19-2248; Rev 2; 5/11 KIT ATION EVALU E L B A AVAIL Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs The MAX1878 dual power supply contains a step-down and step-up DC-DC converter in a small 12-pin TQFN package for use in PDAs. The step-down DC-DC converter delivers over 500mA to an output as low as 1.25V for logic power. The step-up DC-DC converter delivers over 15mA and an output as high as 28V for a liquid crystal display (LCD). With an input voltage from 2.0V to 5.5V the MAX1878 is intended for use in systems powered by a 2-cell alkaline or 1-cell lithium-ion (Li+) battery. Fast switching frequency allows the use of small inductors and capacitors, and the low 19µA typical quiescent current allows high efficiency when the system is in standby mode. Each output can be independently enabled. Features o Evaluation Kit Available to Speed Designs o Two Output Voltages Main Output: 1.25V to VIN LCD Output: Up to 28V o 2.0V to 5.5V Input Range o Low 19µA Quiescent Supply Current o 1µA Shutdown Supply Current o High Switching Frequency for Small External Components o Small 0.75mm High 4mm x 4mm 12-Pin TQFN Package The MAX1878 is available in a small 0.75mm high 4mm x 4mm 12-pin TQFN package and requires no external FETs. The MAX1878 evaluation kit is available to speed designs. Ordering Information ________________________Applications Personal Digital Assistants (PDA) PART Organizers/Translators TEMP RANGE TOP MARK MAX1878ETC+ -40°C to +85°C 12-TQFN-EP* +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. MP3 Players GPS Receivers IN LXLCD AGND ON TOP VIEW 10μH AAGC Pin Configuration __________Typical Operating Circuit INPUT 2.0V TO 5.5V PINPACKAGE 9 8 7 LCD OUTPUT UP TO 28V LXLCD 0.1μF FBLCD ONLCD 10 6 FBLCD 5 FB 4 AIN2 AIN1 PGNDLCD 11 AIN2 10μH MAX1878 LX FB ON MAIN OFF LCD OFF MAX1878 MAIN OUTPUT 1.25V TO VIN PGND 12 EP* + ON IN PGND 2 3 AIN1 1 ONLCD LX ON TQFN 4mm x 4mm AGND PGNDLCD *CONNECT EP TO AGND. ________________________________________________________________ Maxim Integrated Products 1 For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. MAX1878 General Description MAX1878 Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs ABSOLUTE MAXIMUM RATINGS FB, FBLCD, AIN1, AIN2, ON, ONLCD to AGND ......-0.3V to +6V AIN2 to AIN1..........................................................-0.3V to +0.3V AIN1, AIN2 to IN ....................................................-0.3V to +0.3V IN to PGND...............................................................-0.3V to +6V LX to PGND .................................................-0.3V to (VIN + 0.3V) LXLCD to PGNDLCD..............................................-0.3V to +30V PGND, PGNDLCD to AGND..................................-0.3V to +0.3V LX Current .........................................................................800mA LXLCD Current..................................................................500mA Continuous Power Dissipation (TA = +70°C) 12-Pin TQFN (derate 24.4mW/°C above +70°C) ...........1.95W Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C Soldering Temperature (reflow) .......................................+260°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. PACKAGE THERMAL CHARACTERISTICS (Note 1) TQFN Junction-to-Ambient Thermal Resistance (θJA)...............41°C/W Junction-to-Case Thermal Resistance (θJC)......................6°C/W Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a fourlayer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial. ELECTRICAL CHARACTERISTICS (VIN = VAIN = 2.5V, circuit of Figure 1, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 5.5 V GENERAL Input Voltage Range Undervoltage Lockout Threshold VIN, VAIN VUVLO 2.0 VIN rising 1.92 VIN falling 1.7 Undervoltage Lockout Hysteresis Quiescent Current 2.0 1.82 100 IAIN1 + IAIN2 Shutdown Quiescent Current V mV VFB = VFBLCD = 1.30V, VONLCD = 0V, step-down converter only 19 30 VFB = VFBLCD = 1.30V 24 38 VON = VONLCD = 0V 0 1 µA VIN V µA MAIN OUTPUT (Step-Down Converter) Output Voltage Adjustment Range VMAIN 1.25 FB Regulation Voltage VFB VIN = VAIN = 2V FB Input Bias Current IFB VIN = VAIN = 2V Main Output Current (Note 2) IMAIN VMAIN = 1.8V TA = +25°C to +85°C 1.225 TA = 0°C to +85°C 1.220 1.250 1.275 1.280 10 VIN = VAIN = 2.5V 250 500 VIN = VAIN = 2.0V 200 350 50 V nA mA Line Regulation ILOAD = 150mA, VIN = VAIN = 2V to 3V, FB = GND 1 % Load Regulation VIN = VAIN = 2.5V, ILOAD = 10mA to 150mA 1 % Dropout Voltage 2 VIN = VAIN = 2V, ILOAD = 150mA, VFB = 0.8V 150 VIN = VAIN = 3V, ILOAD = 150mA, VFB = 0.8V 100 _______________________________________________________________________________________ mV Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs (VIN = VAIN = 2.5V, circuit of Figure 1, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX LX Max Duty Cycle VFB = 0.8V LX Leakage Current VON = 0V, VIN = 5.5V 0.1 5 VIN = VAIN = 2V, ILX = 300mA 0.55 0.95 VIN = VAIN = 3V, ILX = 300mA 0.42 0.65 VIN = VAIN = 2V, ILX = 300mA 0.62 0.93 VIN = VAIN = 3V, ILX = 300mA 0.46 0.65 LX P-Channel On-Resistance LX N-Channel On-Resistance 100 UNITS % µA Ω Ω LX Current Limit 330 550 800 mA Idle Mode Threshold 70 135 220 mA 240 440 740 ns 200 390 670 ns LX Minimum On-Time tLXON LX Minimum Off-Time tLXOFF ON Input Low Voltage 2V < VIN < 5.5V ON Input High Voltage 2V < VIN < 5.5V 0.4 1.3 ON Input Leakage Current V V -1 1 µA VIN + 1V 28 V LCD OUTPUT (Step-Up Converter) LCD Output Voltage Adjust Range FBLCD Regulation Voltage VLCD VFBLCD LXLCD On-Resistance VIN = VAIN = 2V TA = +25°C to +85°C 1.225 TA = 0°C to +85°C 1.220 LCD Output Current (Note 3) FBLCD Input Bias Current 5.0 VAIN = VIN = 3V, ILXLCD = 150mA 1.7 3.0 280 440 mA 0 1 µA 140 VAIN = VIN = 2.5V, VLCD = 18V 1.5 7.6 VAIN = VIN = 2V, VLCD = 18V 1.4 6.6 Ω mA VAIN = VIN = 2V 10 LCD Line Regulation VAIN = VIN = 2V to 3V, ILOAD = 5mA, VLXLCD = 18V 1 % LCD Load Regulation VAIN = VIN = 2.5V, ILOAD = 1mA to 5mA, VLXLCD = 18V 1.3 % LXLCD Maximum On-Time LXLCD Minimum Off-Time IFBLCD V 2.8 VLXLCD = 28V ILCD 1.275 1.280 VAIN = VIN = 2V, ILXLCD = 150mA LXLCD Current Limit LXLCD Leakage Current 1.250 tLXLCDON tLXLCDOFF VFBLCD < 0.9V (soft-start) ONLCD Input Low Voltage 2V < VAIN = VIN < 5.5V ONLCD Input High Voltage 2V < VAIN = VIN < 5.5V ONLCD Input Leakage Current 50 5.1 9.8 17 0.5 1.0 1.7 1.3 2.6 4.4 0.4 1.3 -1 nA µs µs V V 1 µA _______________________________________________________________________________________ 3 MAX1878 ELECTRICAL CHARACTERISTICS (continued) MAX1878 Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs ELECTRICAL CHARACTERISTICS (VIN = VAIN = 2.5V, circuit of Figure 1, TA = -40°C to +85°C, unless otherwise noted.) (Note 4) PARAMETER SYMBOL CONDITIONS MIN MAX UNITS 38 µA GENERAL Quiescent Current from AIN IAIN VFB = VFBLCD = 1.30V MAIN OUTPUT (Step-Down Converter) FB Regulation Voltage VFB VAIN = VIN = 2V LX Current Limit 1.212 1.288 V 310 820 mA LX Minimum On-Time tLXON 240 740 ns LX Minimum Off-Time tLXOFF 200 670 ns 130 450 mA 5.1 17 µs 0.5 1.7 VFBLCD < 0.9V 1.3 4.5 VAIN = VIN = 2V 1.212 1.288 LCD OUTPUT (Step-Up Converter) LXLCD Current Limit LXLCD Maximum On-Time tLXLCDON LXLCD Minimum Off-Time tLXLCDOFF FBLCD Regulation Voltage VFBLCD µs V Note 2: Main output current is guaranteed by LX current limit, LX on resistance, and LX minimum off-time. Note 3: LCD output current is guaranteed by LXLCD current limit, LXLCD on-resistance, and LXLCD minimum off-time, starting into a resistive load. Note 4: Specifications to TA = -40°C are guaranteed by design and not production tested. 4 _______________________________________________________________________________________ Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs EFFICIENCY (%) 80 VIN = 5.0V VIN = 3.6V 60 VIN = 2.5V 90 80 VIN = 5.0V 70 VIN = 3.6V 60 VIN = 5.0V (22μH) 90 EFFICIENCY (%) 90 100 MAX1878 toc02 VIN = 2.5V 80 70 VIN = 3.6V (15μH) 60 VIN = 2.5V (10μH) 50 50 50 ONLCD = PGNDLCD 40 1 10 100 1000 ON = PGND 30 0.1 1 10 100 0.01 1000 0.1 1 10 100 LOAD CURRENT (mA) LOAD CURRENT (mA) LOAD CURRENT (mA) STEP-DOWN CONVERTER OUTPUT VOLTAGE vs. LOAD CURRENT (VMAIN = 1.8V) STEP-DOWN CONVERTER OUTPUT VOLTAGE vs. LOAD CURRENT (VMAIN = 1.5V) STEP-UP CONVERTER OUTPUT VOLTAGE vs. LOAD CURRENT (VLCD = 18V) 1.83 1.530 VMAIN (V) 1.82 1.81 1.80 VIN = 5.0V 18.6 1.520 18.5 0.1 1 100 1000 18.0 0.1 1 VIN = 2.5V (10μH) 17.9 ONLCD = PGNDLCD 1.495 10 VIN = 3.6V (15μH) 18.3 18.1 VIN = 2.5V 1.500 ONLCD = PGNDLCD 1.77 18.4 18.2 VIN = 3.6V 1.505 VIN = 2.5V VIN = 5.0V (22μH) 18.7 1.525 1.515 ONLCD = PGNDLCD 18.8 1.510 VIN = 3.6V 1.79 1.78 18.9 VMAIN (V) VIN = 5.0V MAX1878 toc05 1.535 MAX1878 toc04 1.84 17.8 10 100 0.01 1000 0.1 1 10 100 LOAD CURRENT (mA) LOAD CURRENT (mA) LOAD CURRENT (mA) NO LOAD SUPPLY CURRENT vs. INPUT VOLTAGE (VMAIN = 1.8V, VLCD = 18V) STEP-DOWN CONVERTER SWITCHING FREQUENCY vs. SUPPLY VOLTAGE STEP-UP CONVERTER CURRENT LIMIT vs. INPUT VOLTAGE 100 STEP-UP 80 60 40 1.0 0.8 VMAIN = 1.5V 0.6 VMAIN = 1.8V 0.4 0 0 1 2 3 4 INPUT VOLTAGE (V) 5 6 L2 = 22μH 400 350 300 2 3 4 SUPPLY VOLTAGE (V) 5 L2 = 15μH 250 IMAIN = 150mA ONLCD = PGNDLCD STEP-DOWN 0 450 L2 = 10μH 0.2 20 500 MAX1878 toc09 120 1.2 CURRENT LIMIT (mA) STEP-UP AND STEP-DOWN SWITCHING FREQUENCY (MHz) MAX1878 toc07 140 MAX1878 toc08 VMAIN (V) 40 40 0.1 SUPPLY CURRENT (μA) ONLCD = PGNDLCD MAX1878 toc06 EFFICIENCY (%) 100 MAX1878 toc01 100 70 STEP-UP CONVERTER EFFICIENCY vs. LOAD CURRENT (VLCD = 18V) STEP-DOWN CONVERTER EFFICIENCY vs. LOAD CURRENT (VMAIN = 1.5V) MAX1878 toc03 STEP-DOWN CONVERTER EFFICIENCY vs. LOAD CURRENT (VMAIN = 1.8V) 200 6 2 3 4 5 SUPPLY VOLTAGE (V) _______________________________________________________________________________________ 6 5 MAX1878 Typical Operating Characteristics (VIN = VAIN = 2.5V, circuit of Figure 1, TA = +25°C, unless otherwise noted.) MAX1878 Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs Typical Operating Characteristics (continued) (VIN = VAIN = 2.5V, circuit of Figure 1, TA = +25°C, unless otherwise noted.) STEP-DOWN LIGHT-LOAD SWITCHING WAVEFORMS STEP-UP LIGHT-LOAD SWITCHING WAVEFORMS STEP-DOWN HEAVY-LOAD SWITCHING WAVEFORMS MAX1878 toc12 MAX1878 toc11 MAX1878 toc10 VMAIN AC-COUPLED 50mV/div VMAIN AC-COUPLED 20mV/div VOUT AC-COUPLED 100mV/div VLX 1V/div VLX 1V/div VLXLCD 10V/div 0 0 0 4μs/div IMAIN = 20mA, VMAIN = +1.8V, VIN = +2.5V, ONLCD = PGNDLCD 1μs/div IMAIN = 250mA, VMAIN = +1.8V, VIN = +2.5V, ONLCD = PGNDLCD 1μs/div ILCD = 2mA, VLCD = +18V, VIN = +2.5V, ON = PGND STEP-DOWN LOAD TRANSIENT RESPONSE STEP-UP LOAD TRANSIENT RESPONSE STEP-UP HEAVY-LOAD SWITCHING WAVEFORMS MAX1878 toc15 MAX1878 toc14 MAX1878 toc13 VOUT AC-COUPLED 100mV/div VLXLCD 10V/div IMAIN 200mA/div ILCD 2mA/div VMAIN AC-COUPLED 100mV/div VLCD AC-COUPLED 200mV/div VLX 2V/div VLXLCD 10V/div 0 1μs/div ILCD = 4.5mA, VLCD = +18V, VIN = +2.5V, ON = PGND 10μs/div IMAIN = 10mA to 250mA, VMAIN = +1.8V, VIN = +2.5V, ONLCD = PGNDLCD 20μs/div ILCD = 1mA to 4mA, VLCD = +18V, VIN = +2.5V, ON = PGND SOFT-START AND SHUTDOWN RESPONSE LINE TRANSIENT RESPONSE MAX1878 toc17 MAX1878 toc16 VIN 3V TO 2V IIN 200mA/div VLCD AC-COUPLED 200mV/div VLCD 10V/div VMAIN 1V/div VMAIN AC-COUPLED 20mV/div 100μs/div VMAIN = +1.8V, IMAIN = 150mA, VLCD =18.0V, ILCD = 2.5mA 6 VON = VONLCD 5V/div 400μs/div RMAIN = 5.1Ω, RLCD = 9.09kΩ _______________________________________________________________________________________ Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs PIN NAME FUNCTION 1 IN Step-Down Converter Power Input. Connect IN to the step-down converter power source. Bypass IN to PGND with a 10µF or greater low-ESR capacitor. 2 LX Step-Down Converter Switching Node. Connect LX to the step-down converter output LC filter. LX swings between IN and PGND. 3 AIN1 Analog Input Power 1. AIN1 supplies power to the MAX1878 internal circuitry. Connect AIN1 to the 2.0V to 5.5V input power source. Bypass AIN1 to AGND with a 1µF or greater low-ESR capacitor. 4 AIN2 Analog Input Power 2. Connect AIN1 and AIN2 together as close to the MAX1878 as possible. 5 FB Step-Down Converter Feedback Input. Connect a resistive voltage-divider from the step-down converter output voltage to FB. The regulation threshold is 1.25V at FB. 6 FBLCD LCD Step-Up Converter Feedback Input. Connect a resistive voltage-divider from the step-up converter output voltage to FBLCD. The regulation threshold is 1.25V at FBLCD. 7 ON 8 AGND 9 LXLCD LCD Step-Up Converter Switching Node. Connect LXLCD to the step-up converter inductor and rectifier. 10 ONLCD LCD Step-Up Converter On/Off Input. Drive ONLCD high to turn on the step-up converter. Drive ONLCD low to turn off the converter. For automatic startup, connect ONLCD to AIN1. 11 PGNDLCD 12 PGND — EP Step-Down Converter On/Off Input. Drive ON high to turn on the step-down converter. Drive ON low to turn off the converter. For automatic startup, connect ON to AIN1. Analog (Low-Noise) Ground. The exposed pad and the corner tabs on the TQFN package are internally connected to analog ground. See the PC Board Layout and Grounding section. LCD Step-Up Converter Power Ground. PGNDLCD is the source of the step-up converter’s internal N-channel MOSFET switch. Connect PGNDLCD to PGND as close to the MAX1878 as possible. Power Ground. PGND is the source of the step-down converter’s internal N-channel MOSFET synchronous rectifier. Connect PGND to PGNDLCD as close to the MAX1878 as possible. Exposed Pad. Internally connected to AGND. Connect to a large analog ground (AGND) plane to maximize thermal performance. Not intended to use as an electrical connection point. Detailed Description The MAX1878 step-down and step-up DC-DC converter operates from a 2.0V to 5.5V supply. Consuming only 19µA of quiescent supply current, the main stepdown converter delivers over 500mA to an output as low as 1.25V and the LCD step-up converter delivers over 15mA and an output as high as 28V. The MAX1878 uses a unique proprietary current-limited control scheme that provides excellent performance and high efficiency. Step-Down Converter Control Scheme The MAX1878 step-down converter uses a proprietary, current-limited control scheme to ensure high efficiency, fast transient response, and physically small external components. This control scheme is simple: when the output voltage is out of regulation, the error comparator begins a switching cycle by turning on the high-side switch. This switch remains on until the minimum ontime of 440ns expires and the output voltage regulates or the current-limit threshold is exceeded. Once off, the high-side switch remains off until the minimum off-time of 390ns expires and the output voltage falls out of regulation. During this period, the low-side synchronous rectifier turns on and remains on until either the high-side switch turns on again or the inductor current approaches zero. The internal synchronous rectifier eliminates the need for an external Schottky diode. This control scheme allows the MAX1878 step-down converter to provide excellent performance throughout the entire load-current range. When delivering light loads, the high-side switch turns off after the minimum on-time and after the inductor current reaches the 135mA ideal mode threshold to reduce peak inductor _______________________________________________________________________________________ 7 MAX1878 Pin Description MAX1878 Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs current, resulting in increased efficiency and reduced output voltage ripple. When delivering medium and higher output currents, the MAX1878 extends either the on-time or the off-time, as necessary to maintain regulation, resulting in nearly constant frequency operation with high efficiency and low output voltage ripple. enters a high-impedance state and the output remains connected to the input through the inductor and rectifier holding the output voltage to a diode drop below VIN. The LCD output capacitance and load determine the rate at which V LCD decays. Connect ON and ONLCD to IN for normal operation. Step-Up Converter Control Scheme Soft-Start The MAX1878 step-up converter features a minimum off-time, current-limited control scheme. The duty cycle is governed by a pair of one-shots that set a minimum off-time and a maximum on-time. The switching frequency can be up to 500kHz and depends upon the load and input voltage. The peak current limit of the internal N-channel MOSFET is 280mA. The MAX1878 internal soft-start circuitry limits current drawn at startup, reducing transients on the input source. Soft-start is particularly useful for higher impedance input sources, such as lithium ion and alkaline cells. Step-down converter soft-start is implemented with current limit. At startup the step-down converter current limit is set to 25% of its full current limit. The current limit is increased by 25% every 256 switching cycles until full current limit is reached. Step-up converter soft-start is implemented with LXLCD minimum off-time. At startup the LXLCD minimum off-time is 2.6µs allowing the LCD output voltage to build up gradually. When the output reaches approximately 80% of its final output voltage the LXLCD minimum offtime is decreased to its final value of 1µs. See Soft-Start and Shutdown Response in the Typical Operating Characteristics section. On/Off Control Pulling ON low places the MAX1878 step-down converter in shutdown mode and reduces step-down converter supply current to less than 1µA. In shutdown, the internal switching MOSFETs and synchronous rectifier turn off and LX goes high impedance. Pulling ONLCD low places the MAX1878 step-up converter in shutdown mode and reduces step-up converter supply current to less than 1µA. In shutdown, LXLCD L2 10μH* VIN 2.0V TO 3.3V* D1 LCD OUTPUT UP TO 28V C1 10μF TWO SERIES ALKALINE CELLS 1 IN LXLCD R5 10Ω FBLCD 3 4 C2 1μF ON OFF ON LCD OFF 7 10 C5 5pF R3 3.6MΩ C3 0.1μF 6 R4 270kΩ AIN1 AIN2 L1 10μH MAX1878 MAIN 9 LX 2 R6 2MΩ ON FB ONLCD PGND 12 MAIN OUTPUT 1.25V TO VIN C6 20pF R1 28kΩ C4 22μF 5 R2 63.4kΩ AGND PGNDLCD 8 11 *FOR INPUT VOLTAGES GREATER THAN 3.3V USE A HIGHER VALUE INDUCTOR L2. SEE INDUCTOR SELECTION Figure 1. MAX1878 Standard Application Circuit 8 _______________________________________________________________________________________ Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs FBLCD (Figure 1). Select an R4 from 30kΩ to 300kΩ. Calculate R3 with the following equation: Setting the Output Voltage Set the MAX1878 step-down converter output voltage by connecting a resistive voltage-divider from VMAIN to FB (Figure 1). Select an R2 from 30kΩ to 300kΩ. Calculate R1 with the following equation: R1 = R2 xR6(VMAIN − VFB ) VFB (R6 + R2) − VMAIN xR2 where VFB = 1.25V, R6 = 2MΩ and VMAIN may range from 1.25V to VIN. Set the MAX1878 step-up converter output voltage by connecting a resistive voltage-divider from V LCD to ⎛ V ⎞ R3 = R4 ⎜ LCD − 1⎟ ⎝ VFBLCD ⎠ where VFBLCD = 1.25V and VLCD may range from (VIN + 1V) to 28V. The FB and FBLCD input bias currents are a maximum of 50nA. These small bias currents allow for large-value feedback resistors that improve light-load efficiency. For less than 1% output voltage error due to bias current, feedback resistors should be chosen such that the current through R2 is 100 times greater than IFB and the current through R4 is 100 times greater than IFBLCD. VIN 1 IN 3 AIN1 4 AIN2 MAX1878 LXLCD 9 VLCD CURRENT LIMIT N P 2 LX VMAIN CONTROL LOGIC CURRENT LIMIT N TIMERS FBLCD 6 5 FB SOFTSTART ON OFF ONLCD 10 7 ON PGND 12 AGND 8 ON OFF PGNDLCD 11 Figure 2. Simplified Functional Diagram _______________________________________________________________________________________ 9 MAX1878 Design Procedure MAX1878 Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs Inductor Selection The MAX1878 is optimized to use a 10µH inductor over the entire operating range. Smaller inductance values typically offer smaller physical size for a given series resistance or saturation current. Circuits using larger inductance values may startup at lower input voltages and exhibit less ripple, but also provide reduced output power. This occurs when the inductance is sufficiently large to prevent the maximum current limit from being reached before the maximum on-time expires. The inductor’s saturation current rating should be greater than the peak switching current. However, it is generally acceptable to bias the inductor into saturation by as much as 20%, although this will slightly reduce efficiency. Choose a low DC-resistance inductor to improve efficiency. For the above reasons choose the step-up converter inductor in the range of 10µH to 33µH depending on the input voltage (4µH per volt of VIN). Step-Up Converter Diode Selection The high maximum switching frequency of 500kHz requires a high-speed rectifier such as the 1N4148. To maintain high efficiency, the average current rating of the diode should be greater than the peak switching current. Choose a reverse breakdown voltage greater than the output voltage. A Schottky diode is not recommended as the lower forward voltage does little to improve efficiency whereas the higher reverse leakage current decreases efficiency. Input Bypass Capacitors Bypass VIN with a 10µF low-ESR surface-mount ceramic capacitor to PGND and PGNDLCD as close to the IC as possible. This input bypass capacitor reduces peak currents and noise at the input voltage source. Connect AIN1 and AIN2 together and bypass with a low-ESR 1µF surface-mount ceramic capacitor to AGND. A low resistance (10Ω) from IN to AIN1 and AIN2 creates a lowpass RC filter and provides low-noise analog input power to the MAX1878. Output Filter Capacitors The MAX1878 is a voltage mode converter and requires ripple at FB and FBLCD for stable regulation. For most applications, bypass VLCD with a 0.1µF small ceramic surface-mount capacitor to PGNDLCD. For small 10 ceramic capacitors, the output ripple voltage is dominated by the capacitance value. If tantalum or electrolytic capacitors are used, the higher ESR increases the output ripple voltage. Decreasing the ESR reduces the output ripple voltage and the peak-to-peak transient voltage. Surface-mount capacitors are generally preferred because they lack the inductance and resistance of their through-hole equivalents. Bypass VMAIN with a 10µF to 47µF tantalum capacitor to PGND. Choose a capacitor with 200mΩ to 300mΩ ESR to provide stable switching while minimizing output ripple. A 22µF filter capacitor works well for most applications. Ripple Regulation For proper switching control the ripple at FB and FBLCD must be greater than 25mV. Use R6 and C6 as shown in Figure 1 to inject ripple into FB. To insure sufficient ripple on FBLCD, connect C5 as shown in Figure 1. PC Board Layout and Grounding High switching frequencies make PC board layout a very important part of design. Good design minimizes excessive EMI on the feedback paths and voltage gradients in the ground plane, both of which can result in instability or regulation errors. Connect the inductors, input filter capacitors, and output filter capacitors as close to the device as possible, and keep their traces short, direct, and wide. The external voltage-feedback networks should be very close to the feedback pins, within 0.2 inches (5mm). Keep noisy traces, such as LX and LXLCD, away from the voltage feedback networks; also keep them separate, using grounded copper. The exposed backside pad and corner tabs of the TQFN package are internally connected to analog ground. For heat dissipation, connect the exposed backside pad to a large analog ground plane, preferably on a surface of the board that receives good airflow. Connect all power grounds and all analog grounds to separate ground planes in a star ground configuration. Connect the analog ground plane and the power ground plane together at a single point. The MAX1878 evaluation kit data sheet includes a proper PC board layout and routing scheme. ______________________________________________________________________________________ Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs EXPOSED PAD CONNECTED TO AGND PROCESS: BiCMOS For the latest package outline information and land patterns (footprints), go to www.maxim-ic.com/packages. Note that a "+", "#", or "-" in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE 12 TQFN PACKAGE CODE OUTLINE NO. LAND PATTERN NO. T1244+4 21-0139 90-0068 ______________________________________________________________________________________ 11 MAX1878 Package Information Chip Information MAX1878 Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs Revision History REVISION NUMBER REVISION DATE 2 5/11 DESCRIPTION Replaced QFN package with TQFN package and added exposed pad to Pin Description section PAGES CHANGED 1, 7 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. 12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2011 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.