Advance Easy Step-Down SwitcherInformationVoltage ™3.0 RegulatorAThe LM2576 series of regulators are monolithic integrated circuits ideallysuited for easy and convenient design of a step–down switching regulator(buck converter). All circuits of this series are capable of driving a 3.0 A loadwith excellent line and load regulation. These devices are available in fixedoutput voltages of 3.3 V, 5.0 V, 12 V, 15 V, and an adjustable output version.These regulators were designed to minimize the number of externalcomponents to simplify the power supply design. Standard series ofinductors optimized for use with the LM2576 are offered by several differentinductor manufacturers.Since the LM2576 converter is a switch–mode power supply, its efficiencyis significantly higher in comparison with popular three–terminal linearregulators, especially with higher input voltages. In many cases, the powerdissipated is so low that no heatsink is required or its size could be reduceddramatically.A standard series of inductors optimized for use with the LM2576 areavailable from several different manufacturers. This feature greatly simplifiesthe design of switch–mode power supplies.The LM2576 features include a guaranteed ±4% tolerance on outputvoltage within specified input voltages and output load conditions, and ±10%on the oscillator frequency (±2% over 0°C to 125°C). External shutdown isincluded, featuring 80 µA (typical) standby current. The output switchincludes cycle–by–cycle current limiting, as well as thermal shutdown for fullprotection under fault conditions.Features•3.3 V, 5.0 V, 12 V, 15 V, and Adjustable Output Versions•Adjustable Version Output Voltage Range, 1.23 to 37 V ±4% MaximumOver Line and Load Conditions•Guaranteed 3.0 A Output Current•Wide Input Voltage Range•Requires Only 4 External Components•52 kHz Fixed Frequency Internal Oscillator•TTL Shutdown Capability, Low Power Standby Mode•High Efficiency•Uses Readily Available Standard Inductors•Thermal Shutdown and Current Limit ProtectionApplications•Simple High–Efficiency Step–Down (Buck) Regulator•Efficient Pre–Regulator for Linear Regulators•On–Card Switching Regulators•Positive to Negative Converter (Buck–Boost)•Negative Step–Up Converters•Power Supply for Battery ChargersThis document contains information on a new product. Specifications and information hereinare subject to change without notice.MOTOROLA ANALOG IC DEVICE DATAOrder this document by LM2576/DLM2576EASY SWITCHER™3.0 A STEP–DOWNVOLTAGE REGULATORSEMICONDUCTORTECHNICAL DATAT SUFFIXPLASTIC PACKAGECASE 314DPin1.V12.Outputin3.Ground54.Feedback5.ON/OFFTV SUFFIX1PLASTIC PACKAGECASE 314B5Heatsink surfaceconnected to Pin 3.D2T SUFFIXPLASTIC PACKAGECASE 936A1(D2PAK)5Heatsink surface (shown as terminal 6 in case outlinedrawing) is connected to Pin 3.DEVICE TYPE/NOMINAL OUTPUT VOLTAGELM2576–3.33.3 VLM2576–55.0 VLM2576–1212 VLM2576–1515 VLM2576–ADJ1.23 V to 37 VORDERING INFORMATIONOperatingDeviceTemperature RangePackageLM2576T–XXStraight LeadLM2576TV–XXTJ = –40° to +125°CVertical MountLM2576D2T–XXSurface MountXX = Voltage Option, i.e. 3.3, 5, 12, 15 V; and ADJ for Adjustable Output.©Motorola, Inc. 1997Rev 01LM2576Figure 1. Block Diagram and Typical ApplicationTypical Application (Fixed Output Voltage Versions)Feedback7.0 V – 40 VUnregulated DC Input+VinCin100 µF13Gnd5LM25764Output2ON/OFFL1100 µHD11N5822Cout1000 µF5.0 V Regulated Output 3.0 A LoadRepresentative Block Diagram and Typical Application+Vin1Cin4FeedbackR2Fixed GainError AmplifierComparatorFreqShift18 kHz52 kHzOscillatorCurrentLimitON/OFF5UnregulatedDC Input3.1 V InternalRegulatorON/OFFOutputVoltage Versions3.3 V5.0 V12 V15 VFor adjustable versionR1 = open, R2 = 0 ΩR2(Ω)1.7 k3.1 k8.84 k11.3 kR11.0 kDriverLatchOutput1.0 AmpSwitchResetThermalShutdown2Gnd3D1L1RegulatedOutputVoutCoutLoad1.235 VBand–GapReferenceABSOLUTE MAXIMUM RATINGS (Absolute Maximum Ratings indicate limits beyondwhich damage to the device may occur.)RatingMaximum Supply VoltageON/OFF Pin Input VoltageOutput Voltage to Ground (Steady–State)Power DissipationCase 314B and 314D (TO–220, 5–Lead)Thermal Resistance, Junction–to–AmbientThermal Resistance, Junction–to–CaseCase 936A (D2PAK)Thermal Resistance, Junction–to–AmbientThermal Resistance, Junction–to–CaseStorage Temperature RangeMinimum ESD Rating (Human Body Model:C = 100 pF, R = 1.5 kΩ)Lead Temperature (Soldering, 10 seconds)Maximum Junction TemperatureNOTE: ESD data available upon request.SymbolVin––PDRθJARθJCPDRθJARθJCTstg––TJValue45–0.3 V ≤ V ≤ +Vin–1.0Internally Limited655.0Internally Limited705.0–65 to +1502.0260150UnitVVVW°C/W°C/WW°C/W°C/W°CkV°C°C2MOTOROLA ANALOG IC DEVICE DATALM2576OPERATING RATINGS (Operating Ratings indicate conditions for which the device isintended to be functional, but do not guarantee specific performance limits. For guaranteedspecifications and test conditions, see the Electrical Characteristics.)RatingOperating Junction Temperature RangeSupply VoltageSymbolTJVinValue–40 to +12540Unit°CVSYSTEM PARAMETERS ([Note 1] Test Circuit Figure 15)the 12 V version, and Vin = 30 V for the 15 V version. ILoad = 500 mA. For typical values TJ = 25°C, for min/max values TJ is the operatingjunction temperature range that applies [Note 2], unless otherwise noted.)CharacteristicsLM2576–3.3 ([Note 1] Test Circuit Figure 15)Output Voltage (Vin = 12 V, ILoad = 0.5 A, TJ = 25°C)Output Voltage (6.0 V ≤ Vin ≤ 40 V, 0.5 A ≤ ILoad ≤ 3.0 A)TJ = 25°CTJ = –40 to +125°CEfficiency (Vin = 12 V, ILoad = 3.0 A)LM2576–5 ([Note 1] Test Circuit Figure 15)Output Voltage (Vin = 12 V, ILoad = 0.5 A, TJ = 25°C)Output Voltage (8.0 V ≤ Vin ≤ 40 V, 0.5 A ≤ ILoad ≤ 3.0 A)TJ = 25°CTJ = –40 to +125°CEfficiency (Vin = 12 V, ILoad = 3.0 A)LM2576–12 ([Note 1] Test Circuit Figure 15)Output Voltage (Vin = 25 V, ILoad = 0.5 A, TJ = 25°C)Output Voltage (15 V ≤ Vin ≤ 40 V, 0.5 A ≤ ILoad ≤ 3.0 A)TJ = 25°CTJ = –40 to +125°CEfficiency (Vin = 15 V, ILoad = 3.0 A)LM2576–15 ([Note 1] Test Circuit Figure 15)Output Voltage (Vin = 30 V, ILoad = 0.5 A, TJ = 25°C)Output Voltage (18 V ≤ Vin ≤ 40 V, 0.5 A ≤ ILoad ≤ 3.0 A)TJ = 25°CTJ = –40 to +125°CEfficiency (Vin = 18 V, ILoad = 3.0 A)LM2576 ADJUSTABLE VERSION ([Note 1] Test Circuit Figure 15)Feedback Voltage (Vin = 12 V, ILoad = 0.5 A, Vout = 5.0 V, TJ = 25°C)Feedback Voltage (8.0 V ≤ Vin ≤ 40 V, 0.5 A ≤ ILoad ≤ 3.0 A, Vout = 5.0 V)TJ = 25°CTJ = –40 to +125°CEfficiency (Vin = 12 V, ILoad = 3.0 A, Vout = 5.0 V)VoutVout1.1931.18η–1.23–771.2671.28–%1.2171.231.243VVVoutVout14.414.25η–15–8815.615.75–%14.71515.3VVVoutVout11.5211.4η–12–8812.4812.6–%11.761212.24VVVoutVout4.84.75η–5.0–775.25.25–%4.95.05.1VVVoutVout3.1683.135η–3.3–753.4323.465–%3.2343.33.366VVSymbolMinTypMaxUnitELECTRICAL CHARACTERISTICS (Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and Adjustable version, Vin = 25 V forNOTES:1.External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2576 is used as shown in the Figure 15 test circuit, system performance will be as shown in system parameters section.2.Tested junction temperature range for the LM2576: Tlow = –40°C Thigh = +125°CMOTOROLA ANALOG IC DEVICE DATA3LM2576DEVICE PARAMETERSthe 12 V version, and Vin = 30 V for the 15 V version. ILoad = 500 mA. For typical values TJ = 25°C, for min/max values TJ is the operatingjunction temperature range that applies [Note 2], unless otherwise noted.)CharacteristicsALL OUTPUT VOLTAGE VERSIONSFeedback Bias Current (Vout = 5.0 V [Adjustable Version Only])TJ = 25°CTJ = –40 to +125°COscillator Frequency [Note 3]TJ = 25°CTJ = 0 to +125°CTJ = –40 to +125°CSaturation Voltage (Iout = 3.0 A [Note 4])TJ = 25°CTJ = –40 to +125°CMax Duty Cycle (“on”) [Note 5]Current Limit (Peak Current [Notes 3 and 4])TJ = 25°CTJ = –40 to +125°COutput Leakage Current [Notes 6 and 7], TJ = 25°COutput = 0 VOutput = –1.0 VQuiescent Current [Note 6]TJ = 25°CTJ = –40 to +125°CStandby Quiescent Current (ON/OFF Pin = 5.0 V (“off”))TJ = 25°CTJ = –40 to +125°CON/OFF Pin Logic Input Level (Test Circuit Figure 15)Vout = 0 VTJ = 25°CTJ = –40 to +125°CVout = Nominal Output VoltageTJ = 25°CTJ = –40 to +125°CON/OFF Pin Input Current (Test Circuit Figure 15)ON/OFF Pin = 5.0 V (“off”), TJ = 25°CON/OFF Pin = 0 V (“on”), TJ = 25°CIb––fosc–4742Vsat––DCICL4.23.5IL––IQ––Istby––VIH2.22.4VIL––IIHIIL––1.2–1501.00.8µA305.01.4–––80–200400V5.0–9.011µA0.86.02.020mA5.8–6.97.5mA941.5–981.82.0–%A52–––5863V25–100200kHznASymbolMinTypMaxUnitELECTRICAL CHARACTERISTICS (Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and Adjustable version, Vin = 25 V forNOTES:3.The oscillator frequency reduces to approximately 18 kHz in the event of an output short or an overload which causes the regulated output voltage todrop approximately 40% from the nominal output voltage. This self protection feature lowers the average dissipation of the IC by lowering the minimum duty cycle from 5% down to approximately 2%.4.Output (Pin 2) sourcing current. No diode, inductor or capacitor connected to output pin.5.Feedback (Pin 4) removed from output and connected to 0 V.6.Feedback (Pin 4) removed from output and connected to +12 V for the Adjustable, 3.3 V, and 5.0 V versions, and +25 V for the 12 V and 15 V versions, to force the output transistor “off”.7.Vin = 40 V.4MOTOROLA ANALOG IC DEVICE DATALM2576TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)Figure 2. Normalized Output Voltage1.0Vout, OUTPUT VOLTAGE CHANGE (%)0.80.60.40.20–0.2–0.4–0.6–0.8–1.0–50 Vin = 20 VILoad = 500 mANormalized at TJ = 25°CVout, OUTPUT VOLTAGE CHANGE (%)1.41.21.00.80.60.40.20–0.2–0.4–0.605.01015202530354012 V and 15 V3.3 V, 5.0 V and ADJILoad = 500 mATJ = 25°CFigure 3. Line Regulation–250255075100125TJ, JUNCTION TEMPERATURE (°C)Vin, INPUT VOLTAGE (V)Figure 4. Dropout Voltage2.0INPUT – OUTPUT DIFFERENTIAL (V)IO, OUTPUT CURRENT (A)ILoad = 3.0 A1.56.56.05.55.04.54.0–50Figure 5. Current LimitVin = 25 V1.0ILoad = 500 mA0.5L1 = 150 µHRind = 0.1 Ω0–50–250255075100125–250255075100125TJ, JUNCTION TEMPERATURE (°C)TJ, JUNCTION TEMPERATURE (°C)Figure 6. Quiescent CurrentIQ, QUIESCENT CURRENT (mA)18161412108.06.04.005.010152025303540ILoad = 200 mAILoad = 3.0 AVout = 5.0 VMeasured atGround PinTJ = 25°CIstby, STANDBY QUIESCENT CURRENT (µA)20200180160140120100806040200–50Figure 7. Standby Quiescent CurrentVON/OFF = 5.0 VVin = 40 VVin = 12 V–250255075100125Vin, INPUT VOLTAGE (V)TJ, JUNCTION TEMPERATURE (°C)MOTOROLA ANALOG IC DEVICE DATA5LM2576TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)Figure 8. Standby Quiescent CurrentIstby, STANDBY QUIESCENT CURRENT (µA)200160140120100806040200TJ = 25°CVsat, SATURATION VOLTAGE (V)1801.61.41.21.00.825°C0.60.40.2000.51.01.52.02.53.0125°C–40°CFigure 9. Switch Saturation Voltage0510152025303540Vin, INPUT VOLTAGE (V)SWITCH CURRENT (A)Figure 10. Oscillator Frequency8.0NORMALIZED FREQUENCY (%)6.0Vin, INPUT VOLTAGE (V)4.02.00–2.0–4.0–6.0–8.0–10–50–250255075100125Vin = 12 VNormalized at25°C5.04.54.03.53.02.52.01.51.00.50–50Figure 11. Minimum Operating VoltageAdjustable Version OnlyVout ' 1.23 VILoad = 500 mA–250255075100125TJ, JUNCTION TEMPERATURE (°C)TJ, JUNCTION TEMPERATURE (°C)Figure 12. Feedback Pin Current100Ib, FEEDBACK PIN CURRENT (nA)806040200–20–40–60–80–250255075100125Adjustable Version Only–100–50TJ, JUNCTION TEMPERATURE (°C)6MOTOROLA ANALOG IC DEVICE DATALM2576TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)Figure 13. Switching WaveformsA50 V04.0 AB2.0 A04.0 ACD2.0 A05 µs/DIVVout = 15 VA: Output Pin Voltage, 10 V/DIVB: Inductor Current, 2.0 A/DIVC: Inductor Current, 2.0 A/DIV, AC–CoupledD: Output Ripple Voltage, 50mV/dDIV, AC–CoupledHorizontal Time Base: 5 µs/DIV3.0 ALoad2.0 ACurrent1.0 A0100 µs/DIV100 mVOutput0VoltageChange– 100 mVFigure 14. Load Transient ResponseMOTOROLA ANALOG IC DEVICE DATA7LM2576Figure 15. Typical Test CircuitFixed Output Voltage VersionsFeedbackVin1LM2576Fixed Output4Output2ON/OFFD1MBR360Cout1000 µFLoadL1100 µHVout37.0 V – 40 VUnregulatedDC InputCin100 µFGnd5CinCoutD1L1R1R2––––––100 µF, 75 V, Aluminium Electrolytic1000 µF, 25 V, Aluminium ElectrolyticSchottky, MBR360100 µH, Pulse Eng. PE–921082.0 k, 0.1%6.12 k, 0.1%Adjustable Output Voltage VersionsFeedbackVin137.0 V – 40 VUnregulatedDC InputCin100 µFLM2576Adjustable4Output2ON/OFFD1MBR360Cout1000 µFR2LoadR1L1100 µHVout5,000 VGnd5Vout+VR2+R1refǒ1.0)R2ǓR1–1.0refǒVoutVǓWhere Vref = 1.23 V, R1 between 1.0 k and 5.0 kPCB LAYOUT GUIDELINESAs in any switching regulator, the layout of the printedcircuit board is very important. Rapidly switching currentsassociated with wiring inductance, stray capacitance andparasitic inductance of the printed circuit board traces cangenerate voltage transients which can generateelectromagnetic interferences (EMI) and affect the desiredoperation. As indicated in the Figure 15, to minimizeinductance and ground loops, the length of the leadsindicated by heavy lines should be kept as short as possible.For best results, single–point grounding (as indicated) orground plane construction should be used.On the other hand, the PCB area connected to the Pin 2(emitter of the internal switch) of the LM2576 should be keptto a minimum in order to minimize coupling to sensitivecircuitry.Another sensitive part of the circuit is the feedback. It isimportant to keep the sensitive feedback wiring short. Toassure this, physically locate the programming resistors nearto the regulator, when using the adjustable version of theLM2576 regulator.8MOTOROLA ANALOG IC DEVICE DATALM2576PIN FUNCTION DESCRIPTION Pin1VinSymbolDescription (Refer to Figure 1)This pin is the positive input supply for the LM2576 step–down switching regulator. In order to minimizevoltage transients and to supply the switching currents needed by the regulator, a suitable input bypasscapacitor must be present (Cin in Figure 1).This is the emitter of the internal switch. The saturation voltage Vsat of this output switch is typically 1.5 V.It should be kept in mind that the PCB area connected to this pin should be kept to a minimum in order tominimize coupling to sensitive circuitry.Circuit ground pin. See the information about the printed circuit board layout.This pin senses regulated output voltage to complete the feedback loop. The signal is divided by theinternal resistor divider network R2, R1 and applied to the non–inverting input of the internal error amplifier.In the Adjustable version of the LM2576 switching regulator this pin is the direct input of the error amplifierand the resistor network R2, R1 is connected externally to allow programming of the output voltage.It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the totalinput supply current to approximately 80 µA. The threshold voltage is typically 1.4 V. Applying a voltageabove this value (up to +Vin) shuts the regulator off. If the voltage applied to this pin is lower than 1.4 V orif this pin is left open, the regulator will be in the “on” condition.2Output34GndFeedback5ON/OFFDESIGN PROCEDUREBuck Converter BasicsThe LM2576 is a “Buck” or Step–Down Converter which isthe most elementary forward–mode converter. Its basicschematic can be seen in Figure 16.The operation of this regulator topology has two distincttime periods. The first one occurs when the series switch ison, the input voltage is connected to the input of the inductor.The output of the inductor is the output voltage, and therectifier (or catch diode) is reverse biased. During this period,since there is a constant voltage source connected acrossthe inductor, the inductor current begins to linearly rampupwards, as described by the following equation:+L(on)LDuring this “on” period, energy is stored within the corematerial in the form of magnetic flux. If the inductor is properlydesigned, there is sufficient energy stored to carry therequirements of the load during the “off” period.IDiode VoltageǒVin–VoutǓtonThis period ends when the power switch is once againturned on. Regulation of the converter is accomplished byvarying the duty cycle of the power switch. It is possible todescribe the duty cycle as follows:td+on, where T is the period of switching.TFor the buck converter with ideal components, the dutycycle can also be described as:Voutd+VinFigure 17 shows the buck converter, idealized waveformsof the catch diode voltage and the inductor current.Figure 17. Buck Converter Idealized WaveformsVon(SW)Figure 16. Basic Buck ConverterPowerSwitchLPowerSwitchOffVD(FWD)PowerSwitchOnPowerSwitchOffPowerSwitchOnVinDCoutRLoadTimeThe next period is the “off” period of the power switch.When the power switch turns off, the voltage across theinductor reverses its polarity and is clamped at one diodevoltage drop below ground by the catch diode. The currentnow flows through the catch diode thus maintaining the loadcurrent loop. This removes the stored energy from theinductor. The inductor current during this time is:IL(off)+IpkInductor CurrentILoad(AV)IminDiodePowerSwitchPowerSwitchTimeDiodeǒVout–VDǓtoffLMOTOROLA ANALOG IC DEVICE DATA9LM2576Procedure (Fixed Output Voltage Version) In order to simplify the switching regulator design, a step–by–stepdesign procedure and some examples are provided.ProcedureGiven Parameters:Vout = Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V)Vin(max) = Maximum Input VoltageILoad(max) = Maximum Load Current1.Controller IC SelectionAccording to the required input voltage, output voltage and current, select the appropriate type of the controller IC output voltage version.2.Input Capacitor Selection (Cin)To prevent large voltage transients from appearing at the input and for stable operation of the converter, an aluminium or tantalum electrolytic bypass capacitor is needed between the input pin +Vin and ground pin Gnd. This capacitor should be located close to the IC using short leads. This capacitor should have a low ESR (Equivalent Series Resistance) value.3.Catch Diode Selection (D1)A.Since the diode maximum peak current exceeds the regulator maximum load current the catch diode current rating must be at least 1.2 times greater than the maximum load current. For a robust design the diode should have a current rating equal to the maximum current limit of the LM2576 to be able to withstand a continuous output shortB.The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage.4.Inductor Selection (L1)A.According to the required working conditions, select the correct inductor value using the selection guide from Figures 18 to 22.B.From the appropriate inductor selection guide, identify the inductance region intersected by the Maximum Input Voltage line and the Maximum Load Current line. Each region is identified by an inductance value and an inductor code.C.Select an appropriate inductor from the several different manufacturers part numbers listed in Table 2. The designer must realize that the inductor current rating must be higher than the maximum peak current flowing through the inductor. This maximum peak current can be calculated as follows:Ip(max)+IExampleGiven Parameters:Vout = 5.0 VVin(max) = 15 VILoad(max) = 3.0 A1.Controller IC SelectionAccording to the required input voltage, output voltage, current polarity and current value, use the LM2576–5 controller IC2.Input Capacitor Selection (Cin)A100 µF, 25 V aluminium electrolytic capacitor located near to the input and ground pins provides sufficient bypassing.3.Catch Diode Selection (D1)A.For this example the current rating of the diode is 3.0 A.B.Use a 20 V 1N5820 Schottky diode, or any of the suggested fast recovery diodes shown in Table 1.4.Inductor Selection (L1)A.Use the inductor selection guide shown in Figures 19.B.From the selection guide, the inductance area intersected by the 15 V line and 3.0 A line is L100.C.Inductor value required is 100 µH. From Table 2, choose an inductor from any of the listed manufacturers.Load(max))ǒVin–VoutǓton2Lwhere ton is the “on” time of the power switch andVton+outx1.0VfoscinFor additional information about the inductor, see the inductor section in the “Application Hints” section of this data sheet.10MOTOROLA ANALOG IC DEVICE DATALM2576Procedure (Fixed Output Voltage Version) (continued)In order to simplify the switching regulator design, a step–by–stepdesign procedure and some examples are provided.Procedure5.Output Capacitor Selection (Cout)A.Since the LM2576 is a forward–mode switching regulator with voltage mode control, its open loop 2–pole–1–zero frequency characteristic has the dominant pole–pair determined by the output capacitor and inductor values. For stable operation and an acceptable ripple voltage, (approximately 1% of the output voltage) a value between 680 µF and 2000 µF is recommended.B.Due to the fact that the higher voltage electrolytic capacitors generally have lower ESR (Equivalent Series Resistance) numbers, the output capacitor’s voltage rating should be at least 1.5 times greater than the output voltage. For a 5.0 V regulator, a rating at least 8.0 V is appropriate, and a 10 V or 16 V rating is recommended.Example5.Output Capacitor Selection (Cout)A.Cout = 680 µF to 2000 µF standard aluminium electrolytic.B.Capacitor voltage rating = 20 V.Procedure (Adjustable Output Version: LM2576–ADJ) ProcedureGiven Parameters:Vout = Regulated Output VoltageVin(max) = Maximum DC Input VoltageILoad(max) = Maximum Load Current1.Programming Output VoltageTo select the right programming resistor R1 and R2 value (seeFigure 2) use the following formula:Vout+VrefGiven Parameters:Vout = 8.0 VVin(max) = 25 VILoad(max) = 2.5 A1.Programming Output Voltage (selecting R1 and R2)Select R1 and R2:Vout+1.231.0)R2+R1Exampleǒ1.0)R2ǓR1ǒR2R1ǓSelect R1 = 1.8 kΩ+1.8k8.0Vǒ1.23*1.0ǓVwhere Vref = 1.23 VResistor R1 can be between 1.0 k and 5.0 kΩ. (For best temperature coefficient and stability with time, use 1% metal film resistors).VoutR2+R1–1.0VrefǒVoutVref*1.0ǓǒǓR2 = 9.91 kΩ, choose a 9.88 k metal film resistor.2.Input Capacitor Selection (Cin)To prevent large voltage transients from appearing at the input and for stable operation of the converter, an aluminium or tantalum electrolytic bypass capacitor is needed between the input pin +Vin and ground pin Gnd This capacitor should be located close to the IC using short leads. This capacitor should have a low ESR (Equivalent Series Resistance) value.For additional information see input capacitor section in the “Application Hints” section of this data sheet.3.Catch Diode Selection (D1)A.Since the diode maximum peak current exceeds the regulator maximum load current the catch diode current rating must be at least 1.2 times greater than the maximum load current. For a robust design, the diode should have a current rating equal to the maximum current limit of the LM2576 to be able to withstand a continuous output short.B.The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage.2.Input Capacitor Selection (Cin)A 100 µF, 150 V aluminium electrolytic capacitor located near the input and ground pin provides sufficient bypassing.3.Catch Diode Selection (D1)A.For this example, a 3.0 A current rating is adequate.B.Use a 30 V 1N5821 Schottky diode or any suggested fast recovery diode in the Table 1.MOTOROLA ANALOG IC DEVICE DATA11LM2576Procedure (Adjustable Output Version: LM2576–ADJ) (continued)Procedure4.Inductor Selection (L1)A.Use the following formula to calculate the inductor Volt x microsecond [V x µs] constant:Vout6ExT+V–Voutx10[Vxms]inVF[Hz]inB.Match the calculated E x T value with the corresponding number on the vertical axis of the Inductor Value Selection Guide shown in Figure 22. This E x T constant is a measure of the energy handling capability of an inductor and is dependent upon the type of core, the core area, the number of turns, and the duty cycle.C.Next step is to identify the inductance region intersected by the E x T value and the maximum load current value on the horizontal axis shown in Figure 25.D.From the inductor code, identify the inductor value. Then select an appropriate inductor from Table 2. The inductor chosen must be rated for a switching frequency of 52 kHz and for a current rating of 1.15 x ILoad. The inductor current rating can also be determined by calculating the inductor peak current:Example4.Inductor Selection (L1)A.Calculate E x T [V x µs] constant:ExT+(25–8.0)x8.0x1000+80[Vxms]2552B.E x T = 80 [V x µs]ǒǓC.ILoad(max) = 2.5 AInductance Region = H150D.Proper inductor value = 150 µHChoose the inductor from Table 2.Ip(max)+ILoad(max))ǒVin–VoutǓton2Lwhere ton is the “on” time of the power switch andx1.0VfoscinFor additional information about the inductor, see the inductor section in the “External Components” section of this data sheet.ton+5.Output Capacitor Selection (Cout)A.Since the LM2576 is a forward–mode switching regulator with voltage mode control, its open loop 2–pole–1–zero frequency characteristic has the dominant pole–pair determined by the output capacitor and inductor values.For stable operation, the capacitor must satisfy the following requirement:Vin(max)Coutw13,300[µF]VoutxL[µH]B.Capacitor values between 10 µF and 2000 µF will satisfy the loop requirements for stable operation. To achieve an acceptable output ripple voltage and transient response, the output capacitor may need to be several times larger than the above formula yields.C.Due to the fact that the higher voltage electrolytic capacitorsgenerally have lower ESR (Equivalent Series Resistance) numbers, the output capacitor’s voltage rating should be at least 1.5 times greater than the output voltage. For a 5.0 V regulator, a rating of at least 8.0 V is appropriate, and a 10 V or 16 V rating is recommended.5.Output Capacitor Selection (Cout)A.25Coutw13,300x+332.5µF8x150To achieve an acceptable ripple voltage, selectCout = 680 µF electrolytic capacitor.Vout12MOTOROLA ANALOG IC DEVICE DATALM2576LM2576 Series Buck Regulator Design Procedures (continued)Indicator Value Selection Guide (For Continuous Mode Operation)Figure 18. LM2576–3.3MAXIMUM INPUT VOLTAGE (V)MAXIMUM INPUT VOLTAGE (V)60402015108.07.06.0L68060L470L330L220L150L100L68L475.00.340201512109.08.0H1000L680L470L330L220L150L100L68L470.40.50.60.81.01.52.02.53.07.00.30.40.50.60.81.01.21.52.02.53.0H680Figure 19. LM2576–5H470H330H220H150IL, MAXIMUM LOAD CURRENT (A)IL, MAXIMUM LOAD CURRENT (A)Figure 20. LM2576–1260MAXIMUM INPUT VOLTAGE (V)MAXIMUM INPUT VOLTAGE (V)403530H150025H10002018L6801615140.3L470L330L220604035302522201918170.3L680Figure 21. LM2576–15H1500H1000H680H470H680H470H330H220H150H330H220H150L470L330L150L220L100L68L150L1002.0L682.53.00.40.50.60.81.01.52.02.53.00.40.50.60.81.01.5IL, MAXIMUM LOAD CURRENT (A)IL, MAXIMUM LOAD CURRENT (A)Figure 22. LM2576–ADJ300250200ET, VOLTAGE TIME (Vµ s)15010090807060504540353025200.3H2000H1500H1000H680H470H330H220H150L680L470L330L220L150L100L68L472.53.00.40.50.60.81.01.52.0IL, MAXIMUM LOAD CURRENT (A)MOTOROLA ANALOG IC DEVICE DATA13LM2576Table 1. Diode Selection GuideSchottky3.0 AVR20 VThroughHole1N5820MBR320PSR3021N5821MBR330SR30331DQ031N5822MBR340SR30431DQ04MBR35031DQ05SR305MBR360DQ06SR306SurfaceMountSK324.0 – 6.0 AThroughHole1N5823SR502SB5201N5824SR503SB5301N5825SR504SB540SB55050WQ03MUR32031DF1HER302MBRD640CT50WQ04(all diodesratedto at least100V)100 V)MURS320T3MURD32030WF10(all diodesratedto at least100V)100 V)MUR420HER602MURD620CT50WF10SurfaceMountThroughHole3.0 ASurfaceMountFast Recovery4.0 – 6.0 AThroughHoleSurfaceMount30 VSK3330WQ0340 VSK3430WQ04MBRS340T3MBRD340SK3530WQ05(all diodesratedto at least100V)100 V)(all diodesratedto at least100V)100 V)50 V50WQ0560 VMBRS360T3MBRD36050SQ080MBRD660CTNOTE: Diofes listed inbold are available from Motorola.Table 2. Inductor Selection by Manufacturer’s Part NumberInductorCodeL47L68L100L150L220L330L470L680H150H220H330H470H680H1000H1500H2200NOTE: * Contact ManufacturerInductorValue47 µH68 µH100 µH150 µH220 µH330 µH470 µH680 µH150 µH220 µH330 µH470 µH680 µH1000 µH1500 µH2200 µHTech 3977 21277 26277 31277 36077 40877 456*77 50677 36277 41277 462*77 50877 556**Schott Corp.671 26980671 26990671 27000671 27010671 27020671 27030671 27040671 27050671 27060671 27070671 27080671 27090671 27100671 27110671 27120671 27130Pulse Eng.PE–53112PE–92114PE–92108PE–53113PE–52626PE–52627PE–53114PE–52629PE–53115PE–53116PE–53117PE–53118PE–53119PE–53120PE–53121PE–53122RencoRL2442RL2443RL2444RL1954RL1953RL1952RL1951RL1950RL2445RL2446RL2447RL1961RL1960RL1959RL1958RL244814MOTOROLA ANALOG IC DEVICE DATALM2576Table 3. Example of Several Inductor Manufacturers Phone/Fax NumbersPulse Engineering, Inc.Pulse Engineering, Inc. EuropeRenco Electronics, Inc.Tech 39Schott CorporationPhoneFaxPhoneFaxPhoneFaxPhoneFaxPhoneFax+ 1–619–674–8100+ 1–619–674–8262+ 353–9324–107+ 353–9324–459+ 1–516–645–5828+ 1–516–586–5562+ 33–1–4115–1681+ 33–1–4709–5051+ 1–612–475–1173+ 1–612–475–1786EXTERNAL COMPONENTSInput Capacitor (Cin)The Input Capacitor Should Have a Low ESRFor stable operation of the switch mode converter a lowESR (Equivalent Series Resistance) aluminium or solidtantalum bypass capacitor is needed between the input pinand the ground pin, to prevent large voltage transients fromappearing at the input. It must be located near the regulatorand use short leads. With most electrolytic capacitors, thecapacitance value decreases and the ESR increases withlower temperatures. For reliable operation in temperaturesbelow –25°C larger values of the input capacitor may beneeded. Also paralleling a ceramic or solid tantalumcapacitor will increase the regulator stability at coldtemperatures.An aluminium electrolytic capacitor’s ESR value is relatedto many factors such as the capacitance value, the voltagerating, the physical size and the type of construction. In mostcases, the higher voltage electrolytic capacitors have lowerESR value. Often capacitors with much higher voltageratings may be needed to provide low ESR values that, arerequired for low output ripple voltage.RMS Current Rating of CinThe important parameter of the input capacitor is the RMScurrent rating. Capacitors that are physically large and havelarge surface area will typically have higher RMS currentratings. For a given capacitor value, a higher voltageelectrolytic capacitor will be physically larger than a lowervoltage capacitor, and thus be able to dissipate more heat tothe surrounding air, and therefore will have a higher RMScurrent rating. The consequence of operating an electrolyticcapacitor beyond the RMS current rating is a shortenedoperating life. In order to assure maximum capacitoroperating lifetime, the capacitor’s RMS ripple current ratingshould be:Irms > 1.2 x d x ILoadwhere d is the duty cycle, for a buck regulatorVtd+on+outTVin|Vout|tonandd++forabuck*boostregulator.T|Vout|)VinOutput Capacitor (Cout)For low output ripple voltage and good stability, low ESRoutput capacitors are recommended. An output capacitor hastwo main functions: it filters the output and provides regulatorloop stability. The ESR of the output capacitor and thepeak–to–peak value of the inductor ripple current are themain factors contributing to the output ripple voltage value.Standard aluminium electrolytics could be adequate for someapplications but for quality design, low ESR types arerecommended.MOTOROLA ANALOG IC DEVICE DATAThe Output Capacitor Requires an ESR ValueThat Has an Upper and Lower LimitAs mentioned above, a low ESR value is needed for lowoutput ripple voltage, typically 1% to 2% of the output voltage.But if the selected capacitor’s ESR is extremely low (below0.05 Ω), there is a possibility of an unstable feedback loop,resulting in oscillation at the output. This situation can occurwhen a tantalum capacitor, that can have a very low ESR, isused as the only output capacitor.At Low Temperatures, Put in Parallel AluminiumElectrolytic Capacitors with Tantalum CapacitorsElectrolytic capacitors are not recommended fortemperatures below –25°C. The ESR rises dramatically atcold temperatures and typically rises 3 times at –25°C and asmuch as 10 times at –40°C. Solid tantalum capacitors havemuch better ESR spec at cold temperatures and arerecommended for temperatures below –25°C. They can bealso used in parallel with aluminium electrolytics. The valueof the tantalum capacitor should be about 10% or 20% of thetotal capacitance. The output capacitor should have at least50% higher RMS ripple current rating at 52 kHz than thepeak–to–peak inductor ripple current.Catch DiodeLocate the Catch Diode Close to the LM2576The LM2576 is a step–down buck converter; it requires afast diode to provide a return path for the inductor currentwhen the switch turns off. This diode must be located close tothe LM2576 using short leads and short printed circuit tracesto avoid EMI problems.Use a Schottky or a Soft SwitchingUltra–Fast Recovery DiodeSince the rectifier diodes are very significant sources oflosses within switching power supplies, choosing the rectifierthat best fits into the converter design is an importantprocess. Schottky diodes provide the best performance15LM2576
because of their fast switching speed and low forwardvoltage drop.
They provide the best efficiency especially in low outputvoltage applications (5.0 V and lower). Another choice couldbe Fast–Recovery, or Ultra–Fast Recovery diodes. It has tobe noted, that some types of these diodes with an abruptturnoff characteristic may cause instability or EMI troubles.A fast–recovery diode with soft recovery characteristicscan better fulfill some quality, low noise design requirements.Table 1 provides a list of suitable diodes for the LM2576regulator. Standard 50/60 Hz rectifier diodes, such as the1N4001 series or 1N5400 series are NOT suitable.
Inductor
The magnetic components are the cornerstone of allswitching power supply designs. The style of the core andthe winding technique used in the magnetic component’sdesign has a great influence on the reliability of the overallpower supply.
Using an improper or poorly designed inductor can causehigh voltage spikes generated by the rate of transitions incurrent within the switching power supply, and the possibilityof core saturation can arise during an abnormal operationalmode. Voltage spikes can cause the semiconductors to enteravalanche breakdown and the part can instantly fail if enoughenergy is applied. It can also cause significant RFI (RadioFrequency Interference) and EMI (Electro–MagneticInterference) problems.
different design load currents are selected. For light loads(less than approximately 300 mA) it may be desirable tooperate the regulator in the discontinuous mode, becausethe inductor value and size can be kept relatively low.Consequently, the percentage of inductor peak–to–peakcurrent increases. This discontinuous mode of operation isperfectly acceptable for this type of switching converter. Anybuck regulator will be forced to enter discontinuous mode ifthe load current is light enough.
Figure 23. Continuous Mode Switching Current
Waveforms
VERTRICAL RESOLUTION 1.0 A/DIVHORIZONTAL TIME BASE: 5.0 µs/DIV2.0 AInductorCurrentWaveform0 A2.0 APowerSwitchCurrentWaveform0 AContinuous and Discontinuous Mode of OperationThe LM2576 step–down converter can operate in both thecontinuous and the discontinuous modes of operation. Theregulator works in the continuous mode when loads arerelatively heavy, the current flows through the inductorcontinuously and never falls to zero. Under light loadconditions, the circuit will be forced to the discontinuousmode when inductor current falls to zero for certain period oftime (see Figure 23 and Figure 24). Each mode hasdistinctively different operating characteristics, which canaffect the regulator performance and requirements. In manycases the preferred mode of operation is the continuousmode. It offers greater output power, lower peak currents inthe switch, inductor and diode, and can have a lower outputripple voltage. On the other hand it does require largerinductor values to keep the inductor current flowingcontinuously, especially at low output load currents and/orhigh input voltages.
To simplify the inductor selection process, an inductorselection guide for the LM2576 regulator was added to thisdata sheet (Figures 18 through 22). This guide assumes thatthe regulator is operating in the continuous mode, andselects an inductor that will allow a peak–to–peak inductorripple current to be a certain percentage of the maximumdesign load current. This percentage is allowed to change as
Selecting the Right Inductor StyleSome important considerations when selecting a coretype are core material, cost, the output power of the powersupply, the physical volume the inductor must fit within, andthe amount of EMI (Electro–Magnetic Interference) shieldingthat the core must provide. The inductor selection guidecovers different styles of inductors, such as pot core, E–core,toroid and bobbin core, as well as different core materialssuch as ferrites and powdered iron from differentmanufacturers.
For high quality design regulators the toroid core seems tobe the best choice. Since the magnetic flux is containedwithin the core, it generates less EMI, reducing noiseproblems in sensitive circuits. The least expensive is thebobbin core type, which consists of wire wound on a ferriterod core. This type of inductor generates more EMI due to thefact that its core is open, and the magnetic flux is notcontained within the core.
When multiple switching regulators are located on thesame printed circuit board, open core magnetics can causeinterference between two or more of the regulator circuits,especially at high currents due to mutual coupling. A toroid,pot core or E–core (closed magnetic structure) should beused in such applications.
16MOTOROLA ANALOG IC DEVICE DATA
LM2576
Do Not Operate an Inductor Beyond itsMaximum Rated CurrentExceeding an inductor’s maximum current rating maycause the inductor to overheat because of the copper wirelosses, or the core may saturate. Core saturation occurswhen the flux density is too high and consequently the crosssectional area of the core can no longer support additionallines of magnetic flux.
This causes the permeability of the core to drop, theinductance value decreases rapidly and the inductor beginsto look mainly resistive. It has only the DC resistance of thewinding. This can cause the switch current to rise very rapidlyand force the LM2576 internal switch into cycle–by–cyclecurrent limit, thus reducing the DC output load current. Thiscan also result in overheating of the inductor and/or theLM2576. Different inductor types have different saturationcharacteristics, and this should be kept in mind whenselecting an inductor.
Figure 24. Discontinuous Mode Switching Current
Waveforms
VERTICAL RESOLUTION 200 mA/DIVHORIZONTAL TIME BASE: 5.0 µs/DIV0.4 AInductorCurrentWaveform0 A0.4 APowerSwitchCurrentWaveform0 AGENERAL RECOMMENDATIONS
Output Voltage Ripple and TransientsSource of the Output RippleSince the LM2576 is a switch mode power supplyregulator, its output voltage, if left unfiltered, will contain asawtooth ripple voltage at the switching frequency. Theoutput ripple voltage value ranges from 0.5% to 3% of theoutput voltage. It is caused mainly by the inductor sawtoothripple current multiplied by the ESR of the output capacitor.
to smooth the output by means of an additional LC filter (20 µH,100 µF), that can be added to the output (see Figure 34) tofurther reduce the amount of output ripple and transients.With such a filter it is possible to reduce the output ripplevoltage transients 10 times or more. Figure 25 shows thedifference between filtered and unfiltered output waveformsof the regulator shown in Figure 34.
The lower waveform is from the normal unfiltered output ofthe converter, while the upper waveform shows the outputripple voltage filtered by an additional LC filter.
Heatsinking and Thermal ConsiderationsThe Through–Hole Package TO–220The LM2576 is available in two packages, a 5–pinTO–220(T, TV) and a 5–pin surface mount D2PAK(D2T).Although the TO–220(T) package needs a heatsink undermost conditions, there are some applications that require noheatsink to keep the LM2576 junction temperature within theallowed operating range. Higher ambient temperaturesrequire some heat sinking, either to the printed circuit (PC)board or an external heatsink.
Short Voltage Spikes and How to Reduce ThemThe regulator output voltage may also contain shortvoltage spikes at the peaks of the sawtooth waveform (seeFigure 25). These voltage spikes are present because of thefast switching action of the output switch, and the parasiticinductance of the output filter capacitor. There are someother important factors such as wiring inductance, straycapacitance, as well as the scope probe used to evaluatethese transients, all these contribute to the amplitude of thesespikes. To minimize these voltage spikes, low inductancecapacitors should be used, and their lead lengths must bekept short. The importance of quality printed circuit boardlayout design should also be highlighted.
Figure 25. Output Ripple Voltage Waveforms
Voltage spikescaused byswitching actionof the outputswitch and theparasiticinductance of theoutput capacitorFilteredOutputVoltageUnfilteredOutputVoltageHORIZONTAL TIME BASE: 5.0 µs/DIVMinimizing the Output RippleIn order to minimize the output ripple voltage it is possibleto enlarge the inductance value of the inductor L1 and/or touse a larger value output capacitor. There is also another way
The Surface Mount Package D2PAK and its HeatsinkingThe other type of package, the surface mount D2PAK, isdesigned to be soldered to the copper on the PC board. Thecopper and the board are the heatsink for this package andthe other heat producing components, such as the catchdiode and inductor. The PC board copper area that thepackage is soldered to should be at least 0.4 in2 (or260 mm2) and ideally should have 2 or more square inches(1300 mm2) of 0.0028 inch copper. Additional increases ofcopper area beyond approximately 6.0 in2 (4000 mm2) willnot improve heat dissipation significantly. If further thermalimprovements are needed, double sided or multilayer PCboards with large copper areas should be considered. Inorder to achieve the best thermal performance, it is highlyrecommended to use wide copper traces as well as largeareas of copper in the printed circuit board layout. The onlyexception to this is the OUTPUT (switch) pin, which shouldnot have large areas of copper (see page 8 ’PCB LayoutGuideline’).
VERTRICALRESOLUTION20 mV/DIVMOTOROLA ANALOG IC DEVICE DATA
17
LM2576Thermal Analysis and DesignThe following procedure must be performed to determinewhether or not a heatsink will be required. First determine:1.PD(max) maximum regulator power dissipation in the application.2.TA(max) maximum ambient temperature in the application.3.TJ(max)maximum allowed junction temperature (125°C for the LM2576). For a conservative design, the maximum junction temperature should not exceed 110°C to assure safe operation. For every additional +10°C temperature rise that the junction must withstand, the estimated operating lifetime of the component is halved.4.RθJCpackage thermal resistance junction–case.5.RθJApackage thermal resistance junction–ambient.(Refer to Absolute Maximum Ratings on page 2 of this datasheet or RθJC and RθJA values).The following formula is to calculate the approximate totalpower dissipated by the LM2576:PD = (Vin x IQ) + d x ILoad x Vsatwhere d is the duty cycle and for buck converterVton+O,d+TVinIQ(quiescent current) and Vsat can be found in theLM2576 data sheet,Vinis minimum input voltage applied,VOis the regulator output voltage,ILoadis the load current.The dynamic switching losses during turn–on and turn–offcan be neglected if proper type catch diode is used.If the actual operating temperature is greater than theselected safe operating junction temperature, then a largerheatsink is required.Some Aspects That can Influence Thermal DesignIt should be noted that the package thermal resistance andthe junction temperature rise numbers are all approximate,and there are many factors that will affect these numbers,such as PC board size, shape, thickness, physical position,location, board temperature, as well as whether thesurrounding air is moving or still.Other factors are trace width, total printed circuit copperarea, copper thickness, single– or double–sided, multilayerboard, the amount of solder on the board or even colour ofthe traces.The size, quantity and spacing of other components onthe board can also influence its effectiveness to dissipatethe heat.Figure 26. Inverting Buck–Boost Develops –12 V12 to 40 VUnregulatedDC InputCin100 µFFeedback +Vin13LM2576–12Gnd54Output2ON/OFFL168 µHD11N5822Cout2200 µF–12 V @ 0.7 ARegulatedOutputADDITIONAL APPLICATIONSInverting RegulatorAn inverting buck–boost regulator using the LM2576–12 isshown in Figure 26. This circuit converts a positive inputvoltage to a negative output voltage with a common groundby bootstrapping the regulators ground to the negative outputvoltage. By grounding the feedback pin, the regulator sensesthe inverted output voltage and regulates it.In this example the LM2576–12 is used to generate a–12 V output. The maximum input voltage in this casecannot exceed +28 V because the maximum voltageappearing across the regulator is the absolute sum of theinput and output voltages and this must be limited to amaximum of 40 V.This circuit configuration is able to deliver approximately0.7 A to the output when the input voltage is 12 V or higher. Atlighter loads the minimum input voltage required drops toapproximately 4.7 V, because the buck–boost regulatortopology can produce an output voltage that, in its absolutevalue, is either greater or less than the input voltage.Packages Not on a Heatsink (Free–Standing)For a free–standing application when no heatsink is used,the junction temperature can be determined by the followingexpression:TJ = (RθJA) (PD) + TAwhere (RθJA)(PD) represents the junction temperature risecaused by the dissipated power and TA is the maximumambient temperature.Packages on a HeatsinkIf the actual operating junction temperature is greater thanthe selected safe operating junction temperature determinedin step 3, than a heatsink is required. The junctiontemperature will be calculated as follows:TJ = PD (RθJA + RθCS + RθSA) + TAwhereRθJC is the thermal resistance junction–case,RθCS is the thermal resistance case–heatsink,RθSA is the thermal resistance heatsink–ambient.18MOTOROLA ANALOG IC DEVICE DATALM2576Since the switch currents in this buck–boost configurationare higher than in the standard buck converter topology, theavailable output current is lower.This type of buck–boost inverting regulator can alsorequire a larger amount of start–up input current, even forlight loads. This may overload an input power source with acurrent limit less than 5.0 A.Such an amount of input start–up current is needed for atleast 2.0 ms or more. The actual time depends on the outputvoltage and size of the output capacitor.Because of the relatively high start–up currents requiredby this inverting regulator topology, the use of a delayedstart–up or an undervoltage lockout circuit is recommended.Using a delayed start–up arrangement, the input capacitorcan charge up to a higher voltage before the switch–moderegulator begins to operate.The high input current needed for start–up is now partiallysupplied by the input capacitor Cin.It has been already mentioned above, that in somesituations, the delayed start–up or the undervoltage lockoutfeatures could be very useful. A delayed start–up circuitapplied to a buck–boost converter is shown in Figure 27,Figure 33 in the “Undervoltage Lockout” section describes anundervoltage lockout feature for the same convertertopology.Figure 27. Inverting Buck–Boost Regulatorwith Delayed start–up12 V to 25 VUnregulatedDC InputCin100 µF/50 VC10.1 µFR147 kFeedback +Vin1LM2576–125ON/OFF34Output2GndD11N5822Cout2200 µF/16 VL168 µHR247 k–12 V @ 700 m ARegulatedOutputFigure 28. Inverting Buck–Boost Regulator ShutdownCircuit Using an Optocoupler +Vin +Vin1CinR1100 µF47 k5LM2576–XXDesign Recommendations:The inverting regulator operates in a different manner thanthe buck converter and so a different design procedure has tobe used to select the inductor L1 or the output capacitor Cout.The output capacitor values must be larger than what isnormally required for buck converter designs. Low inputvoltages or high output currents require a large value outputcapacitor (in the range of thousands of µF).The recommended range of inductor values for theinverting converter design is between 68 µH and 220 µH. Toselect an inductor with an appropriate current rating, theinductor peak current has to be calculated.The following formula is used to obtain the peak inductorcurrent:I(V)|V|)O)VinxtonI[LoadinpeakV2L1in|V|Owhereton+x1.0,andfosc+52kHz.V)|V|foscinOUnder normal continuous inductor current operatingconditions, the worst case occurs when Vin is minimal.5.0 V0OnShutdownInputOffR3470ON/OFF3GndR247 k–VoutMOC8101NOTE: This picture does not show the complete circuit.With the inverting configuration, the use of the ON/OFF pinrequires some level shifting techniques. This is caused by thefact, that the ground pin of the converter IC is no longer atground. Now, the ON/OFF pin threshold voltage (1.3 Vapproximately) has to be related to the negative outputvoltage level. There are many different possible shut downmethods, two of them are shown in Figures 28 and 29.MOTOROLA ANALOG IC DEVICE DATA19LM2576Figure 29. Inverting Buck–Boost Regulator ShutdownCircuit Using a PNP Transistor+V0OnR25.6 k +VinCin100 µFQ12N39065 +Vin1LM2576–XXOffShutdownInputcurrents require a large value output capacitor (in the rangeof thousands of µF). The recommended range of inductorvalues for the negative boost regulator is the same as forinverting converter design.Another important point is that these negative boostconverters cannot provide current limiting load protection inthe event of a short in the output so some other means, suchas a fuse, may be necessary to provide the load protection.Delayed Start–upThere are some applications, like the inverting regulatoralready mentioned above, which require a higher amount ofstart–up current. In such cases, if the input power source islimited, this delayed start–up feature becomes very useful.To provide a time delay between the time when the inputvoltage is applied and the time when the output voltagecomes up, the circuit in Figure 31 can be used. As the inputvoltage is applied, the capacitor C1 charges up, and thevoltage across the resistor R2 falls down. When the voltageon the ON/OFF pin falls below the threshold value 1.3 V, theregulator starts up. Resistor R1 is included to limit themaximum voltage applied to the ON/OFF pin. It reduces thepower supply noise sensitivity, and also limits the capacitorC1 discharge current, but its use is not mandatory.When a high 50 Hz or 60 Hz (100 Hz or 120 Hzrespectively) ripple voltage exists, a long delay time cancause some problems by coupling the ripple into the ON/OFFpin, the regulator could be switched periodically on and offwith the line (or double) frequency.Figure 31. Delayed start–up Circuitry +Vin +Vin1ON/OFF3R112 kGnd–VoutNOTE: This picture does not show the complete circuit.Negative Boost RegulatorThis example is a variation of the buck–boost topology andit is called negative boost regulator. This regulatorexperiences relatively high switch current, especially at lowinput voltages. The internal switch current limiting results inlower output load current capability.The circuit in Figure 30 shows the negative boostconfiguration. The input voltage in this application rangesfrom –5.0 V to –12 V and provides a regulated –12 V output.If the input voltage is greater than –12 V, the output will riseabove –12 V accordingly, but will not damage the regulator.LM2576–XXFigure 30. Negative Boost RegulatorC10.1 µFCout2200 µFLow EsrCin100 µF5ON/OFF3Gnd4 Vin1Cin100 µF3Gnd5LM2576–12FeedbackOutput2ON/OFF1N5820R147 kR247 kNOTE: This picture does not show the complete circuit.Vout = –12 V Vin–5.0 V to –12 V100 µHTypical Load Current400 mA for Vin = –5.2 V750 mA for Vin = –7.0 VDesign Recommendations:The same design rules as for the previous invertingbuck–boost converter can be applied. The output capacitorCout must be chosen larger than would be required for a whatstandard buck converter. Low input voltages or high outputUndervoltage LockoutSome applications require the regulator to remain off untilthe input voltage reaches a certain threshold level. Figure 32shows an undervoltage lockout circuit applied to a buckregulator. A version of this circuit for buck–boost converter isshown in Figure 33. Resistor R3 pulls the ON/OFF pin highand keeps the regulator off until the input voltage reaches a20MOTOROLA ANALOG IC DEVICE DATALM2576predetermined threshold level with respect to the groundPin 3, which is determined by the following expression:(Q1)V[V)1.0)R2VBEthZ1R1ǒǓUnder normal continuous inductor current operatingconditions, the worst case occurs when Vin is minimal.Figure 33. Undervoltage Lockout Circuit forBuck–Boost ConverterFigure 32. Undervoltage Lockout Circuit for Buck Converter +Vin +Vin +Vin1R210 kR347 kCin100 µF5 +Vin1LM2576–XXR215 kGndR347 kCin100 µF5LM2576–XXON/OFF3GndON/OFF3Z11N5242BQ12N3904Vth ≈ 13 VZ11N5242BQ12N3904R110 kVth ≈ 13 VR115 kVoutNOTE: This picture does not show the complete circuit.NOTE: This picture does not show the complete circuit.The following formula is used to obtain the peak inductorcurrent:I(V)|V|)O)VinxtonI[LoadinpeakV2L1in|V|Owhereton+x1.0,andfosc+52kHz.V)|V|foscinOAdjustable Output, Low–Ripple Power SupplyA 3.0 A output current capability power supply thatfeatures an adjustable output voltage is shown in Figure 34.This regulator delivers 3.0 A into 1.2 V to 35 V output. Theinput voltage ranges from roughly 3.0 V to 40 V. In order toachieve a 10 or more times reduction of output ripple, anadditional L–C filter is included in this circuit.Figure 34. 1.2 to 35 V Adjustable 3.0 A Power Supply with Low Output Ripple40 V MaxUnregulatedDC InputFeedback +Vin1Cin100 µF34LM2574–AdjOutputGnd52ON/OFFD11N5822Cout2200 µFR11.21 kL1150 µHR250 kC1100 µFL220 µHOutputVoltage1.2 to 35 V @ 3.0 AOptional OutputRipple FilterMOTOROLA ANALOG IC DEVICE DATA21LM2576THE LM2576–5 STEP–DOWN VOLTAGE REGULATOR WITH 5.0 V @ 3.0 A OUTPUT POWER CAPABILITY.TYPICAL APPLICATION WITH THROUGH–HOLE PC BOARD LAYOUTFigure 35. Schematic Diagram of the LM2576–5 Step–Down ConverterFeedbackUnregulated DC Input+Vin = 7.0 to 40 V +Vin13C1100 µF/50 V4LM2576–5OutputGnd52ON/OFFL1150 µHRegulated OutputVout1 = 5.0 V @ 3.0 AON/OFFD11N5822Cout1000 µF/16 VGndoutGndinC1C2D1L1––––100 µF, 50 V, Aluminium Electrolytic1000 µF, 16 V, Aluminium Electrolytic3.0 A, 40 V, Schottky Rectifier, 1N5822150 µH, RL2444, Renco ElectronicsFigure 36. Printed Circuit Board LayoutComponent SideFigure 37. Printed Circuit Board LayoutCopper SideU1D1C2C1+Vout+ON/OFF+VinL1GndinNOTE: Not to scale.GndoutNOTE: Not to scale.2200060_00LM2576MOTOROLA ANALOG IC DEVICE DATALM2576THE LM2576–ADJ STEP–DOWN VOLTAGE REGULATOR WITH 8.0 V @ 1.0 A OUTPUT POWERCAPABILITY. TYPICAL APPLICATION WITH THROUGH–HOLE PC BOARD LAYOUTFigure 38. Schematic Diagram of the 8.0 V @ 3.0 A Step–Down Converter Using the LM2576–ADJ4UnregulatedDC Input+Vin = 10 V to 40 V +Vin1FeedbackLM2576–ADJOutput3Gnd52ON/OFFL1150 µHR210 kD11N5822C21000 µF/16 VRegulatedOutput FilteredVout2 = 8.0 V @ 3.0 AC1100 µF/50 VON/OFFR11.8 kVC1C2D1L1R1R2––––––100 µF, 50 V, Aluminium Electrolytic1000 µF, 16 V, Aluminium Electrolytic3.0 A, 40 V, Schottky Rectifier, 1N5822150 µH, RL2444, Renco Electronics1.8 kΩ, 0.25 W10 kΩ, 0.25 WR2out+Vref)1.0)R1ǒǓVref = 1.23 VR1 is between 1.0 k and 5.0 kFigure 39. Printed Circuit Board LayoutComponent SideFigure 40. Printed Circuit Board LayoutCopper SideLM2576U1D1R1R2ON/OFFC1+C2Vout+VinL1+GndinNOTE: Not to scale.GndoutNOTE: Not to scale.References••••National Semiconductor LM2576 Data Sheet and Application NoteNational Semiconductor LM2595 Data Sheet and Application NoteMarty Brown “Practical Switching Power Supply Design”, Academic Press, Inc., San Diego 1990Ray Ridley “High Frequency Magnetics Design”, Ridley Engineering, Inc. 199523MOTOROLA ANALOG IC DEVICE DATA00059_00LM2576OUTLINE DIMENSIONST SUFFIXPLASTIC PACKAGECASE 314D–03ISSUE DSEATINGPLANE–T––Q–UAL12345CBENOTES:1.DIMENSIONING AND TOLERANCING PER ANSIY14.5M, 1982.2.CONTROLLING DIMENSION: INCH.3.DIMENSION D DOES NOT INCLUDEINTERCONNECT BAR (DAMBAR) PROTRUSION.DIMENSION D INCLUDING PROTRUSION SHALLNOT EXCEED 10.92 (0.043) MAXIMUM.DIMABCDEGHJKLQUSINCHESMINMAX0.5720.6130.3900.4150.1700.1800.0250.0380.0480.0550.067 BSC0.0870.1120.0150.0251.0201.0650.3200.3650.1400.1530.1050.1170.5430.582MILLIMETERSMINMAX14.52915.5709.90610.5414.3184.5720.6350.9651.2191.3971.702 BSC2.2102.8450.3810.63525.90827.0518.1289.2713.5563.8862.6672.97213.79214.783KSGD5 PLJHM0.356 (0.014)MTQTV SUFFIXPLASTIC PACKAGECASE 314B–05ISSUE JNOTES:1.DIMENSIONING AND TOLERANCING PER ANSIY14.5M, 1982.2.CONTROLLING DIMENSION: INCH.3.DIMENSION D DOES NOT INCLUDEINTERCONNECT BAR (DAMBAR) PROTRUSION.DIMENSION D INCLUDING PROTRUSION SHALLNOT EXCEED 0.043 (1.092) MAXIMUM.DIMABCDEFGHJKLNQSUVWINCHESMINMAX0.5720.6130.3900.4150.1700.1800.0250.0380.0480.0550.8500.9350.067 BSC0.166 BSC0.0150.0250.9001.1000.3200.3650.320 BSC0.1400.153–––0.6200.4680.505–––0.7350.0900.110MILLIMETERSMINMAX14.52915.5709.90610.5414.3184.5720.6350.9651.2191.39721.59023.7491.702 BSC4.216 BSC0.3810.63522.86027.9408.1289.2718.128 BSC3.5563.886–––15.74811.88812.827–––18.6692.2862.794QB–P–COPTIONAL CHAMFEREUKFASLWV5XJTHN–T–SEATINGPLANEG5X0.24 (0.610)MDM0.10 (0.254)TPM24MOTOROLA ANALOG IC DEVICE DATALM2576OUTLINE DIMENSIONSD2T SUFFIXPLASTIC PACKAGECASE 936A–02(D2PAK)ISSUE ANOTES:1DIMENSIONING AND TOLERANCING PER ANSIY14.5M, 1982.2CONTROLLING DIMENSION: INCH.3TAB CONTOUR OPTIONAL WITHIN DIMENSIONSA AND K.4DIMENSIONS U AND V ESTABLISH A MINIMUMMOUNTING SURFACE FOR TERMINAL 6.5DIMENSIONS A AND B DO NOT INCLUDE MOLDFLASH OR GATE PROTRUSIONS. MOLD FLASHAND GATE PROTRUSIONS NOT TO EXCEED0.025 (0.635) MAXIMUM.INCHESMINMAX0.3860.4030.3560.3680.1700.1800.0260.0360.0450.0550.067 BSC0.5390.5790.050 REF0.0000.0100.0880.1020.0180.0260.0580.078_5 REF0.116 REF0.200 MIN0.250 MINMILLIMETERSMINMAX9.80410.2369.0429.3474.3184.5720.6600.9141.1431.3971.702 BSC13.69114.7071.270 REF0.0000.2542.2352.5910.4570.6601.4731.981_5 REF2.946 REF5.080 MIN6.350 MIN–T–AKB12345OPTIONALCHAMFERTERMINAL 6EUVSHMLD0.010 (0.254)MTNGRPCDIMABCDEGHKLMNPRSUVMOTOROLA ANALOG IC DEVICE DATA25LM2576Motorola reserves the right to make changes without further notice to any products herein. Motorola makes no warranty, representation or guarantee regardingthe suitability of its products for any particular purpose, nor does Motorola assume any liability arising out of the application or use of any product or circuit, andspecifically disclaims any and all liability, including without limitation consequential or incidental damages. “Typical” parameters which may be provided in Motoroladata sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals”must be validated for each customer application by customer’s technical experts. Motorola does not convey any license under its patent rights nor the rights ofothers. Motorola products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or otherapplications intended to support or sustain life, or for any other application in which the failure of the Motorola product could create a situation where personal injuryor death may occur. Should Buyer purchase or use Motorola products for any such unintended or unauthorized application, Buyer shall indemnify and hold Motorolaand its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney feesarising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges thatMotorola was negligent regarding the design or manufacture of the part. Motorola and are registered trademarks of Motorola, Inc. Motorola, Inc. is an EqualOpportunity/Affirmative Action Employer.26MOTOROLA ANALOG IC DEVICE DATALM2576Mfax is a trademark of Motorola, Inc.How to reach us:USA/EUROPE/Locations Not Listed: Motorola Literature Distribution;P.O. Box 5405, Denver, Colorado 80217. 1–303–675–2140 or 1–800–441–2447Customer Focus Center: 1–800–521–6274Mfax™: RMFAX0@email.sps.mot.com– TOUCHTONE 1–602–244–6609ASIA/PACIFIC: Motorola Semiconductors H.K. Ltd.; 8B Tai Ping Industrial Park,Motorola Fax Back System– US & Canada ONLY 1–800–774–184851 Ting Kok Road, Tai Po, N.T., Hong Kong. 852–26629298– http://sps.motorola.com/mfax/HOME PAGE: http://motorola.com/sps/JAPAN: Nippon Motorola Ltd.: SPD, Strategic Planning Office, 4–32–1,Nishi–Gotanda, Shinagawa–ku, Tokyo 141, Japan. 81–3–5487–8488MOTOROLA ANALOG IC DEVICE DATA◊LM2576/D27