SC4538

SC4538 High Efficiency Boost Converter for up to 10 LEDs POWER MANAGEMENT Features 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 — 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 100 3.6VIN 80 2.8VIN 60 5.5VIN 40 Efficiency vs. Output Voltage 90 86 5.5VIN Efficiency (%) Efficiency (%) 82 2.8VIN 78 3.6VIN 20 74 0 0 6 12 18 70 IBL (mA) 24 30 0 8 16 24 32 40 VOUT (V) Efficiency vs. Input Voltage 90 90 Efficiency vs. Input Voltage 6 LEDs 4 LEDs 86 86 Efficiency (%) 82 VOUT 78 LED Efficiency (%) 82 10 LEDs 78 74 74 70 2.8 3.3 3.9 4.4 70 5.0 5.5 2.8 3.3 3.9 4.4 5.0 5.5 VIN (V) VIN (V) Efficiency vs. Input Voltage 90 20mA 86 Normalized IBL vs. Output Voltage 6 4 Normalized IBL (%) 5.0 5.5 Efficiency (%) 2 82 0 78 30mA 10mA 74 -2 -4 70 2.8 3.3 3.9 4.4 -6 0 8 16 VIN (V) VOUT (V) 24 32 40 5 SC4538 Typical Characteristics (continued) PWM Operation at 200Hz and 1% Duty Cycle PWM Operation at 50kHz and 15% Duty Cycle VLX (20V/div) VLX (20V/div) IOUT (20mA/div) VOUT (10V/div) IOUT (20mA/div) VOUT (20V/div) VEN (2V/div) 2ms/div VEN (2V/div) 4μs/div PWM Operation at 32kHz and 50% Duty Cycle PWM Operation at 32kHz and 10% Duty Cycle VLX (20V/div) VLX (20V/div) IOUT (20mA/div) VOUT (20V/div) IOUT (20mA/div) VOUT (20V/div) VEN (2V/div) 10μs/div VEN (2V/div) 10μs/div IBL vs. Duty Cycle 100 IBL Error vs. PWM Frequency at 50% Duty Cycle 6.0 Percentage of Maximum IBL (%) 80 4.0 2.0 IBL Error (%) 50kHz 32kHz 20 40 60 80 100 60 0.0 40 200Hz -2.0 20 -4.0 0 0 -6.0 0.1 1 10 100 PWM Duty Cycle (%) PWM Frequency (kHz) 6 SC4538 Typical Characteristics (continued) Typical Waveforms at VIN = 2.8V Typical Waveforms at VIN = 5.5V VLX (20V/div) VLX (20V/div) VIN (50mV/div) VIN (50mV/div) VOUT (50V/div) VOUT (50V/div) 1μs/div 1μs/div Typical Waveforms at VIN = 3.6V Start-Up Response 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 Waveforms During Over-Voltage Protection 3.5 Normalized IBL vs. Input Voltage VLX (50V/div) IOUT (20mA/div) 2.3 1.2 IBL Error (%) 0.0 VOUT (20V/div) -1.2 -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 # 1 2 3 4 Pin Name LX EN ISET COMP Pin Function Collector of the internal power transistor — connect to the boost inductor and rectifying Schottky diode. Enable and brightness control pin for LED string Output current set pin — connect a resistor from this pin to GND to set the maximum current. 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. 5 6 7 8 T OUT BL GND IN Thermal Pad 8 SC4538 Block Diagram OUT 5 Thermal Shutdown UVLO OVP 1 IN 8 Reference and Internal Regulator S R Comparator RSENSE 7 OSC Sense Amp Q LX GND Error Signal Selection and Summation 4 COMP EN 2 EN ADJ ADJ LED Setpoint BL 6 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. 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. 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. 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 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 SC4538 provides several protection features to safeguard the device from catastrophic failures. These features include: 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 • • • • 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. D 1 VIN VOUT IIN VOUT IOUT 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 Part Number Value (μH) DCR (Ω) Rated Current (A) Tolerance Dimensions (L x W x H) (mm) 3.9 x 3.9 x 1.7 Coilcraft LPS4018-223ML Murata LQH43CN150K03 Murata LQH32CN150K53 22 0.360 0.70 ±20% 15 15 0.320 0.580 0.570 0.300 ±10% ±10% 4.5 x 3.2 x 2.6 3.2 x 2.5 x 1.55 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 Input Capacitor Value (μF) Rated Voltage (V) Type Case Size 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. 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 2 Murata GRM188C70J225KE20 Output Capacitor Murata GRM21BR71H105KA12L Compensation Capacitor Taiyo Yuden EMK105BJ473KV-F 2.2 6.3 X7S 0603 1.0 50 X7R 0805 IL(Peak) IIN 0.047 16 X7R 0402 where IL VIN D L f osc 11 SC4538 Applications Information (continued) Diode Selection 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. PCB Layout Considerations 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. The following design rules are recommended: Selection of Other Components 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). Refer to Figure 1 for selecting values for other current settings. Notice that the error increases as the desired IBL current decreases. 100 • • • 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. IBL (mA) 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 A D B DIMENSIONS INCHES MILLIMETERS DIM MIN NOM MAX MIN NOM MAX PIN 1 INDICATOR (LASER MARK) E A A1 A2 b D D1 E E1 e L N aaa bbb .020 . 000 . 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 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 A aaa C A1 A2 C SEATING PLANE D1 12 LxN E/2 E1 N e e/2 D/2 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. 13 SC4538 Land Pattern – MLPD-UT-8 2x2 H R DIM C G (C) K G Z H K P R Y DIMENSIONS INCHES (.077) .047 .067 .031 .020 .006 .012 .030 .106 MILLIMETERS (1.95) 1.20 1.70 0.80 0.50 0.15 0.30 0.75 2.70 X Y Z P X NOTES: 1. 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. 2. 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