LM2842

LM2841/ LM2842 300 mA/600 mA up to 42V Input Step-Down DC/DC Regulator in Thin SOT-23 LM2841 LM2842 September 2, 2009 300 mA/600 mA up to 42V Input Step-Down DC/DC Regulator in Thin SOT-23 General Description The LM2841 and LM2842 are PWM DC/DC buck (step-down) regulators. With a wide input range from 4.5V-42V, they are suitable for a wide range of applications such as power conditioning from unregulated sources. They feature a low RDSON (0.9Ω typical) internal switch for maximum efficiency (85% typical). Operating frequency is fixed at 550 kHz (X version) and 1.25 MHz (Y version) allowing the use of small external components while still being able to have low output voltage ripple. Soft-start can be implemented using the shutdown pin with an external RC circuit allowing the user to tailor the soft-start time to a specific application. The LM2842 is optimized for up to 600 mA load currents while the LM2841 is optimized for up to 300 mA load current. Both have a 0.765V nominal feedback voltage. Additional features include: thermal shutdown, VIN under-voltage lockout, and gate drive under-voltage lockout. The LM2841 and LM2842 are available in a low profile TSOT-6L package. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ Input voltage range of 4.5V to 42V Output current options of 300 mA and 600 mA Feedback pin voltage of 0.765V 550 kHz (X) or 1.25 Mhz (Y) switching frequency Low shutdown IQ, 16 µA typical Short circuit protected Internally compensated Soft-start circuitry Small overall solution size (TSOT-6L package) Applications ■ ■ ■ ■ Battery powered equipment Industrial distributed power applications Portable media players Portable hand held instruments Typical Application Circuit 30036702 © 2009 National Semiconductor Corporation 300367 www.national.com LM2841/ LM2842 Connection Diagram Top View 30036704 TSOT 6 Lead NS Package Number MK06A Ordering Information Order Number LM2841XMK-ADJL LM2841XMKX-ADJL LM2841YMK-ADJL LM2841YMKX-ADJL NOPB LM2842XMK-ADJL LM2842XMKX-ADJL LM2842YMK-ADJL LM2842YMKX-ADJL TSOT-6 MK06A STVB 1000 Units, Tape and Reel 3000 Units, Tape and Reel 1000 Units, Tape and Reel 3000 Units, Tape and Reel Spec. Package Type NSC Package Drawing Top Mark Supplied As 1000 Units, Tape and Reel 3000 Units, Tape and Reel 1000 Units, Tape and Reel 3000 Units, Tape and Reel STFB STTB STXB Pin Descriptions Pin 1 2 3 4 5 6 Name CB GND FB SHDN VIN SW Ground connection. Feedback pin: Set feedback voltage divider ratio with VOUT = VFB (1+(R1/R2)). Resistors should be in the 100-10K range to avoid input bias errors. Logic level shutdown input. Pull to GND to disable the device and pull high to enable the device. If this function is not used tie to VIN or leave open. Power input voltage pin: 4.5V to 42V normal operating range. Power FET output: Connect to inductor, diode, and CBOOT cap. Function SW FET gate bias voltage. Connect CBOOT cap between CB and SW. www.national.com 2 LM2841/ LM2842 Block Diagram 30036703 3 www.national.com LM2841/ LM2842 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 SHDN SW Voltage CB Voltage above SW Voltage FB Voltage Maximum Junction Temperature Power Dissipation(Note 2) -0.3V to +45V -0.3V to (VIN+0.3V) SHDN=VIN at 45V max -0.3V to +45V 7V -0.3V to +5V 150°C Internally Limited Lead Temperature Vapor Phase (60 sec.) Infrared (15 sec.) ESD Susceptibility (Note 3) Human Body Model 300°C 215°C 220°C 1.5 kV Operating Conditions Operating Junction Temperature Range (Note 4) Storage Temperature Input Voltage VIN SW Voltage −40°C to +125°C −65°C to +150°C 4.5V to 42V Up to 42V Electrical Characteristics Specifications in standard type face are for TJ = 25°C and those with boldface type apply over the full Operating Temperature Range ( TJ = −40°C to +125°C). Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm at TJ = +25°C, and are provided for reference purposes only. Unless otherwise stated the following conditions apply: VIN = 12V. Symbol IQ Parameter Quiescent current Conditions SHDN = 0V Device On, Not Switching Device On, No Load RDSON ILSW ICL IFB VFB tMIN fSW Switch ON resistance Switch leakage current Switch current limit Feedback pin bias current FB Pin reference voltage Minimum ON time Switching frequency LM2841/42X, VFB = 0.5V LM2841/42X, VFB = 0V LM2841/42Y, VFB = 0.5V LM2841/42Y, VFB = 0V DMAX VUVP VSHDN ISHDN Maximum duty cycle Undervoltage lockout thresholds Shutdown threshold LM2841/42X LM2841/42Y On threshold Off threshold Device on Device off Shutdown pin input bias current VSHDN = 2.3V (Note 8) VSHDN = 0V THERMAL SPECIFICATIONS RθJA Junction-to-Ambient Thermal (Note 9) Resistance, TSOT-6L Package 121 °C/W 2.3 88 81 4.4 0.95 325 (Note 6) VIN = 42V LM2842 (Note 7) LM2841 (Note 7) LM2841/42 (Note 8) 0.747 Min (Note 4) Typ (Note 5) Max (Note 4) 16 1.30 1.35 0.9 0.0 1.15 525 0.1 0.765 100 550 140 1.25 0.35 94 87 3.7 3.5 1.0 0.9 0.05 0.02 0.3 1.5 1.5 3.25 1.50 750 1.0 0.782 40 1.75 1.85 1.6 0.5 Units µA mA Ω µA A mA µA V ns kHz MHz % V V µA www.national.com 4 LM2841/ LM2842 Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature is calculated using: PD (MAX) = (TJ(MAX) − TA)/ θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown. Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=175°C (typ.) and disengages at TJ=155°C (typ). Note 3: Human Body Model, applicable std. JESD22-A114-C. Note 4: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100% production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL). Note 5: Typical numbers are at 25°C and represent the most likely norm. Note 6: Includes the bond wires, RDSON from VIN pin to SW pin. Note 7: Current limit at 0% duty cycle. Note 8: Bias currents flow into pin. Note 9: All numbers apply for packages soldered directly onto a 3" x 3" PC board with 2 oz. copper on 4 layers in still air in accordance to JEDEC standards. Thermal resistance varies greatly with layout, copper thickness, number of layers in PCB, power distribution, number of thermal vias, board size, ambient temperature, and air flow. 5 www.national.com LM2841/ LM2842 Typical Performance Characteristics Efficiency vs. Load Current (LM2842X, VOUT = 3.3V) Efficiency vs. Load Current (LM2841X, VOUT = 3.3V) 30036719 30036718 Efficiency vs. Load Current (LM2841X, VOUT = 8V) Switching Frequency vs. Temperature (X version) 30036720 30036766 Input UVLO Voltage vs. Temperature Switch Current Limit vs. SHDN Pin Voltage (Soft-start Implementation, LM2841) 30036767 30036768 www.national.com 6 LM2841/ LM2842 Switch Current Limit vs. SHDN Pin Voltage (Soft-start Implementation, LM2842) SHDN Pin Current vs. SHDN Pin Voltage 30036769 30036721 Switching Node and Output Voltage Waveforms Load Transient Waveforms 30036770 30036771 VIN = 12V, VOUT = 3.3V, IOUT = 200 mA Top trace: VOUT, 10 mV/div, AC Coupled Bottom trace: SW, 5V/div, DC Coupled T = 1 µs/div VIN = 12V, VOUT = 3.3V, IOUT = 300 mA to 200 mA to 300 mA Top trace: VOUT, 20 mV/div, AC Coupled Bottom trace: IOUT, 100 mA/div, DC Coupled T = 200 µs/div Start-up Waveform 30036772 VIN = 12V, VOUT = 3.3V, IOUT = 50 mA Top trace: VOUT, 1V/div, DC Coupled Bottom trace: SHDN, 2V/div, DC Coupled T = 40 µs/div 7 www.national.com LM2841/ LM2842 Operation PROTECTION The LM2841/42 has dedicated protection circuitry running during normal operation to protect the IC. The thermal shutdown circuitry turns off the power device when the die temperature reaches excessive levels. The UVLO comparator protects the power device during supply power startup and shutdown to prevent operation at voltages less than the minimum input voltage. A gate drive (CB) under-voltage lockout is included to guarantee that there is enough gate drive voltage to drive the MOSFET before the device tries to start switching. The LM2841/42 also features a shutdown mode decreasing the supply current to approximately 16 µA. CONTINUOUS CONDUCTION MODE The LM2841/42 contains a current-mode, PWM buck regulator. A buck regulator steps the input voltage down to a lower output voltage. In continuous conduction mode (when the inductor current never reaches zero at steady state), the buck regulator operates in two cycles. The power switch is connected between VIN and SW. In the first cycle of operation the transistor is closed and the diode is reverse biased. Energy is collected in the inductor and the load current is supplied by COUT and the rising current through the inductor. During the second cycle the transistor is open and the diode is forward biased due to the fact that the inductor current cannot instantaneously change direction. The energy stored in the inductor is transferred to the load and output capacitor. The ratio of these two cycles determines the output voltage. The output voltage is defined approximately as: D=VOUT/VIN and D’ = (1D) where D is the duty cycle of the switch. D and D' will be required for design calculations. DESIGN PROCEDURE This section presents guidelines for selecting external components. SETTING THE OUTPUT VOLTAGE The output voltage is set using the feedback pin and a resistor divider connected to the output as shown on the front page schematic. The feedback pin voltage is 0.762V, so the ratio of the feedback resistors sets the output voltage according to the following equation: VOUT=0.765V(1+(R1/R2)) Typically R2 will be given as 100Ω-10 kΩ for a starting value. To solve for R1 given R2 and VOUT use R1=R2((VOUT/0.765V)-1). INPUT CAPACITOR A low ESR ceramic capacitor (CIN) is needed between the VIN pin and GND pin. This capacitor prevents large voltage transients from appearing at the input. Use a 2.2 µF-10 µF value with X5R or X7R dielectric. Depending on construction, a ceramic capacitor’s value can decrease up to 50% of its nominal value when rated voltage is applied. Consult with the capacitor manufacturer's data sheet for information on capacitor derating over voltage and temperature. INDUCTOR SELECTION The most critical parameters for the inductor are the inductance, peak current, and the DC resistance. The inductance is related to the peak-to-peak inductor ripple current, the input and the output voltages. A higher value of ripple current reduces inductance, but increases the conductance loss, core loss, and current stress for the inductor and switch devices. It also requires a bigger output capacitor for the same output voltage ripple requirement. A reasonable value is setting the ripple current to be 30% of the DC output current. Since the ripple current increases with the input voltage, the maximum input voltage is always used to determine the inductance. The DC resistance of the inductor is a key parameter for the efficiency. Lower DC resistance is available with a bigger winding area. A good tradeoff between the efficiency and the core size is letting the inductor copper loss equal 2% of the output power. See AN-1197 for more information on selecting inductors. A good starting point for most applications is a 10 µH to 22 µH with 1.1A or greater current rating for the LM2842 or a 0.7A or greater current rating for the LM2841. Using such a rating will enable the LM2841/42 to current limit without saturating the inductor. This is preferable to the LM2841/42 going into thermal shutdown mode and the possibility of damaging the inductor if the output is shorted to ground or other longterm overload. OUTPUT CAPACITOR The selection of COUT is driven by the maximum allowable output voltage ripple. The output ripple in the constant frequency, PWM mode is approximated by: VRIPPLE = IRIPPLE (ESR+(1/8fSWCOUT)) The ESR term usually plays the dominant role in determining the voltage ripple. Low ESR ceramic capacitors are recommended. Capacitors in the range of 22 µF-100 µF are a good starting point with an ESR of 0.1Ω or less. BOOTSTRAP CAPACITOR A 0.15 µF ceramic capacitor or larger is recommended for the bootstrap capacitor (CBOOT). For applications where the input voltage is less than twice the output voltage a larger capacitor is recommended, generally 0.15 µF to 1 µF to ensure plenty of gate drive for the internal switches and a consistently low RDSON. SOFT-START COMPONENTS The LM2841/42 has circuitry that is used in conjunction with the SHDN pin to limit the inrush current on start-up of the DC/ DC switching regulator. The SHDN pin in conjunction with a RC filter is used to tailor the soft-start for a specific application. When a voltage applied to the SHDN pin is between 0V and up to 2.3V it will cause the cycle by cycle current limit in the power stage to be modulated for minimum current limit at 0V up to the rated current limit at 2.3V. Thus controlling the output rise time and inrush current at startup. The resistor value should be selected so the current sourced into the SHDN pin will be greater then the leakage current of the SHDN pin (1.5 µA ) when the voltage at SHDN is equal or greater then 2.3V. SHUTDOWN OPERATION The SHDN pin of the LM2841/42 is designed so that it may be controlled using 2.3V or higher logic signals. If the shutdown function is not to be used the SHDN pin may be tied to VIN. The maximum voltage to the SHDN pin should not exceed 42V. If the use of a higher voltage is desired due to system or other constraints it may be used, however a 100 kΩ or larger resistor is recommended between the applied voltage and the SHDN pin to protect the device. SCHOTTKY DIODE The breakdown voltage rating of the diode (D1) is preferred to be 25% higher than the maximum input voltage. The cur- www.national.com 8 LM2841/ LM2842 rent rating for the diode should be equal to the maximum output current for best reliability in most applications. In cases where the input voltage is much greater than the output voltage the average diode current is lower. In this case it is possible to use a diode with a lower average current rating, approximately (1-D)IOUT, however the peak current rating should be higher than the maximum load current. A 0.5A to 1A rated diode is a good starting point. LAYOUT CONSIDERATIONS To reduce problems with conducted noise pick up, the ground side of the feedback network should be connected directly to the GND pin with its own connection. The feedback network, resistors R1 and R2, should be kept close to the FB pin, and away from the inductor to minimize coupling noise into the feedback pin. The input bypass capacitor CIN must be placed close to the VIN pin. This will reduce copper trace resistance which effects input voltage ripple of the IC. The inductor L1 should be placed close to the SW pin to reduce magnetic and electrostatic noise. The output capacitor, COUT should be placed close to the junction of L1 and the diode D1. The L1, D1, and COUT trace should be as short as possible to reduce conducted and radiated noise and increase overall efficiency. The ground connection for the diode, CIN, and COUT should be as small as possible and tied to the system ground plane in only one spot (preferably at the COUT ground point) to minimize conducted noise in the system ground plane. For more detail on switching power supply layout considerations see Application Note AN-1149: Layout Guidelines for Switching Power Supplies. 9 www.national.com LM2841/ LM2842 Application Information 30036705 FIGURE 1. Application Circuit, 3.3V Output Some Recommended Inductors (Others May Be Used) Manufacturer Coilcraft MuRata Coiltronics Inductor LPS4018, DO1608C, DO3308, and LPO2506 series LQH55D and LQH66S series MP2 and MP2A series Contact Information www.coilcraft.com 800-3222645 www.murata.com www.cooperbussman.com Some Recommended Input And Output Capacitors (Others May Be Used) Manufacturer Vishay Sprague Taiyo Yuden Capacitor 293D, 592D, and 595D series tantalum High capacitance MLCC ceramic ESRD seriec Polymer Aluminum Electrolytic SPV and AFK series V-chip series High capacitance MLCC ceramic Contact Information www.vishay.com 407-324-4140 www.t-yuden.com 408-573-4150 www.cde.com www.murata.com Cornell Dubilier MuRata 30036708 FIGURE 2. Application Circuit, 5V Output www.national.com 10 LM2841/ LM2842 30036709 FIGURE 3. Application Circuit, 12V Output 30036716 FIGURE 4. Application Circuit, 15V Output 30036717 FIGURE 5. Application Circuit, 0.8V Output 11 www.national.com LM2841/ LM2842 Physical Dimensions inches (millimeters) unless otherwise noted TSOT 6 Pin Package (MK) For Ordering, Refer to Ordering Information Table NS Package Number MK06A www.national.com 12 LM2841/ LM2842 Notes 13 www.national.com LM2841/ LM2842 300 mA/600 mA up to 42V Input Step-Down DC/DC Regulator in Thin SOT-23 Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage Reference PowerWise® Solutions Temperature Sensors Wireless (PLL/VCO) www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc 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/vref www.national.com/powerwise WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Quality and Reliability Feedback/Support Design Made Easy Solutions Mil/Aero PowerWise® Design University Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/training Serial Digital Interface (SDI) www.national.com/sdi www.national.com/wireless www.national.com/tempsensors SolarMagic™ THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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