PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Features Description n Six Constant-Current Output Channels n Parallel Channels Allow Higher Current per LED String n Maximum 40V Continuous Voltage Output Limit for Each Channel n Adjustable Constant Output Voltage n Adjustable Constant LED Current n Drives 10 or more LEDs Each String as Long as the String Voltage Less Than 40V n Internal 2.5A Power MOSFET n Allows Digital PWM and Analog Dimming n Wide (100:1) PWM Dimming Range without Color Shift n Independent Dimming and Shutdown Control of the LED Driver n ±1% Typical Current Matching between Strings n Short LED Protection n 3 Frequencies Selection: 1.6MHz/1MHz/500kHz n Wide Input Voltage Range: 4.8V to 28V n Over Temperature Protection n Available in 24-pin 4mmx4mm QFN Package n Pb-free Package The PAM2845 is a high-efficiency boost type LED driver. It is designed for large LCD panel that employs an array of LEDs as back light source. The PAM2845 employs a current-mode step-up converter that drives six parallel strings of LEDs connected in multiple series. This built-in stringcurrent-control circuit achieves ±1% typical current matching between strings, which ensures even brightness for all LEDs. Separate feedback loops limit the output voltage if one or more LEDs open or short. The PAM2845 has features cycle-by-cycle current limit to provide consistent operation and soft-start capability. A thermal-shutdown circuit provides another level of protection. The PAM2845 has a wide +4.8V to +28V inputvoltage range and provides adjustable full-scale LED current. The switching frequency of this device can be selected among 500kHz, 1MHz and 1.6MHz according to the application requirements. Applications n White or RGB Backlighting for LCD TV, LCD Monitor, Notebook, Handy Terminals, and Avionics Displays Panels n LED Lighting Devices Typical Application 22 μ H VIN: 4.8V to 28V 3A/40V L 10 μ F SW SW SW VIN ENA PWMD R1 270kΩ PWMD Fsel 10 μ F FB Fsel R2 PAM2845 Total 10 LEDs per string 9.1kΩ VC Vcc-5V Vcc-driver 100nF 1μF LED1 LED2 LED3 LED4 LED5 LED6 Iset 1μF 10kΩ GND PGND PGND PGND Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 1 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Block Diagram VIN Vcc-5V LDO1 Vcc-driver LDO2 ENA FB SOFT START 1.6MHz 800KHz 500KHz Fsel UVLO ENABLE SLOPE COMPENSATION OTP PWM COMPATATOR Bandgap MAIN CONTROL SW Level Shift Driver + 1.2V EA POR CURRENT LIMIT VC CS AMP PWMD PGND PWM DIMMING LED1 LED2 LED3 LED4 LED5 LED6 led short protect led short protect led short protect led short protect led short protect led short protect I_set AMP Iset GND Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 2 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Pin Configuration QFN 4X4 24 23 22 21 20 19 1 18 2 17 P2845 XXXYW 3 16 4 15 5 14 6 13 7 8 9 X: Internal Code Y: Year W: Week 10 11 12 Pin Descriptions Pin Number Name Description 1 VIN Supply input 2 Vcc-driver 3 GND Ground 4 ENA Enable input 5 PWMD PWM dimming control 6 LED1 LED1 cathode terminal 7 LED2 LED2 cathode terminal 8 LED3 LED3 cathode terminal 9 GND Ground 10 GND Ground 11 LED4 LED4 cathode terminal 12 LED5 LED5 cathode terminal 13 LED6 LED6 cathode terminal 14 Iset 15 Vcc-5V 16 VC Boost stage compensation pin 17 Fsel Oscillator frequency selection pin 18 FB 19 PGND Power ground 20 PGND Power ground 21 PGND Power ground 22 SW Power MOS drain 23 SW Power MOS drain 24 SW Power MOS drain 5V linear regulator output for power MOS driver LED current adjustment pin 5V linear regulator output Feedback Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 3 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Absolute Maximum Ratings These are stress ratings only and functional operation is not implied . Exposure to absolute maximum ratings for prolonged time periods may affect device reliability . All voltages are with respect to ground . VIN,ENA.....................................-0.3V To +30V Operating Temperature...............-40 OC to 125 OC SW ,LED.....................................-0.3V To +40V Storage Temperature..................-40 OC to 150 OC Vcc-5V,Vcc-driver,VC....................-0.3V To +6V Maximum Junction Temperature...............150 OC PWMD,Fsel,FB,Iset.......................-0.3V To +6V Soldering Temperature....................300 OC,5sec Recommended Operating Conditions Operation Temperature Range.........-20 OC to 85 OC Supply Voltage Range.......................4.8V to 28V Thermal Information Parameter Thermal Resistance (Junction to Case) Thermal Resistance (Junction to Environment) Package Symbol Maximum Unit 4x4mm QFN θjc 20 °C/W 4x4mm QFN θjA 37 °C/W Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 4 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Electrical Characteristic T A=25°C, V IN=ENA=12V, L=22 μ H, Rset=10k Ω, unless otherwise noted PARAMETER Conditions Min Input Voltage Range Quiescent Current Typ 4.8 Max Units 28 V ENA=high (no switching) 1 2 Fsel=high (1.6M switching frequency) 10 Fsel =high (1M switching frequency) 6 Fsel =high (500k switching frequency) 3 ENA=low 5 20 μA mA LDO Stage Vcc_5V 4.5 5 5.5 V Vcc_5V current_limit 14 74 90 mA Vcc_5V UVLO Threshold 3.9 4.2 4.5 V Vcc_5V UVLO Hysteresis 70 mV Vcc_driver 4.5 5 5.5 V Vcc_driver current_limit 14 74 90 mA Vcc_driver UVLO Threshold 3.9 4.2 4.5 V Vcc_driver UVLO Hysteresis 70 mV 1.2 V 0.2 Ω 2.5 A 1 μA Fsel =Vcc_5V 1.6 MHz Fsel =Open 1.0 MHz Fsel =Gnd 500 kHz Fsel =Vcc_5V 20 % Fsel =Open 10 % Fsel =Gnd 5 % Maximums Duty Cycle 90 % VC Source Current 60 μA VC Sink Current 60 μA Boost Stage Feedback Voltage Switch Rdson Vcc_5V=5V Switch Current Limit Switch Leakage Current Switching Frequency Minimums Duty Cycle Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 5 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Electrical Characteristic T A=25°C, V IN=ENA=12V, L=22 μ H, Rset=10k Ω, unless otherwise noted. PARAMETER Conditions Min Typ Max Units LED Controller Stage Full-Scale LED_Output Current I=190*1.2V/Riset, Riset=7.68k 30 mA I=190*1.2V/Riset, Riset=11.3k 20 mA I=190*1.2V/Riset, Riset=22.6k 10 mA LED current matching -3 1 +3 % Iset Voltage 1.2 V Minimums LED voltage 400 mV Analog Dimming Range I=190*1.2V /Riset PWM Dimming Frequency I/32 I mA 100 1k Hz 5.1 V Fault Protection LED_ Overvoltage Threshold 4.6 LED_ Overvoltage Hysteresis 4.9 1 V Thermal-Shutdown 150 °C Thermal-Shutdown Hysteresis 30 °C Controll Interface EN High 1.5 V EN Low 0.4 PWMD High 1.5 V PWMD Low 0.4 Fsel High 1 Fsel Low EN Min pulse width 2 V 0.5 V 0.5 μs EN Max pulse width 10 EN off delay V V Vcc_5V-0.5 Fsel Midlevel V 100 μs μs Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 6 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Typical Performance Characteristic Input Voltage VS Efficiency 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% LED Current VS PWMD Duty cycle 25 20 LED current(mA) Efficiency(%) T A=25°C, V IN=ENA=12V, L=22 μ H, Rset=10k Ω, unless otherwise noted. 15 10 5 0 5 9 13 17 20% 21 40% 60% Input voltage(V) 100% Duty cycle Shutdown current VS Input Voltage Quiescent Current VS Input Voltage 9 6 8 5 Supply Current (mA) Shutdown current(uA) 80% 4 3 2 1 Fsw=1.6MHz 7 6 5 Fsw=1MHz 4 3 Fsw=500KHz 2 1 0 PWMD=0 0 0 10 20 30 6 10 Input voltage(V) Vout Line Regulation 26 Total LED Current Line regulation 41 133 132.5 LED Current (mA) 40.5 Output Voltage (V) 14 18 22 Input Voltage (V) 40 39.5 39 132 131.5 131 130.5 130 38.5 129.5 38 129 4.5 8 12 16 20 24 28 4.5 Input Voltage (V) 8 12 16 20 24 28 Input Voltage (V) Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 7 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Typical Performance Characteristic T A=25°C, V IN=ENA=12V, L=22 μ H, Rset=10k Ω, unless otherwise noted. Efficiency VS Input Voltage Start up and Shutdown waveforms 100% 8x6 LED 6x6 LED 95% Efficiency 10x6 LED 90% Vout DC coupling 12x6 LED IL DC coupling 85% 80% ENA DC coupling 75% 4.5 8 12 16 20 Input Voltage(V) 24 28 Switching Waveforms Inductor Current Waveforms( PWMD =20%) PWMD DC coupling V LED1 DC coupling Vsw DC coupling IL DC coupling IL DC coupling Inductor Current Waveforms( PWMD =50%) LED Current Waveforms( PWMD =20%) PWMD DC coupling V LED1 DC coupling PWMD DC coupling V LED1 DC coupling I LED DC coupling IL DC coupling Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 8 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Typical Performance Characteristic T A=25°C, V IN=ENA=12V, L=22 μ H, Rset=10k Ω, unless otherwise noted. Inductor Current Waveforms( PWMD =80%) LED Current Waveforms(PWMD=50%) PWMD DC coupling PWMD DC coupling V LED1 DC coupling V LED1 DC coupling I LED DC coupling IL DC coupling Startup Waveforms LED Current Waveforms ( PWMD =80%) PWMD DC coupling V LED1 DC coupling Vout DC coupling IL DC coupling I LED DC coupling ENA DC coupling Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 9 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Application Information Inductor Selection (equivalent series resistance) capacitors should be used at the input. At least 2.2uF input capacitor is recommended for most applications. A minimum output capacitor value of 10uF is recommended under normal operating conditions, while a 22uF or higher capacitor may be required for higher power LED current. A reasonable value of the output capacitor depends on the LED current. The total output voltage ripple has two components: the capacitive ripple caused by the charging and discharging on the output capacitor, and the ohmic ripple due to the capacitor's equivalent series resistance. The ESR of the output capacitor is the important parameter to determine the output voltage ripple of the converter, so low ESR capacitors should be used at the output to reduce the output voltage ripple. The voltage rating and temperature characteristics of the output capacitor must also be considered. So a value of 10uF, voltage rating(50V) capacitor is chosen. The inductance, peak current rating, series resistance, and physical size should all be considered when selecting an inductor. These factors affect the conver ter's operating mode, efficiency, maximum output load capability, transient response time, output voltage ripple, and cost. The maximum output current, input voltage, output voltage, and switching frequency d e t e r m i n e t h e i n d u c t o r v a l u e . Ve r y h i g h inductance minimizes the current rip ple, and therefore reduces the peak current, which 2 decreases core losses in the inductor and I R losses in the entire power path. However, large inductor values also require more energy storage and more turns of wire, which increases physical 2 size and I R copper losses in the inductor. Low inductor values decrease the physical size, but increase the current ripple and peak current. Finding the best inductor involves the compromises among circuit efficiency, inductor size, and cost. Diode Selection PAM2845 is high switching frequency convertor, w h i c h d e m a n d s h i g h s p e e d r e c t i f i e r. I t ' s indispensable to use a Schottky diode rated at 2A, 60V with the PAM2845. Using a Schottky diode with a lower forward voltage drop is better to improve the power LED efficiency, and its voltage rating should be greater than the output voltage. When choosing an inductor, the first step is to determine the operating mode: continuous conduction mode (CCM) or discontinuous conduction mode (DCM). When CCM mode is chosen, the ripple current and the peak current of the inductor can be minimized. If a small-size inductor is required, DCM mode can be chosen. In DCM mode, the inductor value and size can be mini mized but the inductor ripple current and peak current are higher than those in CCM. Methods for Setting LED Current There are three methods for setting and adjusting the LED current outlined here. The methods are: 1) RSET only 2) PWM Input at PWMD 3) PWM Input at ENA Capacitor Selection An input capacitor is required to reduce the input ripple and noise for proper operation of the PAM2845. For good input decoupling, Low ESR >10ms 0 ENA 1 2 3 4 31/32 500ns<t LO <10us 30/32 29/32 28/32 27 5 28 29 30 31 32 t SOFT-START 32/32 32/32 t HI>500ns 5/32 I LED_ 4/32 3/32 2/32 1/32 t 200us(typ) shutdown shutdown shutdown Figure 1 Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 10 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Method 1: LED Current Setting with External Resistor R ISET V OUT=V FB(1+R2/R1) The recommend procedure is to choose R2=300k Ω and R1=9.2k Ω to set V OUT MAX=40V. The most basic means of setting the LED current is connecting a resistor between R ISET and GND. The LED current is decided by I SET Resistor. Generally the Vout must be higher than total LED voltage. For 10-LED application, R1=9.1k Ω, R2=270k Ω, Vout=36.8V, higher than V LED=33V. I LED=228/ R ISET Method 2: LED Current Setting Using PWM Signal to PWMD Pin One or more of the LED1-6 pins could be floating if not used because the PAM2845 uses external resistor to set the output voltage. One or more LEDx pins floating is just like change of output loadings. This circuit uses resistor R ISET to set the on state current and the average LED current, then proportional to the percentage of on-time when the PWMD pin is logic low. Average LED current is approximately equal to: LED Short Protection The PAM2845 uses LED FB function to protect devices when one or more LED(s) is/are shorted. I= ( t on*I)/ (t on+ t off) V LED = V OUT – V f N Also, the recommended PWM frequency is between 100Hz and 1kHz. Frequency <100Hz can cause the LEDs to blink visibly. Normally V LED is around 0.4V and V OUT is decided by LED numbers. When one or more LED(s) is/are shorted, the PAM2845 will clamp V OUT to make sure all LED pins’ voltage is less then 5V. With this function V OUT will be clamped at (5V+ V fN MIN). Method 3: LED Current Setting with single wire logic to ENA Pin When the LEDs are enabled by high level, the LED current initially goes to I LED. Dimming is done by pulsing ENA low (500ns to 10 μ s pulse width). Each pulse reduces the LED current by 1/32, so after one pulse the LED current is 31/32*I LED. The 32th pulse sets the LED current back to I LED. Figure 1 shows a timing diagram for EN. Note: V LED: LED pin voltage V OUT: Output voltage V f : LED forward voltage N MIN: The minimum LED numbers among all strings. Setting the Output Voltage The FB pin is connected to the center tap of a resistive voltage divider (R1 and R2 in Figure 1) from the high-voltage output. Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 11 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source PCB Layout Guidelines noise spikes. Create an analog ground island (GND) consisting of the output voltage detection-divider ground connection, the I SET resistor connections, VCC-5V and VCC-driver capacitor connections, and the device's exposed backside pad. Connect the GND and PGND islands by connecting the GND pins directly to the exposed backside pad. Make sure no other connections between these separate ground planes. Careful PCB layout is important for proper operation. Use the following guidelines for good PCB layout: 1) Minimize the area of the high current switching loop of the rectifier diode and output capacitor to avoid excessive switching noise. 2) Connect high-current input and output components with short and wide connections. The high-current input loop goes from the positive terminal of the input capacitor to the inductor, to the SW pin. The high-current output loop is from the positive terminal of the input capacitor through the inductor, rectifier diode, and positive terminal of the output capacitors, reconnecting between the output capacitor and input capacitor ground terminals. Avoid using vias in the high-current paths. If vias are unavoidable, use multiple vias in parallel to reduce resistance and inductance. 4) Place the output voltage setting-divider resistors as close to the OVP pin as possible. The divider's center trace should be kept short. Avoid running the sensing traces near SW Pin. 5) Place the VIN pin bypass capacitor as close to the device as possible. The ground connection of the VIN bypass capacitor should be connected directly to GND pins with a wide trace. 6) Minimize the size of the SW node while keeping it wide and short. Keep the SW node away from the feedback node and ground. If possible, avoid running the SW node from one side of the PCB to the other. 3) Create a ground island (PGND) consisting of the input and output capacitor ground and PGND pin. Connect all these together with short, wide traces or a small ground plane. Maximizing the width of the power ground traces improves efficiency and reduces output-voltage ripple and 7) Refer to the PAM2845 Evaluation board for an example of proper board layout. PAM2845 Evaluation Board Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 12 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Ordering Information PAM2845 X X X Packing Number of Pins Package Type Part Number Package Type MOQ/Packing PAM2845KHR QFN 4mmx4mm 3,000 Unites /Tape &Reel Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 13 PAM2845 40V High Power Boost Convertor with 6 Channel Constant Current Source Outline Dimensions QFN 4mmx4mm QFN Note:all dimensions are in millimeters. Power Analog Microelectronics , Inc www.poweranalog.com 08/2008 Rev 1.0 14