SC4540 High Efficiency Boost Converter for up to 10 LEDs POWER MANAGEMENT Features Description The SC4540 is designed to regulate current for a series string of white LEDs in LCD backlighting applications where small size and high efficiency are priorities. This device integrates an 800kHz current-mode PWM boost converter and a 30mA programmable low dropout current sink regulator. The wide input voltage range of the SC4540 makes it ideal for use with dual-cell Li-ion powered applications. Input voltage range — 4.5V to 12V Programmable LED current up to 30mA Current-mode PWM control — 800kHz Soft-start to reduce in-rush current PWM dimming — 100Hz to 50kHz PWM dimming at ISET — analog and filtered Over-voltage protection — 38V (minimum) Under-voltage lockout (UVLO) Thermal shutdown Shutdown current — <0.1μA (typical) Ultra-thin package — 2 x 2 x 0.6 (mm) Lead-free package, WEEE and RoHS compliant An external resistor sets the LED current up to 30mA. PWM dimming (100Hz to 50kHz) may be applied directly to the enable (EN) pin, or dimming can be controlled by applying an analog signal to the ISET circuit. The boost circuit can output up to 38V (guaranteed) to drive up to 10 LEDs in series. The current regulator protects against shorts between the BL and OUT pins and also eliminates backlight glow during shutdown when using LEDs with high leakage. Under-voltage lockout and thermal shutdown provide additional protection. A small external capacitor and series resistor control soft-start and loop compensation. Over-voltage detection protects the SC4540 if the BL pin is floating or shorted to ground. Applications DSLR, DSC, and Video Cameras Portable media players Set-top boxes Personal navigation devices The SC4540 is available in an MLPD-8 pin 2 x 2 x 0.6 (mm) package, with a rated temperature range of -40°C to +85°C. Typical Application Circuit D L1 1 μF 5 6 μH LX C O UT 4 .5 to 1 2 V IN OU T C IN 2.2μF SC 4540 E n a b le a n d PWM d im m in g EN R IS E T CC OM P 47nF RC OM P IS E T C OMP 1 .2 7 k Ω BL GN D July 18, 2008 © 2008 Semtech Corporation 1 SC4540 Pin Configuration LX 1 EN 2 IS E T 3 COMP 4 Ordering Information TO P V IE W T 8 IN 7 GND 6 BL 5 OUT Device Package SC4540ULTRT(1) (2) MLPD-UT-8 2x2 SC4540EVB Evaluation Board Note: (1) Available in tape and reel only. A reel contains 3,000 devices. (2) Lead-free package only. Device is WEEE and RoHS compliant. MLPD: 2mm x 2mm 8 Lead θJA = 74°C/W Marking Information FJ0 yw FJ0 = Marking code for SC4540 yw = Date Code 2 SC4540 Absolute Maximum Ratings Recommended Operating Conditions Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +20.0 Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 to 12 LX Voltage, Output Voltage (V) . . . . . . . . . . . . . -0.3 to +45 Output Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 to 38 Current Sink Voltage (V) . . . . . . . . . . . . . . . . . . . . -0.3 to +45 Ambient Temperature Range (°C) . . . . . . . . . . . . . -40 to +85 Enable Voltage (V) . . . . . . . . . . . . . . . . . . . . . . -0.3 to (VIN + 0.3) Compensation Voltage (V) . . . . . . . . . . . . . . . . . . -0.3 to +2.0 Current Set Voltage (V) . . . . . . . . . . . . . . . . . . . . . . -0.3 to +2.0 ESD Protection Level(1) (kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Thermal Information Thermal Resistance, Junction to Ambient(2) (°C/W) . . . . . 74 Maximum Junction Temperature (°C) . . . . . . . . . . . . . . +150 Storage Temperature Range (°C) . . . . . . . . . . . . -65 to +150 Peak IR Reflow Temperature (10s to 30s) (°C) . . . . . . . +260 Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended. NOTES: (1) Tested according to JEDEC standard JESD22-A114-B. (2) Calculated from package in still air, mounted to 3 x 4.5 (in), 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards. Electrical Characteristics Unless otherwise noted: VIN = 7.2V, CIN = 2.2μF, COUT = 1μF, CCOMP = 47nF, RCOMP = 1.27kΩ, RISET = 5.76kΩ, L = 56μH, TA = -40 to 85ºC, typical values are at TA = 25ºC. Parameter Symbol Conditions Min Typ Max Units UVLO Threshold VUVLO VIN rising 2.40 2.60 2.79 V UVLO Hysteresis VUVLO-HYS 120 mV mA Quiescent Supply Current IQ Not switching 1.9 Shutdown Supply Current ISHDN EN tied to GND 0.1 EN Logic High Voltage VIN EN Logic Low Voltage VIL EN Logic Input Current IIL, IIH Thermal Shutdown Temperature TSD 3 1.80 μA V 0.8 V VIN = 12V, VEN = 0V to 1.8V ±0.01 ±1 VIN = 12V, VEN = 1.8V to 12V ±0.01 ±5 TJ rising 155 °C 20 °C μA Thermal Shutdown Hysteresis Boost Converter Characteristics Switching Frequency fSW Maximum Duty Cycle DMAX Minimum On-Time tON(MIN) 680 TA = 25°C 800 92 920 kHz % 100 ns 3 SC4540 Electrical Characteristics (continued) Parameter Symbol Conditions Min Typ Max Units 725 mA Boost Converter Characteristics (continued) 425 Switch Over-Current Protection IOCP Switch Leakage Current IL(LX) VLX = 12V 0.01 1 μA Switch Saturation Voltage VSAT ILX = 0.3A 250 450 mV VCOMP = 0.9V, TA = 25ºC 5 VCOMP = 0.9V, TA = 25ºC 6 COMP Sourcing Current μA ICOMP COMP Sinking Current OUT Over-Voltage Protection VOVP OUT Internal Pull-Down Current IOVP OUT Bias Current 38 43 V During OVP condition 1 mA VEN = VIN, VOUT = 20V 50 70 μA VEN = 0V, VOUT = VIN = 12V 0.01 1 μA IOUT OUT Leakage Current PWM Dimming Frequency Range(1)(2) fEN Applied to EN pin 100 50k Hz PWM Dimming Duty Cycle Range(1)(2) DEN 200Hz on EN pin 0 100 % 1 30 mA -3.5 3.5 % 0.1 μA Current Sink Characteristics BL Current Setting Range(3) IBL TA= 25°C BL Current Setting Accuracy BL Leakage Current BL Current Line Regulation IL(BL) VEN = 0V, VBL = 2V 0.01 ΔIBL/ΔVIN VIN = 3.0 to 5.5V ±0.05 mA/V BL Voltage VBL 0.35 V ISET Bias Voltage VISET 0.5 V ISET-to-IBL Gain AISET 230 A/A Start-Up Time tstart-up 1 μs Notes: (1) Guaranteed by design. (2) See PWM Dimming description in the Applications Information section for limitations at high PWM dimming frequencies and low PWM dimming duty cycles. (3) Not recommended to program below 1mA with RISET due to tolerance stackup. To produce output current less than 1mA, set the current > 1mA and use PWM dimming. 4 SC4540 Typical Characteristics All data taken with VOUT = 26.5V (8 white LEDs), RISET = 5.76kΩ (IOUT = 20mA), VIN = 7.2V, L = 56μH, and efficiency (η) = PLED/PIN unless otherwise noted. Efficiency versus Output Voltage Efficiency vs. IBL 90 90 V IN = 1 2V V IN = 8 .4 V V IN = 7.2 V 86 8 6 V = 8.4 V IN E fficien cy (% ) E ffic ie n c y (% ) V IN = 5.0 V 82 V IN = 1 2V 78 82 78 74 74 70 V IN = 7 .2 V V IN = 5 .0 V 70 10 15 IB L 25 20 (m A ) 30 20 22 Efficiency versus Input Voltage 24 V O U T (V ) 26 28 30 Efficiency versus Input Voltage 90 90 30 m A 88 88 6 LEDs E fficie n c y (% ) E fficien cy (% ) 20 m A 86 84 10 m A 10 LEDs 8 LEDs 84 82 82 80 4 .5 86 6 .0 7.5 V IN (V ) 9 .0 1 0.5 80 4 .5 1 2 .0 7 .5 V IN (V ) 9 .0 1 0 .5 1 2 .0 Normalized IBL versus Output Voltage Efficiency versus Input Voltage 90 3 .5 2 .5 V O U T E fficie n cy 88 1 .5 N o rm a lize d I B L (% ) L E D E fficie n cy E ffic ie n c y (% ) 6.0 86 84 V IN = 1 2 V 0 .5 V IN = 8 .4 V -0.5 V IN = 7 .2V -1.5 V IN = 5 .0V 82 -2.5 80 4 .5 -3.5 6 .0 7 .5 V IN (V ) 9 .0 1 0 .5 1 2 .0 20 22 24 V O U T (V ) 26 28 30 5 SC4540 Typical Characteristics (continued) Typical Waveforms at VIN = 5.0V Normalized IBL versus Duty Cycle P ercen tag e o f M a xim u m I B L (% ) 100 80 VLX (20V/div) 60 VIN (50mV/div) 40 20 2 0 0H z 5 0kH z VOUT (100mV/div) 3 2kH z 0 0 20 40 60 D u ty C y c le (m A ) 80 100 1μs/div Typical Waveforms at VIN = 12V Typical Waveforms at VIN = 7.2V VLX (20V/div) VLX (20V/div) VIN (50mV/div) VIN (50mV/div) VOUT (100mV/div) VOUT (100mV/div) 1μs/div 1μs/div Start-Up Response VLX (20V/div) PWM Operation at 200Hz and 1% Duty Cycle VLX (20V/div) VOUT (20V/div) IOUT (10mA/div) IOUT (10mA/div) VEN (5V/div) VEN (5V/div) 1ms/div 10μs/div 6 SC4540 Typical Characteristics (continued) PWM Operation at 20kHz and 10% Duty Cycle PWM Operation at 32kHz and 17% Duty Cycle VLX (20V/div) VLX (20V/div) IOUT (10mA/div) IOUT (10mA/div) VEN (5V/div) VEN (5V/div) 2μs/div 2μs/div PWM Operation at 50kHz and 20% Duty Cycle VLX (20V/div) IOUT (10mA/div) VEN (5V/div) 1μs/div 7 SC4540 Pin Descriptions Pin # Pin Name Pin Function 1 LX Collector of the internal power transistor — connect to the boost inductor and rectifying Schottky diode. 2 EN Enable and brightness control pin for LED string 3 ISET Output current set pin — connect a resistor from this pin to GND to set the maximum current. 4 COMP Output of the internal transconductance error amplifier — this pin is used for loop compensation and soft-start. Connect a 1.27kΩ resistor and 47nF capacitor in series to GND. 5 OUT Boost output voltage monitor pin — internal over-voltage protection monitors the voltage at this pin. Connect this pin to the output capacitor and the anode of the LED string. 6 BL 7 GND 8 IN T Thermal Pad LED constant current sink — connect this pin to the cathode end of the LED string Ground Power supply pin — bypass this pin with a capacitor close to the pin Pad for heatsinking purposes — connect to the ground plane using multiple vias. Not connected internally. 8 SC4540 Block Diagram OUT 5 U V LO IN 8 T herm al S hutdow n OVP S R eference and Internal R egulator 7 GND 4 COMP R RSENSE S ense A m p OSC E rror S ignal S election and S um m ation 2 ADJ EN BL LX Q C om parator EN 1 6 ADJ LE D S etpoint 3 IS E T 9 SC4540 Applications Information General Description The SC4540 contains an 800kHz fixed-frequency currentmode boost converter and an independent LED current regulator. The LED current set point is chosen using an external resistor, while the PWM controller operates independently to keep the current in regulation. The SC4540 receives information from the internal LED current regulator and drives the output to the proper voltage with no user intervention. The current flowing through the LED string is independently controlled by an internal current regulator, unlike the ballasting resistor methodology that many LED current regulators use. The internal current regulator can be shut off entirely without leaking current from a charged output capacitor or causing false-lighting with low LED count and high VIN. The backlight current (IBL) is programmed using an external resistor. cycle PWM signal used for a few milliseconds provides the additional advantage of reduced in-rush at start up. The start-up delay time between the enable signal going high and the activation of the internal current regulator causes nonlinearity between the IBL current and the duty cycle of the PWM frequency seen by the EN pin. As the PWM signal frequency increases, the total on time per cycle of the PWM signal decreases. Since the start up delay time remains constant, the effect of the delay becomes more noticeable, causing the average IBL to be less predictable at lower duty cycles. Recommended minimum duty cycles are 20% for 50kHz PWM frequency, 10% for 32kHz PWM frequency and 1% for 200Hz PWM frequency. Refer to the Normalized IOUT versus Duty Cycle in the Typical Characteristics section for PWM performance across duty cycle for different PWM frequencies. Zero Duty Cycle Mode The path from the EN pin to the output control is a high bandwidth control loop. This feature allows the PWM dimming frequency to range between 100Hz and 50kHz. In shutdown mode, leakage through the current regulator output is less than 1μA. This keeps the output capacitor charged and ready for instant activation of the LED string. Zero duty cycle mode is activated when the voltage on the BL pin exceeds 1.3V. In this mode, the COMP pin voltage is pulled low, suspending all switching. This allows the VOUT and VBL voltages to fall. The COMP voltage is held low until the VBL falls below 1V, allowing VCOMP to return to its normal operating voltage and switching to resume. Protection Features The 800kHz switching speed provides high output power while allowing the use of a low profile inductor, maximizing efficiency for space constrained and cost-sensitive applications. The converter and output capacitor are protected from open-LED conditions by over-voltage protection. PWM Dimming The enable pin can be toggled to allow PWM dimming. In a typical application, a microcontroller sets a register or counter that varies the pulse width on a GPIO pin. The SC4540 provides dimming between 100Hz and 50kHz. The SC4540 is compatible with a wide range of devices by using dimming technology that avoids the audio band by using high frequency PWM dimming. A wide range of illumination can be generated while keeping the instantaneous LED current at its peak value for luminescent efficiency and color purity. The SC4540 can accommodate any PWM duty cycle between 0 and 100%. A low duty The SC4540 provides several protection features to safeguard the device from catastrophic failures. These features include: • • • • Over-voltage Protection (OVP) Soft-start Thermal Shutdown Current Limit Over-Voltage Protection (OVP) A built-in over-voltage protection circuit prevents damage to the IC and output capacitor in the event of an opencircuit condition. The output voltage of the boost converter is detected at the OUT pin and divided internally. If the voltage at the OUT pin exceeds the OVP limit, the boost converter is shut down and a pull down is applied to the OUT pin to quickly discharge the output capacitor. This additional level of protection prevents a condition where the output capacitor and Schottky diode 10 SC4540 Applications Information (continued) must withstand high voltage for an extended period of time. Soft-Start The soft-start mode reduces in-rush current by utilizing the external compensation network. As the error amplifier slowly charges the COMP node voltage, the duty cycle of the boost switch ramps from 0% to its final value when in regulation. The gradual increase of the duty cycle slowly charges the output capacitor and limits in-rush current during start up. Soft-start is implemented only when the power is cycled on the part. Thermal Shutdown A thermal shutdown mode is included for protection in the event the junction temperature exceeds 155°C. In thermal shutdown, the on-chip power switch is disabled. Switching and sinking resumes when the temperature drops by 20°C. Current Limit The power switch of the boost converter is protected by an internal current limit function. The switch is opened when the current exceeds the maximum switch current value. Inductor Selection The inductor value should be within the range of 22μH to 68μH. The DCR needs to be considered when selecting the inductor to ensure optimum efficiency. The largest inductor package that can be accommodated in the circuit area should be used since the DCR generally decreases with increasing package size. The saturation current of the inductor should be much higher than the peak current of the internal boost switch to ensure that the inductor never enters saturation during normal operation of the part. The equation to calculate the peak inductor current is: IL(Peak) IIN ǻIL 2 where VIN u D L u f osc ǻIL D IIN 1 VIN VOUT VOUT u IOUT Ș u VIN D is the duty cycle for continuous operation. Efficiency (η) can be approximated by using the curves provided in the Typical Characteristics section. Table 1 lists inductors that have been proven to work with SC4540. Table 1 — Recommended Inductors Value (μH) DCR (Ω) Rated Current (A) Tolerance Dimensions (L x W x H) (mm) Coilcraft LPS6235 56 0.28 1.1 ±20% 6.2 x 6.2 x 3.5 Coilcraft LPS4018-223ML 22 0.360 0.70 ±20% 3.9 x 3.9 x 1.7 Part Number Capacitor Selection The input capacitor should be at least 2.2μF. A larger capacitor will reduce the voltage ripple on the input. The output capacitor values can range from 0.22μF to 1μF. The compensation capacitor value should be 47nF. Capacitors of X5R type material or better can be used for any of the capacitors. See Table 2 for recommended capacitors. Table 2 — Recommended Capacitors Part Number Value (μF) Rated Voltage (V) Type Case Size 2.2 25 X5R 0805 1.0 50 X7R 0805 0.047 16 X7R 0402 Input Capacitor Murata GRM219R61E225KA12 Output Capacitor Murata GRM21BR71H105KA12L Compensation Capacitor Taiyo Yuden EMK105BJ473KV-F 11 SC4540 Applications Information (continued) Diode Selection PCB Layout Considerations A Schottky diode with a reverse voltage of 60V and a forward current rating of 1A should be used with this device for optimum performance. The ST Microelectronics STPS05602 is a recommended diode for this application. Poor layout can degrade the performance of the DC-DC converter and can be a contributory factor in EMI problems, ground bounce, thermal issues, and resistive voltage losses. Poor regulation and instability can result. A typical layout is shown in Figure 4. Selection of Other Components RISET sets the maximum load current for the SC4540. Use the following equation to select the proper value: The following design rules are recommended: • RISET = 230×VISET/ILOAD • where VISET = 0.5V (typ). Refer to Figure 1 for selecting values for other current settings. Note that the error increases as the desired IBL current decreases. • Place the inductor and filter capacitors as close to the device as possible and use short, wide traces between the power components. Route the output voltage feedback path away from the inductor and LX node to minimize noise and magnetic interference. Use a ground plane to further reduce noise interference on sensitive circuit nodes. IB L (m A ) 100 10 R IS E T T yp ica l C u rre n t T o le ra n ce 1 1 10 R IS E T (k Ω ) 100 Figure 1 – Set Resistor Value Selection Graph Figure 4– Layout 12 SC4540 Applications Information (continued) Application Circuit for Alternate Input voltages The typical applications circuit for the SC4540 is shown in Figure 2. The performance data and specifications included in this document are for the typical application circuit (Figure 2). The SC4540 can be used with input voltages lower than 4.5V (see Figure 3). The Inductor, L1, will need to be adjusted to allow proper operation for input voltages between to 2.8V to 5.5V. The SC4538 datasheet should be consulted for the performance of the lower input voltage circuit. D L1 2 2 μH 1 μF 5 6 μH LX C O UT 4 .5 to 1 2 V IN OU T 1 μF OU T IN C IN 2.2μF 2.2μF SC 4540 SC 4540 E n a b le a n d PWM d im m in g E n a b le a n d PWM d im m in g EN R IS E T 47nF C O UT LX 2 .8 V to 5 .5 V C IN CC OM P D L1 RC OM P EN R IS E T IS E T C COM P C OMP 1 .2 7 k Ω 47nF RC OM P IS E T C OMP 1 .2 7 k Ω BL BL GN D GN D Figure 2 — Typical Application Circuit Figure 3 — Low Input Voltage Application Circuit 13 SC4540 Outline Drawing – MLPD-UT-8 2x2 B D A D IM E P IN 1 IN D IC A T O R (LA S E R M A R K ) A aaa C A2 A1 S E A T IN G P LA N E C A A1 A2 b D D1 E E1 e L N aaa bbb D IM E N S IO N S IN C H E S M IL LIM E T E R S M IN N O M M A X M IN N O M M A X .0 2 4 .0 0 2 (.0 0 6) .0 0 7 .0 1 0 .0 1 2 .0 7 5 .0 7 9 .0 8 3 .0 6 1 .0 6 7 .0 7 1 .0 7 5 .0 7 9 .0 8 3 .0 2 6 .0 3 1 .0 3 5 .0 2 0 B S C .0 1 2 .0 1 4 .0 1 6 8 .0 0 3 .0 0 4 .0 2 0 .0 0 0 0 .5 0 0 .0 0 - - 0 .6 0 0 .0 5 (0 .1524) 0 .1 8 0 .2 5 0 .3 0 1 .9 0 2 .0 0 2 .1 0 1 .5 5 1 .7 0 1 .8 0 1 .9 0 2 .0 0 2 .1 0 0 .6 5 0 .8 0 0 .9 0 0 .5 0 B S C 0 .3 0 0 .3 5 0 .4 0 8 0 .0 8 0 .1 0 D1 1 E /2 2 L xN E1 N b xN bbb e C A B e /2 D /2 NOTES: 1 . C O N T R O L L IN G D IM E N S IO N S A R E IN M ILLIM E T E R S (A N G LE S IN D E G R E E S ). 2 . C O P L A N A R IT Y A P P L IE S T O T H E E X P O S E D P A D A S W E LL A S T H E T E R M IN A LS . 14 SC4540 Land Pattern – MLPD-UT-8 2x2 H R D IM (C ) K G Y P Z C G H K P R X Y Z D IM E N S IO N S IN C H E S M IL L IM E T E R S (.0 7 7 ) .047 .067 .031 .020 .0 0 6 .012 .030 .106 (1.95) 1 .2 0 1 .70 0 .80 0 .5 0 0 .1 5 0 .3 0 0 .7 5 2 .7 0 X NOTES: 1. C O N T R O L L IN G D IM E N S IO N S A R E IN M IL L IM E T E R S (A N G LE S IN D E G R E E S ). 2. T H IS L A N D P A T T E R N IS F O R R E F E R E N C E P U R P O S E S O N LY . C O N S U L T Y O U R M A N U F A C T U R IN G G R O U P T O E N S U R E Y O U R C O M P A N Y 'S M A N U F A C T U R IN G G U ID E LIN E S A R E M E T . 3. T H E R M A L V IA S IN T H E L A N D P A T T E R N O F T H E E X P O S E D P A D S H A L L B E C O N N E C T E D T O A S Y S T E M G R O U N D P LA N E . F A IL U R E T O D O S O M A Y C O M P R O M IS E T H E T H E R M A L A N D /O R F U N C T IO N A L P E R F O R M A N C E O F T H E D E V IC E . 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 15