, ILED=15mA, ROUT=16Ω, the VTH will be 3.24V. Select R2=100kΩ, then R1=158kΩ. CAPACITOR SELECTION Ceramic capacitors are recommended for their inherently low ESR, which will help produce low peak to peak output ripple, and reduce high frequency spikes. The fly capacitor controls the strength of the charge pump. Selection of the fly capacitor is a trade-off between the output voltage ripple and the output current capability. Decreasing the fly capacitor will reduce the output voltage ripple because less charge will be delivered to the output capacitor. However, smaller fly capacitors lead to larger output resistance, thus decreasing the out-put current capability and the circuit effi-ciency. Place all the capacitors as close to the SP6682 as possible for layout. Increasing 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 Voltage Capacitor Size/Type ESR @ 100KHz TDK C2012X5R1A225K 2.2µF/10V 0805/X5R 0.030Ω TDK C2012X5R0J475K 4.7µF/6.3V 0805/X5R 0.020Ω Murata GRM188R60J225KE01D 2.2µF/6.3V 0805/X5R 0.030Ω Murata GRM219R60J475KE01D 4.7µF/6.3V 0805/X5R 0.020Ω Table 1: Suggested Low ESR Capacitors 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 con-trolled 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 maxim-um brightness frequency range of the PWM signal is from 60Hz to 200Hz. A repetition rate of at least 60Hz is required to prevent flicker. |
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Similar Description - SP6682 |
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