FLOWLB03

LM2576 NSC | Alldatasheet

Document overview

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Technical content

Features

n 3.3V, 5V, 12V, 15V, and adjustable output versions n Adjustable version output voltage range, 1.23V to 37V (57V for HV version) ±4% max over line and load conditions n Guaranteed 3A output current n Wide input voltage range, 40V up to 60V for HV version n Requires only 4 external components n 52 kHz fixed frequency internal oscillator n TTL shutdown capability, low power standby mode n High efficiency n Uses readily available standard inductors n Thermal shutdown and current limit protection n P+ Product Enhancement tested

Applications

n Simple high-efficiency step-down (buck) regulator n Efficient pre-regulator for linear regulators n On-card switching regulators n Positive to negative converter (Buck-Boost) Typical Application (Fixed Output Voltage Versions) SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation. 01147601 FIGURE 1. August 2004 LM2576/LM2576HV Series SIMPLE SWITCHER 3A Step-Down Voltage Regulator © 2004 National Semiconductor Corporation DS011476 www.national.com

3.3V R2 = 1.7k 5V, R2 = 3.1k 12V, R2 = 8.84k 15V, R2 = 11.3k For ADJ. Version R1 = Open, R2 = 0 Ω Patent Pending

Ordering Information

Output Voltage NS Package Package Type3.3 5.0 12 15 ADJ Number −40˚C ≤ TA ≤ 125˚C LM2576HVS-3.3 LM2576HVS-5.0 LM2576HVS-12 LM2576HVS-15 LM2576HVS-ADJ TS5B TO-263 LM2576S-3.3 LM2576S-5.0 LM2576S-12 LM2576S-15 LM2576S-ADJ LM2576HVSX-3.3 LM2576HVSX-5.0 LM2576HVSX-12 LM2576HVSX-15 LM2576HVSX-ADJ TS5B Tape & ReelLM2576SX-3.3 LM2576SX-5.0 LM2576SX-12 LM2576SX-15 LM2576SX-ADJ LM2576HVT-3.3 LM2576HVT-5.0 LM2576HVT-12 LM2576HVT-15 LM2576HVT-ADJ T05A TO-220 LM2576T-3.3 LM2576T-5.0 LM2576T-12 LM2576T-15 LM2576T-ADJ LM2576HVT-3.3 LM2576HVT-5.0 LM2576HVT-12 LM2576HVT-15 LM2576HVT-ADJ T05D Flow LB03 Flow LB03 Flow LB03 Flow LB03 Flow LB03 LM2576T-3.3 LM2576T-5.0 LM2576T-12 LM2576T-15 LM2576T-ADJ Flow LB03 Flow LB03 Flow LB03 Flow LB03 Flow LB03 LM2576/LM2576HV www.national.com 2

Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Maximum Supply Voltage LM2576 45V LM2576HV 63V ON /OFF Pin Input Voltage −0.3V ≤ V ≤ +VIN Output Voltage to Ground (Steady State) −1V Power Dissipation Internally Limited Storage Temperature Range −65˚C to +150˚C Maximum Junction Temperature 150˚C Minimum ESD Rating (C = 100 pF, R = 1.5 k Ω)2 k V Lead Temperature (Soldering, 10 Seconds) 260˚C Operating Ratings Temperature Range LM2576/LM2576HV −40˚C ≤ TJ ≤ +125˚C Supply Voltage LM2576 40V LM2576HV 60V LM2576-3.3, LM2576HV-3.3

Electrical Characteristics

Specifications with standard type face are for T J = 25˚C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions LM2576-3.3 Units (Limits)LM2576HV-3.3 Typ Limit (Note 2) SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage V IN = 12V, ILOAD = 0.5A 3.3 V Circuit of Figure 2 3.234 V(Min)

3.366 V(Max)

VOUT Output Voltage 6V ≤ VIN ≤ 40V, 0.5A ≤ ILOAD ≤ 3A 3.3 V LM2576 Circuit of Figure 2 3.168/3.135 V(Min) 3.432/3.465 V(Max) VOUT Output Voltage 6V ≤ VIN ≤ 60V, 0.5A ≤ ILOAD ≤ 3A 3.3 V LM2576HV Circuit of Figure 2 3.168/3.135 V(Min) 3.450/3.482 V(Max) η Efficiency V IN = 12V, ILOAD =3 A 7 5 % LM2576-5.0, LM2576HV-5.0 Specifications with standard type face are for T J = 25˚C, and those with Figure 2 boldface type apply over full Operating Tem- perature Range. Symbol Parameter Conditions LM2576-5.0 Units (Limits)LM2576HV-5.0 Typ Limit (Note 2) SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage V IN = 12V, ILOAD = 0.5A 5.0 V Circuit of Figure 2 4.900 V(Min)

5.100 V(Max)

VOUT Output Voltage 0.5A ≤ ILOAD ≤ 3A, 5.0 V LM2576 8V ≤ VIN ≤ 40V 4.800/ 4.750 V(Min) Circuit of Figure 2 5.200/5.250 V(Max) VOUT Output Voltage 0.5A ≤ ILOAD ≤ 3A, 5.0 V LM2576HV 8V ≤ VIN ≤ 60V 4.800/ 4.750 V(Min) Circuit of Figure 2 5.225/5.275 V(Max) LM2576/LM2576HV www.national.com3

LM2576-5.0, LM2576HV-5.0 Electrical Characteristics (Continued) Specifications with standard type face are for T J = 25˚C, and those with Figure 2 boldface type apply over full Operating Tem- perature Range. Symbol Parameter Conditions LM2576-5.0 Units (Limits)LM2576HV-5.0 Typ Limit (Note 2) SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 η Efficiency V IN = 12V, ILOAD =3 A 7 7 % LM2576-12, LM2576HV-12 Specifications with standard type face are for T J = 25˚C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions LM2576-12 Units (Limits)LM2576HV-12 Typ Limit (Note 2) SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage V IN = 25V, ILOAD = 0.5A 12 V Circuit of Figure 2 11.76 V(Min)

12.24 V(Max)

VOUT Output Voltage 0.5A ≤ ILOAD ≤ 3A, 12 V LM2576 15V ≤ VIN ≤ 40V 11.52/ 11.40 V(Min) Circuit of Figure 2 12.48/12.60 V(Max) VOUT Output Voltage 0.5A ≤ ILOAD ≤ 3A, 12 V LM2576HV 15V ≤ VIN ≤ 60V 11.52/ 11.40 V(Min) Circuit of Figure 2 12.54/12.66 V(Max) η Efficiency V IN = 15V, ILOAD =3 A 8 8 % LM2576-15, LM2576HV-15 Specifications with standard type face are for T J = 25˚C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions LM2576-15 Units (Limits)LM2576HV-15 Typ Limit (Note 2) SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage V IN = 25V, ILOAD = 0.5A 15 V Circuit of Figure 2 14.70 V(Min)

15.30 V(Max)

VOUT Output Voltage 0.5A ≤ ILOAD ≤ 3A, 15 V LM2576 18V ≤ VIN ≤ 40V 14.40/ 14.25 V(Min) Circuit of Figure 2 15.60/15.75 V(Max) VOUT Output Voltage 0.5A ≤ ILOAD ≤ 3A, 15 V LM2576HV 18V ≤ VIN ≤ 60V 14.40/ 14.25 V(Min) Circuit of Figure 2 15.68/15.83 V(Max) η Efficiency V IN = 18V, ILOAD =3 A 8 8 % LM2576/LM2576HV www.national.com 4

LM2576-ADJ, LM2576HV-ADJ Specifications with standard type face are for T J = 25˚C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions LM2576-ADJ Units (Limits)LM2576HV-ADJ Typ Limit (Note 2) SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Feedback Voltage V IN = 12V, ILOAD = 0.5A 1.230 V VOUT = 5V, 1.217 V(Min) Circuit of Figure 2 1.243 V(Max) VOUT Feedback Voltage 0.5A ≤ ILOAD ≤ 3A, 1.230 V LM2576 8V ≤ VIN ≤ 40V 1.193/ 1.180 V(Min) VOUT = 5V, Circuit of Figure 2 1.267/1.280 V(Max) VOUT Feedback Voltage 0.5A ≤ ILOAD ≤ 3A, 1.230 V LM2576HV 8V ≤ VIN ≤ 60V 1.193/ 1.180 V(Min) VOUT = 5V, Circuit of Figure 2 1.273/1.286 V(Max) η Efficiency V IN = 12V, ILOAD = 3A, VOUT =5 V 7 7 % All Output Voltage Versions Specifications with standard type face are for T J = 25˚C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, V IN = 12V for the 3.3V, 5V, and Adjustable version, V IN = 25V for the 12V version, and V IN = 30V for the 15V version. I LOAD = 500 mA. Symbol Parameter Conditions LM2576-XX Units (Limits)LM2576HV-XX Typ Limit (Note 2) DEVICE PARAMETERS I b Feedback Bias Current V OUT = 5V (Adjustable Version Only) 50 100/ 500 nA fO Oscillator Frequency (Note 11) 52 kHz 47/42 kHz (Min) 58/63 kHz (Max) VSAT Saturation Voltage I OUT = 3A (Note 4) 1.4 V 1.8/2.0 V(Max) DC Max Duty Cycle (ON) (Note 5) 98 % 93 %(Min) ICL Current Limit (Notes 4, 11) 5.8 A 4.2/3.5 A(Min) 6.9/7.5 A(Max) IL Output Leakage Current (Notes 6, 7): Output = 0V 2 mA(Max) Output = −1V 7.5 mA Output = −1V 30 mA(Max) I Q Quiescent Current (Note 6) 5 mA 10 mA(Max) ISTBY Standby Quiescent ON /OFF Pin = 5V (OFF) 50 µA Current 200 µA(Max) LM2576/LM2576HV www.national.com5

All Output Voltage Versions Electrical Characteristics (Continued) Specifications with standard type face are for T J = 25˚C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, V IN = 12V for the 3.3V, 5V, and Adjustable version, V IN = 25V for the 12V version, and V IN = 30V for the 15V version. I LOAD = 500 mA. Symbol Parameter Conditions LM2576-XX Units (Limits)LM2576HV-XX Typ Limit (Note 2) DEVICE PARAMETERS θ JA Thermal Resistance T Package, Junction to Ambient (Note 8) 65 θJA T Package, Junction to Ambient (Note 9) 45 ˚C/W θJC T Package, Junction to Case 2 θJA S Package, Junction to Ambient (Note 10) 50 ON /OFF CONTROL Test Circuit Figure 2 VIH ON /OFF Pin VOUT = 0V 1.4 2.2/ 2.4 V(Min) VIL Logic Input Level V OUT = Nominal Output Voltage 1.2 1.0/ 0.8 V(Max) IIH ON /OFF Pin Input ON /OFF Pin = 5V (OFF) 12 µA Current 30 µA(Max) IIL ON /OFF Pin = 0V (ON) 0µ A 10 µA(Max) Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100% production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. Note 3: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2576/LM2576HV is used as shown in the Figure 2test circuit, system performance will be as shown in system parameters section of Electrical Characteristics. Note 4: Output pin sourcing current. No diode, inductor or capacitor connected to output. Note 5: Feedback pin removed from output and connected to 0V. Note 6: Feedback pin removed from output and connected to +12V for the Adjustable, 3.3V, and 5V versions, and +25V for the 12V and 15V versions, to force the output transistor OFF. Note 7: VIN = 40V (60V for high voltage version). Note 8: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with 1⁄2 inch leads in a socket, or on a PC board with minimum copper area. Note 9: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with1⁄4 inch leads soldered to a PC board containing approximately 4 square inches of copper area surrounding the leads. Note 10: If the TO-263 package is used, the thermal resistance can be reduced by increasing the PC board copper area thermally connected to the package. Using 0.5 square inches of copper area,θJA is 50˚C/W, with 1 square inch of copper area,θJA is 37˚C/W, and with 1.6 or more square inches of copper area,θJA is 32˚C/W. Note 11: The oscillator frequency reduces to approximately 11 kHz in the event of an output short or an overload which causes the regulated output voltage to drop approximately 40% from the nominal output voltage. This self protection feature lowers the average power dissipation of the IC by lowering the minimum duty cycle from 5% down to approximately 2%. Typical Performance Characteristics (Circuit of Figure 2) Normalized Output Voltage Line Regulation 01147627 01147628 LM2576/LM2576HV www.national.com 6

Typical Performance Characteristics (Circuit of Figure 2) (Continued) Dropout Voltage Current Limit 01147629 01147630 Quiescent Current Standby Quiescent Current 01147631 01147632 Oscillator Frequency Switch Saturation Voltage 01147633 01147634 LM2576/LM2576HV www.national.com7

Typical Performance Characteristics (Circuit of Figure 2) (Continued) Efficiency Minimum Operating Voltage 01147635 01147636 Quiescent Current vs Duty Cycle Feedback Voltage vs Duty Cycle 01147637 01147638 Minimum Operating Voltage Quiescent Current vs Duty Cycle 01147636 01147637 LM2576/LM2576HV www.national.com 8

Typical Performance Characteristics (Circuit of Figure 2) (Continued) Feedback Voltage vs Duty Cycle Feedback Pin Current 01147638 01147604 Maximum Power Dissipation (TO-263) (See Note 10) Switching Waveforms 01147624 01147606 VOUT = 15V A: Output Pin Voltage, 50V/div B: Output Pin Current, 2A/div C: Inductor Current, 2A/div D: Output Ripple Voltage, 50 mV/div, AC-Coupled Horizontal Time Base: 5 µs/div Load Transient Response 01147605 LM2576/LM2576HV www.national.com9

Test Circuit and Layout Guidelines As in any switching regulator, layout is very important. Rap- idly switching currents associated with wiring inductance generate voltage transients which can cause problems. For minimal inductance and ground loops, the length of the leads indicated by heavy lines should be kept as short as possible. Single-point grounding (as indicated) or ground plane con- struction should be used for best results. When using the Adjustable version, physically locate the programming resis- tors near the regulator, to keep the sensitive feedback wiring short. Fixed Output Voltage Versions 01147607 CIN — 100 µF, 75V, Aluminum Electrolytic COUT — 1000 µF, 25V, Aluminum Electrolytic D1 — Schottky, MBR360 L1 — 100 µH, Pulse Eng. PE-92108 R1 — 2k, 0.1% R2 — 6.12k, 0.1% Adjustable Output Voltage Version 01147608 where VREF = 1.23V, R1 between 1k and 5k. FIGURE 2. LM2576/LM2576HV www.national.com 10

LM2576 Series Buck Regulator Design Procedure PROCEDURE (Fixed Output Voltage Versions) EXAMPLE (Fixed Output Voltage Versions) Given: VOUT = Regulated Output Voltage (3.3V, 5V, 12V, or 15V) VIN(Max) = Maximum Input Voltage I LOAD(Max) = Maximum Load Current Given: VOUT =5 VV IN(Max) = 15V I LOAD(Max) = 3A 1. Inductor Selection (L1) A. Select the correct Inductor value selection guide from Figures 3, 4, 5or Figure 6. (Output voltages of 3.3V, 5V, 12V or 15V respectively). For other output voltages, see the design procedure for the adjustable version. B. From the inductor value selection guide, identify the inductance region intersected by V IN(Max) and ILOAD(Max), and note the inductor code for that region. C. Identify the inductor value from the inductor code, and select an appropriate inductor from the table shown in Figure 3. Part numbers are listed for three inductor manufacturers. The inductor chosen must be rated for operation at the LM2576 switching frequency (52 kHz) and for a current rating of 1.15 x I LOAD. For additional inductor information, see the inductor section in the Application Hints section of this data sheet. 1. Inductor Selection (L1) A. Use the selection guide shown in Figure 4. B. From the selection guide, the inductance area intersected by the 15V line and 3A line is L100. C. Inductor value required is 100 µH. From the table in Figure 3. Choose AIE 415-0930, Pulse Engineering PE92108, or Renco RL2444. 2. Output Capacitor Selection (C OUT)A . The value of the output capacitor together with the inductor defines the dominate pole-pair of the switching regulator loop. For stable operation and an acceptable output ripple voltage, (approximately 1% of the output voltage) a value between 100 µF and 470 µF is recommended. B. The capacitor’s voltage rating should be at least 1.5 times greater than the output voltage. For a 5V regulator, a rating of at least 8V is appropriate, and a 10V or 15V rating is recommended. Higher voltage electrolytic capacitors generally have lower ESR numbers, and for this reason it may be necessary to select a capacitor rated for a higher voltage than would normally be needed. 2. Output Capacitor Selection (C OUT)A . COUT = 680 µF to 2000 µF standard aluminum electrolytic. B.Capacitor voltage rating = 20V. 3. Catch Diode Selection (D1) A. The catch-diode current rating must be at least 1.2 times greater than the maximum load current. Also, if the power supply design must withstand a continuous output short, the diode should have a current rating equal to the maximum current limit of the LM2576. The most stressful condition for this diode is an overload or shorted output condition. B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 3. Catch Diode Selection (D1) A. For this example, a 3A current rating is adequate. B. Use a 20V 1N5823 or SR302 Schottky diode, or any of the suggested fast-recovery diodes shown in Figure 8. 4. Input Capacitor (C IN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation. 4. Input Capacitor (C IN) A 100 µF, 25V aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing. LM2576/LM2576HV www.national.com11

1. Programming Output Voltage (Selecting R1 and R2,

the appropriate resistor values.

1. Programming Output Voltage (Selecting R1 and R2)

FIGURE 7. LM2576(HV)-ADJ

LM2576 Series Buck Regulator Design Procedure (Continued) PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions) 2. Inductor Selection (L1) A. Calculate the inductor Volt  microsecond constant, E  T( V  µs), from the following formula: B. Use the E  T value from the previous formula and match it with the E  T number on the vertical axis of the Inductor Value Selection Guide shown in Figure 7. C. On the horizontal axis, select the maximum load current. D. Identify the inductance region intersected by the E  T value and the maximum load current value, and note the inductor code for that region. E. Identify the inductor value from the inductor code, and select an appropriate inductor from the table shown in Figure 9. Part numbers are listed for three inductor manufacturers. The inductor chosen must be rated for operation at the LM2576 switching fre- quency (52 kHz) and for a current rating of 1.15 x I LOAD. For additional inductor information, see the inductor sec- tion in the application hints section of this data sheet. 2. Inductor Selection (L1) A. Calculate E  T( V  µs) B. E  T = 115 V  µs C. ILOAD(Max) = 3A D. Inductance Region = H150 E. Inductor Value = 150 µH Choose from AIE part #415-0936Pulse Engineeringpart #PE-531115, or Renco part #RL2445. 3. Output Capacitor Selection (C OUT)A . The value of the output capacitor together with the inductor defines the dominate pole-pair of the switching regulator loop. For stable operation, the capacitor must satisfy the following requirement: The above formula yields capacitor values between 10 µF and 2200 µF that will satisfy the loop requirements for stable operation. But to achieve an acceptable output ripple voltage, (approximately 1% of the output voltage) and transient response, the output capacitor may need to be several times larger than the above formula yields. B. The capacitor’s voltage rating should be at last 1.5 times greater than the output voltage. For a 10V regulator, a rating of at least 15V or more is recommended. Higher voltage electrolytic capacitors generally have lower ESR numbers, and for this reason it may be necessary to select a capacitor rate for a higher voltage than would normally be needed. 3. Output Capacitor Selection (C OUT) However, for acceptable output ripple voltage select COUT ≥ 680 µF COUT = 680 µF electrolytic capacitor 4. Catch Diode Selection (D1) A. The catch-diode current rating must be at least 1.2 times greater than the maximum load current. Also, if the power supply design must withstand a continuous output short, the diode should have a current rating equal to the maximum current limit of the LM2576. The most stressful condition for this diode is an overload or shorted output. See diode selection guide in Figure 8. B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 4. Catch Diode Selection (D1) A. For this example, a 3.3A current rating is adequate. B. Use a 30V 31DQ03 Schottky diode, or any of the suggested fast-recovery diodes in Figure 8. 5. Input Capacitor (C IN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation. 5. Input Capacitor (C IN) A 100 µF aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing. To further simplify the buck regulator design procedure, Na- tional Semiconductor is making available computer design software to be used with the SIMPLE SWITCHER line of switching regulators.Switchers Made Simple(Version 3.3) is available on a (3 1⁄2") diskette for IBM compatible comput- ers from a National Semiconductor sales office in your area. LM2576/LM2576HV www.national.com 14

Application Hints (Continued) INDUCTOR SELECTION All switching regulators have two basic modes of operation: continuous and discontinuous. The difference between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a period of time in the normal switching cycle. Each mode has distinctively different operating characteristics, which can affect the regulator per- formance and requirements. The LM2576 (or any of the SIMPLE SWITCHER family) can be used for both continuous and discontinuous modes of operation. The inductor value selection guides in Figure 3 through Figure 7 were designed for buck regulator designs of the continuous inductor current type. When using inductor val- ues shown in the inductor selection guide, the peak-to-peak inductor ripple current will be approximately 20% to 30% of the maximum DC current. With relatively heavy load cur- rents, the circuit operates in the continuous mode (inductor current always flowing), but under light load conditions, the circuit will be forced to the discontinuous mode (inductor current falls to zero for a period of time). This discontinuous mode of operation is perfectly acceptable. For light loads (less than approximately 300 mA) it may be desirable to operate the regulator in the discontinuous mode, primarily because of the lower inductor values required for the discon- tinuous mode. The selection guide chooses inductor values suitable for continuous mode operation, but if the inductor value chosen is prohibitively high, the designer should investigate the possibility of discontinuous operation. The computer design software Switchers Made Simplewill provide all component values for discontinuous (as well as continuous) mode of operation. Inductors are available in different styles such as pot core, toriod, E-frame, bobbin core, etc., as well as different core materials, such as ferrites and powdered iron. The least expensive, the bobbin core type, consists of wire wrapped on a ferrite rod core. This type of construction makes for an inexpensive inductor, but since the magnetic flux is not com- pletely contained within the core, it generates more electro- magnetic interference (EMI). This EMI can cause problems in sensitive circuits, or can give incorrect scope readings because of induced voltages in the scope probe. The inductors listed in the selection chart include ferrite pot core construction for AIE, powdered iron toroid for Pulse Engineering, and ferrite bobbin core for Renco. An inductor should not be operated beyond its maximum rated current because it may saturate. When an inductor begins to saturate, the inductance decreases rapidly and the inductor begins to look mainly resistive (the DC resistance of the winding). This will cause the switch current to rise very rapidly. Different inductor types have different saturation characteristics, and this should be kept in mind when select- ing an inductor. The inductor manufacturer’s data sheets include current and energy limits to avoid inductor saturation. INDUCTOR RIPPLE CURRENT When the switcher is operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the input voltage). For a given input voltage and output voltage, the peak-to- peak amplitude of this inductor current waveform remains constant. As the load current rises or falls, the entire saw- tooth current waveform also rises or falls. The average DC value of this waveform is equal to the DC load current (in the buck regulator configuration). If the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and the switcher will change to a discontinuous mode of operation. This is a perfectly acceptable mode of operation. Any buck switching regulator (no matter how large the inductor value is) will be forced to run discontinuous if the load current is light enough. OUTPUT CAPACITOR An output capacitor is required to filter the output voltage and is needed for loop stability. The capacitor should be located near the LM2576 using short pc board traces. Standard aluminum electrolytics are usually adequate, but low ESR types are recommended for low output ripple voltage and good stability. The ESR of a capacitor depends on many factors, some which are: the value, the voltage rating, physi- cal size and the type of construction. In general, low value or low voltage (less than 12V) electrolytic capacitors usually have higher ESR numbers. The amount of output ripple voltage is primarily a function of the ESR (Equivalent Series Resistance) of the output ca- pacitor and the amplitude of the inductor ripple current (∆I IND). See the section on inductor ripple current in Applica- tion Hints. The lower capacitor values (220 µF–1000 µF) will allow typically 50 mV to 150 mV of output ripple voltage, while larger-value capacitors will reduce the ripple to approxi- mately 20 mV to 50 mV. Output Ripple Voltage = ( ∆I IND) (ESR of C OUT) To further reduce the output ripple voltage, several standard electrolytic capacitors may be paralleled, or a higher-grade capacitor may be used. Such capacitors are often called “high-frequency,” “low-inductance,” or “low-ESR.” These will reduce the output ripple to 10 mV or 20 mV. However, when operating in the continuous mode, reducing the ESR below 0.03Ω can cause instability in the regulator. Tantalum capacitors can have a very low ESR, and should be carefully evaluated if it is the only output capacitor. Be- cause of their good low temperature characteristics, a tan- talum can be used in parallel with aluminum electrolytics, with the tantalum making up 10% or 20% of the total capaci- tance. The capacitor’s ripple current rating at 52 kHz should be at least 50% higher than the peak-to-peak inductor ripple cur- rent. LM2576/LM2576HV www.national.com 16

Application Hints (Continued) CATCH DIODE Buck regulators require a diode to provide a return path for the inductor current when the switch is off. This diode should be located close to the LM2576 using short leads and short printed circuit traces. Because of their fast switching speed and low forward volt- age drop, Schottky diodes provide the best efficiency, espe- cially in low output voltage switching regulators (less than 5V). Fast-Recovery, High-Efficiency, or Ultra-Fast Recovery diodes are also suitable, but some types with an abrupt turn-off characteristic may cause instability and EMI prob- lems. A fast-recovery diode with soft recovery characteristics is a better choice. Standard 60 Hz diodes (e.g., 1N4001 or 1N5400, etc.) are also not suitable. See Figure 8for Schot- tky and “soft” fast-recovery diode selection guide. OUTPUT VOLTAGE RIPPLE AND TRANSIENTS The output voltage of a switching power supply will contain a sawtooth ripple voltage at the switcher frequency, typically about 1% of the output voltage, and may also contain short voltage spikes at the peaks of the sawtooth waveform. The output ripple voltage is due mainly to the inductor saw- tooth ripple current multiplied by the ESR of the output capacitor. (See the inductor selection in the application hints.) The voltage spikes are present because of the the fast switching action of the output switch, and the parasitic induc- tance of the output filter capacitor. To minimize these voltage spikes, special low inductance capacitors can be used, and their lead lengths must be kept short. Wiring inductance, stray capacitance, as well as the scope probe used to evalu- ate these transients, all contribute to the amplitude of these spikes. An additional small LC filter (20 µH & 100 µF) can be added to the output (as shown in Figure 15) to further reduce the amount of output ripple and transients. A 10 x reduction in output ripple voltage and transients is possible with this filter. FEEDBACK CONNECTION The LM2576 (fixed voltage versions) feedback pin must be wired to the output voltage point of the switching power supply. When using the adjustable version, physically locate both output voltage programming resistors near the LM2576 to avoid picking up unwanted noise. Avoid using resistors greater than 100 k Ω because of the increased chance of noise pickup. ON /OFF INPUT For normal operation, the ON /OFF pin should be grounded or driven with a low-level TTL voltage (typically below 1.6V). To put the regulator into standby mode, drive this pin with a high-level TTL or CMOS signal. The ON /OFF pin can be safely pulled up to +V IN without a resistor in series with it. The ON /OFF pin should not be left open. GROUNDING To maintain output voltage stability, the power ground con- nections must be low-impedance (see Figure 2). For the 5-lead TO-220 and TO-263 style package, both the tab and pin 3 are ground and either connection may be used, as they are both part of the same copper lead frame. HEAT SINK/THERMAL CONSIDERATIONS In many cases, only a small heat sink is required to keep the LM2576 junction temperature within the allowed operating range. For each application, to determine whether or not a heat sink will be required, the following must be identified: 1. Maximum ambient temperature (in the application). 2. Maximum regulator power dissipation (in application). 3. Maximum allowed junction temperature (125˚C for the LM2576). For a safe, conservative design, a tempera- ture approximately 15˚C cooler than the maximum tem- peratures should be selected. 4. LM2576 package thermal resistances θ JA and θJC. Total power dissipated by the LM2576 can be estimated as follows: PD =( VIN)(IQ)+( V O/VIN)(ILOAD)(VSAT) where IQ (quiescent current) and V SAT can be found in the Characteristic Curves shown previously, V IN is the applied minimum input voltage, V O is the regulated output voltage, and I LOAD is the load current. The dynamic losses during turn-on and turn-off are negligible if a Schottky type catch diode is used. When no heat sink is used, the junction temperature rise can be determined by the following: J =( PD)( θJA) To arrive at the actual operating junction temperature, add the junction temperature rise to the maximum ambient tem- perature. T J = ∆TJ +T A If the actual operating junction temperature is greater than the selected safe operating junction temperature determined in step 3, then a heat sink is required. When using a heat sink, the junction temperature rise can be determined by the following: J =( PD)( θJC + θinterface + θHeat sink) The operating junction temperature will be: T J =T A + ∆TJ As above, if the actual operating junction temperature is greater than the selected safe operating junction tempera- ture, then a larger heat sink is required (one that has a lower thermal resistance). Included on the Switcher Made Simple design software is a more precise (non-linear) thermal model that can be used to determine junction temperature with different input-output parameters or different component values. It can also calcu- late the heat sink thermal resistance required to maintain the regulators junction temperature below the maximum operat- ing temperature. Additional Applications INVERTING REGULATOR Figure 10shows a LM2576-12 in a buck-boost configuration to generate a negative 12V output from a positive input voltage. This circuit bootstraps the regulator’s ground pin to the negative output voltage, then by grounding the feedback pin, the regulator senses the inverted output voltage and regulates it to −12V. For an input voltage of 12V or more, the maximum available output current in this configuration is approximately 700 mA. At lighter loads, the minimum input voltage required drops to approximately 4.7V. LM2576/LM2576HV www.national.com17

turning the output switch OFF). ing in the continuous mode (vs. discontinuous mode). tance of the wire and the available source current. number of turns, and the duty cycle. FIGURE 16. Simple Model of a Real Capacitor

Physical Dimensions inches (millimeters) unless otherwise noted 5-Lead TO-220 (T) Order Number LM2576T-3.3, LM2576HVT-3.3, LM2576T-5.0, LM2576HVT-5.0, LM2576T-12, LM2576HVT-12, LM2576T-15, LM2576HVT-15, LM2576T-ADJ or LM2576HVT-ADJ LM2576/LM2576HV www.national.com21

Physical Dimensions inches (millimeters) unless otherwise noted (Continued) Bent, Staggered 5-Lead TO-220 (T) Order Number LM2576T-3.3 Flow LB03, LM2576T-XX Flow LB03, LM2576HVT-3.3 Flow LB03, LM2576T-5.0 Flow LB03, LM2576HVT-5.0 Flow LB03, LM2576T-12 Flow LB03, LM2576HVT-12 Flow LB03, LM2576T-15 Flow LB03, LM2576HVT-15 Flow LB03, LM2576T-ADJ Flow LB03 or LM2576HVT-ADJ Flow LB03 LM2576/LM2576HV www.national.com 22

Physical Dimensions inches (millimeters) unless otherwise noted (Continued) 5-Lead TO-263 (S) Order Number LM2576S-3.3, LM2576S-5.0, LM2576S-12,LM2576S-15, LM2576S-ADJ, LM2576HVS-3.3, LM2576HVS-5.0, LM2576HVS-12, LM2576HVS-15, or LM2576HVS-ADJ 5-Lead TO-263 in Tape & Reel (SX) Order Number LM2576SX-3.3, LM2576SX-5.0, LM2576SX-12, LM2576SX-15, LM2576SX-ADJ, LM2576HVSX-3.3, LM2576HVSX-5.0, LM2576HVSX-12, LM2576HVSX-15, or LM2576HVSX-ADJ LM2576/LM2576HV www.national.com23

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. BANNED SUBSTANCE COMPLIANCE National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2. National Semiconductor Americas Customer Support Center Email: new.feedback@nsc.com Tel: 1-800-272-9959 National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: ap.support@nsc.com National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: jpn.feedback@nsc.com Tel: 81-3-5639-7560 www.national.com LM2576/LM2576HV Series SIMPLE SWITCHER 3A Step-Down Voltage Regulator National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the righ t at any time without notice to change said circuitry and specifications.