NJU6050F-TE2

NJU6050 PRELIMINARY High Efficiency White LED Driver PACKAGE OUTLINE GENERAL DESCRIPTION The NJU6050 is a high-efficiency white LED driver. It contains a high-efficiency step-up DC/DC converter and an output driver. The IC ensures a 28V maximum capacity which allows series connection of 8 white LEDs with a constant current so that the LED currents are identical for uniform brightness. The high frequency of the step-up converter permits the use of small, low-profile inductors and capacitors to minimize the footprint in space-conscious applications. And additional features are a low operating voltage of 2.5V and the small package SOT-23 (MTP-5). All of these benefits make the NJU6050 suitable for the battery-powered portable applications such as a cellular phone, a camcorder, PDA, etc. FEATURES Drives Up to 8 White LEDs in Series VSW = 28V, IOUT = 20mA Uses Small Inductor and Capacitors 2.5V to 6.5V Operating Voltage for Step-up Circuits (VDD) CMOS Technology Package : SOT-23 5 pin (MTP-5) PIN CONFIGURATION (TOP VIEW) SW 1 VSS 2 FB 3 Ver.2004-02-25 5 VDD 4 SHDNb -1- NJU6050 PIN DESCRIPTIONS No. SYMBOL TYPE DESCRIPTION 1 SW Input Switch Terminal 2 VSS Power Ground Terminal 3 FB Input 4 SHDNb Input 5 VDD Power Feedback Terminal Shutdown Terminal Active “L” “L” : Shutdown “H” : Normal Operation Power Supply terminal BLOCK DIAGRAM D1 L1 VIN VDD VOUT SW C1 DC/DC Control Bias SHDNb + FB C2 D2 VREF Shutdown circuit 1.2V RLED ILED VSS FUNCTIONAL DESCRIPTONS (1) LED Current Control and Resistor RLED Selection The NJU6050 incorporates the LED current control to regulate the LED current (ILED), which is programmed by the feedback resistor (RLED) connected between the FB and the VSS terminals. The reference voltage VREF is internally regulated to 1.2V typical and connected to the positive input of the built-in comparator. Formula (1) is used to choose the value of the RLED, as shown below. RLED = VREF I LED --- Formula (1) VREF=1.2V (typ.) The ILED is the constant current programmed by the RLED. When the feedback voltage on the FB terminal reaches above the reference voltage VREF (i.e., ILED is above the level programmed by RLED), the output capacitor C2 delivers the ILED. Once the feedback voltage drops below the reference voltage (i.e., ILED drops below the level programmed by the RLED), the MOS switch is turned on, then the current of the inductor L1 begins increasing. When the switch current reaches 350mA, the MOS switch is turned off, then the L1 delivers current to the output through the diode D1 as the inductor current drops. After that, the MOS switch is turned on again and the switch current increases up to 350mA. This switching cycle continues until the ILED reaches the level programmed by the RLED, then the ILED current is maintained constant. -2- Ver.2004-02-25 NJU6050 (2) Inductor Selection The following equation gives the optimum inductor value: L= VOUT-VIN(MIN)+VD ICL tOFF VD : Schottky diode Voltage ICL :Switch current limit Formula (2) The power supply voltage VIN may fluctuate in battery-powered applications. For this reason, the minimum voltage should be applied to the VIN in Formula (2). The NJU6050 has about 100ns of delay time (TDELAY), which is defined as the period from the reach of the current limit 350mA to the MOS-switch-off. The TDELAY may cause an overshoot-inductor-current, which is called the peak current IL,PEAK, and calculated by Formula (3). Therefore, it is recommended that an inductor with a rating twice of the IL,PEAK and a low DCR (DC resistance) be used for high efficiency. IL,PEAK = ICL + VIN(MAX)+VDS L tDELAY Formula (3) VDS : Drain-Source voltage of MOS switch (=ILIMIT*RON) VIN(MAX) : Maximum of VIN Voltage (3) Diode Selection A schottky diode with a low forward-voltage-drop and a fast switching-speed is ideal for the D1. And the D1 must have a rating greater than the output voltage and output current in the system. (4) Capacitor Selection A low ESR (Equivalent Series Resistance) capacitor should be used as the output capacitor C2 to minimize output ripples. A multi-layer ceramic capacitor is the best selection for the NJU6050 application because of not only the low ESR but its small package. A ceramic capacitor is also recommended for the input decoupling-capacitor C1, and should be placed as close to the NJU6050 as possible. (5) Open-circuit Protection The zener diode D2 works as an open-circuit protection. When the LED is disconnected from the circuit, the feedback voltage on the FB terminal is pulled down to 0V, which makes the switching frequency maximum. Without the diode D2, the SW voltage exceeds the absolute maximum rating, and this voltage stress may cause a permanent damage to the IC. The zener voltage must be greater than the maximum forward-voltage of the LED string. Ver.2004-02-25 -3- NJU6050 ABSOLUTE MAXIMUMN RATINGS Ta=25°C PARAMETERS SYMBOL VDD Power Supply SHDNb Terminal Voltage SW Terminal Voltage FB Terminal Voltage Power Dissipation Operating Temperature Storage Temperature Note1) Note2) VDD VSHDNb VSW VFB PD Topr Tstg CONDITIONS SHDNb terminal SW terminal FB terminal MTP-5 RATINGS UNIT -0.3 to +7.0 -0.3 to +7.0 -0.3 to +30.0 -0.3 to VDD 200 -40 to +85 -65 to +150 V V V V mW °C °C All voltages are relative to VSS = 0V reference. Do not exceed the absolute maximum ratings, otherwise the stress may cause a permanent damage to the IC. It is also recommended that the IC be used in the range specified in the DC electrical characteristics, or the electrical stress may cause mulfunctions and impact on the reliability. DC ELECTRICAL CHARACTERISTICS (VDD=3.6V, VSHDNb=3.6V, VSS=0V, Ta=25°C) PARAMETERS VDD Power Supply Quiescent Current FB Comparator Trip point Output Voltage Line Regulation FB Terminal Bias Current SYMBOL VDD ISTBY1 ISTBY2 VFBP VK IFB Switch Off Time tOFF Switch VDS(ON) Switch Current Limit SHDNb Terminal Current SHDNb Input Voltage High SHDNb Input Voltage Low Switch Leakage Current VDS ICL -4- ISHDNb VSHDNH VSHDNL IL CONDITIONS MIN. TYP. 2.5 No switching VSHDNb=0V 1.18 2.5V