EUP2538 10X2 Strings WLED Boost Converter DESCRIPTION FEATURES The EUP2538 is a constant current step-up converter specifically designed to drive white LEDs. The Step-up converter topology allows series connection of the white LEDs so the LED currents are identical for uniform brightness. The EUP2538 switches at 1MHz, allowing the use of tiny external components. The output capacitor can be as small as 0.47µF, saving space and cost versus alternative solutions. A low 0.3V feedback voltage minimizes power loss in the current setting resistor for better efficiency. The EUP2538 high-voltage output stage is perfect for driving mid-size and large panel displays containing up to ten white LEDs in series. The 1.2A current limit and 0.65Ω RDSON allows the part using for up to 10 × 2 string WLED lighting. LED dimming can be done by using a DC voltage, a logic signal, or a pulse width modulation(PWM) signal. The enable input pin allows the device to be placed in shutdown mode with “zero” quiescent current. z z z z z z z z z 2.6V to 5.5V Input Range 38V Output with Over Voltage Protection Internal Soft-Start PWM Dimming Control Internal High Power 40V MOSFET Switch Fast 1MHz Switching Frequency Using Low-Profile Inductors and Capacitors Available in 3mm × 3mm TDFN-8 Package RoHS Compliant and 100% Lead (Pb)-Free APPLICATIONS z z z z GPS Navigation Systems Portable Media Players Handheld Devices, Digital Camera Portable Game Machines The EUP2538 is available in low profile 8pin 3mm × 3mm TDFN package. Typical Application Circuit Figure 1. White LED Application DS2538 Ver1.3 Nov. 2008 1 EUP2538 Pin Configurations Package Type Pin Configurations TDFN-8 Pin Description PIN TDFN-8 PGND 1,2 IN 3 Input Supply Voltage EN 4 Chip Enable Pin. Connect to 1.4V or higher to enable device, 0.3V or less to disable device. GND 5 FB 6 LX 7,8 Common Ground. Connect the pin to the ground plane. Feedback Pin. Reference voltage is 0.3V. Connect cathode of lowest LED and resistor here. Calculate resistor value according to the formula: RFB=0.3/ILED Switch Pin. This is the drain of the internal power switch. Connect inductor/diode here. Minimize trace area at this pin to reduce EMI. DS2538 Ver1.3 Nov. 2008 DESCRIPTION Power Ground. Connect to GND and the exposed pad directly under the IC. 2 EUP2538 Ordering Information Order Number Package Type Marking Operating Temperature Range EUP2538JIR1 TDFN-8 xxxxx P2538 -40 °C to +85°C EUP2538 □ □ □ □ Lead Free Code 1: Lead Free 0: Lead Packing R: Tape & Reel Operating temperature range I: Industry Standard Package Type J: TDFN Block Diagram Figure 2. DS2538 Ver1.3 Nov. 2008 3 EUP2538 Absolute Maximum Ratings (1) Supply Voltage ,VIN ----------------------------------------------------------------SW ----------------------------------------------------------------------------------The Other Pins ---------------------------------------------------------------------Package Thermal Resistance TDFN-8,θJA ------------------------------------------------------------------------Maximum Junction Temperature --------------------------------------------------Lead Temperature (Soldering, 10sec.) --------------------------------------------Storage Temperature Range --------------------------------------------------------- -0.3V to 6V -0.3V to 40V -0.3V to 6V 39.1°C/W 125°C 260°C -65°C to +150°C Recommend Operating Conditions (2) Junction Temperature Range ------------------------------------------------------- -40°C to +85°C Supply Voltage , VIN----------------------------------------------------------------- 2.6V to 5.5V Note (1): Stress beyond those listed under “Absolute Maximum Ratings” may damage the device. Note (2): The device is not guaranteed to function outside the recommended operating conditions. Electrical Characteristics (VIN =3.6V, VOUT=34V, COUT=0.22µF, CIN=2.2µF, L1=22µH, RFB=15Ω,TA=-40°C to +85°C. Unless otherwise noted. Typical values are at TA= +25°C) Symbol UVLO Parameter Under Voltage Lock Out Maximum Output Voltage Supply Current Quiescent Current Shutdown current ICC1 ICC2 ICC3 Conditions Rising No Switching VCC=5.5V, Continuous Switching VCC=5.5V, FB=1.3V, No Switching VCC=5.5V, VEN<0.4V Min 2.2 EUP2538 Typ Max. 2.4 Unit 1.7 130 0.1 2.6 40 2.6 170 1 V V mA µA µA 1 1.3 MHz Oscillator Fosc Operation Frequency Dmax Maximum Duty Cycle 0.8 92 % Reference Voltage VFB Feedback Voltage 285 300 315 mV 0.65 1.3 Ω 0.85 1.2 1.6 A 0.4 0.7 MOSFET Rds (on) On resistance of MOSFET ILX Current Limit Control and Protection VEN1 Shutdown Voltage VEN2 Enable Voltage 0.7 1.2 V EN Pin Pull Low Current 0.1 1 µA OVP Threshold 38 IEN OVP DS2538 Ver1.3 Nov. 2008 4 V V EUP2538 Typical Operating Characteristic Quiescent Current vs. VIN (Not Switching) Quiescent Current vs. VIN (Switching) 2 Quiescent Current (mA) Quiescent Current (µA) 150 125 VFB=1.3V 100 75 1.5 1 0.5 0 50 3 3.5 4 4.5 5 Input Voltage (V) 3 5.5 4 4.5 5 5.5 Input Voltage (V) FB PIN Voltage vs. Temperature 310 3.5 310 FB PIN Voltage vs. Output Current FB PIN Voltage (mV) FB PIN Voltage (mV) 308 306 304 302 305 300 295 300 10WLEDs 298 -50 290 0 50 100 150 5 Temperature (℃) 15 20 25 30 Output Current (mA) Switching Frequency vs. Supply Voltage LED Current vs. Input Voltage 40 1.1 35 LED Current (mA) Switching Frequency (MHz) 10 1 0.9 RFB=10Ω 30 25 RFB=15Ω 20 15 RFB=20Ω 10 5 0 0.8 3 3.5 4 4.5 5 5.5 3 Input Voltage (V) DS2538 Ver1.3 Nov. 2008 5 3.5 4 4.5 Input Voltage (V) 5 5.5 EUP2538 Efficiency vs. Load Current (10 WLEDs) 1 100 0.5 90 Efficiency (%) LED Current Variation (%) Typical Operating Characteristics (continued) LED Current Regulation (10mA) 0 80 70 -0.5 vin=3.6v vin=5v 60 -1 3 3.5 4 4.5 5 5 5.5 10 Input Voltage (V) 25 30 Efficiency vs. Input Voltage (20WLEDs) 100 83 95 82 90 81 Efficiency(%) Efficiency (%) 20 LED Current (mA) Efficiency vs. Input Voltage (10WLEDs) 85 80 75 80 79 78 70 15mA 65 77 20mA 76 60 3 3.5 4 4.5 Input Voltage (V) 5 3.4 5.5 3.9 4.4 4.9 5.4 VIN(V) Maximum Output Current vs. Input Voltage (10WLEDs) Switch On Resistance vs. Input Voltage 2 120 100 1.5 Output Current (mA) Switch Resistance (Ω) 15 1 0.5 80 60 40 20 0 3 DS2538 Ver1.3 3.5 4 4.5 Input Voltage (V) Nov. 2008 5 0 5.5 3 6 3.5 4 4.5 Input Voltage (V) 5 5.5 EUP2538 Typical Operating Characteristics (continued) Light Load Switching Waveform VIN=5V, ILED=1.5mA, L=22µH Shutdown Voltage vs. Input Voltage 1 SW Shutdown Voltage (V) 20V/div 0.8 Inductor Current 0.6 100mA/d -40℃ 0.4 VOUT -25℃ AC Coupled 85℃ 20mV/div 125℃ 0.2 3 3.5 4 4.5 5 500ns/div Input Voltage (V) Typical Switching Waveforms Power-up With 10 LEDs at 20mA SW VEN 20V/div 5V/div Inductor VOUT Current 20V/div 100mA/d Input VOUT Current AC Coupled 100mA/div 100mV/div 500ns/div DS2538 Ver1.3 Nov. 2008 50.0µs/div 7 EUP2538 Application Information Switching Waveform with 1kHz PWM on EN VEN LED Current Control The EUP2538 regulates the LED current by setting the current sense resistor (RFB) connecting to feedback and ground. The internal feedback reference voltage is 0.3V. The LED current can be set from following equation easily. I LED = 0.3 R FB 5V/div VOUT 20V/div LED --------------------------------------(1) Current In order to have an accurate LED current, precision resistors are preferred (1% is recommended). The table for RFB selection is shown below. RFB Resistor Value selection LED Current (mA) RFB (Ω) 5 60 10 30 15 20 20 15 25 12 30 10 20mA/div 250µs/div Figure 4. 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 5. 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 selection of resistors in Figure 5 sets dimming control of LED current from 20mA to 0mA. Dimming Control a. Using a PWM Signal to EN Pin For controlling the LED brightness, the EUP2538 can perform the dimming control by applying a PWM signal to EN pin, and the PWM signal frequency range is from 100Hz to 100KHz. The average LED current is proportional to the PWM signal duty cycle. The magnitude of the PWM signal should be higher than the maximum enable voltage of EN pin, in order to let the dimming control perform correctly. Figure 5. DC Voltage Dimming Control 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 recommended 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 6, output current is from 20.5mA to 5.5mA by adjusting the PWM duty cycle from 10% to 90%. Figure 3. Direct PWM Dimming Control DS2538 Ver1.3 Nov. 2008 8 EUP2538 Shutdown Voltage (V) 1 Shutdown Voltage vs. Input Voltage 0.8 0.6 -40℃ 0.4 -25℃ 85℃ 125℃ 0.2 3 3.5 4 4.5 5 Input Voltage (V) Figure 6. Filtered PWM Dimming Control Figure 9. 25 Filtered PWM Dimming (0V to 2.5V) Enable to Open LED Waveforms VEN LED Current (mA) 20 5V/div 15 VOUT 10 20V/div 5 SW 0 0 10 20 30 40 50 60 70 80 90 20V/div 100 PWM Duty Cycle (%) 100µs/div Figure 7. Figure 10. Open Load Shutdown In the event of an “Open LED” fault condition, the EUP2538 will continue to boost the output voltage with maximum power until the output voltage reaches approximately 38V. Once the output exceeds this level, the device will cease operation until the EN pin is cycled off and on. Thermal Shutdown Thermal overload protection circuitry has been included to prevent the device from operating at unsafe junction temperatures above 150ºC. In the event of a thermal overload condition the device will automatically shutdown and wait till the junction temperatures cools to 130ºC before normal operation is resumed. Capacitors Selection A 4.7µF to 10µF ceramic input capacitor (CIN) and a 0.22µF to 4.7µF ceramic output capacitor (COUT) are sufficient for most applications. During Direct PWM Dimming control, a larger output capacitor will significantly reduce audio noise induced by output capacitor, and a smaller will enlarge the audio noise, a 2.2uF COUT is recommended. Under normal condition, a 4.7µF input capacitor is sufficient. For applications with higher output power, a larger input capacitor of 10µF may be appropriate. X5R and X7R capacitor types are ideal due to their stability across temperature range. Figure 8. Open LED Protection DS2538 Ver1.3 Nov. 2008 9 EUP2538 Inductor Selection The recommended value of inductor for 2 to 10 WLEDs applications are 4.7µH to 47µH. Small size and better efficiency are the major concerns for portable device, such as EUP2538 used for mobile phone. The inductor should have low core loss at 1MHz and low DCR for better efficiency. To avoid inductor saturation current rating should be considered. Schottky Diode Selection The current rating of the Schottky diode must exceed the peak current flowing through it. The Schottky diode performance is rated in terms of its forward voltage at a given current. In order to achieve the best efficiency, this forward voltage should be as low as possible. The response time is also critical since the driver is operating at 1MHz. Board Layout Careful PC board layout is required due to fast switching. All components must be placed as close to the device as possible. Keep the path between the inductor L1, diode D1, and output capacitor COUT extremely short for minimal noise and ringing. The feedback components such as the sense resistor RFB must be kept close to the FB pin to prevent noise injection on the FB pin trace. The ground return of CIN and COUT should be tied close to the GND pin. See the EUP2538 demo board layout for reference. DS2538 Ver1.3 Nov. 2008 10 EUP2538 Packaging Information TDFN-8 DETAIL A A MILLIMETERS MIN. MAX. 0.70 0.80 MIN. 0.028 MAX. 0.031 A1 0.00 0.05 0.000 0.002 b 0.20 0.40 0.008 0.016 D 2.90 3.10 0.114 0.122 SYMBOLS D1 E Ver1.3 Nov. 2008 2.30 0.090 2.90 3.10 0.114 0.122 E1 1.50 0.059 e 0.65 0.026 L DS2538 INCHES 0.25 0.45 11 0.010 0.018