SC440A

SC440A High Efficiency Integrated Driver for 6-Strings of 30mA LEDs POWER MANAGEMENT Features Wide input range 4.5V to 27V 42V maximum output voltage with adjustable OVP for smaller output capacitor Drives up to 72 WLEDs in 6 strings Programmable WLED current for up to 30mA per string 1% string-to-string current matching 2A integrated power switch Up to 91% efficiency 0.2% to 100% PWM dimming Analog dimming 800KHz switching frequency for small external component Open/short LED protection Short LED protection disable Lboundary. Generally the converter has higher efficiency under CCM and the inductor peak current is, IL -peak IIN VIN D 2 FS L IIN - Input current; IOUT – Output current; VOUT – Boost output voltage; VIN – Input voltage; η – Efficiency of the boost converter. Then the duty ratio is, For many applications, an inductor with value of 4.7μH to 22μH should be fine, such as for the typical case shown on page 1. The inductor peak current must be less than its saturation rating. When the inductor current is close to the saturation level, its inductance can decrease 20% to 35% from the 0A value depending on the vendor specifications. Using a small value inductor forces the converter under DCM in which case the inductor current ramps down to zero before the end of each switching cycle. It reduces the boost converter’s maximum output current, and produces large input voltage ripple. An inductor with larger inductance will reduce the bandwidth of the feedback loop, possibly higher DC resistance (DCR). Inductor’s DCR plays a significant role for the total efficiency since the power transistor is integrated inside the SC440A. Of course, there is a trade-off between the DCR and inductor size. Table 2 lists recommended inductors and their vendors. Table 2. Recommended Inductors D VOUT VIN VD VD Inductor DR74, 4.7μH ~ 15μH IHLP-2525CZ-01, 4.7μ ~ 10μH Website www.cooperet.com www.vishay.com www.tokoam.com VOUT VD – Forward conduction drop of the output rectifying diode When the boost converter runs in DCM ( L < Lboundary), it © 2010 Semtech Corp. DS85LC, 6.8μH ~ 10μH www.semtech.com 14 SC440A Applications Information (continued) Output Capacitor Selection The next task in SC440A design is targeting the proper amount of ripple voltage due to the constant-current LED loads. The two error amplifiers that control the PWM converter sense the delta between requested current and actual current in each output current regulator. On a cycle-by-cycle basis, a small amount of output ripple ensures good sensing and tight regulation, while the output current regulators keep each LED current at a fixed value. Overall, this allows usage of small output caps while ensuring precision LED current regulation. Although the mechanics of regulation and frequency dependence may be complex, actual selection of output capacitor can be simplified because this capacitor is mainly selected for the output ripple of the converter. Assume a ceramic capacitor is used. The minimum capacitance needed for a given ripple can be estimated by, C OUT (VOUT VOUT VIN ) IOUT FS VRIPPLE Output Rectifying Diode Selection Schottky diodes are the ideal choice for SC440A due to their low forward voltage drop and fast switching speed. Table 4 shows several different Schottky diodes that work well with the SC440A. Make sure that the diode has a voltage rating greater that the possible maximum ouput voltage. The diode conducts current only when the power switch is turned off. A diode of 1A will be sufficient for most designs. Layout Guidelines The SC440A contains a boost converter and the placements of the power components outside the SC440A should follow the layout guidelines of a general boost converter. The application circuit on page 17 will be used as an example. The layout illustration diagram is shown on page 19. R5 and C7 form a decoupling filter for the SC440A. C7 should be placed as close as possible to the VIN and PGND to achieve the best performance. C6 is the input power filtering capacitor for the boost converter power train. L1 is the boost converter input inductor. D1 is the output rectifying diode and it is recommended that a Schottky diode be used for fast reverse recovery. To minimize switching noise for the boost converter, the output capacitor, C2, should be placed at the bottom, as displayed on page 19, so that the loop formed by C2, D1, and the internal switch, is the smallest. The output of the boost converter is used to power up the LEDs. R6, C9 and C10 (open, not used), form the compensation network for the boost converter. C9 should return to analog ground. Table 4. Recommended Rectifying Diodes Part Vendor Vishay www.vishay.com VRIPPLE – Peak to peak output ripple; IOUT – Output current; VOUT – Boost output voltage; VIN – Input voltage; FS – Switching frequency. During load transient, the output capacitor supplies or absorbs additional current before the inductor current reaches its steady state value. Larger capacitance helps with the overshoot and undershoots during load transient, and loop stability. Recommended ceramic capacitor manufacturers are listed in Table 3. Table 3. Recommended Ceramic Capacitor Manufacturers Vendor Kemet Murata Taiyo Yuden SS13 SS14 Phone 408-986-0424 814-237-1431 408-573-4150 Website www.kemet.com www.murata.com www.t-yuden.com C8 determines the soft-start time and should be connected to analog ground. R8 is the output current programming resistor for IO1 through IO6 and should return to analog ground. IOGND should also be connected to AGND. Since there is pad at the bottom of the SC440A for heat dissipation, a copper area right underneath the pad is used for better heat spreading. On the bottom layer of the © 2010 Semtech Corp. www.semtech.com 15 SC440A Applications Information (continued) board another copper area, connected through vias to the top layer, is used for better thermal performance. The pad at the bottom of the SC440A should be tied to the analog ground. The analog ground should be connected to the power ground at one point for better noise immunity. © 2010 Semtech Corp. www.semtech.com 16 SC440A Case 1: Schematic for 6 strings 8 LEDs per string application L1 C6 2.2u R2 221k LED 25 12 PGND SINK 6.8u D1 10uF C2 8 LEDs per channel LED LED LED 10 SW 9 SW 8 PGND 7 OVPIN R7 10k R3 20k C8 47n R4 10k CR4 1n LED LED VIN=5V ~ 20V R5 1 2.2u C7 C9 12n R8 2k 13 14 15 R6 1.5k R9 20k 16 17 18 VIN IOSET SCP_EN COMP VOUT IOGND OVPRTN SS 6 5 4 LED LED GND SC440A EN FFLAG AGND IO1 LED 3 2 1 LED LED LED PWM IO6 IO5 IO4 IO3 GND 19 20 21 22 23 24 Case 2: Schematic for 5 strings 8 LEDs per string application 0 SS14 D1 C1 L1 10u C6 2.2u IO2 PWM 8 LEDs per channel 10uF Coilcraft, MOS6020 R2 0 0 221k P1 Input 12V 13 2.2u C7 R8 3k 14 15 16 R9 20k 17 18 OVPIN PGND SW PGND SINK SW NC R7 20k 25 12 11 10 R3 20k 9 8 7 R4 CR4 1nF 10k VIN IOSET SCP_EN COMP VOUT IOGND OVPRTN SS EN FFLAG AGND IO1 IO3 IO2 6 5 4 3 2 1 C8 47nF P2 U1 0 0 C9 R6 SC440A 12n 1.5k PWM IO6 IO5 20 IO4 21 19 22 23 © 2010 Semtech Corp. 24 www.semtech.com 17 SC440A BOM for 6 string 8-LED Application Case ITEM 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 QUANTITY 1 1 2 1 1 1 1 1 1 2 2 1 1 1 48 1 REFERENCE CR4 C2 C6, C7 C8 C9 D1 L1 R4 R2 R3, R9 R7 R8 U1 R5 LEDs R6 PART 1nF 10μF, 50V, 1210 2.2μ, 25V, 0805 47nF 12nF SS14 6.8μ, IHLP-2525CZ1 10k 221k 20k 10k 2k SC440A, Semtech 1, 0805 SML-LX0603UWD 1.5k Case 3: Schematic for 6 strings 12 low Vf LEDs per string application C1 4.7uF 50V C2 4.7uF 50V C3 4.7uF 50V LED R2 249k LED 25 12 10 9 8 7 R7 10k R4 10k R3 20k C8 47n LED L1 C6 2.2u 10u D1 SS15 12 low Vf LEDs per string LED Toko, DS86C PGND PGND CR4 1n VIN=12V R5 1 2.2u C7 C9 12n R8 3k OVPIN SINK SW SW LED LED 13 14 15 R6 6.04k R9 20k 16 17 18 VIN IOSET SCP_EN COMP VOUT OVPRTN SS 6 5 4 LED LED GND SC440A EN FFLAG AGND LED 3 2 1 LED LED LED IOGND PWM PWM IO1 IO6 IO5 IO4 IO3 23 GND 19 20 21 22 24 © 2010 Semtech Corp. IO2 www.semtech.com 18 SC440A Layout Illustration Diagrams PCB TOP PCB B OTTOM COMPONENTS TOP COMPONENTS BOTTOM © 2010 Semtech Corp. www.semtech.com 19 SC440A Outline Drawing - MLPQ-24 A D B DIM A A1 A2 b D D1 E E1 e L N aaa bbb SEATING PLANE A1 D1 LxN E/2 E1 2 1 N e D/2 C PIN 1 INDICATOR (LASER MARK) E A2 A aaa C .031 .035 .039 .000 .001 .002 - (.008) .007 .010 .012 .152 .157 .163 .100 .106 .110 .152 .157 .163 .100 .106 .110 .020 BSC .012 .016 .020 24 .004 .004 DIMENSIONS INCHES MILLIMETERS MIN NOM MAX MIN NOM MAX 0.80 0.00 0.18 3.85 2.55 3.85 2.55 0.90 1.00 0.02 0.05 (0.20) 0.25 0.30 4.00 4.15 2.70 2.80 4.00 4.15 2.70 2.80 0.50 BSC 0.30 0.40 0.50 24 0.10 0.10 bxN bbb CAB NOTES: 1. 2. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS. © 2010 Semtech Corp. www.semtech.com 20 SC440A Land Pattern - MLPQ-24 K DIMENSIONS DIM C G H K P X Y Z INCHES (.156) .122 .106 .106 .020 .010 .033 .189 MILLIMETERS (3.95) 3.10 2.70 2.70 0.50 0.25 0.85 4.80 (C) H G Z X P NOTES: 1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET. THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD SHALL BE CONNECTED TO A SYSTEM GROUND PLANE. FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR FUNCTIONAL PERFORMANCE OF THE DEVICE. 2. Contact Information Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805) 498-2111 Fax: (805) 498-3804 www.semtech.com © 2010 Semtech Corp. www.semtech.com 21