Preliminary Datasheet LP3385 High Performance, Constant Current Switching Regulator For 39PCS White LED Driver General Description Features The LP3385 is a 1.2MHz PWM boost switching regulator designed for constant- current white LED driver applications. The LP3385 can drive a string of up to 10 white LEDs in series, ensuring uniform brightness and eliminating several ballast resistors. The LP3385 implements a constant frequency 1.2MHz PWM control scheme. The high frequency PWM operation also saves board space by reducing external component sizes. To improve efficiency, the feedback voltage is set to 300 mV, which reduces the power dissipation in the current setting resistor. Highly integration and internal compensation network minimizes as 5 external component counts. Optimized operation frequency can meet the requirement of small LC filters value and low operation current with high efficiency. High Efficiency: 92% 1.2MHzFixed-Frequency PWM Operation Maximum Output Voltage up to 36V Operating Range : 2.7V to 6V Shutdown Supply Current:<1uA Available in SOT23-6 Package Built-in 36V Over Voltage Protection Minimize the External Component RoHS Compliant and 100% Lead (Pb)-Free Applications WLED Backlight driver OLED Backlight driver PDA MID/PTV Typical Application Circuit Ordering Information Vin 22uH 6 F: Pb-Free PWM 4 Vin EN 1 Cout 1uF OVP FB 5 3 2 Package: B6: SOT23-6 U80 LX Cin 10uF GND LP3385 LP3385 Pin Configurations VFB=300mV R1 15 Marking information Part Number LP3385B6F Top Marking F8 Package SOT23-6 Other Information please view website: www.lowpowersemi.com. (SOT23-6) LP3385 – 00 Version 1.0 Datasheet FEB.-2012 www.lowpowersemi.com Page 1 of 9 Preliminary Datasheet LP3385 Functional pin description SOT-23-6 Pin Name Pin Function 1 LX Switch Pin. Connect this Pin to inductor and catch diode. Minimize the track area to reduce EMI. 2 GND Ground Pin FB Feedback Reference Voltage Pin. Series connect a resistor between WLED and ground as a current sense. Sense the current feedback voltage to set the current rating. 4 EN Chip Enable (Active High). Voltage sensing input to trigger the function of over voltage protection. Note that this pin is high impedance. There should be a pull low 100kΩ resistor connected to GND when the control signal is floating. 5 OVP Over voltage pin. 6 Vin Supply Input Voltage Pin. Bypass 1uF capacitor to GND to reduce the input noise. 3 Function Block Diagram LP3385 – 00 Version 1.0 Datasheet FEB.-2012 www.lowpowersemi.com Page 2 of 9 Preliminary Datasheet LP3385 Typical Application Circuit Vin 22uH 6 4 EN 1 OVP FB 5 3 2 PWM Vin Cout 1uF LX U80 GND Cin 10uF LP3385 VFB=300mV R1 15 Vin 3S13P 6 4 EN 1 OVP FB 5 3 VFB=300mV R1 1.2 2 PWM Vin Cout 4.7uF LX U79 GND Cin17 10uF 22uH LP3385 LP3385 – 00 Version 1.0 Datasheet FEB.-2012 www.lowpowersemi.com Page 3 of 9 Preliminary Datasheet LP3385 Absolute Maximum Ratings Supply Input Voltage---------------------------------------------------------------------------------------------------−0.3V to 6.0V SW Input Voltage ---------------------------------------------------------------------------------------------------------−0.3V to 45V The Other Pins -----------------------------------------------------------------------------------------------------------−0.3V to 5.5V Power Dissipation, PD @ TA = 25°C SOT-23-6----------------------------------------------------------------------------0.455W Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------------------- 280°C Operation Temperature Range ------------------------------------------------------------------------------ −40°C to 80°C Storage Temperature Range ----------------------------------------------------------------------------------- −65°C to 150°C ESD Susceptibility HBM(Human Body Mode) ---------------------------------------------------------------------------------------MM((Machine Mode)) -------------------------------------------------------------------------------------------- 4KV 200V Electrical Characteristics Parameter Test Condition Symbol Min Typ. Max Units 6 V System Supply Input Operation voltage Range VDD Under Voltage Lock Out VDD Supply Current IDD Shut Down Current IDD 2.7 Line Regulation 2.4 V FB=0.2V, Switch 90 uA VEN < 0.4V 0.1 VIN : 3.0~4.3V 1 uA 2 % 1.2 MHz Oscillator Operation Frequency FOSC Maximum Duty Cycle Feedback Voltage 93 LP3385 285 % 300 315 mV MOSFET On Resistance of MOSFET RDS(ON) 0.2 Ω SW Current Limit ILM 2 A VOVP 36 V 5 μA Protection OVP Threshold OVP Sink Current Shut Down Voltage VEN Enable on Voltage VEN Minimum EN Dimming Threshold Minimum EN Dimming Threshold 0.4 0.7 V VFB=0 V 0.7 V VFB=300mV 1.4 V PWM EN Dimming range LP3385 – 00 Version 1.0 Datasheet V 100 FEB.-2012 www.lowpowersemi.com 500K Hz Page 4 of 9 Preliminary Datasheet LP3385 Typical Operating Characteristics LP3385 – 00 Version 1.0 Datasheet FEB.-2012 www.lowpowersemi.com Page 5 of 9 Preliminary Datasheet LP3385 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. 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. 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. 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. 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. Dimming control 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 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 LP3385 – 00 Version 1.0 Datasheet FEB.-2012 www.lowpowersemi.com Page 6 of 9 Preliminary Datasheet LP3385 Vin 22uH Vin 4 Vin EN 22uH 6 4 EN 1 2 PWM Vin OVP FB FB 5 3 2 Cout 1uF LX U80 GND Cin 10uF OVP LP3385 5 R3 R2 90k VDC 3 VFB=300mV 6 Cout 1uF 1 U77 LX Cin 10uF GND PWM signal should be higher than the maximum enable voltage of EN pin, in order to let the dimming control perform correctly. 5k R1 15 LP3385 VFB=300mV R1 15 c. Using a Filtered PWM signal Another common application is using a filtered PWM signal as an adjustable DC voltage for LED dimming control. A filtered PWM signal acts as the DC voltage to regulate the output current. The recomended application circuit is shown in the Figure 6. In this circuit, the output ripple depends on the frequency of PWM signal. For smaller output voltage ripple (<100mV), the recommended frequency of 2.8V PWM signal should be above 2kHz. To fix the frequency of PWM signal and change the duty cycle of PWM signal can get different output current. According to the application circuit of Figure 5, output current is from 20.5mA to 5.5mA by adjusting the PWM duty cycle from 10% to 90%. Vin 4 Vin EN 1 PWM OVP FB LP3385 R4 100K 5 3 VFB=195mV 6 Cout 1uF LX U78 2 b. Using a DC Voltage Using a variable DC voltage to adjust the brightness is a popular method in some applications. The dimming control using a DC voltage circuit is shown in Figure 4. According to the Superposition Theorem, as the DC voltage increases, the voltage contributed to VFB increases and the voltage drop on R2 decreases, i.e. the LED current decreases. For example, if the VDC range is from 0V to 2.8V, the 22uH GND Cin 10uF R2 R3 5k 90k Cout 0.1uF selection of resistors in Figure 4 sets dimming control of LED current from 20mA to 0mA. LP3385 – 00 Version 1.0 Datasheet FEB.-2012 www.lowpowersemi.com Page 7 of 9 R1 15 Preliminary Datasheet LP3385 Layout Consideration For best performance of the LP3385, the following guidelines must be strictly followed. ※ Input and Output capacitors should be placed close to the IC and connected to ground plane to reduce noise coupling. ※ The GND and Exposed Pad should be connected to a strong ground plane for heat sinking and noise protection. ※ Keep the main current traces as possible as short and wide. ※ LX node of DC-DC converter is with high frequency voltage swing. It should be kept at a small area. ※ Place the feedback components as close as possible to the IC and keep away from the noisy devices. LP3385 – 00 Version 1.0 Datasheet FEB.-2012 www.lowpowersemi.com Page 8 of 9 Preliminary Datasheet LP3385 Packing information LP3385 – 00 Version 1.0 Datasheet FEB.-2012 www.lowpowersemi.com Page 9 of 9