SC4538 High Efficiency Boost Converter for up to 10 LEDs POWER MANAGEMENT Features Description The SC4538 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. Input voltage range — 2.8V to 5.5V 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 — 2mm x 2mm x 0.6mm 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 SC4538 if the BL pin is floating or shorted to ground. Applications DSLR, DSC, and Video Cameras Cellular handsets Portable media players Personal navigation systems Satellite radio Handheld video games The SC4538 is available in an MLPD-8 pin 2mm x 2mm x 0.6mm package, with a rated temperature range of -40°C to +85°C. Typical Application Circuit D L1 LX 2.8 to 5.5V IN COUT OUT CIN SC4538 Enable and PWM dimming EN RISET CCOMP ISET RCOMP COMP BL GND June 20, 2008 © 2008 Semtech Corporation 1 SC4538 Pin Configuration LX 1 EN 2 ISET 3 COMP 4 Ordering Information TOP VIEW T 8 IN 7 GND 6 BL 5 OUT Device Package SC4538ULTRT(1) (2) MLPD-UT-8 2x2 SC4538EVB 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 FE0 yw FE0 = Marking code for SC4538 yw = Date Code 2 SC4538 Absolute Maximum Ratings Recommended Operating Conditions Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +20.0 Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 to 5.5 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 = 3.6V, CIN = 2.2μF, COUT = 1μF, CCOMP = 47nF, RCOMP = 1.27kΩ, RISET = 5.76kΩ, L = 22μH, 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 1.8 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 VIN = 5.5V, VEN = 0V or 5.5V ±0.01 Thermal Shutdown Temperature TSD TJ rising 155 °C 20 °C 1 1.80 Thermal Shutdown Hysteresis μA V 0.8 V ±1 μA Boost Converter Characteristics Switching Frequency fSW Maximum Duty Cycle DMAX Minimum On-Time Switch Over-Current Protection 680 VIN = 3.2V, TA = 25°C 920 92 tON(MIN) IOCP 800 % 100 425 kHz ns 725 mA 3 SC4538 Electrical Characteristics (continued) Parameter Symbol Conditions Switch Leakage Current IL(LX) Switch Saturation Voltage VSAT Min Typ Max Units VLX = 5.5V 0.01 1 μA ILX = 0.3A 250 450 mV VCOMP = 0.9V, TA = 25ºC 5 VCOMP = 0.9V, TA = 25ºC 6 Boost Converter Characteristics (continued) COMP Sourcing Current ICOMP COMP Sinking Current OUT Over-Voltage Protection VOVP OUT Internal Pull-Down Current IOVP OUT Bias Current μA 38 43 V During OVP condition 1 mA VEN = VIN, VOUT = 20V 50 70 μA VEN = 0V, VOUT = VIN = 5.5V 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 1.3 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 SC4538 Typical Characteristics All data taken with VOUT = 20V (6 white LEDs), RISET = 5.76kΩ (IBL = 20mA), VIN = 3.6V, L = 22μH, and efficiency (η) = PLED/PIN unless otherwise noted. Efficiency vs. IBL Efficiency vs. Output Voltage 90 100 3.6VIN 86 80 Efficiency (%) Efficiency (%) 5.5VIN 2.8VIN 60 5.5VIN 40 82 78 20 74 70 0 0 6 12 18 24 IBL (mA) 0 30 8 16 40 Efficiency vs. Input Voltage 90 90 86 86 Efficiency (%) 6 LEDs 82 VOUT 32 24 VOUT (V) Efficiency vs. Input Voltage Efficiency (%) 3.6VIN 2.8VIN LED 78 74 4 LEDs 82 10 LEDs 78 74 70 2.8 3.3 70 4.4 3.9 5.0 5.5 2.8 3.3 4.4 3.9 5.0 5.5 VIN (V) VIN (V) Efficiency vs. Input Voltage Normalized IBL vs. Output Voltage 90 6 20mA 4 Normalized IBL (%) Efficiency (%) 86 82 78 30mA 10mA 2 0 -2 74 -4 70 2.8 3.3 3.9 4.4 VIN (V) -6 5.0 5.5 0 8 16 VOUT (V) 24 32 40 5 SC4538 Typical Characteristics (continued) PWM Operation at 50kHz and 15% Duty Cycle PWM Operation at 200Hz and 1% Duty Cycle VLX (20V/div) VLX (20V/div) IOUT (20mA/div) IOUT (20mA/div) VOUT (10V/div) VOUT (20V/div) VEN (2V/div) VEN (2V/div) 2ms/div 4μs/div PWM Operation at 32kHz and 50% Duty Cycle VLX (20V/div) PWM Operation at 32kHz and 10% Duty Cycle VLX (20V/div) IOUT (20mA/div) IOUT (20mA/div) VOUT (20V/div) VOUT (20V/div) VEN (2V/div) VEN (2V/div) 10μs/div 10μs/div IBL vs. Duty Cycle IBL Error vs. PWM Frequency at 50% Duty Cycle 6.0 4.0 80 2.0 60 IBL Error (%) Percentage of Maximum IBL (%) 100 40 200Hz 0.0 -2.0 20 -4.0 50kHz 32kHz -6.0 0 0 20 40 60 PWM Duty Cycle (%) 80 100 0.1 1 10 100 PWM Frequency (kHz) 6 SC4538 Typical Characteristics (continued) Typical Waveforms at VIN = 5.5V Typical Waveforms at VIN = 2.8V VLX (20V/div) VLX (20V/div) VIN (50mV/div) VIN (50mV/div) VOUT (50V/div) VOUT (50V/div) 1μs/div 1μs/div Start-Up Response Typical Waveforms at VIN = 3.6V VLX (20V/div) VLX (20V/div) VIN (50mV/div) IOUT (20mA/div) VOUT (10V/div) VOUT (50mV/div) VEN (2V/div) 1μs/div 100μs/div Normalized IBL vs. Input Voltage Waveforms During Over-Voltage Protection 3.5 2.3 VLX (50V/div) 1.2 IBL Error (%) IOUT (20mA/div) 0.0 -1.2 VOUT (20V/div) -2.3 VEN (2V/div) -3.5 400μs/div 2.8 3.25 3.7 4.15 4.6 5.05 5.5 VIN (V) 7 SC4538 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 5 OUT 6 BL 7 GND 8 IN T Thermal Pad 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. 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. LED constant current sink — connect this pin to the cathode 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 SC4538 Block Diagram OUT 5 UVLO IN 8 Thermal Shutdown OVP S Reference and Internal Regulator 7 GND 4 COMP R RSENSE Sense Amp OSC Error Signal Selection and Summation 2 ADJ EN BL LX Q Comparator EN 1 6 ADJ LED Setpoint 3 ISET 9 SC4538 Applications Information General Description The SC4538 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 SC4538 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 scheme 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, 15% for 32kHz PWM frequency and 2% for 200Hz PWM frequency. Refer to the IBL vs. 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 SC4538 provides dimming between 100Hz and 50kHz. The SC4538 is compatible with a wide range of devices, allowing dimming strategies that avoid 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 SC4538 can accommodate any PWM duty cycle between 0 and 100%. A low duty The SC4538 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 strong 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 SC4538 Applications Information (continued) must endure 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 once 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 input power is cycled. Thermal Shutdown A thermal shutdown system 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 4.7μH to 22μH. The DCR needs to be considered when selecting the inductor to ensure optimum efficiency. The largest inductor package that can be tolerated in the circuit area should be used since the DCR generally decreases with increasing package size. VIN VOUT D 1 IIN 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 SC4538. Table 1 -- Recommended Inductors Value (μH) DCR (Ω) Rated Current (A) Tolerance Dimensions (L x W x H) (mm) Coilcraft LPS4018-223ML 22 0.360 0.70 ±20% 3.9 x 3.9 x 1.7 Murata LQH43CN150K03 15 0.320 0.570 ±10% 4.5 x 3.2 x 2.6 Murata LQH32CN150K53 15 0.580 0.300 ±10% 3.2 x 2.5 x 1.55 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 the recommended capacitors. Table 2 -- Recommended Capacitors Part Number Value (μF) Rated Voltage (V) Type Case Size 2.2 6.3 X7S 0603 1.0 50 X7R 0805 0.047 16 X7R 0402 Input Capacitor 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 Murata GRM188C70J225KE20 Output Capacitor Murata GRM21BR71H105KA12L Compensation Capacitor Taiyo Yuden EMK105BJ473KV-F where ǻIL VIN u D L u f osc 11 SC4538 Applications Information (continued) Diode Selection PCB Layout Considerations A Schottky diode with a reverse voltage of 40V and a forward current rating of 1A should be used with this device for optimum per formance. The Central Semiconductor Corporation CMOSH-4E is a suitable 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 2. Selection of Other Components The following design rules are recommended: RISET sets the maximum load current for the SC4538. Use the following equation to select the proper value: RISET = 230 × VISET/ILOAD • • where • VISET = 0.5V (typ). 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. Refer to Figure 1 for selecting values for other current settings. Notice that the error increases as the desired IBL current decreases. IBL (mA) 100 10 RISET Typical Current Tolerance 1 1 10 RISET (kΩ) 100 Figure 1 – Set Resistor Value Selection Graph Figure 2– Layout 12 SC4538 Outline Drawing – MLPD-UT-8 2x2 B D A DIMENSIONS INCHES MILLIMETERS DIM MIN NOM MAX MIN NOM MAX E PIN 1 INDICATOR (LASER MARK) A aaa C A2 A1 SEATING PLANE C A A1 A2 b D D1 E E1 e L N aaa bbb . 024 . 002 (.006) .007 . 010 . 012 . 075 .079 . 083 . 061 .067 . 071 . 075 .079 . 083 . 026 .031 . 035 . 020 BSC . 012 .014 . 016 8 .003 .004 .020 . 000 0.60 0.50 0.00 0.05 (0. 1524) 0.18 0.25 0.30 1.90 2.00 2.10 1.55 1.70 1.80 1.90 2.00 2.10 0.65 0.80 0.90 0. 50 BSC 0.30 0.35 0.40 8 0.08 0.10 D1 1 2 LxN E/2 E1 N bxN bbb e C A B e/2 D/2 NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS ( ANGLES IN DEGREES). 2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS. 13 SC4538 Land Pattern – MLPD-UT-8 2x2 H DIMENSIONS R (C) G K Z Y P DIM INCHES C (.077) G .047 1.20 H .067 1.70 K .031 0.80 P .020 0.50 R .006 0.15 X .012 0.30 Y .030 0.75 Z .106 2.70 MILLIMETERS (1.95) X NOTES: 1. 2. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET. 3. 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. 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 14