PL6T611 High Efficiency LED Driver

Transcription

1 High Efficiency LED Driver Description The is a current-mode and fixed frequency 600kHz boost converter with an integrated N-FET to drive white LEDs. The series connection allows the LED current to be identical for uniform brightness. Its low on-resistance of NFET and low feedback voltage reduce power loss and achieve high efficiency. 600kHz constant switching frequency allows using small-size inductor and both of input and output capacitors. An over voltage protection function, which monitors the output voltage via LX pin, stops switching of the IC if the LX voltage exceeds the over voltage threshold. An internal soft-start circuit eliminates the inrush current during start-up. The also integrates under-voltage lockout and over-temperature protection to protect the IC in abnormal conditions. The is available in SOT-23-6L package. Feature 2.7V to 5.5V input voltage range 200mV reference voltage with ±2% accuracy Flexible digital and PWM brightness control Up to 85% efficiency Built-in soft start SOT-23-6L package Application Cellular phones Portable media players Ultra mobile devices GPS receivers White LED backlighting for media form factor display Device Information Top View Pin Meaning 1 LX Switch pin. Connect this pin to inductor/diode here. 2 GND Power and signal ground pin. 3 FB Feedback Pin. Connect this pin to cathode of the lowest LED and current-sense resistor (R1). Calculate resistor value according to R1=V REF /I LED 4 EN Enable Control Input. Forcing this pin above 1.0V enables the device, or forcing this pin below 0.4V to shut it down. In shutdown, all functions are disabled to decrease the supply current below 1uA. 5 NC No Commend. 6 VIN Main Supply Pin. Must be closely decoupled to GND with a 1uF or greater ceramic capacitor. Rev

2 Typical Application of V IN C1 1μF L1 22μH V IN LX GND C2 1μF V OUT 10 strings PWM Dimming EN FB R1=10Ω 20mA Figure 1. Typical Application of Output Current Setting In figure 1, the converter regulates the voltage on FB pin, connected with the cathode of the lowest LED and the currentsense resistor R1 at V REF. Therefore, the current (I LED ), flowing via the LEDs and the R1, is calculated by the following equation: I LED = V REF R1 Rev

3 Absolute maximum Parameter Symbol Value Units Input Voltage V IN -0.3~6.5 V LX Voltage V LX -0.3~40 V Feed Back Voltage V FB -0.3~V IN +0.3 V EN Voltage V EN -0.3~V IN +0.3 V Storage Temperature T STG -65~150 Maximum Junction Temperature T J 150 Junction-to-Ambient Resistance in free air Θ JA 165 /W Junction-to-Case Resistance Θ JC 20 /W Recommended Operating Conditions Parameter Symbol Value Units Input Voltage V IN 2.7~5.5 V Input Capacitor C IN 1~ μf Output Capacitor C OUT 1~ μf Converter Output Inductor L1 10~22 μh Ambient Temperature T A -40~85 Junction Temperature T J -40~125 Rev

4 Electrical characteristics per (unless otherwise specified) Parameter Symbol Conditions Min. Typ. Max. Unit s Input Voltage V IN V I DD1 V FB =0.4V,no switching 420 μa Input DC Bias Current I DD2 V FB =GND, switching ma I SD EN=GND 1 μa UVLO Threshold Voltage V IN Rising V UVLO Hysteresis Voltage V IN Falling mv Regulated Feedback Voltage V REF V IN =2.7V~5.5V, T A = V V IN =2.7V~5.5V, T A = V Feedback Input Current I FB V FB =1.23V na Switching Frequency F SW khz Power Switch On Resistance R ON V IN =3.6V V IN =3.0V 0.7 Ω LX Leakage Current V EN =0V,V LX =6V, V IN =5.5V -1 1 μa V EN =0V,V LX =35V, V IN =5.5V 10 μa LX Maximum Duty Cycle D MAX % Over Voltage Threshold V LX_OVP V LX Rising V N-Channel MOSFET Current Limit I LIM Duty=D MAX A Enable Voltage Threshold V EN Rising 1 V Shutdown Voltage Threshold V EN Falling 0.4 V EN Pulled Low Resistance kω EN Dimming Minimum Enable Pulsed Width In Normal Operation 100 ns EN Minimum On Pulsed Width Use VEN=3V to enable to device, PWM Dimming Frequency=5kHz to 100kHz 1 μs Duty Cycle of PWM Dimming In Normal Operation % Rev

5 Block Diagram Function Description Main Control Loop The is a constant frequency current-mode switching regulator. During normal operation, the internal N-channel power MOSFET is turned on each cycle when the oscillator sets an internal RS latch and turned off when an internal comparator (ICMP) resets the latch. The peak inductor current at which ICMP resets the RS latch is controlled by the voltage on the internal COMP node, which is the output of the error amplifier (EAMP). An external current-sense resistor connected between cathode of the lowest LED and ground allows the EAMP to receive a current feedback voltage V FB at FB pin. When the LEDs voltage decreases to cause the LEDs current to decrease, it causes a slightly decrease in V FB relative to the reference voltage, which in turn causes the internal COMP voltage to increase until the LEDs current reaches the set point. Rev

6 V IN Under-Voltage Lockout (UVLO) The Under-Voltage Lockout (UVLO) circuit compares the input voltage at V IN with the UVLO threshold (2.2V rising, typical) to ensure the input voltage is high enough for reliable operation. The 100mV (typ) hysteresis prevents supply transients from causing a restart. Once the input voltage exceeds the UVLO rising threshold, startup begins. When the input voltage falls below the UVLO falling threshold, the controller turns off the converter. Soft-Start The has a built-in soft-start to control the N channel MOSFET current raises during start-up. During softstart, an internal ramp voltage connected to one of the inverting inputs of the current limit comparator. The inductor current limit is proportional to the voltage. When the threshold voltage of the internal soft-start comparator is reached, the full current limit is released. Current-Limit Protection The monitors the inductor current flowing through the N-channel MOSFET, and limits the current peak at current-limit level to prevent loads and the device from damages in overload conditions. Over-Temperature Protection (OTP) The over-temperature circuit limits the junction temperature of the. When the junction temperature exceeds 150, a thermal sensor turns off the power MOSFET, allowing the device to cool. The thermal sensor allows the converter to start a soft-start process and regulate the LEDs current again after the junction temperature cools by 40. The OTP is designed with a 40 hysteresis to lower the average Junction Temperature (T J ) during continuous thermal overload conditions, increasing the lifetime of the device. Enable/Shutdown Driving EN to ground places the in shutdown mode. When in shutdown, the internal power MOSFET turns off, all internal circuitry shuts down and the quiescent supply current reduces to 1mA maximum. This pin also could be used as a digital input allowing brightness controlled by using a PWM signal with frequency from 5kHz to 100kHz. The 0% duty cycle of PWM signal corresponds to zero LEDs current and 100% corresponds to full one. If use EN Pin to enable the device, suggestion dimmimg duty range is from 3% to 100% at 100kHz dimming frequency. Open-LED Protection In driving LED applications, the feedback voltage on FB pin falls down if one of the LEDs, in series, is failed. Meanwhile, the converter unceasingly boosts the output voltage like an open-loop operation. Therefore, an overvoltage protection monitoring the output voltage via LX pin prevents the LX and the output voltages from exceeding their maximum voltage ratings. Once the voltage on the LX pin rises above the OVP threshold, the converter stops switching and prevents the output voltage from rising. The converter can work again when the LX voltage falls below the falling of OVP voltage threshold. Rev

7 Application Information Input Capacitor Selection The input capacitor (C IN ) reduces the ripple of the input current drawn from the input supply and reduces noise injection into the IC. The reflected ripple voltage will be smaller when an input capacitor with larger capacitance is used. For reliable operation, it is recommended to select the capacitor with maximum voltage rating at least 1.2 times of the maximum input voltage. The capacitors should be placed close to the VIN and the GND. Inductor Selection Selecting an inductor with low dc resistance reduces conduction losses and achieves high efficiency. The efficiency is moderated whilst using small chip inductor which operates with higher inductor core losses. Therefore, it is necessary to take further consideration while choosing an adequate inductor. Mainly, the inductor value determines the inductor ripple current: larger inductor value results in smaller inductor ripple current and lower conduction losses of the converter. However, larger inductor value generates slower load transient response. A reasonable design rule is to set the ripple current, I L, to be 30% to 50% of the maximum average inductor current, I L(AVG). The inductor value can be obtained as below, L ( V IN V OUT ) 2 * V OUT -V IN F SW *I OUT(MAX) * η I L I L(AVG) where V IN = input voltage V OUT = output voltage F SW = switching frequency in MHz I OUT = maximum output current in amp. η = efficiency I L /I L(AVG) = inductor ripple current/average current (0.3 to 0.5 typical) To avoid the saturation of the inductor, the inductor should be rated at least for the maximum input current of the converter plus the inductor ripple current. The maximum input current is calculated as below: I IN(MAX) = I OUT(MAX)*V OUT V IN *η The peak inductor current is calculated as the following equation: I PEAK = I IN(MAX) * V IN*(V OUT -V IN ) V OUT *L*F SW Rev

8 Output Capacitor Selection The current-mode control scheme of the allows the usage of tiny ceramic capacitors. The higher capacitor value provides good load transients response. Ceramic capacitors with low ESR values have the lowest output voltage ripple and are recommended. If required, tantalum capacitors may be used as well. The output ripple is the sum of the voltages across the ESR and the ideal output capacitor. V OUT = V ESR + V COUT I OUT C OUT * V OUT-V IN V OUT *F SW V ESR I PEAK * R ESR where I PEAK is the peak inductor current. For ceramic capacitor application, the output voltage ripple is dominated by the V OUT. When choosing the input and output ceramic capacitors, the small size ceramic capacitors are ideal for application. A 1uF input and output capacitor are suggested for 10-Series LED applications. Diode Selection Using a schottky diode is recommended in applications because of its low forward voltage drop and fast reverse recovery time, The current rating of the schottky diode should exceed the peak current of the boost converter, The voltage rating should also exceed the target output voltage. LED Dimming Control With the PWM signal applied to the EN pin, the is correspondingly turned ON or OFF by the PWM signal. The LEDs alternate between zero and full programmed current. The average LED current increases proportionally with the duty cycle of the PWM signal. A 0%duty cycle PWM signal will turn off the and corresponds to zero LED current. A 100% duty cycle PWM signal turns on the LEDs continuously at full current. The typical frequency range of the PWM signal is 5kHz to 100kHz. The magnitude of the PWM signal should be higher than the minimum EN voltage HIGH specification. Rev

9 Recommended Inductor Selection Designator Manufacturer Part Number Inductance (u H) Max DCR(ohm) Saturation Current(mA) Dimensions (mm3) L101 TDK VLCF5020T-220MR *5*2.0 Recommended Capacitor Selection Designator Manufacturer Part Number Capacitance(u Rated Case size F) Voltage(V) C101 Murata GRM185R61E105KA C102 Murata GRM21BR71H105KA Recommended Diode Selection Designator Manufacturer Part Number Maximum average forward rectified Maximum repetitive peak reverse voltage(v) Case size current(a) D101 Prisemi PSBD3D60V SOD323 D101 Prisemi PSBD3D40V SOD323 Layout Considerations As for all switching power supplies, especially those high frequency and high current ones,layout is an important design step. If layout is not carefully done, the regulator could suffer from instability as well as noise problems. To reduce switching losses, the LX pin rise and fall times are made as short as possible. To prevent radiation of high frequency resonance problems, proper layout of the high frequency switching path is essential. Minimize the length and area of all traces connected to the LX pin and always use a ground plane under the switching regulator to minimize inter-plane coupling, The loop including the PWM switch, Schottky diode, and output capacitor, contains high current rising and falling in nanosecond and should be kept as short as possible. The input capacitor needs not only to be close to the VIN pin, but also to the GND pin order to reduce the IC supply ripple. Rev

10 Typical Characteristics Figure 2 Figure 3 Figure 4 Figure 5 Efficiency vs. Input Voltage Efficiency (%) mA 2.5mA V IN (V) Figure 6 Rev

11 Product dimension (SOT-23-6L) A (6) θ C B (1) D E J K F H G Dim Millimeters Inches MIN MAX MIN MAX A B C D 0.950(BSC) 0.037(BSC) E F G H J K θ Rev

12 IMPORTANT NOTICE and are registered trademarks of Prisemi Electronics Co., Ltd (Prisemi),Prisemi reserves the right to make changes without further notice to any products herein. Prisemi makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Prisemi assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. Typical parameters which may be provided in Prisemi data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including Typicals must be validated for each customer application by customer s technical experts. Prisemi does not convey any license under its patent rights nor the rights of others. The products listed in this document are designed to be used with ordinary electronic equipment or devices, Should you intend to use these products with equipment or devices which require an extremely high level of reliability and the malfunction of with would directly endanger human life (such as medical instruments, aerospace machinery, nuclear-reactor controllers, fuel controllers and other safety devices), please be sure to consult with our sales representative in advance. Website: For additional information, please contact your local Sales Representative. Copyright 2009, Prisemi Electronics is a registered trademark of Prisemi Electronics. All rights are reserved. Rev