19-2296; Rev 3; 8/05 Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 The MAX1920/MAX1921 step-down converters deliver over 400mA to outputs as low as 1.25V. These converters use a unique proprietary current-limited control scheme that achieves over 90% efficiency. These devices maintain extremely low quiescent supply current (50µA), and their high 1.2MHz (max) operating frequency permits small, low-cost external components. This combination makes the MAX1920/MAX1921 excellent highefficiency alternatives to linear regulators in spaceconstrained applications. Internal synchronous rectification greatly improves efficiency and eliminates the external Schottky diode required in conventional step-down converters. Both devices also include internal digital soft-start to limit input current upon startup and reduce input capacitor requirements. The MAX1920 provides an adjustable output voltage (1.25V to 4V). The MAX1921 provides factory-preset output voltages (see the Selector Guide). Both are available in space-saving 6-pin SOT23 packages. The MAX1920 is also available in a 6-pin TDFN package. Features ♦ 400mA Guaranteed Output Current ♦ Internal Synchronous Rectifier for >90% Efficiency ♦ Tiny 6-Pin SOT23 Package ♦ Available in 6-Pin TDFN Package (MAX1920) ♦ Up to 1.2MHz Switching Frequency for Small External Components ♦ 50µA Quiescent Supply Current ♦ 0.1µA Logic-Controlled Shutdown ♦ 2V to 5.5V Input Range ♦ Fixed 1.5V, 1.8V, 2.5V, 3V, and 3.3V Output Voltages (MAX1921) ♦ Adjustable Output Voltage (MAX1920) ♦ ±1.5% Initial Accuracy ♦ Soft-Start Limits Startup Current Ordering Information PART Applications Next-Generation Wireless Handsets PDAs, Palmtops, and Handy-Terminals Battery-Powered Equipment CDMA Power Amplifier Supply MAX1920EUT-T TEMP RANGE PIN-PACKAGE -40°C to +85°C 6 SOT23-6 MAX1920EUT+T -40°C to +85°C 6 SOT23-6 MAX1920ETT-T -40°C to +85°C 6 TDFN MAX1920ETT+T -40°C to +85°C 6 TDFN MAX1921EUT_ _-T -40°C to +85°C 6 SOT23-6 MAX1921EUT_ _+T -40°C to +85°C 6 SOT23-6 Note: The MAX1921 offers five preset output voltage options. See the Selector Guide, and then insert the proper designator into the blanks above to complete the part number. +Denotes lead-free package. Typical Operating Circuit TOP VIEW LX 4.75kΩ 5600pF CIN MAX1921 AGND PGND 4.7µF IN 1 AGND 2 LX IN OUTPUT 1.5V UP TO 400mA FB 4.7µH AGND INPUT 2V TO 5.5V Pin Configuration 6 5 4 6 LX MAX1920 MAX1921 5 PGND MAX1920 ON 4 OUT (FB) 1 SOT23-6 2 3 PGND SHDN 3 IN OUT SHDN SHDN OFF TDFN ( ) ARE FOR MAX1920 ONLY A "+" sign will replace the first pin indicator on lead-free packages. ________________________________________________________________ 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 MAX1920/MAX1921 General Description MAX1920/MAX1921 Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 ABSOLUTE MAXIMUM RATINGS IN, FB, SHDN to AGND . . . . . . . . . . . . . . . . . . . . .-0.3V to +6V OUT to AGND, LX to PGND . . . . . . . . . . . .-0.3V to (IN + 0.3V) AGND to PGND . . . . . . . . . . . . . . . . . . . . . . . . . .-0.3V to +0.3V OUT Short Circuit to GND . . . . . . . . . . . . . . . . . . . . . . . . . . .10s Continuous Power Dissipation (TA = +70°C) 6-Pin SOT23-6 (derate 8.7mW/°C above +70°C) . . . .695mW 6-Pin TDFN (derate 18.2mW/°C above +70°C) . . .1454.5mW Operating Temperature Range . . . . . . . . . . . . . .-40°C to +85°C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . .+150°C Storage Temperature . . . . . . . . . . . . . . . . . . . .-65°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. ELECTRICAL CHARACTERISTICS (VIN = 3.6V, SHDN = IN, TA = 0°C to +85°C. Typical parameters are at TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER Input Voltage Range SYMBOL VIN CONDITIONS MIN TYP I(LX) < 400mA 2.5 5.5 I(LX) < 200mA (MAX1921EUT15, MAX1921EUT18) 2.0 2.5 Startup Voltage UVLO Threshold MAX 2.0 UVLO VIN rising VIN falling 1.85 1.50 UVLO Hysteresis 1.95 1.65 200 UNITS V V V mV Quiescent Supply Current IIN No switching, no load 50 70 µA Quiescent Supply Current Dropout IIN SHDN = IN, OUT/FB = 0 220 300 µA Shutdown Supply Current ISHDN SHDN = GND IOUT = 0, TA = +25°C 0.1 4.0 µA Output Voltage Accuracy (MAX1921) OUT BIAS Current IOUT Output Voltage Range (MAX1920) -1.5 +1.5 IOUT = 0 to 400mA, TA = -40°C to +85°C -3 +3 IOUT = 0 to 200mA, TA = -40°C to +85°C -3 +3 OUT at regulation voltage Figure 4, IN = 4.5V TA= 25°C FB Feedback Threshold (MAX1920) VFB TA = -40°C to +85°C FB Feedback Hysteresis (MAX1920) FB Bias Current (MAX1920) 1 SHDN = 0 8 1.25 4.00 1.231 1.25 1.269 1.220 1.25 1.280 1.210 VHYS IFB 16 % µA V V 1.280 5 mV FB = 1.5V 0.01 Load Regulation IOUT = 0 to 400mA 0.005 %/mA Line Regulation VIN = 2.5V to 5.5V 0.2 %/V SHDN Input Voltage High VIH SHDN Input Voltage Low VIL SHDN Leakage Current High-Side Current Limit ISHDN 2 ILIMP 0.20 1.6 SHDN = GND or IN 525 µA V 0.4 V 0.001 1.000 µA 730 950 mA _______________________________________________________________________________________ Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 (VIN = 3.6V, SHDN = IN, TA = 0°C to +85°C. Typical parameters are at TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 350 550 800 mA Ω ILIMN Low-Side Current Limit High-Side On-Resistance RONHS ILX = -40mA, VIN = 3V 0.6 1.1 Rectifier On-Resistance RONSR ILX = 40mA, VIN = 3V 0.5 0.9 Rectifier Off-Current Threshold ILXOFF LX Leakage Current ILXLEAK IN = SHDN = 5.5V, LX = 0 to IN ILXLKR IN unconnected, VLX = 5.5V, SHDN = GND Ω 60 mA 0.1 5.0 µA 0.1 5.0 µA Minimum On-Time tON(MIN) 0.28 0.4 0.5 µs Minimum Off-Time tOFF(MIN) 0.28 0.4 0.5 µs LX Reverse Leakage Current Note 1: All devices are 100% production tested at TA = +25°C. Limits over the operating temperature range are guaranteed by design. Typical Operating Characteristics (CIN = 2.2µF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.) 50 40 50 40 70 50 30 20 20 10 10 10 30 0 1 10 100 1000 VIN = 5V 40 20 0.1 VIN = 3.3V 60 30 0 0 0.1 1 10 100 1000 0.1 1 10 100 1000 LOAD CURRENT ( mA) LOAD CURRENT ( mA) LOAD CURRENT ( mA) OUTPUT VOLTAGE ACCURACY vs. LOAD (VOUT = 3.3V) OUTPUT VOLTAGE ACCURACY vs. LOAD (VOUT = 2.5V) OUTPUT VOLTAGE ACCURACY vs. LOAD (VOUT = 1.5V) VIN = 5V 3.300 VIN = 4.2V 3.267 VIN = 5V 1.530 OUTPUT VOLTAGE 3.333 2.550 OUTPUT VOLTAGE 3.366 1.545 MAX1920 toc05 2.575 MAX1920 toc04 3.399 OUTPUT VOLTAGE 80 VIN = 5V 60 VIN = 2.5V 90 2.525 2.500 VIN = 3V 2.475 VIN = 5V 1.515 1.500 VIN = 3.3V 1.485 VIN = 2.5V VIN = 3.6V 3.234 2.450 3.201 1.470 2.425 0 50 100 150 200 250 300 350 400 LOAD (mA) MAX1920 toc06 60 VIN = 3.3V 70 100 MAX1920 toc03 80 VIN = 5V VIN = 4.2V 70 VIN = 2.7V 90 EFFICIENCY (%) EFFICIENCY (%) 80 EFFICIENCY vs. LOAD CURRENT (VOUT = 1.5V) EFFICIENCY (%) VIN = 3.6V 90 100 MAX1920 toc01 100 EFFICIENCY vs. LOAD CURRENT (VOUT = 2.5V) MAX1920 toc02 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V) 1.455 0 50 100 150 200 250 300 350 400 LOAD (mA) 0 50 100 150 200 250 300 350 400 LOAD (mA) _______________________________________________________________________________________ 3 MAX1920/MAX1921 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (continued) (CIN = 2.2µF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.) SWITCHING FREQUENCY vs. LOAD (VOUT = 1.5V) 1000 100 10 1000 100 10 10,000 1 10 100 VOUT = 3.3V 100 VOUT = 1.5V 10 1 1 1 VOUT = 2.5V 1000 VIN = 3.3 VIN = 3.3 0.1 MAX1920 toc09 MAX1920 toc08 10,000 SWITCHING FREQUENCY (kHz) MAX1920 toc07 10,000 NO LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE NO-LOAD SUPPLY CURRENT (µA) SWITCHING FREQUENCY vs. LOAD (VOUT = 1.8V) SWITCHING FREQUENCY (kHz) MAX1920/MAX1921 Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 1000 LOAD (mA) 0.1 1 10 100 1000 LOAD (mA) LIGHT-LOAD SWITCHING WAVEFORM 2.0 2.5 3.0 3.5 4.0 4.5 5..0 5.5 SUPPLY VOLTAGE (V) MEDIUM-LOAD SWITCHING WAVEFORM MAX1920 toc10 1.5 SOFT-START AND SHUTDOWN RESPONSE MAX1920 toc11 MAX1920 toc12 VOUT 1V/div VOUT AC-COUPLED 5mV/div VOUT AC-COUPLED 5mV/div IIN 100mA/div VLX 2V/div VLX 2V/div VSHDN 5V/div VIN = 3.3V, VOUT = 1.5V, ILOAD = 40mA VIN = 3.3V, VOUT = 1.5V, RLOAD = 6Ω VIN = 3.3V, VOUT = 1.5V, ILOAD = 250mA 1µs/div 1µs/div MEDIUM-LOAD LINE-TRANSIENT RESPONSE LIGHT-LOAD LINE-TRANSIENT RESPONSE MAX1920 toc13 200µs/div LOAD-TRANSIENT RESPONSE MAX1920 toc15 MAX1920 toc14 VIN = 3.3V, VOUT = 1.5V, ILOAD = 20mA TO 320mA VIN AC-COUPLED 200mV/div VIN AC-COUPLED 200mV/div VOUT AC-COUPLED 100mV/div IL 200mA/div VOUT AC-COUPLED 5mV/div 4 VOUT AC-COUPLED 5mV/div VIN = 3.8V to 4.2V, VOUT = 1.5V, ILOAD = 250mA VIN = 3.8V to 4.2V, VOUT = 1.5V, ILOAD = 20mA 4µs/div 4µs/div ILOAD 200mA/div 40µs/div _______________________________________________________________________________________ Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 PIN NAME FUNCTION 1 IN Supply voltage input for MAX1921EUT15 and MAX1921EUT18 is 2V to 5.5V. Supply voltage input for MAX1920 and other voltage versions of MAX1921 is 2.5V to 5.5V. Bypass IN to GND with a 2.2µF ceramic capacitor as close to IN as possible. 6 2 AGND Analog Ground. Connect to PGND. 1 3 SHDN Active-Low Shutdown Input. Connect SHDN to IN for normal operation. In shutdown, LX becomes high-impedance and quiescent current drops to 0.1µA. — 4 OUT 5 4 FB 3 5 PGND 4 6 LX TDFN* SO 2 MAX1921 Voltage Sense Input. OUT is connected to an internal voltage-divider. MAX1920 Voltage Feedback Input. FB regulates to 1.25V nominal. Connect FB to an external resistive voltage-divider between the output voltage and GND. Power Ground. Connect to AGND. Inductor Connection *MAX1920 only. Detailed Description The MAX1920/MAX1921 step-down DC-DC converters deliver over 400mA to outputs as low as 1.25V. They use a unique proprietary current-limited control scheme that maintains extremely low quiescent supply current (50µA), and their high 1.2MHz (max) operating frequency permits small, low-cost external components. Control Scheme The MAX1920/MAX1921 use 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 on-time of 400ns 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 400ns expires and the output voltage falls out of regulation. During this period, the low-side synchronous rectifier turns on and INPUT 2V TO 5.5V 1 IN LX L 6 OUTPUT UP TO 400mA R1 CIN 2 ON 3 MAX1921 AGND PGND SHDN OUT 5 COUT CFF 4 OFF Figure 1. Typical Output Application Circuit (MAX1921) 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 MAX1920/MAX1921 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 to reduce peak inductor current, resulting in increased efficiency and reduced output voltage ripple. When delivering medium and higher output currents, the MAX1920/MAX1921 extend either the on-time or the offtime, as necessary to maintain regulation, resulting in nearly constant frequency operation with high-efficiency and low-output voltage ripple. Shutdown Mode Connecting SHDN to GND places the MAX1920/ MAX1921 in shutdown mode and reduces supply current to 0.1µA. In shutdown, the control circuitry, internal switching MOSFET, and synchronous rectifier turn off and LX becomes high impedance. Connect SHDN to IN for normal operation. Soft-Start The MAX1920/MAX1921 have internal soft-start circuitry that limits current draw at startup, reducing transients on the input source. Soft-start is particularly useful for higher impedance input sources, such as Li+ and alkaline cells. Soft-start is implemented by starting with the current limit at 25% of its full current value and gradually increasing it in 25% steps until the full current limit is reached. See Soft-Start and Shutdown Response in the Typical Operating Characteristics. _______________________________________________________________________________________ 5 MAX1920/MAX1921 Pin Description MAX1920/MAX1921 Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 Design Procedure The MAX1920/MAX1921 are optimized for small external components and fast transient response. There are several application circuits (Figures 1 through 4) to allow the choice between ceramic or tantalum output capacitor and internally or externally set output voltages. The use of a small ceramic output capacitor is preferred for higher reliability, improved voltage-positioning transient response, reduced output ripple, and the smaller size and greater availability of ceramic versus tantalum capacitors. In order to calculate the smallest inductor, several calculations are needed. First, calculate the maximum duty cycle of the application as: DutyCycle(MAX) = Second, calculate the critical voltage across the inductor as: if DutyCycle(MAX) < 50%, then VCRITICAL = (VIN(MIN) - VOUT), else VCRITICAL = VOUT Last, calculate the minimum inductor value as: Voltage Positioning Figures 1 and 2 are the application circuits that utilize small ceramic output capacitors. For stability, the circuit obtains feedback from the LX node through R1, while load transients are fed-forward through CFF. Because there is no D.C. feedback from the output, the output voltage exhibits load regulation that is equal to the output load current multiplied by the inductor’s series resistance. This small amount of load regulation is similar to voltage positioning as used by high-powered microprocessor supplies intended for personal computers. For the MAX1920/MAX1921, voltage positioning eliminates or greatly reduces undershoot and overshoot during load transients (see the Typical Operating Characteristics), which effectively halves the peak-to-peak output voltage excursions compared to traditional step-down converters. For convenience, Table 1 lists the recommended external component values for use with the MAX1921 application circuit of Figure 1 with various input and output voltages. VOUT × 100% VIN (MIN) L(MIN) = 2.5 × 10 −6 × VCRITICAL Select the next standard value larger than L(MIN). The L(MIN) calculation already includes a margin for inductance tolerance. Although values much larger than L(MIN) work, transient performance, efficiency, and inductor size suffer. A 550mA rated inductor is enough to prevent saturation for output currents up to 400mA. Saturation occurs when the inductor’s magnetic flux density reaches the maximum level the core can support and inductance falls. Choose a low DC-resistance inductor to improve efficiency. Tables 2 and 3 list some suggested inductors and suppliers. Capacitor Selection For nearly all applications, the input capacitor, CIN, may be as small as 2.2µF ceramic with X5R or X7R Table 2. Suggested Inductors PART NUMBER Table 1. MAX1921 Suggested Components for Figure 1 Coilcraft LPO1704 INPUT SOURCE OUTPUT 5V 3.3V, 1 Li+, 3 x AA 3.3V 3.0V L = 10µH, COUT = 10µF, R1 = 8.25kΩ, CFF = 3300pF 2.5V L = 6.8µH, COUT = 6.8µF, R1 = 5.62kΩ, CFF = 4700pF 1.8V 1.5V L = 10µH, COUT = 10µF, R1 = 8.25kΩ, CFF = 3300pF 2.5V, 2 x AA N/A L = 4.7µH, COUT = 4.7µF, R1 = 4.75kΩ, CFF = 5600pF Inductor Selection 6 Sumida CDRH3D16 L (µH) RL Isat (A) (ohms max) 4.7 0.200 1.10 6.8 0.320 0.90 10 0.410 0.80 4.7 0.080 0.90 6.8 0.095 0.73 10 0.160 0.55 Sumida CDRH2D18 4.7 0.081 0.63 6.8 0.108 0.57 Toko D312F 4.7 0.38 0.74 10 0.79 0.50 Toko D412F 4.7 0.230 0.84 10 0.490 0.55 4.7 0.087 1.14 6.8 0.105 0.95 10 0.150 0.76 Toko D52LC _______________________________________________________________________________________ SIZE 6.6 x 5.5 x 1.0 = 36.3mm3 3.8 x 3.8 x 1.8 = 26.0mm3 3.2 x 3.2 x 2.0 = 20.5mm3 3.6 x 3.6 x 1.2 = 15.6mm3 4.6 x 4.6 x 1.2 = 25.4mm3 5.0 x 5.0 x 2.0 = 50.0mm3 Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 IIN (RMS) = IOUT (MAX) × VOUT (VIN − VOUT ) VIN The output capacitor, COUT, may be either ceramic or tantalum depending upon the chosen application circuit (see Figures 1 through 4). Table 3 lists some suggested capacitor suppliers. Ceramic Output Capacitor For ceramic COUT, use the application circuit of Figure 1 or Figure 2. Calculate the minimum capacitor value as: COUT (MIN) = 2.5 × 10 −6 × VCRITICAL 1 IN LX L 6 R1 CIN 2 ON 3 MAX1920 AGND PGND SHDN FB ESRCOUT (MIN) = 8.0 × 10 −2 × VOUT Because tantalum capacitors rarely specify minimum ESR, choose a capacitor with typical ESR that is about twice as much as ESR COUT (MIN). Although ESRs greater than this work, output ripple becomes larger. For tantalum C OUT , calculate the minimum output capacitance as: COUT (MIN) = 1.25 × Select the next standard value larger than COUT(MIN). The COUT(MIN) calculation already includes a margin for capacitor tolerance. Values much larger than COUT(MIN) always improve transient performance and stability, but capacitor size and cost increase. INPUT 2V TO 5.5V Tantalum Output Capacitor For tantalum COUT, use the application circuit of Figure 3 or Figure 4. With tantalum COUT, the equivalent series resistance (ESR) of COUT must be large enough for stability. Generally, 25mV of ESR-ripple at the feedback node is sufficient. The simplified calculation is: OUTPUT UP TO 400mA COUT The 1.25 multiplier is for capacitor tolerance. Select any standard value larger than COUT(MIN). Feedback and Compensation The MAX1921 has factory preset output voltages of 1.5V, 1.8V, 2.5V, 3V, and 3.3V, while the MAX1920 is externally adjusted by connecting FB to a resistive voltage-divider. When using a ceramic output capacitor, the feedback network must include a compensation feed-forward capacitor, CFF. INPUT 2V TO 5.5V OFF IN LX 6 L OUTPUT UP TO 400mA COUT 2 ON 4 1 CIN CFF 5 L × IOUT (MAX) ESRCOUT (MIN) × VCRITICAL 3 MAX1921 AGND PGND SHDN OUT 5 4 OFF R2 Figure 2. Typical Application Circuit (MAX1920) Figure 3. MAX1921 Application Circuit Using Tantalum Output Capacitor _______________________________________________________________________________________ 7 MAX1920/MAX1921 dielectric. The input capacitor filters peak currents and noise at the voltage source and, therefore, must meet the input ripple requirements and voltage rating. Calculate the maximum RMS input current as: MAX1920/MAX1921 Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 Table 3. Component Suppliers SUPPLIER Coilcraft PHONE WEBSITE 847-639-6400 www.coilcraft.com Kemet 408-986-0424 www.kemet.com Murata 814-237-1431 www.murata.com Sumida Taiyo Yuden USA 847-956-0666 Japan 81-3-3607-5111 USA Japan USA Toko Japan www.sumida.com 408-573-4150 www.T-Yuden.com 81-3-3833-5441 www.yuden.co.jp 847-297-0070 www.tokoam.com 81-3-3727-1161 www.toko.co.jp MAX1920 Using Ceramic COUT When using the application circuit of Figure 2, the inductor’s series resistance causes a small amount of load regulation, as desired for a voltage-positioning load transient response. Choose R1 and R2 such that VOUT is high at no load by about half of this load regulation: V + RL × IOUT (MAX) / 2 R1 = R2 × OUT − 1 VREF where R2 is chosen in the 50kΩ to 500kΩ range, VREF = 1.25V and RL is the typical series resistance of the inductor. Use 1% or better resistors. Next, calculate the equivalent resistance at the FB node as: MAX1921 Using Ceramic COUT When using the application circuit of Figure 1, the inductor’s series resistance causes a small amount of load regulation, as desired for a voltage-positioning load transient response. Choose R1 such that VOUT is high at no load by about half of this load regulation. The simplified calculation is: R1 = 5 × 104 × RL (MAX) where RL(MAX) is the maximum series resistance of the inductor. Select a standard resistor value that is within 20% of this calculation. Next, calculate CFF for 25mV ripple at the internal feedback node. The simplified calculation is: CFF = 2.5 × 10 −5 R1 where R1 is the standard resistor value that is used. Select a standard capacitor value that is within 20% of the calculated CFF. INPUT 2V TO 5.5V 1 IN LX 6 OUTPUT UP TO 400mA L CIN COUT 2 ON 3 MAX1920 AGND PGND SHDN FB 5 R1 4 OFF R2 Figure 4. MAX1920 Application Circuit Using Tantalum Output Capacitor 8 R1 × R2 R1 + R2 Then, calculate CFF for 25mV ripple at FB. The simplified calculation is: Re q = R1 || R2 = CFF = 2.5 × 10 −5 Re q Select a standard capacitor value that is within 20% of the calculated CFF. MAX1920 Using Tantalum COUT When using the application circuit of Figure 4, choose R1 and R2 such as to obtain the desired VOUT: V R1 = R2 × OUT − 1 V REF where R2 is chosen to be less than 50kΩ and VREF = 1.25V. Use 1% or better resistors. Layout Considerations 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 inductor, input filter capacitor, and output filter capacitor as close to the device as possible, and keep their traces short, direct, and wide. Connect their ground pins at a single common node in a star ground configuration. The external voltage-feedback network should be very close to the FB pin, within 0.2in (5mm). Keep noisy traces, such as the LX trace, away from the voltagefeedback network; also keep them separate, using grounded copper. The MAX1920/MAX1921 evaluation kit data sheet includes a proper PC board layout and routing scheme. _______________________________________________________________________________________ Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 VOUT (V) TOP MARK MAX1920EUT Adjustable ABCO MAX1920ETT Adjustable ADR PART MAX1921EUT33 3.3 ABCJ MAX1921EUT30 3.0 ABCK MAX1921EUT25 2.5 ABCL MAX1921EUT18 1.8 ABCM MAX1921EUT15 1.5 ABCN Chip Information TRANSISTOR COUNT: 1467 Package Information 6, 8, &10L, DFN THIN.EPS (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.) D2 D A2 PIN 1 ID N 0.35x0.35 b PIN 1 INDEX AREA E [(N/2)-1] x e REF. E2 DETAIL A e k A1 CL CL A L L e e PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm -DRAWING NOT TO SCALE- 21-0137 G 1 2 _______________________________________________________________________________________ 9 MAX1920/MAX1921 Selector Guide MAX1920/MAX1921 Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 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.) COMMON DIMENSIONS MIN. MAX. D 0.70 2.90 0.80 3.10 E A1 2.90 0.00 3.10 0.05 L k 0.20 0.40 0.25 MIN. A2 0.20 REF. SYMBOL A PACKAGE VARIATIONS PKG. CODE N D2 E2 e JEDEC SPEC b [(N/2)-1] x e DOWNBONDS ALLOWED T633-1 6 1.50±0.10 2.30±0.10 0.95 BSC MO229 / WEEA 0.40±0.05 1.90 REF NO T633-2 6 1.50±0.10 2.30±0.10 0.95 BSC MO229 / WEEA 0.40±0.05 1.90 REF NO T833-1 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF NO T833-2 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF NO T833-3 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF YES T1033-1 10 1.50±0.10 2.30±0.10 0.50 BSC MO229 / WEED-3 0.25±0.05 2.00 REF NO T1433-1 14 1.70±0.10 2.30±0.10 0.40 BSC ---- 0.20±0.05 2.40 REF YES T1433-2 14 1.70±0.10 2.30±0.10 0.40 BSC ---- 0.20±0.05 2.40 REF NO PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm -DRAWING NOT TO SCALE- 10 21-0137 ______________________________________________________________________________________ G 2 2 Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 6LSOT.EPS PACKAGE OUTLINE, SOT 6L BODY 21-0058 G 1 1 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 ____________________ 11 © 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc. MAX1920/MAX1921 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.)