19-1621; Rev 2; 2/07 High-Efficiency Step-Up Current Regulator for LEDs Features The MAX1698 and MAX1698A are efficient drivers for white or color LEDs. They are ideal for large LED backlit displays in PDAs and laptop computers. Numerous benefits include greater simplicity, lower cost, higher efficiency, longer bulb life, and greater reliability when compared to fluorescent (CCFL) and electroluminescent (EL) lamps. ♦ Over 90% Efficiency The MAX1698/MAX1698A are switch-mode boost controllers in which LED current, rather than output voltage, is regulated The devices drive series-connected LEDs with a controlled current that is measured with a typically 15Ω sense resistor, not an expensive fractional-ohm value. LED current control and dimming are accomplished with an adjust input (ADJ). For larger light output, multiple LED banks can be connected in parallel with up to 5W total output power. The MAX1698A has a higher, and more accurately specified, CS current limit than the MAX1698 for higherpower applications of 5W or more. Both devices are supplied in a space-saving 10-pin µMAX package that occupies half the space of an 8-pin SO. An evaluation kit (MAX1698EVKIT) is available to speed designs. ♦ Up to 5W Output Power ♦ Lossless, Adjustable LED Brightness ♦ Space-Saving 10-Pin µMAX Package ♦ Simpler, Lower Cost, More Reliable Compared to CCFL or EL Backlights Ordering Information PART TEMP RANGE PINPACKAGE PKG CODE MAX1698EUB -40°C to +85°C 10µMAX U10-2 MAX1698AEUB -40°C to +85°C 10µMAX U10-2 Pin Configuration TOP VIEW VCC 1 Applications SHDN 10 EXT 2 MAX1698 MAX1698A 9 CS Battery-Powered Backlight Applications REF 3 8 PGND Backlight for LCD Panels ADJ 4 7 GND Notebook PCs GND 5 6 FB Handy Terminals µMAX PDAs Typical Operating Circuit L D1 VBATT VOUT COUT VCC MAX1698 MAX1698A ON OFF EXT VCC SHDN REF ILED CS PGND ADJ FB GND RFB RN ________________________________________________________________ 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 MAX1698/MAX1698A General Description MAX1698/MAX1698A High-Efficiency Step-Up Current Regulator for LEDs ABSOLUTE MAXIMUM RATINGS VCC, SHDN to GND ..................................................-0.3V to +6V EXT, FB, CS, ADJ, REF to GND..................-0.3V to (VCC + 0.3V) GND to PGND.....................................................................±0.3V Continuous Power Dissipation (TA = +70°C) 10-Pin µMAX (derate 5.6mW/°C above +70°C) ............444mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature Range (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. ELECTRICAL CHARACTERISTICS (VCC = +3.3V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL Supply Voltage VCC Quiescent Supply Current ICC Undervoltage Lockout CONDITIONS MIN 5.5 V 260 500 µA 2.55 V Rising edge 2.25 2.4 ADJ = REF 285 300 315 VADJ = 100mV 18 24 30 MAX1698A 2.0 2.4 2.8 MAX1698 0.8 1.5 2.5 80 VFB CS Trip Current CS Input Current MAX 2.7 VFB = 0.3V Undervoltage Hysteresis FB Regulation Voltage TYP FB = GND ICS UNITS mV mV A 0.8 ARMS µs Minimum Off Time tOFF(MIN) FB = GND, ADJ = REF 0.8 1.0 1.2 Maximum On Time tON(MAX) FB = GND, ADJ = REF, CS = GND 10 15 20 µs 30 50 70 mV ADJ Start Threshold ADJ Input Range 0.03 REF V FB Input Bias Current VADJ IFB VFB = 300mV -15 15 nA ADJ Input Bias Current IADJ ADJ = REF -50 50 nA µA SHDN = GND Shutdown Supply Current REF Output Voltage REF Load Regulation VREF ∆VREF REF Short-Circuit Current REF Power-Supply Rejection Ratio PSRR EXT Driver Sink/Source Current EXT Driver On-Resistance 0.01 1 1.25 1.30 V -2 -25 mV REF = GND 0.45 1 mA VCC = 2.7V to 5.5V +0.3 +2 mV/V VCC = 5V, EXT = 2V 0.2 IREF = 0 IREF = 0 to 150µA VCC = 5V SHDN Input High Voltage VIH VCC = 2.7V to 5.5V SHDN Input Low Voltage VIL VCC = 2.7V to 5.5V SHDN Input Bias Current I SHDN VCC = 2.7V to 5.5V 2 1.20 A 8 2 -1 _______________________________________________________________________________________ Ω V 0.8 V 1 µA High-Efficiency Step-Up Current Regulator for LEDs MAX1698/MAX1698A ELECTRICAL CHARACTERISTICS (VCC = +3.3V, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL Supply Voltage VCC Quiescent Supply Current ICC Undervoltage Lockout FB Regulation Voltage VFB CS Trip Current CONDITIONS MIN 2.7 VFB = 0.3V MAX UNITS 5.5 V 500 µA V Rising edge 2.20 2.65 ADJ = REF 280 320 VADJ = 100mV 13 35 MAX1698A 2.0 3.0 MAX1698 0.8 3.0 0.6 1.4 µs 9 21 µs FB = GND Minimum Off Time tOFF(MIN) FB = GND, ADJ = REF Maximum On Time tON(MAX) FB = GND, ADJ = REF, CS = GND ADJ Start Threshold mV A 25 75 mV FB Input Bias Current IFB VFB = 300mV -20 20 nA ADJ Input Bias Current IADJ ADJ = REF -50 50 nA 1 µA 1.17 1.33 V -30 mV 1 mA +2 mV/V 8 Ω SHDN = GND Shutdown Supply Current REF Output Voltage REF Load Regulation VREF ∆VREF REF Short-Circuit Current REF Power-Supply Rejection Ratio IREF = 0 IREF = 0 to 150µA REF = GND PSRR EXT On-Resistance VCC = 2.7V to 5.5V VCC = 5V SHDN Input High Voltage VIH VCC = 2.7V to 5.5V SHDN Input Low Voltage VIL VCC = 2.7V to 5.5V SHDN Input Bias Current I SHDN VCC = 2.7V to 5.5V 2 -1 V 0.8 V 1 µA Note 1: Specifications to -40°C are guaranteed by design, not production tested. _______________________________________________________________________________________ 3 Typical Operating Characteristics (Circuit of Figure 2, VCC = VBATT = 3.3V, VADJ = VREF, TA = +25°C, unless otherwise noted.) NORMALIZED OUTPUT CURRENT vs. ADJ VOLTAGE vs. VCC 15 10 5 22.0 1.0 OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) 20 1.2 MXA1698-02 vs. VBATT NORMALIZED OUTPUT CURRENT MXA1698-01 25 OUTPUT CURRENT vs. INPUT VOLTAGE 0.8 0.6 0.4 MXA1698-03 OUTPUT CURRENT vs. VBATT AND VCC 21.5 21.0 20.5 0.2 1 CHAIN OF 4 LEDs 1 CHAIN OF 4 LEDs 0 0 2 3 4 5 6 7 8 9 10 0 0.2 VBATT, VCC (V) 0.6 1.2 0 RISE/FALL TIME (ns) IBATT 600 500 400 300 200 IOUT 0 60 FALL TIME 40 2 3 4 5 6 7 8 9 60 50 40 20 20 10 10 IOUT = 80mA 4 CHAINS OF 4 LEDs 0 100 400 700 1000 1300 1600 1900 2200 0 2 4 6 EFFICIENCY vs. OUTPUT CURRENT SHUTDOWN/SOFT-START TIMING 10 LED AND INDUCTOR CURRENT WAVEFORMS MXA1698-08 MXA1698-07 MXA1698-09 IOUT 50mA/div 80 LED CURRENT 50mA/div 70 60 50 IBATT 100mA/div 40 INDUCTOR CURRENT 500mA/div 30 20 VBATT = 5V 4 CHAINS OF 4 LEDs 10 3.3V 0 IOUT SET TO 80mA, 4 CHAINS OF 4 LEDs SHDN 5V/div 0 5 15 25 35 45 55 65 OUTPUT CURRENT (mA) 4 8 BATTERY VOLTAGE (V) CEXT (pF) 90 70 30 VBATT (V) 100 10 12 14 16 18 20 80 30 10 8 90 RISE TIME 70 50 6 EFFICIENCY vs. BATTERY VOLTAGE 0 1 4 100 MXA1698-05 80 0 2 INPUT VOLTAGE (V) 90 800 EFFICIENCY (%) 1.4 100 MXA1698-04 4 CHAINS OF 4 LEDs 100 1.0 EXT RISE/FALL TIME vs. CEXT 1000 700 0.8 ADJ VOLTAGE (V) BATTERY CURRENT vs. BATTERY VOLTAGE 900 0.4 MXA1698-06 1 20.0 EFFICIENCY (%) 0 IOUT, IBATT (mA) MAX1698/MAX1698A High-Efficiency Step-Up Current Regulator for LEDs 75 85 5ms/div 1µs/div _______________________________________________________________________________________ High-Efficiency Step-Up Current Regulator for LEDs PIN NAME FUNCTION 1 VCC 2 SHDN 3 REF 1.25V Reference Output. Capable of sourcing 150µA for external loads. This pin is internally compensated. Do not connect any bypass capacitors at REF. 4 ADJ Adjust Input. Allows dynamic adjustment of the output current. FB regulates to 300mV when ADJ = REF. 5, 7 GND Ground 6 FB 8 PGND 9 CS FET Current-Sense Input 10 EXT Gate Driver Output IC Supply Voltage Input. Power for internal circuitry. Input range is 2.7V to 5.5V. Active-Low Shutdown Input. In shutdown, the MOSFET turns off, but a current path still exists between the input and output. The minimum forward voltage of the LED array must exceed the maximum VBATT to ensure that the LEDs are off in shutdown. Feedback Input. Connect to the external LED current-sense feedback resistor. Power Ground CS MAX1698 SHDN VCC UNDERVOLTAGE LOCKOUT/ SHUTDOWN VCC TRIG MAX tON Q EXT ADJ TRIG MIN tOFF PGND Q FB SOFT-START GND 1.25V REFERENCE REF Figure 1. Functional Diagram _______________________________________________________________________________________ 5 MAX1698/MAX1698A Pin Description MAX1698/MAX1698A High-Efficiency Step-Up Current Regulator for LEDs 10µH VBATT = 3.3V MBR0540 10µF 1µF VCC = 3.3V EXT VCC 1µF MAX1698 MAX1698A SHDN CS REF LEDs OPTIONAL DZ ADJ FB GND PGND RFB 15Ω 15Ω 15Ω 15Ω Figure 2. Typical Operating Circuit Detailed Description The MAX1698/MAX1698As' high efficiency and small size make them ideally suited to drive LEDs. They operate as a boost DC-DC converter that controls output current rather than voltage. Losses are minimized by a low, 300mV current-sense threshold. In the standard configuration, a feedback resistor, RFB, sets the current through the primary chain of LEDs. Additional chains of matching LEDs can be added with an equivalent resistor. In matched LED arrays, the secondary chain currents closely track the primary chain. An optional zener diode, D2, prevents overvoltage in the event that one of the LEDs in the primary chain becomes an open circuit. The LED brightness can be adjusted dynamically by a voltage input at ADJ. Shutdown In shutdown, the supply current is reduced below 1µA. EXT goes low in shutdown, shutting off the external Nchannel FET. This leaves a current path between the input and the LEDs through the boost inductor and catch diode. The minimum forward voltage of the LED array must exceed the maximum VBATT to ensure that the LEDs remain off in shutdown. Typical shutdown timing characteristics are shown in the Typical Operating Characteristics. 6 Soft-Start The MAX1698/MAX1698A include a soft-start function that eliminates input current surges at turn-on. They do this by extending the external FET driver (EXT) minimum off-time during start-up. During the first 512 switching cycles, the minimum off-time is 5µs. It is then allowed to drop to 2µs for the next 1500 switching cycles. After that time, the minimum off-time falls to the 1µs value used during normal operation. (See Shutdown/Soft-Start Timing in the Typical Operating Characteristics section.) Design Procedure Setting the Maximum LED Current Resistor RFB sets the maximum current in the primary chain of LEDs: RFB = 300mV I LEDMAX where ILEDMAX is the maximum LED current. Adjusting LED Current RFB sets the maximum LED current. This current can be reduced proportional to the voltage at the ADJ pin (see Normalized Output Current vs. ADJ Voltage in the Typical Operating Characteristics section). Figure 3 _______________________________________________________________________________________ High-Efficiency Step-Up Current Regulator for LEDs REF MAX1698 MAX1698A 500k ADJ GND Catch Diode (D1) Selection Figure 3. Adjusting LED Current shows the standard method of setting the ADJ voltage. Use the following equation to determine ILED: VADJ I LED = 4.16 ⋅ RFB where VADJ is the voltage at ADJ. Note that ADJ voltages below 50mV turn the LEDs off. Inductor Selection Choose an inductor with low DC resistance (in the neighborhood of 100mΩ) to minimize losses. A typical inductance value for L is 10µH; however, values from 3.3µH to 100µH can also be used. Higher inductor values reduce the MAX1698’s switching frequency. The typical operating frequency is given by: f= 0.67 ⋅ VBATT L The MAX1698/MAX1698A limit peak inductor current to 1.5A, but also contains a control loop that reduces inductor current as a function of output power. For a given output power, the required inductor peak current rating is approximately set by: IL(PEAK) = 1.0 · POUT where POUT is the output power to all LED banks in watts and IL(PEAK) is in amperes. Capacitor Selection The exact value of output capacitance is not critical. Typical values for the output capacitor are 0.1µF to 10µF. Larger values help reduce output ripple at the expense of size and higher cost. The requirements of the input capacitor depend on the type of the input voltage source. However, in many applications, the same capacitor type and value are used for both the input and output capacitors. The MAX1698/MAX1698As' high-switching frequency demands a high-speed rectifier. Schottky diodes are recommended for most applications, due to their fast recovery time and low forward-voltage drop. Ensure that the diode’s average and peak current ratings exceed the average output current and peak inductor current, respectively. In addition, the diode’s reverse breakdown voltage must exceed VOUT. For output voltages exceeding 40V, high-speed silicon rectifiers may be required for their higher breakdown voltages. Zener Diode For applications requiring open-circuit protection if one of the LEDs in the primary chain opens, add a zener diode as shown in Figure 2. The zener diode protects the MOSFET and output capacitor if the current feedback signal is lost. The zener voltage should exceed the maximum forward voltage of the LED network by at least 2V. Applications Information PCB Layout Due to fast switching waveforms and high-current paths, careful PCB layout is required. Protoboards and wire-wrap boards should not be used for evaluation. An EV kit (MAX1698EVKIT) is available to aid most designs. When laying out a board, minimize trace lengths to CS, the inductor, diode, input capacitor, and output capacitor. Keep traces short, direct, and wide. Keep noisy traces, such as the inductor’s traces, away from FB. VCC’s bypass capacitor should be placed as close to the IC as possible. Refer to the MAX1698 EV kit for an example of proper layout. Chip Information TRANSISTOR COUNT: 2180 _______________________________________________________________________________________ 7 MAX1698/MAX1698A Transistor Selection The MAX1698/MAX1698A drive an external N-channel MOSFET. Since the gate drive voltage is derived from VCC, best performance is achieved with low-threshold NFETs that specify on-resistance with gate-source voltages (VGS) at the voltage supplied at VCC or less. For best results, minimize the FET’s RDS(ON). The external NFET’s maximum drain-to-source voltage (VDS(MAX)) must exceed the output voltage. 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.) e 10LUMAX.EPS MAX1698/MAX1698A High-Efficiency Step-Up Current Regulator for LEDs 4X S 10 10 INCHES H Ø0.50±0.1 0.6±0.1 1 1 0.6±0.1 BOTTOM VIEW TOP VIEW D2 MILLIMETERS MAX DIM MIN 0.043 A 0.006 A1 0.002 A2 0.030 0.037 0.120 D1 0.116 0.118 0.114 D2 0.116 0.120 E1 0.118 E2 0.114 0.199 H 0.187 L 0.0157 0.0275 L1 0.037 REF b 0.007 0.0106 e 0.0197 BSC c 0.0035 0.0078 0.0196 REF S α 0° 6° MAX MIN 1.10 0.15 0.05 0.75 0.95 3.05 2.95 3.00 2.89 3.05 2.95 2.89 3.00 4.75 5.05 0.40 0.70 0.940 REF 0.177 0.270 0.500 BSC 0.090 0.200 0.498 REF 0° 6° E2 GAGE PLANE A2 c A b A1 α E1 L D1 L1 FRONT VIEW SIDE VIEW PROPRIETARY INFORMATION TITLE: PACKAGE OUTLINE, 10L uMAX/uSOP APPROVAL DOCUMENT CONTROL NO. 21-0061 REV. 1 1 Note: The MAX1698 does not have an exposed pad. Revision History Pages changed at Rev 2: 1, 2, 8 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. 8 _____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products. Inc.