LM2678 SIMPLE SWITCHER ® High Efficiency 5A Step-Down Voltage Regulator General Description Features The LM2678 series of regulators are monolithic integrated circuits which provide all of the active functions for a stepdown (buck) switching regulator capable of driving up to 5A loads with excellent line and load regulation characteristics. High efficiency ( > 90%) is obtained through the use of a low ON-resistance DMOS power switch. The series consists of fixed output voltages of 3.3V, 5V and 12V and an adjustable output version. The SIMPLE SWITCHER concept provides for a complete design using a minimum number of external components. A high fixed frequency oscillator (260KHz) allows the use of physically smaller sized components. A family of standard inductors for use with the LM2678 are available from several manufacturers to greatly simplify the design process. The LM2678 series also has built in thermal shutdown, current limiting and an ON/OFF control input that can power down the regulator to a low 50µA quiescent current standby condition. The output voltage is guaranteed to a ± 2% tolerance. The clock frequency is controlled to within a ± 11% tolerance. n Efficiency up to 92% n Simple and easy to design with (using off-the-shelf external components) n 120 mΩ DMOS output switch n 3.3V, 5V and 12V fixed output and adjustable (1.2V to 37V ) versions n 50µA standby current when switched OFF n ± 2%maximum output tolerance over full line and load conditions n Wide input voltage range: 8V to 40V n 260 KHz fixed frequency internal oscillator n −40 to +125˚C operating junction temperature range Applications n Simple to design, high efficiency ( > 90%) step-down switching regulators n Efficient system pre-regulator for linear voltage regulators n Battery chargers Typical Application 10088603 SIMPLE SWITCHER ® is a registered trademark of National Semiconductor Corporation. © 2005 National Semiconductor Corporation DS100886 www.national.com LM2678 SIMPLE SWITCHER High Efficiency 5A Step-Down Voltage Regulator April 2005 LM2678 Connection Diagrams and Ordering Information TO-263 Package Top View TO-220 Package Top View 10088601 10088602 Order Number LM2678S-3.3, LM2678S-5.0, LM2678S-12 or LM2678S-ADJ See NSC Package Number TS7B Order Number LM2678T-3.3, LM2678T-5.0, LM2678T-12 or LM2678T-ADJ See NSC Package Number TA07B Top View 10088641 LLP-14 See NS package Number SRC14A Ordering Information for LLP Package Output Voltage Order Information Package Marking Supplied As 12 LM2678SD-12 S0003BB 250 Units on Tape and Reel 12 LM2678SDX-12 S0003BB 2500 Units on Tape and Reel 3.3 LM2678SD-3.3 S0003CB 250 Units on Tape and Reel 3.3 LM2678SDX-3.3 S0003CB 2500 Units on Tape and Reel 5.0 LM2678SD-5.0 S0003DB 250 Units on Tape and Reel 5.0 LM2678SDX-5.0 S0003DB 2500 Units on Tape and Reel ADJ LM2678SD-ADJ S0003EB 250 Units on Tape and Reel ADJ LM2678SDX-ADJ S0003EB 2500 Units on Tape and Reel www.national.com 2 Storage Temperature Range If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Soldering Temperature Input Supply Voltage 45V ON/OFF Pin Voltage −0.1V to 6V Switch Voltage to Ground −1V to VIN Boost Pin Voltage 10 sec, 240˚C Vapor Phase 75 sec, 219˚C Supply Voltage −0.3V to 14V 8V to 40V Junction Temperature Range (TJ) Internally Limited ESD (Note 2) 4 sec, 260˚C Infrared Operating Ratings VSW + 8V Feedback Pin Voltage Power Dissipation Wave −65˚C to 150˚C −40˚C to 125˚C 2 kV Electrical Characteristics Limits appearing in bold type face apply over the entire junction temperature range of operation, −40˚C to 125˚C. Specifications appearing in normal type apply for TA = TJ = 25˚C. LM2678-3.3 Symbol Parameter Conditions Typical Min Max (Note 3) (Note 4) (Note 4) 3.234/3.201 3.366/3.399 VOUT Output Voltage VIN = 8V to 40V, 100mA ≤ IOUT ≤ 5A 3.3 η Efficiency VIN = 12V, ILOAD = 5A 82 Units V % LM2678-5.0 Symbol Parameter Conditions Typical Min Max (Note 3) (Note 4) (Note 4) 4.900/4.850 5.100/5.150 VOUT Output Voltage VIN = 8V to 40V, 100mA ≤ IOUT ≤ 5A 5.0 η Efficiency VIN = 12V, ILOAD = 5A 84 Units V % LM2678-12 Symbol Parameter Conditions Typical Min Max (Note 3) (Note 4) (Note 4) 11.76/11.64 12.24/12.36 VOUT Output Voltage VIN = 15V to 40V, 100mA ≤ IOUT ≤ 5A 12 η Efficiency VIN = 24V, ILOAD = 5A 92 Units V % LM2678-ADJ Symbol Parameter Conditions VFB Feedback Voltage VIN = 8V to 40V, 100mA ≤ IOUT ≤ 5A VOUT Programmed for 5V η Efficiency VIN = 12V, ILOAD = 5A Typ Min Max (Note 3) (Note 4) (Note 4) 1.21 1.186/1.174 1.234/1.246 84 3 Units V % www.national.com LM2678 Absolute Maximum Ratings (Note 1) LM2678 All Output Voltage Versions Electrical Characteristics Limits appearing in bold type face apply over the entire junction temperature range of operation, −40˚C to 125˚C. Specifications appearing in normal type apply for TA = TJ = 25˚C. Unless otherwise specified VIN =12V for the 3.3V, 5V and Adjustable versions and VIN =24V for the 12V version. Symbol Parameter Conditions Typ Min Max Units 4.2 6 mA 50 100/150 µA 8.3/8.75 A DEVICE PARAMETERS IQ Quiescent Current VFEEDBACK = 8V For 3.3V, 5.0V, and ADJ Versions VFEEDBACK = 15V For 12V Versions ISTBY Standby Quiescent Current ON/OFF Pin = 0V ICL Current Limit IL Output Leakage Current VIN = 40V, ON/OFF Pin = 0V VSWITCH = 0V VSWITCH = −1V 16 200 15 RDS(ON) Switch On-Resistance ISWITCH = 5A 0.12 0.14/0.225 Ω fO Oscillator Frequency Measured at Switch Pin 260 280 kHz D Duty Cycle Maximum Duty Cycle 91 Minimum Duty Cycle 0 % VFEEDBACK = 1.3V ADJ Version Only 85 nA IBIAS Feedback Bias Current VON/OFF ON/OFF Threshold Voltage 7 1.4 ION/OFF ON/OFF Input Current ON/OFF Input = 0V θJA Thermal Resistance T Package, Junction to Ambient θJA 6.1/5.75 225 µA mA % 0.8 20 2.0 V 45 µA 65 (Note 5) T Package, Junction to Ambient 45 (Note 6) θJC T Package, Junction to Case 2 θJA S Package, Junction to Ambient 56 ˚C/W (Note 7) θJA S Package, Junction to Ambient 35 (Note 8) θJA S Package, Junction to Ambient 26 (Note 9) θJC S Package, Junction to Case 2 θJA SD Package, Junction to Ambient 55 (Note 10) θJA SD Package, Junction to Ambient 29 ++ ˚C/W (Note 11) Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings indicate conditions under which of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test condition, see the electrical Characteristics tables. Note 2: ESD was applied using the human-body model, a 100pF capacitor discharged through a 1.5 kΩ resistor into each pin. Note 3: Typical values are determined with TA = TJ = 25˚C and represent the most likely norm. www.national.com 4 LM2678 All Output Voltage Versions Electrical Characteristics (Continued) Note 4: All limits are guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100% tested during production with TA = TJ = 25˚C. All limits at temperature extremes are guaranteed via correlation using standard standard Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL). Note 5: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with 1⁄2 inch leads in a socket, or on a PC board with minimum copper area. Note 6: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with 1⁄2 inch leads soldered to a PC board containing approximately 4 square inches of (1 oz.) copper area surrounding the leads. Note 7: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.136 square inches (the same size as the TO-263 package) of 1 oz. (0.0014 in. thick) copper. Note 8: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.4896 square inches (3.6 times the area of the TO-263 package) of 1 oz. (0.0014 in. thick) copper. Note 9: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board copper area of 1.0064 square inches (7.4 times the area of the TO-263 package) of 1 oz. (0.0014 in. thick) copper. Additional copper area will reduce thermal resistance further. See the thermal model in Switchers Made Simple ® software. Note 10: Junction to ambient thermal resistance for the 14-lead LLP mounted on a PC board copper area equal to the die attach paddle. Note 11: Junction to ambient thermal resistance for the 14-lead LLP mounted on a PC board copper area using 12 vias to a second layer of copper equal to die attach paddle. Additional copper area will reduce thermal resistance further. For layout recommendations, refer to Application Note AN-1187. 5 www.national.com LM2678 Typical Performance Characteristics Normalized Output Voltage Line Regulation 10088609 10088610 Efficiency vs Input Voltage Efficiency vs ILOAD 10088611 10088612 Switch Current Limit Operating Quiescent Current 10088605 10088604 www.national.com 6 LM2678 Typical Performance Characteristics (Continued) Standby Quiescent Current ON/OFF Threshold Voltage 10088640 10088613 ON/OFF Pin Current (Sourcing) Switching Frequency 10088614 10088615 Feedback Pin Bias Current 10088616 7 www.national.com LM2678 Typical Performance Characteristics (Continued) Discontinuous Mode Switching Waveforms VIN = 20V, VOUT = 5V, ILOAD = 500 mA L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ Continuous Mode Switching Waveforms VIN = 20V, VOUT = 5V, ILOAD = 5A L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ 10088617 10088618 A: VSW Pin Voltage, 10 V/div. B: Inductor Current, 2 A/div A: VSW Pin Voltage, 10 V/div. B: Inductor Current, 1 A/div C: Output Ripple Voltage, 20 mV/div AC-Coupled C: Output Ripple Voltage, 20 mV/div AC-Coupled Horizontal Time Base: 1 µs/div Horizontal Time Base: 1 µs//iv Load Transient Response for Continuous Mode VIN = 20V, VOUT = 5V L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ Load Transient Response for Discontinuous Mode VIN = 20V, VOUT = 5V, L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ 10088619 10088620 A: Output Voltage, 100 mV//div, AC-Coupled. A: Output Voltage, 100 mV/div, AC-Coupled. B: Load Current: 500 mA to 5A Load Pulse B: Load Current: 200 mA to 3A Load Pulse Horizontal Time Base: 100 µs/div www.national.com Horizontal Time Base: 200 µs/div 8 LM2678 Block Diagram 10088606 * Active Inductor Patent Number 5,514,947 † Active Capacitor Patent Number 5,382,918 9 www.national.com LM2678 GROUND Application Hints This is the ground reference connection for all components in the power supply. In fast-switching, high-current applications such as those implemented with the LM2678, it is recommended that a broad ground plane be used to minimize signal coupling throughout the circuit The LM2678 provides all of the active functions required for a step-down (buck) switching regulator. The internal power switch is a DMOS power MOSFET to provide power supply designs with high current capability, up to 5A, and highly efficient operation. The LM2678 is part of the SIMPLE SWITCHER family of power converters. A complete design uses a minimum number of external components, which have been predetermined from a variety of manufacturers. Using either this data sheet or a design software program called LM267X Made Simple (version 2.0) a complete switching power supply can be designed quickly. The software is provided free of charge and can be downloaded from National Semiconductor’s Internet site located at http://www.national.com. FEEDBACK This is the input to a two-stage high gain amplifier, which drives the PWM controller. It is necessary to connect pin 6 to the actual output of the power supply to set the dc output voltage. For the fixed output devices (3.3V, 5V and 12V outputs), a direct wire connection to the output is all that is required as internal gain setting resistors are provided inside the LM2678. For the adjustable output version two external resistors are required to set the dc output voltage. For stable operation of the power supply it is important to prevent coupling of any inductor flux to the feedback input. SWITCH OUTPUT This is the output of a power MOSFET switch connected directly to the input voltage. The switch provides energy to an inductor, an output capacitor and the load circuitry under control of an internal pulse-width-modulator (PWM). The PWM controller is internally clocked by a fixed 260KHz oscillator. In a standard step-down application the duty cycle (Time ON/Time OFF) of the power switch is proportional to the ratio of the power supply output voltage to the input voltage. The voltage on pin 1 switches between Vin (switch ON) and below ground by the voltage drop of the external Schottky diode (switch OFF). ON/OFF This input provides an electrical ON/OFF control of the power supply. Connecting this pin to ground or to any voltage less than 0.8V will completely turn OFF the regulator. The current drain from the input supply when OFF is only 50µA. Pin 7 has an internal pull-up current source of approximately 20µA and a protection clamp zener diode of 7V to ground. When electrically driving the ON/OFF pin the high voltage level for the ON condition should not exceed the 6V absolute maximum limit. When ON/OFF control is not required pin 7 should be left open circuited. INPUT The input voltage for the power supply is connected to pin 2. In addition to providing energy to the load the input voltage also provides bias for the internal circuitry of the LM2678. For guaranteed performance the input voltage must be in the range of 8V to 40V. For best performance of the power supply the input pin should always be bypassed with an input capacitor located close to pin 2. DAP (LLP PACKAGE) The Die Attach Pad (DAP) can and should be connected to PCB Ground plane/island. For CAD and assembly guidelines refer to Application Note AN-1187 at http:// power.national.com. C BOOST A capacitor must be connected from pin 3 to the switch output, pin 1. This capacitor boosts the gate drive to the internal MOSFET above Vin to fully turn it ON. This minimizes conduction losses in the power switch to maintain high efficiency. The recommended value for C Boost is 0.01µF. DESIGN CONSIDERATIONS 10088607 FIGURE 1. Basic circuit for fixed output voltage applications. www.national.com 10 LM2678 Application Hints (Continued) 10088608 FIGURE 2. Basic circuit for adjustable output voltage applications Power supply design using the LM2678 is greatly simplified by using recommended external components. A wide range of inductors, capacitors and Schottky diodes from several manufacturers have been evaluated for use in designs that cover the full range of capabilities (input voltage, output voltage and load current) of the LM2678. A simple design procedure using nomographs and component tables provided in this data sheet leads to a working design with very little effort. Alternatively, the design software, LM267X Made Simple (version 6.0), can also be used to provide instant component selection, circuit performance calculations for evaluation, a bill of materials component list and a circuit schematic. The individual components from the various manufacturers called out for use are still just a small sample of the vast array of components available in the industry. While these components are recommended, they are not exclusively the only components for use in a design. After a close comparison of component specifications, equivalent devices from other manufacturers could be substituted for use in an application. Important considerations for each external component and an explanation of how the nomographs and selection tables were developed follows. input and output voltage and load current. Several part types are offered for a given amount of inductance. Both surface mount and through-hole devices are available. The inductors from each of the three manufacturers have unique characteristics. Renco: ferrite stick core inductors; benefits are typically lowest cost and can withstand ripple and transient peak currents above the rated value. These inductors have an external magnetic field, which may generate EMI. Pulse Engineering: powdered iron toroid core inductors; these also can withstand higher than rated currents and, being toroid inductors, will have low EMI. Coilcraft: ferrite drum core inductors; these are the smallest physical size inductors and are available only as surface mount components. These inductors also generate EMI but less than stick inductors. OUTPUT CAPACITOR The output capacitor acts to smooth the dc output voltage and also provides energy storage. Selection of an output capacitor, with an associated equivalent series resistance (ESR), impacts both the amount of output ripple voltage and stability of the control loop. The output ripple voltage of the power supply is the product of the capacitor ESR and the inductor ripple current. The capacitor types recommended in the tables were selected for having low ESR ratings. In addition, both surface mount tantalum capacitors and through-hole aluminum electrolytic capacitors are offered as solutions. Impacting frequency stability of the overall control loop, the output capacitance, in conjunction with the inductor, creates a double pole inside the feedback loop. In addition the capacitance and the ESR value create a zero. These frequency response effects together with the internal frequency compensation circuitry of the LM2678 modify the gain and phase shift of the closed loop system. As a general rule for stable switching regulator circuits it is desired to have the unity gain bandwidth of the circuit to be limited to no more than one-sixth of the controller switching INDUCTOR The inductor is the key component in a switching regulator. For efficiency the inductor stores energy during the switch ON time and then transfers energy to the load while the switch is OFF. Nomographs are used to select the inductance value required for a given set of operating conditions. The nomographs assume that the circuit is operating in continuous mode (the current flowing through the inductor never falls to zero). The magnitude of inductance is selected to maintain a maximum ripple current of 30% of the maximum load current. If the ripple current exceeds this 30% limit the next larger value is selected. The inductors offered have been specifically manufactured to provide proper operation under all operating conditions of 11 www.national.com LM2678 Application Hints The average current through the catch diode is dependent on the switch duty cycle (D) and is equal to the load current times (1-D). Use of a diode rated for much higher current than is required by the actual application helps to minimize the voltage drop and power loss in the diode. (Continued) frequency. With the fixed 260KHz switching frequency of the LM2678, the output capacitor is selected to provide a unity gain bandwidth of 40KHz maximum. Each recommended capacitor value has been chosen to achieve this result. In some cases multiple capacitors are required either to reduce the ESR of the output capacitor, to minimize output ripple (a ripple voltage of 1% of Vout or less is the assumed performance condition), or to increase the output capacitance to reduce the closed loop unity gain bandwidth (to less than 40KHz). When parallel combinations of capacitors are required it has been assumed that each capacitor is the exact same part type. The RMS current and working voltage (WV) ratings of the output capacitor are also important considerations. In a typical step-down switching regulator, the inductor ripple current (set to be no more than 30% of the maximum load current by the inductor selection) is the current that flows through the output capacitor. The capacitor RMS current rating must be greater than this ripple current. The voltage rating of the output capacitor should be greater than 1.3 times the maximum output voltage of the power supply. If operation of the system at elevated temperatures is required, the capacitor voltage rating may be de-rated to less than the nominal room temperature rating. Careful inspection of the manufacturer’s specification for de-rating of working voltage with temperature is important. INPUT CAPACITOR Fast changing currents in high current switching regulators place a significant dynamic load on the unregulated power source. An input capacitor helps to provide additional current to the power supply as well as smooth out input voltage variations. Like the output capacitor, the key specifications for the input capacitor are RMS current rating and working voltage. The RMS current flowing through the input capacitor is equal to one-half of the maximum dc load current so the capacitor should be rated to handle this. Paralleling multiple capacitors proportionally increases the current rating of the total capacitance. The voltage rating should also be selected to be 1.3 times the maximum input voltage. Depending on the unregulated input power source, under light load conditions the maximum input voltage could be significantly higher than normal operation and should be considered when selecting an input capacitor. The input capacitor should be placed very close to the input pin of the LM2678. Due to relative high current operation with fast transient changes, the series inductance of input connecting wires or PCB traces can create ringing signals at the input terminal which could possibly propagate to the output or other parts of the circuitry. It may be necessary in some designs to add a small valued (0.1µF to 0.47µF) ceramic type capacitor in parallel with the input capacitor to prevent or minimize any ringing. CATCH DIODE When the power switch in the LM2678 turns OFF, the current through the inductor continues to flow. The path for this current is through the diode connected between the switch output and ground. This forward biased diode clamps the switch output to a voltage less than ground. This negative voltage must be greater than −1V so a low voltage drop (particularly at high current levels) Schottky diode is recommended. Total efficiency of the entire power supply is significantly impacted by the power lost in the output catch diode. www.national.com During the switch ON time the diode will be reversed biased by the input voltage. The reverse voltage rating of the diode should be at least 1.3 times greater than the maximum input voltage. BOOST CAPACITOR The boost capacitor creates a voltage used to overdrive the gate of the internal power MOSFET. This improves efficiency by minimizing the on resistance of the switch and associated power loss. For all applications it is recommended to use a 0.01µF/50V ceramic capacitor. ADDITIONAL APPLICATION INFORMATION When the output voltage is greater than approximately 6V, and the duty cycle at minimum input voltage is greater than approximately 50%, the designer should exercise caution in selection of the output filter components. When an application designed to these specific operating conditions is subjected to a current limit fault condition, it may be possible to observe a large hysteresis in the current limit. This can affect the output voltage of the device until the load current is reduced sufficiently to allow the current limit protection circuit to reset itself. Under current limiting conditions, the LM267x is designed to respond in the following manner: 1. At the moment when the inductor current reaches the current limit threshold, the ON-pulse is immediately terminated. This happens for any application condition. 2. However, the current limit block is also designed to momentarily reduce the duty cycle to below 50% to avoid subharmonic oscillations, which could cause the inductor to saturate. 3. Thereafter, once the inductor current falls below the current limit threshold, there is a small relaxation time during which the duty cycle progressively rises back above 50% to the value required to achieve regulation. If the output capacitance is sufficiently ‘large’, it may be possible that as the output tries to recover, the output capacitor charging current is large enough to repeatedly retrigger the current limit circuit before the output has fully settled. This condition is exacerbated with higher output voltage settings because the energy requirement of the output capacitor varies as the square of the output voltage (1⁄2CV2), thus requiring an increased charging current. A simple test to determine if this condition might exist for a suspect application is to apply a short circuit across the output of the converter, and then remove the shorted output condition. In an application with properly selected external components, the output will recover smoothly. Practical values of external components that have been experimentally found to work well under these specific operating conditions are COUT = 47µF, L = 22µH. It should be noted that even with these components, for a device’s current limit of ICLIM, the maximum load current under which the possibility of the large current limit hysteresis can be minimized is ICLIM/2. For example, if the input is 24V and the set output voltage is 18V, then for a desired maximum current of 1.5A, the current limit of the chosen switcher must be confirmed to be at least 3A. SIMPLE DESIGN PROCEDURE 12 2 x 560µF/35V Panasonic HFQ (code C5) Step 5: Use Table 4 to select an input capacitor. With 3.3V output and 15µH there are three through-hole solutions. These capacitors provide a sufficient voltage rating and an rms current rating greater than 2A (1/2 Iload max). Again using Table 2 for specific component characteristics the following choices are suitable: (Continued) Using the nomographs and tables in this data sheet (or use the available design software at http://www.national.com) a complete step-down regulator can be designed in a few simple steps. Step 1: Define the power supply operating conditions: Required output voltage Maximum DC input voltage 2 x 680µF/63V Sanyo MV-GX (code C13) 1 x 1200µF/63V Nichicon PL (code C25) 1 x 1500µF/63V Panasonic HFQ (code C16) Maximum output load current Step 2: Set the output voltage by selecting a fixed output LM2678 (3.3V, 5V or 12V applications) or determine the required feedback resistors for use with the adjustable LM2678−ADJ Step 6: From Table 5 a 5A or more Schottky diode must be selected. For through-hole components only 40V rated diodes are indicated and 4 part types are suitable: 1N5825 MBR745 80SQ045 Step 3: Determine the inductor required by using one of the four nomographs, Figure 3 through Figure 6. Table 1 provides a specific manufacturer and part number for the inductor. Step 4: Using Table 3 (fixed output voltage) or Table 6 (adjustable output voltage), determine the output capacitance required for stable operation. Table 2 provides the specific capacitor type from the manufacturer of choice. 6TQ045 Step 7: A 0.01µF capacitor will be used for Cboost. ADJUSTABLE OUTPUT DESIGN EXAMPLE In this example it is desired to convert the voltage from a two battery automotive power supply (voltage range of 20V to 28V, typical in large truck applications) to the 14.8VDC alternator supply typically used to power electronic equipment from single battery 12V vehicle systems. The load current required is 3.5A maximum. It is also desired to implement the power supply with all surface mount components. Step 1: Operating conditions are: Step 5: Determine an input capacitor from Table 4 for fixed output voltage applications. Use Table 2 to find the specific capacitor type. For adjustable output circuits select a capacitor from Table 2 with a sufficient working voltage (WV) rating greater than Vin max, and an rms current rating greater than one-half the maximum load current (2 or more capacitors in parallel may be required). Step 6: Select a diode from Table 5. The current rating of the diode must be greater than I load max and the Reverse Voltage rating must be greater than Vin max. Step 7: Include a 0.01µF/50V capacitor for Cboost in the design. FIXED OUTPUT VOLTAGE DESIGN EXAMPLE A system logic power supply bus of 3.3V is to be generated from a wall adapter which provides an unregulated DC voltage of 13V to 16V. The maximum load current is 4A. Through-hole components are preferred. Step 1: Operating conditions are: Vout = 3.3V Vin max = 16V Iload max = 4A Step 2: Select an LM2678T-3.3. The output voltage will have a tolerance of ± 2% at room temperature and ± 3% over the full operating temperature range. Step 3: Use the nomograph for the 3.3V device ,Figure 3. The intersection of the 16V horizontal line (Vin max) and the 4A vertical line (Iload max) indicates that L46, a 15µH inductor, is required. From Table 1, L46 in a through-hole component is available from Renco with part number RL-1283-15-43. Step 4: Use Table 3 to determine an output capacitor. With a 3.3V output and a 15µH inductor there are four through-hole output capacitor solutions with the number of same type capacitors to be paralleled and an identifying capacitor code given. Table 2 provides the actual capacitor characteristics. Any of the following choices will work in the circuit: 2 x 220µF/10V Sanyo OS-CON (code C5) 2 x 820µF/16V Sanyo MV-GX (code C5) 1 x 3900µF/10V Nichicon PL (code C7) Vout = 14.8V Vin max = 28V Iload max = 3.5A Step 2: Select an LM2678S-ADJ. To set the output voltage to 14.9V two resistors need to be chosen (R1 and R2 in Figure 2). For the adjustable device the output voltage is set by the following relationship: Where VFB is the feedback voltage of typically 1.21V. A recommended value to use for R1 is 1K. In this example then R2 is determined to be: R2 = 11.23KΩ The closest standard 1% tolerance value to use is 11.3KΩ This will set the nominal output voltage to 14.88V which is within 0.5% of the target value. Step 3: To use the nomograph for the adjustable device, Figure 6, requires a calculation of the inductor Volt • microsecond constant (E • T expressed in V • µS) from the following formula: where VSAT is the voltage drop across the internal power switch which is Rds(ON) times Iload. In this example this would be typically 0.12Ω x 3.5A or 0.42V and VD is the voltage drop across the forward bisased Schottky diode, typically 0.5V. 13 www.national.com LM2678 Application Hints LM2678 Application Hints Step 4: Use Table 6 to determine an output capacitor. With a 14.8V output the 12.5 to 15V row is used and with a 47µH inductor there are three surface mount output capacitor solutions. Table 2 provides the actual capacitor characteristics based on the C Code number. Any of the following choices can be used: (Continued) The switching frequency of 260KHz is the nominal value to use to estimate the ON time of the switch during which energy is stored in the inductor. For this example E • T is found to be: 1 x 33µF/20V AVX TPS (code C6) 1 x 47µF/20V Sprague 594 (code C8) 1 x 47µF/20V Kemet T495 (code C8) Important Note: When using the adjustable device in low voltage applications (less than 3V output), if the nomograph, Figure 6, selects an inductance of 22µH or less, Table 6 does not provide an output capacitor solution. With these conditions the number of output capacitors required for stable operation becomes impractical. It is recommended to use either a 33µH or 47µH inductor and the output capacitors from Table 6. Using Figure 6, the intersection of 27V • µS horizontally and the 3.5A vertical line (Iload max) indicates that L48 , a 47µH inductor, or L49, a 33µH inductor could be used. Either inductor will be suitable, but for this example selecting the larger inductance will result in lower ripple current. From Table 1, L48 in a surface mount component is available from Pulse Engineering with part number P0848. www.national.com 14 MBRD1545CT 6TQ045S (Continued) Step 5: An input capacitor for this example will require at least a 35V WV rating with an rms current rating of 1.75A (1/2 Iout max). From Table 2 it can be seen that C12, a 33µF/35V capacitor from Sprague, has the highest voltage/ current rating of the surface mount components and that two of these capacitor in parallel will be adquate. Step 7: A 0.01µF capacitor will be used for Cboost. LLP PACKAGE DEVICES The LM2678 is offered in the 14 lead LLP surface mount package to allow for a significantly decreased footprint with equivalent power dissipation compared to the TO-263. For details on mounting and soldering specifications, refer to Application Note AN-1187. Step 6: From Table 5 a 5A or more Schottky diode must be selected. For surface mount diodes with a margin of safety on the voltage rating one of two diodes can be used: 15 www.national.com LM2678 Application Hints LM2678 Inductor Selection Guides For Continuous Mode Operation 10088621 10088622 FIGURE 3. LM2678-3.3 FIGURE 4. LM2678-5.0 10088624 10088623 FIGURE 6. LM2678-ADJ FIGURE 5. LM2678-12 www.national.com 16 LM2678 Inductor Selection Guides For Continuous Mode Operation (Continued) TABLE 1. Inductor Manufacturer Part Numbers Inductor Inductance Reference (µH) Number Renco Pulse Engineering Current (A) Through Hole Surface Mount Through Hole Surface Mount Coilcraft Surface Mount L23 33 1.35 RL-5471-7 RL1500-33 PE-53823 PE-53823S DO3316-333 L24 22 1.65 RL-1283-22-43 RL1500-22 PE-53824 PE-53824S DO3316-223 L25 15 2.00 RL-1283-15-43 RL1500-15 PE-53825 PE-53825S DO3316-153 L29 100 1.41 RL-5471-4 RL-6050-100 PE-53829 PE-53829S DO5022P-104 L30 68 1.71 RL-5471-5 RL6050-68 PE-53830 PE-53830S DO5022P-683 L31 47 2.06 RL-5471-6 RL6050-47 PE-53831 PE-53831S DO5022P-473 L32 33 2.46 RL-5471-7 RL6050-33 PE-53932 PE-53932S DO5022P-333 L33 22 3.02 RL-1283-22-43 RL6050-22 PE-53933 PE-53933S DO5022P-223 L34 15 3.65 RL-1283-15-43 — PE-53934 PE-53934S DO5022P-153 L38 68 2.97 RL-5472-2 — PE-54038 PE-54038S L39 47 3.57 RL-5472-3 — PE-54039 PE-54039S — L40 33 4.26 RL-1283-33-43 — PE-54040 PE-54040S — L41 22 5.22 RL-1283-22-43 — PE-54041 P0841 — L44 68 3.45 RL-5473-3 — PE-54044 L45 10 4.47 RL-1283-10-43 — — P0845 DO5022P-103HC L46 15 5.60 RL-1283-15-43 — — P0846 DO5022P-153HC L47 10 5.66 RL-1283-10-43 — — P0847 DO5022P-103HC L48 47 5.61 RL-1282-47-43 — — P0848 — L49 33 5.61 RL-1282-33-43 — — P0849 — — — — Inductor Manufacturer Contact Numbers Coilcraft Coilcraft, Europe Pulse Engineering Phone (800) 322-2645 FAX (708) 639-1469 Phone +44 1236 730 595 FAX +44 1236 730 627 Phone (619) 674-8100 FAX (619) 674-8262 Pulse Engineering, Phone +353 93 24 107 Europe FAX +353 93 24 459 Renco Electronics Phone (800) 645-5828 FAX (516) 586-5562 17 www.national.com LM2678 Capacitor Selection Guides TABLE 2. Input and Output Capacitor Codes Capacitor Reference Code www.national.com Surface Mount AVX TPS Series C (µF) WV (V) C1 330 6.3 C2 100 C3 220 C4 Irms (A) Sprague 594D Series C (µF) WV (V) 1.15 120 6.3 10 1.1 220 10 1.15 68 47 16 0.89 C5 100 16 C6 33 C7 68 C8 22 Irms (A) Kemet T495 Series C (µF) WV (V) Irms (A) 1.1 100 6.3 0.82 6.3 1.4 220 6.3 1.1 10 1.05 330 6.3 1.1 150 10 1.35 100 10 1.1 1.15 47 16 1 150 10 1.1 20 0.77 100 16 1.3 220 10 1.1 20 0.94 180 16 1.95 33 20 0.78 25 0.77 47 20 1.15 47 20 0.94 C9 10 35 0.63 33 25 1.05 68 20 0.94 C10 22 35 0.66 68 25 1.6 10 35 0.63 C11 15 35 0.75 22 35 0.63 C12 33 35 1 4.7 50 0.66 C13 15 50 0.9 18 LM2678 Capacitor Selection Guides (Continued) Input and Output Capacitor Codes (continued) Through Hole Capacitor Sanyo OS-CON SA Series Sanyo MV-GX Series Nichicon PL Series Reference Irms Irms Irms Code C (µF) WV (V) (A) C (µF) WV (V) (A) C (µF) WV (V) (A) Panasonic HFQ Series C (µF) WV (V) Irms (A) C1 47 6.3 1 1000 6.3 0.8 680 10 0.8 82 35 0.4 C2 150 6.3 1.95 270 16 0.6 820 10 0.98 120 35 0.44 C3 330 6.3 2.45 470 16 0.75 1000 10 1.06 220 35 0.76 C4 100 10 1.87 560 16 0.95 1200 10 1.28 330 35 1.01 C5 220 10 2.36 820 16 1.25 2200 10 1.71 560 35 1.4 C6 33 16 0.96 1000 16 1.3 3300 10 2.18 820 35 1.62 C7 100 16 1.92 150 35 0.65 3900 10 2.36 1000 35 1.73 C8 150 16 2.28 470 35 1.3 6800 10 2.68 2200 35 2.8 C9 100 20 2.25 680 35 1.4 180 16 0.41 56 50 0.36 C10 47 25 2.09 1000 35 1.7 270 16 0.55 100 50 0.5 C11 220 63 0.76 470 16 0.77 220 50 0.92 C12 470 63 1.2 680 16 1.02 470 50 1.44 C13 680 63 1.5 820 16 1.22 560 50 1.68 C14 1000 63 1.75 1800 16 1.88 1200 50 2.22 C15 220 25 0.63 330 63 1.42 C16 220 35 0.79 1500 63 2.51 C17 560 35 1.43 C18 2200 35 2.68 C19 150 50 0.82 C20 220 50 1.04 C21 330 50 1.3 C22 100 63 0.75 C23 390 63 1.62 C24 820 63 2.22 C25 1200 63 2.51 Capacitor Manufacturer Contact Numbers Nichicon Panasonic AVX Sprague/Vishay Sanyo Kemet Phone (847) 843-7500 FAX (847) 843-2798 Phone (714) 373-7857 FAX (714) 373-7102 Phone (845) 448-9411 FAX (845) 448-1943 Phone (207) 324-4140 FAX (207) 324-7223 Phone (619) 661-6322 FAX (619) 661-1055 Phone (864) 963-6300 FAX (864) 963-6521 19 www.national.com LM2678 Capacitor Selection Guides (Continued) TABLE 3. Output Capacitors for Fixed Output Voltage Application Surface Mount Output Inductance Voltage (V) (µH) 3.3 5 12 AVX TPS Series Sprague 594D Series Kemet T495 Series No. C Code No. C Code No. C Code 10 5 C1 5 C1 5 C2 15 4 C1 4 C1 4 C3 22 3 C2 2 C7 3 C4 33 1 C1 2 C7 3 C4 10 4 C2 4 C6 4 C4 15 3 C3 2 C7 3 C5 22 3 C2 2 C7 3 C4 33 2 C2 2 C3 2 C4 47 2 C2 1 C7 2 C4 10 4 C5 3 C6 5 C9 15 3 C5 2 C7 4 C9 22 2 C5 2 C6 3 C8 33 2 C5 1 C7 3 C8 47 2 C4 1 C6 2 C8 68 1 C5 1 C5 2 C7 100 1 C4 1 C5 1 C8 Through Hole Output Inductance Voltage (V) (µH) 3.3 5 12 Sanyo OS-CON SA Series Sanyo MV-GX Series Nichicon PL Series No. C Code No. C Code No. C Code No. C Code 10 2 C5 2 C6 1 C8 2 C6 15 2 C5 2 C5 1 C7 2 C5 22 1 C5 1 C10 1 C5 1 C7 33 1 C5 1 C10 1 C5 1 C7 10 2 C4 2 C5 1 C6 2 C5 15 1 C5 1 C10 1 C5 1 C7 22 1 C5 1 C9 1 C5 1 C5 33 1 C4 1 C5 1 C4 1 C4 47 1 C4 1 C4 1 C2 2 C4 10 2 C7 1 C10 1 C14 2 C4 15 1 C8 1 C6 1 C17 1 C5 22 1 C7 1 C5 1 C13 1 C5 33 1 C7 1 C4 1 C12 1 C4 47 1 C7 1 C3 1 C11 1 C3 68 1 C6 1 C2 1 C10 1 C3 100 1 C6 1 C2 1 C9 1 C1 No. represents the number of identical capacitor types to be connected in parallel C Code indicates the Capacitor Reference number in Table 2 for identifying the specific component from the manufacturer. www.national.com Panasonic HFQ Series 20 LM2678 Capacitor Selection Guides (Continued) TABLE 4. Input Capacitors for Fixed Output Voltage Application (Assumes worst case maximum input voltage and load current for a given inductance value) Surface Mount Output Inductance Voltage (V) (µH) 3.3 5 12 AVX TPS Series Sprague 594D Series Kemet T495 Series No. C Code No. C Code No. 10 3 C7 2 C10 3 C Code C9 15 * * 3 C13 4 C12 22 * * 2 C13 3 C12 33 * * 2 C13 3 C12 10 3 C4 2 C6 3 C9 15 4 C9 3 C12 4 C10 22 * * 3 C13 4 C12 33 * * 2 C13 3 C12 47 * * 1 C13 2 C12 10 4 C9 2 C10 4 C10 15 4 C8 2 C10 4 C10 22 4 C9 3 C12 4 C10 33 * * 3 C13 4 C12 47 * * 2 C13 3 C12 68 * * 2 C13 2 C12 100 * * 1 C13 2 C12 Through Hole Output Inductance Voltage (V) (µH) 3.3 5 12 Sanyo OS-CON SA Series Sanyo MV-GX Series Nichicon PL Series Panasonic HFQ Series No. C Code No. C Code No. C Code No. C Code 10 2 C9 2 C8 1 C18 1 C8 15 * * 2 C13 1 C25 1 C16 22 * * 1 C14 1 C24 1 C16 33 * * 1 C14 1 C24 1 C16 10 2 C7 2 C8 1 C25 1 C8 15 * * 2 C8 1 C25 1 C8 22 * * 2 C13 1 C25 1 C16 33 * * 1 C14 1 C23 1 C13 47 * * 1 C12 1 C19 1 C11 10 2 C10 2 C8 1 C18 1 C8 15 2 C10 2 C8 1 C18 1 C8 22 * * 2 C8 1 C18 1 C8 33 * * 2 C12 1 C24 1 C14 47 * * 1 C14 1 C23 1 C13 68 * * 1 C13 1 C21 1 C15 100 * * 1 C11 1 C22 1 C11 * Check voltage rating of capacitors to be greater than application input voltage. No. represents the number of identical capacitor types to be connected in parallel C Code indicates the Capacitor Reference number in Table 2 for identifying the specific component from the manufacturer. 21 www.national.com LM2678 Capacitor Selection Guides (Continued) TABLE 5. Schottky Diode Selection Table Reverse Voltage (V) 3A 20V SK32 30V SK33 Surface Mount 5A or More Through Hole 3A 5A or More 1N5820 SR302 MBRD835L 30WQ03F 40V 1N5821 31DQ03 SK34 MBRD1545CT 1N5822 30BQ040 6TQ045S 1N5825 MBR340 MBR745 30WQ04F 31DQ04 80SQ045 MBRS340 SR403 6TQ045 MBRD340 50V or More SK35 MBR350 30WQ05F 31DQ05 SR305 Diode Manufacturer Contact Numbers International Rectifier Motorola General Semiconductor Diodes, Inc. www.national.com Phone (310) 322-3331 FAX (310) 322-3332 Phone (800) 521-6274 FAX (602) 244-6609 Phone (516) 847-3000 FAX (516) 847-3236 Phone (805) 446-4800 FAX (805) 446-4850 22 LM2678 Capacitor Selection Guides (Continued) TABLE 6. Output Capacitors for Adjustable Output Voltage Applications Surface Mount Output Voltage (V) 1.21 to 2.50 2.5 to 3.75 3.75 to 5 5 to 6.25 6.25 to 7.5 7.5 to 10 10 to 12.5 12.5 to 15 15 to 20 20 to 30 30 to 37 Inductance (µH) AVX TPS Series Sprague 594D Series Kemet T495 Series No. C Code No. C Code No. C Code 33* 7 C1 6 C2 7 C3 47* 5 C1 4 C2 5 C3 33* 4 C1 3 C2 4 C3 47* 3 C1 2 C2 3 C3 22 4 C1 3 C2 4 C3 33 3 C1 2 C2 3 C3 47 2 C1 2 C2 2 C3 22 3 C2 3 C3 3 C4 33 2 C2 2 C3 2 C4 47 2 C2 2 C3 2 C4 68 1 C2 1 C3 1 C4 22 3 C2 1 C4 3 C4 33 2 C2 1 C3 2 C4 47 1 C3 1 C4 1 C6 68 1 C2 1 C3 1 C4 33 2 C5 1 C6 2 C8 47 1 C5 1 C6 2 C8 68 1 C5 1 C6 1 C8 100 1 C4 1 C5 1 C8 33 1 C5 1 C6 2 C8 47 1 C5 1 C6 2 C8 68 1 C5 1 C6 1 C8 100 1 C5 1 C6 1 C8 33 1 C6 1 C8 1 C8 47 1 C6 1 C8 1 C8 68 1 C6 1 C8 1 C8 100 1 C6 1 C8 1 C8 33 1 C8 1 C10 2 C10 47 1 C8 1 C9 2 C10 68 1 C8 1 C9 2 C10 100 1 C8 1 C9 1 C10 33 2 C9 2 C11 2 C11 47 1 C10 1 C12 1 C11 68 1 C9 1 C12 1 C11 100 1 C9 1 C12 1 C11 10 4 C13 8 C12 15 3 C13 5 C12 2 C13 4 C12 33 1 C13 3 C12 47 1 C13 2 C12 68 1 C13 2 C12 22 No Values Available 23 www.national.com LM2678 Capacitor Selection Guides (Continued) Output Capacitors for Adjustable Output Voltage Applications (continued) Through Hole Output Voltage (V) 1.21 to 2.50 2.5 to 3.75 3.75 to 5 5 to 6.25 6.25 to 7.5 7.5 to 10 10 to 12.5 12.5 to 15 15 to 20 Inductance (µH) Sanyo OS-CON SA Series 30 to 37 Nichicon PL Series No. C Code No. C Code No. C Code No. 2 C3 5 C1 5 C3 3 C 47* 2 C2 4 C1 3 C3 2 C5 33* 1 C3 3 C1 3 C1 2 C5 47* 1 C2 2 C1 2 C3 1 C5 22 1 C3 3 C1 3 C1 2 C5 33 1 C2 2 C1 2 C1 1 C5 47 1 C2 2 C1 1 C3 1 C5 22 1 C5 2 C6 2 C3 2 C5 33 1 C4 1 C6 2 C1 1 C5 47 1 C4 1 C6 1 C3 1 C5 68 1 C4 1 C6 1 C1 1 C5 22 1 C5 1 C6 2 C1 1 C5 33 1 C4 1 C6 1 C3 1 C5 47 1 C4 1 C6 1 C1 1 C5 68 1 C4 1 C2 1 C1 1 C5 33 1 C7 1 C6 1 C14 1 C5 47 1 C7 1 C6 1 C14 1 C5 68 1 C7 1 C2 1 C14 1 C2 100 1 C7 1 C2 1 C14 1 C2 33 1 C7 1 C6 1 C14 1 C5 47 1 C7 1 C2 1 C14 1 C5 C Code 68 1 C7 1 C2 1 C9 1 C2 100 1 C7 1 C2 1 C9 1 C2 33 1 C9 1 C10 1 C15 1 C2 47 1 C9 1 C10 1 C15 1 C2 68 1 C9 1 C10 1 C15 1 C2 100 1 C9 1 C10 1 C15 1 C2 33 1 C10 1 C7 1 C15 1 C2 47 1 C10 1 C7 1 C15 1 C2 68 1 C10 1 C7 1 C15 1 C2 100 1 C10 1 C7 1 C15 1 C2 47 No Values 68 Available 1 C7 1 C16 1 C2 1 C7 1 C16 1 C2 1 C7 1 C16 1 C2 100 1 C7 1 C16 1 C2 10 1 C12 1 C20 1 C10 15 1 C11 1 C20 1 C11 1 C11 1 C20 1 C10 22 No Values 33 Available 1 C11 1 C20 1 C10 47 1 C11 1 C20 1 C10 68 1 C11 1 C20 1 C10 * Set to a higher value for a practical design solution. See Applications Hints section No. represents the number of identical capacitor types to be connected in parallel C Code indicates the Capacitor Reference number in Table 2 for identifying the specific component from the manufacturer. www.national.com Panasonic HFQ Series 33* 33 20 to 30 Sanyo MV-GX Series 24 LM2678 Physical Dimensions inches (millimeters) unless otherwise noted TO-263 Surface Mount Power Package Order Number LM2678S-3.3, LM2678S-5.0, LM2678S-12 or LM2678S-ADJ NS Package Number TS7B 25 www.national.com LM2678 Physical Dimensions inches (millimeters) unless otherwise noted (Continued) TO-220 Power Package Order Number LM2678T-3.3, LM2678T-5.0, LM2678T-12 or LM2678T-ADJ NS Package Number TA07B 14-Lead LLP Package NS Package Number SRC14A www.national.com 26 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. 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