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LP3385 Datasheet(PDF) 6 Page - Lowpower Semiconductor inc |
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LP3385 Datasheet(HTML) 6 Page - Lowpower Semiconductor inc |
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6 / 9 page  Preliminary Datasheet LP3385 LP3385 – 00 Version 1.0 Datasheet FEB.-2012 www.lowpowersemi.com Page 6 of 9 Applications Information LED Current Control The LP3385 regulates the LED current by setting the current sense resistor (R1) connecting to feedback and ground. The internal feedback reference voltage is 300mV. The LED current can be set from following equation easily. ILED=300mV/R1 In order to have an accurate LED current, precision resistors are preferred (1% is recommended). The table for R1 selection is shown below. Power Sequence In order to assure the normal soft start function for suppressing the inrush current the input voltage should be ready before EN pulls high. Soft-Start The function of soft-start is made for suppressing the inrush current to an acceptable value at the beginning of power-on. The LP3385 provides a built-in soft-start function by clamping the output voltage of error amplifier so that the duty cycle of the PWM will be increased gradually in the soft-start period. Current Limiting The current flow through inductor as charging period is detected by a current sensing circuit. As the value comes across the current limiting threshold, the N-MOSFET will be turned off so that the inductor will be forced to leave charging stage and enter discharging stage. Therefore, the inductor current will not increase over the current limiting threshold. OVP/UVLO/OTP The Over Voltage Protection is detected by a junction breakdown detecting circuit. Once VOUT goes over the detecting voltage, LX pin stops switching and the power N-MOSFET will be turned off. Then, the VOUT will be clamped to be near VOVP. As the output voltage is higher than a specified value or input voltage is lower than a specified value, the chip will enter protection mode to prevent abnormal function. As the die temperature is higher then 160℃, the chip also will enter protection mode. The power MOSFET will be turned off during protection mode to prevent abnormal operation. Inductor Selection The recommended value of inductor for 2 to 8WLEDs applications are 4.7 to 22µH. Small size and better efficiency are the major concerns for portable device, such as LP3385 used for mobile phone. The inductor should have low core loss at 1.3MHz and low DCR for better efficiency. To avoid inductor saturation current rating should be considered. Capacitor Selection Input ceramic capacitor of 2.2uF and output ceramic capacitor of 1uF are recommended for the LP3385 applications for driving 8 series WLEDs. For better voltage filtering, ceramic capacitors with low ESR are recommended. X5R and X7R types are suitable because of their wider voltage and temperature ranges. Dimming control a. Using a PWM Signal to EN Pin For controlling the LED brightness, the LP3385 can perform the dimming control by applying a PWM signal to EN pin. The internal soft-start and wide range dimming frequency from 100Hz to 500KHz can insignificantly reduce audio noise when dimming. The average LED current is proportional to the PWM signal duty cycle. The magnitude of the |
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