LM3678SDE-1.2

LM3678 High-Performance, Miniature 1.5A Step-Down DC-DC Converter for Handheld Applications April 30, 2008 LM3678 High-Performance, Miniature 1.5A Step-Down DCDC Converter for Handheld Applications General Description The LM3678 step-down DC-DC converter is optimized for powering low voltage circuits from a single Li-Ion cell battery and input voltage rails from 2.5V to 5.5V. It provides up to 1.5A load current, over the entire input voltage range. LM3678 offers a 0.8V/1.2V option. One of the pair of voltages is set through the VSELECT pin. LM3678 operates in PWM mode with a fixed frequency of 3.3MHz. Internal synchronous rectification provides high efficiency during PWM mode operation. In shutdown mode, the device turns off and reduces battery consumption to 0.01µA (typ). The LM3678 is available in a 3mm x 3mm LLP-10 package. A high switching frequency of 3.3MHz (typ.) allows use of tiny surface-mount components. Only three external surfacemount components, an inductor and two ceramic capacitors, are required (solution size less than 33 mm2). For voltages other than the voltage shown, please refer to ordering information section or contact National Semiconductor. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ VOUT = 0.8V/1.2V VIN = 2.5V to 5.5V 1.5A maximum load capability 3.3MHz PWM fixed switching frequency (typ.) allows the use of 1µH inductor +/- 3% DC output voltage precision 0.01µA typical shutdown current Internal synchronous rectification for high efficiency Internal soft start Current overload and thermal shutdown protection Applications ■ ■ ■ ■ ■ ■ PDAs and Smart Phones Personal Media Players W-LAN USB Modem Applications Digital still cameras Portable Hard disk drives Typical Application Circuit Efficiency vs. Output Current ( VOUT = 1.2V) 20204501 Note: VSEL H = 1.2V, VSEL L = 0.8V 20204504 FIGURE 1. © 2008 National Semiconductor Corporation 202045 www.national.com LM3678 Functional Block Diagram 20204505 FIGURE 2. LM3678 Block Diagram www.national.com 2 LM3678 Connection Diagram and Package Mark Information 20204502 20204503 Top View Bottom View Pin Descriptions Pin # 1 2 3 4 5 6 Name GND GND SW VDD VDD VSELECT Description Power Ground pin. Analog Ground Pin Switching node connection to the internal PFET switch and NFET synchronous rectifier Analog supply input. Connect to the input filter capacitor (Figure 1). Power supply Input. Connect to the input filter capacitor (Figure 1). Output voltage select ( For example) VSELECT = LOW, VOUT = 0.8V VSELECT = HIGH , VOUT = 1.2V Power Good Flag. This commom drain logic output is pulled to ground when the output voltage is not within +/-7.5% of regulation. Connect PWM pin to VIN. Enable pin. The device is in shutdown mode when voltage to this pin is 1.0V. Do not leave this pin floating. Feedback analog input. Connect directly to the output filter capacitor for fixed voltage versions. Die Attach Pad, connect the DAP to GND on PCB layout to enhance thermal performance. It should not be used as a primary ground connection. 7 8 9 10 DAP PGOOD PWM EN FB DAP Ordering Information Voltage Option 0.8V/1.2V Order Number LM3678SDE-1.2 LM3678SD-1.2 LM3678SDX-1.2 Package Marking S021B S021B S021B Supplied As 250 units, Tape-and-Reel 1000 units, Tape-and-Reel 4500 units, Tape-and-Reel 3 www.national.com LM3678 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN Pin: Voltage to GND EN Pin: FB, SW Pin: Continuous Power Dissipation (Note 3) Junction Temperature (TJ-MAX) −0.2V to 6.0V −0.2V to 6.0V (GND−0.2V) to (VIN + 0.2V) Internally Limited +150°C Storage Temperature Range Maximum Lead Temperature (Soldering, 10 sec.) −65°C to +150°C 260°C Operating Ratings (Notes 1, 2) Input Voltage Range 2.5V to 5.5V Recommended Load Current 0mA to 1.5A Junction Temperature (TJ) Range −30°C to +125°C Ambient Temperature (TA) Range (Note −30°C to +85°C 4) Thermal Properties Junction-to-Ambient Thermal Resistance (θJA) (LLP-10) for 4 layer board (Note 5) 49.8°C/W Electrical Characteristics Symbol VFB VREF RDSON (P) RDSON (N) ILIM ISHDN ENIH ENIL IEN FOSC Parameter Feedback Voltage Internal Reference Voltage Pin-Pin Resistance for PFET Pin-Pin Resistance for NFET Switch Peak Current Limit Shutdown Supply Current Logic High Input Logic Low Input Enable (EN) Input Current Internal Oscillator Frequency (Notes 2, 6, 8) Limits in standard typeface are for TJ = 25°C. Limits in boldface type apply over the full operating ambient temperature range (−30°C ≤ TA ≤ +85°C). Unless otherwise noted, specifications apply to the LM3678 Typical Application Circuit (pg. 1) with VIN = EN = 3.6V Condition VSELECT = Low & High (Note 7) VIN= VGS= 3.6V VIN= VGS= 3.6V Open loop EN = 0V VIN = 3.6V VIN = 3.6V 0.01 PWM Mode 2.7 3.3 1.2 0.4 1 3.6 1.9 Min -3 0.5 150 110 2.15 200 150 2.4 1 Typ Max +3 Units % V mΩ mΩ A µA V V µA MHz Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the Electrical Characteristics tables. Note 2: All voltages are with respect to the potential at the GND pin. Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ= 150°C (typ.) and disengages at TJ= 130°C (typ.). Note 4: In Applications where high power dissipation and/or poor package resistance is present, the maximum ambient temperature may have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX), the maximum power dissipation of the device in the application (PD-MAX) and the junction to ambient thermal resistance of the package (θJA) in the application, as given by the following equation: TA-MAX= TJ-MAX − (θJAx PD-MAX). Note 5: Junction to ambient thermal resistance is highly application and board layout dependent. In applications where high power dissipation exists, special care must be given to thermal dissipation issues in board design. Note 6: Refer to datasheet curves for closed loop data and its variation with regards to supply voltage and temperature. Electrical Characteristic table reflects open loop data (FB=0V and current drawn from SW pin ramped up until cycle by cycle current limit is activated). Closed loop current limit is the peak inductor current measured in the application circuit by increasing output current until output voltage drops by 10%. Note 7: Min and Max limits are guaranteed by design, test or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm. Note 8: The parameters in the electrical characteristic table are tested at VIN= 3.6V unless otherwise specified. For performance over the input voltage range refer to datasheet curves. www.national.com 4 LM3678 Typical Performance Characteristics LM3678SD, Circuit of Figure 1, VIN= 3.6V, VOUT= 1.2V, CIN = 10µF, COUT = 22µF, and TA= 25°C, unless otherwise noted. Quiescent Supply Current vs. Temperature Switching Frequency vs. Temperature 20204533 20204535 NFET_ RDSON vs. Temperature PFET_RDSON vs. Temperature 20204536 20204537 ILIMIT vs. Temperature (Open Loop) Efficiency PWM Mode vs. ILOAD (0.8V) 20204522 20204538 5 www.national.com LM3678 Efficiency PWM Mode vs. ILOAD (1.2V) Line Transient Response (VOUT = 0.8V, LOAD = 500mA) 20204552 20204523 Line Transient Response (VOUT = 1.2V, LOAD = 500mA) Load Transient Response (VIN = 3.6V, VOUT = 1.2V, Load Step 0 ↔ 500mA) 20204553 20204513 Load Transient Response (VIN = 3.6V, VOUT = 0.8V, Load Step 0 ↔ 500mA) Load Transient Response (VIN = 3.6V, VOUT = 0.8V, Load Step 500mA ↔ 1A) 20204515 20204514 www.national.com 6 LM3678 Load Transient Response (VIN = 3.6V, VOUT = 1.2V, Load Step 500mA ↔ 1A) VSELECT Transient Response (VIN = 3.6V, LOAD = 500mA) 20204541 20204520 VSELECT Transient Response (VIN = 3.6V, No LOAD) VSELECT Transient Response (VIN = 3.6V, LOAD = 1A) 20204542 20204547 Start Up (VIN = 3.6V, VOUT = 1.2V, LOAD = 1A) Start Up (VIN = 3.6V, VOUT = 1.2V, LOAD = 500mA) 20204550 20204551 7 www.national.com LM3678 Switching Waveform (VOUT = 1.2V, LOAD = 1A) 20204556 www.national.com 8 LM3678 Operation Description DEVICE INFORMATION The LM3678, a high efficiency step down DC-DC switching buck converter, delivers a constant voltage from a single LiIon battery and input voltage rails from 2.5V to 5.5V to portable devices such as cell phones and PDAs. Using a voltage mode architecture with synchronous rectification, the LM3678 has the ability to deliver up to 1.5A depending on the input voltage, output voltage, ambient temperature and the inductor chosen. Additional features include soft-start, under voltage protection, current overload protection, and thermal shutdown protection. As shown in Figure 1, only three external power components are required for implementation. The part uses an internal reference voltage of 0.5V. It is recommended to keep the part in shutdown until the input voltage is 2.5V or higher. CIRCUIT OPERATION During the first portion of each switching cycle, the control block in the LM3678 turns on the internal PFET switch. This allows current to flow from the input through the inductor to the output filter capacitor and load. The inductor limits the current to a ramp with a slope of (VIN–VOUT)/L, by storing energy in a magnetic field. During the second portion of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the NFET to the output filter capacitor and load, which ramps the inductor current down with a slope of - VOUT/L. The output filter stores charge when the inductor current is high, and releases it when inductor current is low, smoothing the voltage across the load. The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and synchronous rectifier at the SW pin to a low-pass filter formed by the inductor and output filter capacitor. The output voltage is equal to the average voltage at the SW pin. PWM OPERATION During device operation the converter operates as a voltagemode controller with input voltage feed forward. This allows the converter to achieve good load and line regulation. The DC gain of the power stage is proportional to the input voltage. To eliminate this dependence, feed forward inversely proportional to the input voltage is introduced. The output voltage is regulated by switching at a constant frequency and then modulating the energy per cycle to control power to the load. At the beginning of each clock cycle the PFET switch is turned on and the inductor current ramps up until the comparator trips and the control logic turns off the switch. The current limit comparator can also turn off the switch in case the current limit of the PFET is exceeded. Then the NFET switch is turned on and the inductor current ramps down. The next cycle is initiated by the clock turning off the NFET and turning on the PFET. 20204555 FIGURE 3. Typical PWM Operation INTERNAL SYNCHRONOUS RECTIFICATION The LM3678 uses an internal NFET as a synchronous rectifier to reduce rectifier forward voltage drop and associated power loss. Synchronous rectification provides a significant improvement in efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier diode. CURRENT LIMITING A current limit feature allows the LM3678 to protect itself and external components during overload conditions by implementing current limiting with an internal comparator that trips at 2.15A (typ). If the output is shorted to ground the device enters a timed current limit mode where the NFET is turned on for a longer duration until the inductor current falls below a low threshold. This allows the inductor current more time to decay, thereby preventing runaway. SHUTDOWN MODE Setting the EN input pin low (1.0V) enables normal operation. It is recommended to set EN pin low to turn off the LM3678 during system power up and undervoltage conditions when the supply is less than 2.5V. Do not leave the EN pin floating. SOFT START The LM3678 has a soft-start circuit that limits in-rush current during start-up. During start-up the switch current limit is increased in steps. Soft start is activated only if EN goes from logic low to logic high after VIN reaches 2.5V. Soft start is implemented by increasing switch current limit in steps of 250mA, 500mA, 1A and 2A (typical switch current limit). The start-up time thereby depends on the output capacitor and load current demanded at start-up. 9 www.national.com LM3678 Inductor Selection There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current rating specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of application should be requested from the manufacturer. Shielded inductors radiate less noise and should be preferred. There are two methods to choose the inductor saturation current rating. Method 1: The saturation current should be greater than the sum of the maximum load current and the worst case average to peak inductor current. This can be written as • VOUT: output voltage For a more conservative approach, a 1µH inductor with a saturation current rating of at least 2.5A is recommended for most applications. Input Capacitor Selection A ceramic input capacitor of 10uF, 6.3V is sufficient for most applications. Place the input capacitor as close as possible to the VIN pin of the device. A larger value may be used for improved input voltage filtering. Use X7R or X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. The input filter capacitor supplies current to the PFET switch of the LM3678 in the first half of each cycle and reduces voltage ripple imposed on the input power source. A ceramic capacitor’s low ESR provides the best noise filtering of the input voltage spikes due to this rapidly changing current. Select a capacitor with sufficient ripple current rating. The input current ripple can be calculated as: • • • • • IRIPPLE: average to peak inductor current IOUTMAX: maximum load current (1.5A) VIN: maximum input voltage in application L : minimum inductor value including worst case tolerances (30% drop can be considered for method 1) f : minimum switching frequency (2.7Mhz) TABLE 1. Suggest Inductors and Their Suppliers Model NR4012T1R0N LPS4012-102L LPS4012-102L Vendor Taiyo Yuden Coilcraft Coilcraft Dimensions LxWxH (mm) 4 x 4 x 1.2 3.9 x 3.9 x 1.2 3.9 x 3.9 x 1.8 D.C.R (max) 60mΩ 100mΩ 40mΩ ISAT 2.5A 2.5A 3.4A Output Capacitor Selection A ceramic output capacitor of 22µF, 6.3V is sufficient for most applications. Use X7R or X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. DC bias characteristics vary from manufacturer to manufacturer and dc bias curves should be requested from them as part of the capacitor selection process. The output filter capacitor smooths out current flow from the inductor to the load, helps maintain a steady output voltage during transient load changes and reduces output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low ESR to perform these functions. The output voltage ripple is caused by the charging and discharging of the output capacitor and by the RESR and can be calculated as: Voltage peak-to-peak ripple due to capacitance can be expressed as follow: Voltage peak-to-peak ripple due to ESR can be expressed as follow: VPP-ESR = (2 * IRIPPLE) * RESR Because these two components are out of phase the rms (root mean squared) value can be used to get an approximate value of peak-to-peak ripple. The peak-to-peak ripple voltage rms value can be expressed as follow: Note that the output voltage ripple is dependent on the inductor current ripple and the equivalent series resistance of the output capacitor (RESR). The RESR is frequency dependent (as well as temperature dependent); make sure the value used for calculations is at the switching frequency of the part. www.national.com 10 LM3678 TABLE 2. Suggested Capacitors and Their Suppliers Model 10µF for CIN GRM21BR60J106K JMK212BJ106K C2012X5R0J106K 22µF for COUT JMK212BJ226MG Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012) Ceramic, X5R Ceramic, X5R Ceramic, X5R Murata Taiyo-Yuden TDK 6.3V 6.3V 6.3V 0805 (2012) 0805 (2012) 0805 (2012) Type Vendor Voltage Rating Case Size Inch (mm) Board Layout Considerations PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance of a DCDC converter and surrounding circuitry by contributing to EMI, ground bounce, and resistive voltage loss in the traces. These can send erroneous signals to the DC-DC converter IC, resulting in poor regulation or instability. Good layout for the LM3678 can be implemented by following a few simple design rules below. 1. Place the LM3678, inductor and filter capacitors close together and make the traces short. The traces between these components carry relatively high switching currents and act as antennas. Following this rule reduces radiated noise. Special care must be given to place the input filter capacitor very close to the VIN and GND pin. 2. Arrange the components so that the switching current loops curl in the same direction. During the first half of each cycle, current flows from the input filter capacitor through the LM3678 and inductor to the output filter capacitor and back through ground, forming a current loop. In the second half of each cycle, current is pulled up from ground through the LM3678 by the inductor to the output filter capacitor and then back through ground forming a second current loop. Routing these loops so the current curls in the same direction prevents magnetic field reversal between the two half-cycles and reduces radiated noise. 3. Connect the ground pins of the LM3678 and filter capacitors together using generous component-side copper fill as a pseudo-ground plane. Then, connect this to the ground-plane (if one is used) with several vias. This reduces ground-plane noise by preventing the switching currents from circulating through the ground plane. It also reduces ground bounce at the LM3678 by giving it a lowimpedance ground connection. 4. Use wide traces between the power components and for power connections to the DC-DC converter circuit. This reduces voltage errors caused by resistive losses across the traces. 5. Route noise sensitive traces, such as the voltage feedback path, away from noisy traces between the power components. The voltage feedback trace must remain close to the LM3678 circuit and should be direct but should be routed opposite to noisy components. This reduces EMI radiated onto the DC-DC converter’s own voltage feedback trace. A good approach is to route the feedback trace on another layer and to have a ground plane between the top layer and layer on which the feedback trace is routed. In the same manner for the adjustable part it is desired to have the feedback dividers on the bottom layer. 6. Place noise sensitive circuitry, such as radio IF blocks, away from the DC-DC converter, CMOS digital blocks and other noisy circuitry. Interference with noisesensitive circuitry in the system can be reduced through distance. For detailed layout information, refer to Application Note 1722 LM3678 Evaluation Board. In mobile phones, for example, a common practice is to place the DC-DC converter on one corner of the board, arrange the CMOS digital circuitry around it (since this also generates noise), and then place sensitive preamplifiers and IF stages on the diagonally opposing corner. Often, the sensitive circuitry is shielded with a metal pan and power to it is postregulated to reduce conducted noise, using low-dropout linear regulators. 11 www.national.com LM3678 Physical Dimensions inches (millimeters) unless otherwise noted Non Pullback LLP-10, 0.5mm Pitch NS Package Number SDA10A The dimensions for X1, X2, and X3 are as given: X1 = 3mm +/- 0.030mm X2 = 3mm +/- 0.030mm X3 = 0.800mm +/- 0.075mm www.national.com 12 LM3678 Notes 13 www.national.com LM3678 High-Performance, Miniature 1.5A Step-Down DC-DC Converter for Handheld Applications Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers Audio Clock Conditioners Data Converters Displays Ethernet Interface LVDS Power Management Switching Regulators LDOs LED Lighting PowerWise Serial Digital Interface (SDI) Temperature Sensors Wireless (PLL/VCO) www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/displays www.national.com/ethernet www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/powerwise www.national.com/sdi www.national.com/tempsensors www.national.com/wireless WEBENCH Analog University App Notes Distributors Green Compliance Packaging Design Support www.national.com/webench www.national.com/AU www.national.com/appnotes www.national.com/contacts www.national.com/quality/green www.national.com/packaging www.national.com/quality www.national.com/refdesigns www.national.com/feedback Quality and Reliability Reference Designs Feedback THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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