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SP6682 Datasheet(PDF) 6 Page - Exar Corporation |
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SP6682 Datasheet(HTML) 6 Page - Exar Corporation |
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6 / 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 6/13 Rev. 2.0.0 THEORY OF OPERATION OVERVIEW The SP6682 is a current regulated charge pump ideal for converting a Li-Ion battery input for driving white LEDs used in back- lighting color displays, Cell Phone Camera Flash, PDAs, digital cameras and MP3 players. The SP6682’s proprietary AutoBoost feature enables the IC to automatically trans-ition from X1.5 boost mode to X2 boost mode based on battery input voltage for optimal efficiency and performance. The SP6682 is able to efficiently drive up to six 20mA white LEDs in parallel and maintain a constant brightness over a very wide operating voltage range (2.7V to 5.5V). The SP6682 operates with an internal 600kHz clock, enabling the use of small external components. Other features of SP6682 include PWM dimming control as well as complete input/out disconnect in shutdown. In shutdown mode the IC draws less than 1.5μA current. The output regulation is achieved by sensing the voltage at the feedback pin and modulating the switcher between the charge pump and output capacitor. OPERATION The SP6682 regulated charge pump block diagram consists of four main blocks (Voltage Reference, Mode Control, Clock Manager, Start-up and Charge-Pump Switches) and two comparators (VMODE Comparator and VOUT Comparator). Voltage Reference. This block provides the 306mV and 1.25V reference voltages needed for the two comparators. Mode Control An external voltage divider connected to the VMODE pin will define an input voltage to the mode comparator which sets the logic state of the mode selec-tion outputs to the x2 or x1.5 modes. VMODE is compared to a 1.25V bandgap voltage. For example, if one makes a 158KΩ/100KΩ divider, the mode will change at 2.58 x 1.25 V = 3.23V. A comparator-based cycle-by-cycle regulation ensures that no mode change occurs during cycles. Clock Manager An internal 600 kHz clock is generated in this block. Depending on the mode control, the appropriate clock phasing is generated here and sent to the start-up and charge-pump switches block. Start-up and Charge Pump Switches During start-up, until the reference is established, this block keeps the charge pump inactive. During this period the output stays floating, and by consequence the charge pump drivers are now referenced to VOUT.Charging of the output will occur (e.g. when VIN is ramped up to 4.2V, VOUT ramps only up to about 3V), but not to the value of VIN,protecting the White LEDs from experiencing high input voltages. Another important operation of this block is the PWM/EN dim-ming control, which is implemented in the delay of each pump driver, so that the enable high pulse width is proportional to the delay of the individual pump switches. VOUT Comparator and Output Control A 306mV reference voltage is compared to feedback output voltage to control the VOUTneeded for the application. Output current is set by a bias resistor from FB pin to GND pin chosen by the relationship: FB FB OUT R V I = where VFB = 306mV. CONFIGURING THE SP6682 AS VOLTAGE OR CURRENT SOURCE The white LED load configuration used by customers can be discrete white LEDs or a white LED module. Inside the white LED module, there may or may not be resistors in series with the white LEDs. According to the different application requirements, the SP6682 can be configured as either a voltage source or a current source to provide solutions for these |
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