GM19371 BOOST WHITE LED DRIVER WITH OVP Description Features The GM19371 is a step up DC/DC convert specifically designed to drive white LED;s with a constant current. The device can drive up to 27 LEDs from a 5V supply. Additional feature includes output voltage limiting when LEDs are disconnected. GM19371 is available in SOT-26 package. Inherently matched LED current Drives up to 126LEDs (9S14P) from a 12V supply 36V rugged bipolar switch Fast 1.2MHz switching frequency Over Voltage Protection, 30V Application Cellular phones MP3 Player PDAs, Handheld Computers GPS Receivers Digital Cameras MR16 Lighting Typical Application Circuits 10µH 14 Strings in Parallel 9 LED in Series D1 VIN 6V to 18V C1 4.7µF 6 VIN 1 SW OVP 5 C2 4.7µF GM19371 FB 3 4 CTRL GND 2 RSET 20mA x 14 GM19371V2.01 DIM Figure 1 GM19371 typical application www.gammamicro.com.tw 1 GM19371 BOOST WHITE LED DRIVER WITH OVP Marking Information and Pin Configurations (Top View) SOT26 VIN 6 OVP 5 CTRL 4 70YWW 70: Device Code, Green Product Y: Year 1 SW 2 GND 3 WW: Week code FB Pin Descriptions Pin Number Pin Name 1 SW 2 GND 3 FB Pin Function This is the collector of the internal NPN power switch. Connect to inductor and diode. Minimize the metal trace area connected to this pin to reduce EMI. Ground Reference voltage is 95mV. Connect LEDs and a resistor at this pin. LED current is determined by the resistance and CTRL voltage. RSET resistor value is calculated as RSET = 95mV/ILED 4 CTRL This Pin could be used as either shut down or dimming functions: When VCTRL > 1.8V, generates full scale LED current VCTRL < 0.4V, chip is OFF 0.4V ≤ VCTRL ≤ 1.8V, PWM duty cycle controls the LED current 5 OVP Over Voltage Protection, 30V 6 VIN The input supply pin, bypass this pin with a capacitor as close to the device as possible GM19371V201 Ordering Information – Green Products 2 Ordering Number Package Shipping GM19371-ST26RG SOT-26 3,000 Units/Tape and Reel GM19371 BOOST WHITE LED DRIVER WITH OVP Absolute Maximum Ratings (Note 1) PARAMETER SYMBOL RATINGS UNITS Input Voltage VIN 20 V SW Voltage VSW 36 V FB Voltage VFB 10 V VCTRL 10 V Junction Temperature TJ -40 to 125 °C Storage Temperature Tstg - 65 to 150 °C 260 °C /SHDN Voltage Lead Temperature (soldering, 5 sec) .Note 1: Stresses greater than those listed under Maximum Ratings may cause permanent damage to the device. Recommended Operating Conditions Rating Symbol Value Unit VIN 2.5 to 18 V Output Voltage VOUT 28 V Operating Temperature Range TOPR 0 to 70 °C Maximum Supply Voltage Parameter Symbol Condition Min Typ 2.5 Max Unit 18 V Operating Voltage Range VIN Feedback Voltage VFB ILOAD = 180mA 80 95 107 mV FB Pin Bias Current IFB VIN = VOUT + 1V to 16V 10 45 100 nA Supply Current ICC 2.1 3.0 mA 0.1 1.0 μA 1.6 MHz Standby Current ISTBY V/SHDN = 0V Switching Frequency fOSC 1.1 1.3 Maximum Duty Cycle DMAX 85 90 % Switch Current Limit ICL 650 mA mV Switch VCESAT VSAT ISW = 250mA 350 Switch Leakage Current ILEAK VSW = 5V 0.01 CTRL Voltage High VTH CTRL Voltage Low VTL CTRL Pin Bias Current ICTRL OVP Threshold 5 1.5 μA V 0.4 V 65 μA 29 V GM19371V2.01 Electrical Characteristics (VIN = 5V, V/SHDN = 5V, TA = 25˚C, unless otherwise noted) 3 GM19371 BOOST WHITE LED DRIVER WITH OVP VIN FB VREF 1.25V 95mV SW Comparator A1 Driver Q1 A2 RC Q R S CC 0.3 OVP /SHDN Over voltage protection Ramp Generator 1.2MHz Oscillator GM19371V201 Figure 2 GM19371 Block Diagram 4 GND Q2 GM19371 BOOST WHITE LED DRIVER WITH OVP Typica; Application Circuits 14 Strings in Parallel 6 - 9 LED in Series D1 L VIN 5V to 12V 1 SW OVP 5 6 CIN 4.7µF VIN COUT 4.7µF GM19371 FB 3 4 CTRL OFF ON GND 2 RSET 20mA x 14 Figure 3 GM19371, typical application D1 L1 VIN 12V to 15V CIN 6 4.7µF VIN 68µH SS14 1 SW OVP 5 COUT 4.7µF GM19371 Dimming Control 4 CTRL FB 3 GND 2 RSET 0.22 350mA Figure 4. GM19371 – application for 1W LED x 6 D1 L1 VIN 12V AC CIN 6 VIN 5 LED (1W) in Series COUT 1 SW OVP 5 GM19371 GND 2 FB 3 RSET Figure 5. GM19371 – application for 1MR16 with 1W LED in series GM19371V2.01 4 CTRL 5 GM19371 BOOST WHITE LED DRIVER WITH OVP Application Information General Operation The GM19371 uses a constant frequency, current mode control scheme to provide excellent line and load regulations. See Fig 2 for GM19371operation. At the start of each oscillator cycle, the RS latch is set, which turns on the power switch Q1. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the RS latch is reset turning off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 95mV. In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. Over Voltage Protection and Open Circuit Protection In case of output open circuit, while the LED strings are disconnected from the circuit or the LEDs fail open, the feedback voltage, VFB, will be zero. The GM19371 will then switch at a high duty cycle resulting in a high output voltage, which may cause the SW pin voltage to exceed its maximum voltage rating. In order to prevent this condition occur, the GM19317 builds a latch-mode open-circuit protection circuit. Based on the result from the sensing pin, OVP pin, the chip can detect the output open condition when the OVP voltage is higher than 30V. The chip will shutdown until VIN reset Inductor Selection A 10uH inductor is recommended for most GM19371 applications. Although small size and high efficiency are major concerns, the inductor should have low core losses at 1.2MHz and low DCR (copper wire resistance). Capacitor Selection The small size of ceramic capacitors makes them ideal for GM19371 applications. X5R and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V or Z5U. A 4.7uF input capacitor and a 4.7uF output capacitor are sufficient for most GM19371 applications. Diode Selection Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for GM19371 applications. The forward voltage drop of a Schottky diode represents the conduction losses in the diode while the diode capacitance (CT or CD) represents the switching losses. For diode selection, both forward voltage drop and diode capacitance need to be considered. Schottky diodes with higher current ratings usually have lower forward voltage drop and larger diode capacitance, which can cause significant switching losses at the 1.2MHz switching frequency of the GM19371. A Schottky diode rated at 1000mA is sufficient for most GM19371 applications. Soft Start and Current Limit GM19371V201 The internal soft start circuit minimizes the inrush current during turning on GM19371. The Typical switch current is limited to about 650mA by the chip. 6 GM19371 BOOST WHITE LED DRIVER WITH OVP LED Current Control The LED current is controlled by the feedback resistor (R1, in Fig 1). The feedback reference is 95mV. The LED current is 95mV/R1. The formula and table 1 for R1 selection are shown below. R1=95mV/ILED Table 1 RSET resistor value selection R1 (Ω) ILED 5 19.1 10 9.53 15 6.34 20 4.75 60 1.56 350 0.22 Dimming Control 1. Using a PWM Signal to CTRL Pin With the PWM signal applied to the CTRL pin, the GM19371 is turned on or off by the PWM signal. The LED operate at either zero or full current. The average LED current increases proportionally with the duty cycle of the PWM signal. A 0% duty cycle will turn off the GM19371 and corresponds to zero LED current. A 100% duty cycle corresponds to full current. The typical frequency range of the PWM signal is 100Hz to 1KHz. The magnitude of the PWM signal should be higher than the minimum CTRL voltage high. 2. Using a DC Voltage For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The dimming control using a DC voltage is shown in below figure 6. As the DC voltage increases, the voltage drop on R1 increases and the voltage drop on RSET decreases. Thus, the LED current decreases. The selection of R1 and R2 should make the current from the variable DC source much smaller than the LED current and much larger than the FB pin bias current. For VDC range from 0V to 2V, the selection of resistors in below figure gives dimming control of LED current from 0mA to 15mA. GM19371 VDC R2 R1 90K 5K RSET 6.3Ω Figure 6 For The filtered PWM signal can be considered as an adjustable DC voltage. It can be used to replace the variable DC voltage source in dimming control. The circuit is show in figure 7 PWM GM19371 100K R2 R1 90K 5K 0.1μF Figure 7 RSET 6.3Ω GM19371V2.01 3. Using a DC Voltage 7 GM19371 BOOST WHITE LED DRIVER WITH OVP 4. Using a Logic Signal For applications that need to adjust the LED current in discrete steps, a logic signal can be used as shown in figure 8. RSET sets the minimum LED current (when the NMOS is off). RINC sets how much the LED current increases when the NMOS is turned on. The selection of RSET and RINC follows the formula and Table in “LED Current Control” section. GM19371 RINC RSET 6.3Ω Logic Signal Figure 8 Board Layout Consideration GM19371V201 As with all switching regulators, careful attention must be paid to the PCB board layout and component placement. To maximize efficiency, the rising and falling times of the switch are made as short as possible. To prevent electromagnetic interference (EMI) problems, proper layout of the high frequency switching path is essential. The voltage signal of the SW pin has sharp rising and falling edges. Minimize the length and area of all traces connected to the SW pin and always use a ground plane under the switching regulator to minimize inter-plane coupling. In addition, the ground connection for the feedback resistor RSET should be tied directly to the GND pin and not shared with any other component, ensuring a clean, noise-free connection. 8 GM19371 BOOST WHITE LED DRIVER WITH OVP Package Outline Dimensions – SOT 26 0.111 - 0.119 (2.820 - 3.020) 0.037 BSC (0.950) 0.071 - 0.079 (1.800 - 2.000) 0.012 - 0.024 (0.300 - 0.600) 0.059 - 0.067 (1.500 - 1.700) 0.004 - 0.008 (0.100 - 0.200) 0.000 - 0.004 (0.000 - 0.100) 0.041 - 0.049 (1.050 - 1.250) 0.041 - 0.045 (1.050 - 1.150) inch (mm) GM19371V2.01 0.104 - 0.116 (2.650 - 2.950) 0-8 9 GM19371 BOOST WHITE LED DRIVER WITH OVP Ordering Number GM 19371 APM Gamma Micro Circuit Type ST26 R Package Type Shipping Type ST26: SOT 26 R: Tape & Reel G Blank:Pb-free G:Green Note: Pb-free products: ♦ RoHS compliant and compatible with the current require-ments of IPC/JEDEC J-STD-020. ♦ Suitable for use in SnPb or Pb-free soldering processes with 100% matte tin (Sn) plating. GM19371V201 Green products: 10 ♦ Lead-free (RoHS compliant) ♦ Halogen free(Br or Cl does not exceed 900ppm by weight in homogeneous material and total of Br and Cl does not exceed 1500ppm by weight)