19-2028; Rev 1; 9/02 White LED Step-Up Converter in SOT23 Features ♦ Constant Current Regulation for Uniform Illumination This step-up PWM converter includes an internal, highvoltage, low RDSON N-channel MOSFET switch for high efficiency and maximum battery life. A single analog voltage Dual Mode™ input provides a simple means of brightness adjustment and on/off control. Fast 1.2MHz current-mode PWM control allows for small input and output capacitors and a small inductor while minimizing ripple on the input supply/battery. Programmable softstart eliminates inrush current during startup. The MAX1848 is available in space-saving 8-pin thin QFN (3mm ✕ 3mm) and 8-pin SOT23 packages. ♦ 2.6V to 5.5V Input Range ♦ High 87% Efficiency ♦ Analog or Logic Control of LED Intensity ♦ 0.8W Output Power with Internal High-Voltage MOSFET Switch ♦ Small, Low-Profile External Components ♦ 13V Maximum Output with Overvoltage Protection ♦ Optimized for Low Input Ripple ♦ Programmable Soft-Start ♦ 0.3µA Shutdown Current ♦ Small 8-pin Thin QFN (3mm ✕ 3mm) and 8-Pin SOT23 Packages Ordering Information Applications Cell Phones and Smart Phones PART PDAs, Palmtops, and Wireless Handhelds e-Books and Subnotebooks MAX1848EKA MAX1848ETA White LED Display Backlighting TEMP RANGE PIN-PACKAGE TOP MARK -40°C to +85°C 8 SOT23 AAIM -40°C to +85°C 8 Thin QFN (3mm ✕ 3mm) ACR Note: Hand soldering is not recommended for the MAX1848 SOT23 package. Pin Configuration Typical Application Circuit L1 33µH 2.6V to 5.5V CIN 3.3µF TOP VIEW D1 V+ LX COMP 1 8 V+ COMP 1 8 OUT CTRL 2 7 CS CTRL 2 7 CS 3 6 GND OUT 3 6 GND LX 4 5 PGND LX 4 5 PGND COUT 1µF MAX1848 LOGIC OR DAC CCOMP 0.15µF CTRL PGND GND COMP MAX1848 OUT SOT23 V+ MAX1848 THIN QFN 3mm x 3mm CS RSENSE 5Ω Dual Mode is a trademark of Maxim Integrated Products, Inc. ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1848 General Description The MAX1848 drives white LEDs with a constant current to provide backlight in cell phones, PDAs, and other hand-held devices. The step-up converter topology allows series connection of the white LEDs so that the LED currents are identical for uniform brightness. This configuration eliminates the need for ballast resistors and expensive factory calibration. Other benefits include greater simplicity, lower cost, higher efficiency, and greater reliability. MAX1848 White LED Step-Up Converter in SOT23 ABSOLUTE MAXIMUM RATINGS V+ to GND ................................................................-0.3V to +6V PGND to GND .......................................................-0.3V to +0.3V LX, OUT to GND .....................................................-0.3V to +14V LX to OUT ...............................................................-14V to +0.3V CTRL to GND.......................................-0.3V to +6V or (V+ + 2V) COMP, CS to GND .......................................-0.3V to (V+ + 0.3V) LX Current ....................................................................0.45ARMS Continuous Power Dissipation (TA = +70°C) 8-Pin SOT23 (derate 9.7mW/°C above +70°C).............777mW 8-Pin Thin QFN 3mm ✕ 3mm (derate 24.4mW/°C above +70°C)..............................................................1951mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-60°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. DC ELECTRICAL CHARACTERISTICS (V+ = 3V, VOUT = 11V, L = 33µH, COUT = 1µF, CCOMP = 0.15µF, RSENSE = 5Ω, VCTRL = 1V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS Supply Voltage Undervoltage Lockout Threshold Quiescent Current Shutdown Supply Current Overvoltage Threshold OUT Input Bias Current MIN TYP 2.6 MAX UNITS 5.5 V V+ rising, 40mV hysteresis typical 2.15 2.38 2.59 V+ falling 2.10 2.34 2.56 0.25 0.40 1 2 TA = +25°C 0.3 1 TA = +85°C 0.3 Not switching, VCTRL = VCS = V+ Switching, VCTRL = V+, VCS = GND VCTRL = GND V+ rising, 1V hysteresis typical 12.5 13.25 14.0 V+ falling 11.5 12.25 13 20 30 0.01 1 VOUT = 13V VCTRL > 0.25V 10 VCTRL = GND V+ VDIODE Output Voltage Range V mA µA V µA 12.5 V 81 mV/V ERROR AMPLIFIER CTRL to CS Regulation VCTRL = 2V, V+ = 2.6V to 5.5V CS Input Bias Current VCS = VCTRL /13.33 CTRL Input Resistance CTRL Dual Mode Threshold Hysteresis = 25mV typical CS Line Regulation V+ = 2.6V to 5.5V, VCTRL = 3V COMP Pin Resistance to Ground Device in shutdown or overvoltage COMP Output Voltage Swing 71 75 0.01 1 µA 450 670 1100 kΩ 100 170 240 mV 0.05 7.5 15 0.3 %/V 50 kΩ 2.4 V 1.5 MHz OSCILLATOR Operating Frequency Minimum Duty Cycle Maximum Duty Cycle 2 1.0 1.2 PWM mode 12 Pulse skipping 0 VCTRL = V+, VCS = GND 85 90 _______________________________________________________________________________________ % 97 % White LED Step-Up Converter in SOT23 (V+ = 3V, VOUT = 11V, L = 33µH, COUT = 1µF, CCOMP = 0.15µF, RSENSE = 5Ω, VCTRL = 1V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS MIN TYP MAX UNITS Ω N-CHANNEL SWITCH LX On-Resistance ILX = 100mA LX Leakage Current VOUT = VLX = 13V LX Current Limit Duty cycle = 65% 1.4 2.2 TA = +25°C 0.01 2 TA = +85°C 0.05 300 500 750 µA mA DC ELECTRICAL CHARACTERISTICS (V+ = 3V, VOUT = 11V, L = 33µH, COUT = 1µF, CCOMP = 0.15µF, RSENSE = 5Ω, VCTRL = 1V, TA = -40°C to +85°C, unless otherwise noted. (Note 1) PARAMETER CONDITIONS MIN MAX UNITS 2.6 5.5 V V+ rising, 40mV hysteresis typical 2.15 2.59 V+ falling 2.10 2.56 Supply Voltage Undervoltage Lockout Threshold Not switching, VCTRL = VCS = V+ Quiescent Current 0.4 Switching, VCTRL = V+, VCS = GND Overvoltage Threshold OUT Input Bias Current 2 V+ rising, 1V hysteresis typical 12.25 14.00 V+ falling 11.25 13.25 10 30 VOUT = 13V VCTRL > 0.25V VCTRL = GND Output Voltage Range 1 V mA V µA V+ VDIODE 12.25 65 85 mV/V 1 µA V ERROR AMPLIFIER CTRL to CS Regulation VCTRL = 2V, V+ = 2.6V to 5.5V CS Input Bias Current VCS = VCTRL/13.33 CTRL Input Resistance 450 1100 kΩ CTRL Dual Mode Threshold Hysteresis = 25mV typical 100 240 mV COMP Pin Resistance to Ground Device in shutdown or overvoltage 7.5 50.0 kΩ 0.3 2.4 V 0.9 1.6 MHz 85 97 % 2.2 Ω 225 850 mA COMP Output Voltage Swing OSCILLATOR Operating Frequency Maximum Duty Cycle VCTRL = V+, VCS = GND N-CHANNEL SWITCH LX On-Resistance ILX = 100mA LX Current Limit Duty cycle = 65% Note 1: Limits are 100% production tested at TA = +25°C. Limits over the operating temperature range are guaranteed through correlation using statistical quality control (SQC) methods. _______________________________________________________________________________________ 3 MAX1848 DC ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (See Typical Application Circuit, V+ = 3V, ILED = 15mA, L = 33µH, COUT = 1µF, CCOMP = 0.15µF, RSENSE = 5Ω, CTRL = 1V, 2 LEDs, TA = +25°C, unless otherwise noted.) SWITCHING FREQUENCY vs. SUPPLY VOLTAGE EFFICIENCY vs. LOAD CURRENT 88 2 LEDs 85 EFFICIENCY (%) 2 LEDs 86 84 3 LEDs 3 LEDs 80 75 82 80 1.4 3.0 3.5 4.0 4.5 5.0 5.5 0 SUPPLY VOLTAGE (V) 1.3 TA = +25°C TA = -40°C 1.2 10 20 30 40 50 60 2.5 LOAD CURRENT (mA) POWER-UP WAVEFORMS POWER-UP WAVEFORMS 3.0 3.5 4.0 4.5 SUPPLY VOLTAGE (V) 0 38mA VCTRL MAX1848 toc06 0 IIN 0 tRISE = 15ms ILED 0 38mA IIN 0 15mA tRISE = 110ms 15mA ILED 0 CCOMP = 0.15µF 1V VCTRL 38mA IIN 0 15mA CCOMP = 1µF 0 0 50ms/div 50ms/div VCTRL TRANSIENT RESPONSE MAX1848 toc09 MAX1848 toc08 MAX1848 toc07 3.50V 1V 0 VIN VCTRL 2.0V 3.00V VOUT 6.25V 500mV/div 6.3V 4.9V VOUT 20µs/div LINE-TRANSIENT RESPONSE SHUTDOWN WAVEFORMS VCTRL 1.0V 6.7V 6.3V VOUT VOUT EVENTUALLY DECAYS TO V+ = 3V CCOMP = 0.15µF 1.5V VCOMP 0 2µs/div 4 5.5 1V 1V VCTRL 5.0 SHUTDOWN WAVEFORMS MAX1848 toc05 MAX1848 toc04 ILED TA = +85°C 1.1 70 2.5 MAX1848 toc03 IOUT = 15mA MAX1848 toc02 90 MAX1848 toc01 90 SWITCHING FREQUENCY (MHz) EFFICIENCY vs. SUPPLY VOLTAGE EFFICIENCY (%) MAX1848 White LED Step-Up Converter in SOT23 ILED 15mA 10mA/div 30mA ILED 15mA CCOMP = 0.15µF 5ms/div _______________________________________________________________________________________ 20ms/div White LED Step-Up Converter in SOT23 VCTRL TRANSIENT RESPONSE SWITCHING WAVEFORMS MAX1848 toc10 MAX1848 toc11 2.0V VCTRL 6.5V VLX 0 1.0V 61mA IL 17mA 6.7V 6.3V VOUT VIN_RIPPLE 10mV/div VOUT_RIPPLE 10mV/div 30mA ILED CCOMP = 1µF 20ms/div 15mA 200ns/div Pin Description PIN NAME 1 COMP FUNCTION Compensation Pin for Error Amplifier. Connect capacitor from COMP to GND. Startup time is set by the capacitance connected to this pin (0.833ms for each 0.01µF). VCOMP passively discharges to GND when in shutdown. Brightness/Shutdown Dual Mode Control Input. LED brightness and IC shutdown are controlled by the voltage on CTRL. Voltages between 250mV and 5.5V or (V+ + 2V), whichever is less, adjust the brightness from dim to bright, respectively. To put the IC into shutdown, drive below 100mV or connect to GND. Overvoltage Sense. When VOUT is greater than 13.25V, the internal N-channel MOSFET is turned off and VCOMP decays to GND. When VOUT drops below 12.25V, the IC will re-enter soft-start. Connect a 1µF capacitor from OUT to GND. 2 CTRL 3 OUT 4 LX 5 PGND 6 GND 7 CS Current-Sense Feedback Input. Connect a resistor from this pin to GND to set the LED bias current. This pin regulates to 7.5% of VCTRL. 8 V+ Supply Voltage Input. The IC is powered from this pin. Input range is 2.6V to 5.5V. Bypass with a ceramic capacitor to GND. Inductor Connection. Drain of the internal high-voltage N-channel MOSFET. Power Ground. Source of the internal high-voltage N-channel MOSFET. Ground _______________________________________________________________________________________ 5 MAX1848 Typical Operating Characteristics (continued) (See Typical Application Circuit, V+ = 3V, ILED = 15mA, L = 33µH, COUT = 1µF, CCOMP = 0.15µF, RSENSE = 5Ω, CTRL = 1V, 2 LEDs, TA = +25°C, unless otherwise noted.) Detailed Description The MAX1848’s high efficiency and small size make it ideally suited to drive series-connected LEDs. It operates as a boost DC-DC converter that controls output current rather than voltage. The MAX1848 provides even illumination by sending the same output current through each LED, eliminating the need for expensive factory calibration. The fast 1.2MHz internal oscillator allows for a small inductor and small input and output capacitors while minimizing input and output ripple. The single analog control input allows easy adjustment of LED brightness and on/off control. This allows either simple logic-level on/off control or a DAC to control both brightness and on/off. In shutdown, supply current is reduced to a low 0.3µA. A programmable soft-start gradually illuminates the LEDs, reducing the inrush current during startup. Soft-Start The MAX1848 attains soft-start by charging C COMP gradually with a constant 12µA current. When VCOMP rises above 1.25V, the internal MOSFET begins switching, but at a reduced duty cycle. When VCOMP rises above 2.25V, the duty cycle will be at its maximum. The maximum startup time is determined by the value of CCOMP. For every 0.01µF connected to COMP, the startup time will increase by 0.833ms. The start time can be calculated by: t SOFT- START (MAX) = CCOMP × Shutdown Overvoltage Protection Overvoltage protection occurs when VOUT is above 13.25V. The protection circuitry stops the internal MOSFET from switching and causes VCOMP to decay to GND. The device comes out of overvoltage lockout and into soft-start when VOUT falls below 12.25V. Design Procedure Adjusting LED Current LX PWM CONTROL PGND 13.25V OUT 1.2MHz CS SLOPE CURRENT SENSE COMP CTRL 617kΩ 50kΩ SHUTDOWN 170mV MAX1848 GND Adjusting the MAX1848’s output current will change the brightness of the LEDs. An analog input (CTRL) and the sense resistor value set the output current. Output current is given by: ILED = VCTRL 13.33 × RSENSE The VCTRL voltage range for adjusting output current is 250mV to (V+ + 2V) or 5.5V, whichever is less. To set the maximum current, calculate RSENSE when VCTRL is at its maximum. Power dissipation in RSENSE is typically less than 5mW; therefore, a standard chip resistor is sufficient. Capacitor Selection The exact values of input and output capacitors are not critical. The typical value for the input capacitor is 3.3µF, and the typical value for the output capacitor is 1.0µF. Larger value capacitors can be used to reduce input and output ripple, but at the expense of size and higher cost. Figure 1. Functional Diagram 6 1V 12µA The MAX1848 is put into shutdown when VCTRL is less than 100mV. In shutdown, supply current is reduced to 0.3µA by powering down the entire IC except for the CTRL voltage detection circuitry. CCOMP is passively discharged during shutdown, allowing the device to reinitiate a soft-start whenever the device is enabled. When in shutdown, the internal N-channel FET does not switch, which leaves a current path between the input and the LEDs through the boost inductor and Schottky diode. The minimum forward voltage of the LED array must exceed the maximum V+ to ensure that the LEDs remain off in shutdown. Typical shutdown timing characteristics are shown in the Typical Operating Characteristics. V+ 8 MAX1848 White LED Step-Up Converter in SOT23 _______________________________________________________________________________________ White LED Step-Up Converter in SOT23 Inductor Selection The value of the inductor depends on the maximum output current to the LEDs. See Table 1 for inductance values and peak current ratings for the inductor. Schottky Diode Selection The MAX1848’s high-switching frequency demands a high-speed rectification diode. A Schottky diode is required due to their fast recovery time and low forward-voltage drop. Ensure that the diode’s average and peak current rating exceed the average output current and peak inductor current, respectively. In addition, the diode’s reverse breakdown voltage must exceed VOUT. IDIODE(RMS) ≅ PC Board Layout Due to fast-switching waveforms and high-current paths, careful PC board layout is required. Protoboards and wire-wrap boards should not be used for evaluation. An evaluation kit (MAX1848EV kit) is available to aid design. When laying out a board, minimize trace lengths between the IC and RSENSE, the inductor, the diode, the input capacitor, and the output capacitor. Keep traces short, direct, and wide. Keep noisy traces, such as the inductor's traces, away from CS. V+’s bypass capacitor (CIN) should be placed as close to the IC as possible. PGND and GND should be connected in only one place as close to the IC as possible. The ground connections of C IN and C OUT should be as close together as possible. The traces from V+ to the inductor and from the Schottky diode to the LEDs may be longer. Refer to the MAX1848 EV kit for an example of proper layout. IOUT × IPEAK Applications Information Connecting Four or Six LEDs The MAX1848 can drive one, two, or three legs of LEDs (Figure 2) as long as the total number of LEDs does not exceed six. Each leg must contain the same number of LEDs and the same sense-resistor value. Adding the second or third leg does not affect the sense-resistor value (see the Adjusting LED Current section). Three legs of two LEDs is more efficient than two legs of three LEDs (see Efficiency Graphs in the Typical Operating Characteristics); however, a third sense resistor is needed. Multiple legs can have slight current mismatches due to component tolerances. 10µH 2.6V to 5.5V D1 3.3µF LX V+ OUT 1µF MAX1848 LOGIC OR DAC CTRL PGND GND COMP 0.01µF 25Ω CS RSENSE 5Ω 30Ω Figure 2. Six LEDs in 3 x 2 Configuration Table 1. Component Selection ILED (mA) 12 20 40 60 NO. OF LEDs CCOMP (µF) 3 0.220 2 0.100 3 0.150 2 0.068 3 0.100 2 0.047 3 0.068 2 0.01 Chip Information INDUCTOR L (µH) IPEAK(mA) 56 80 33 130 15 260 10 375 TRANSISTOR COUNT: 1290 _______________________________________________________________________________________ 7 MAX1848 The output current and the number of LEDs in each leg affect the capacitance of CCOMP. Table 1 shows the minimum CCOMP values needed to stabilize the converter in worst-case conditions. If further stability analysis is required, note that the error amplifier has 50µA/V transconductance. Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.) SOT23, 8L.EPS MAX1848 White LED Step-Up Converter in SOT23 8 _______________________________________________________________________________________ White LED Step-Up Converter in SOT23 6, 8, &10L, QFN THIN.EPS PACKAGE OUTLINE, 6, 8 & 10L, QFN THIN (DUAL), EXPOSED PAD, 3x3x0.80 mm 21-0137 C COMMON DIMENSIONS SYMBOL A MIN. 0.70 MAX. 0.80 D 2.90 3.10 E 2.90 3.10 A1 0.00 0.05 L k 0.20 0.40 0.25 MIN A2 0.20 REF. PACKAGE VARIATIONS PKG. CODE N D2 E2 e JEDEC SPEC b T633-1 6 1.50–0.10 2.30–0.10 0.95 BSC MO229 / WEEA 0.40–0.05 1.90 REF T833-1 8 1.50–0.10 2.30–0.10 0.65 BSC MO229 / WEEC 0.30–0.05 1.95 REF T1033-1 10 1.50–0.10 2.30–0.10 0.50 BSC MO229 / WEED-3 0.25–0.05 2.00 REF [(N/2)-1] x e PACKAGE OUTLINE, 6, 8 & 10L, QFN THIN (DUAL), EXPOSED PAD, 3x3x0.80 mm 21-0137 C Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________ 9 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX1848 Package Information (continued) (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)