LM1575 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 1A 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) 01147501 Note: Pin numbers are for the TO-220 package. SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation. August 2004 LM1575/LM2575/LM2575HV Series SIMPLE SWITCHER 1A Step-Down Voltage Regulator © 2004 National Semiconductor Corporation DS011475 www.national.com
Block Diagram and Typical Application Connection Diagrams (XX indicates output voltage option. See Ordering Information table for complete part number.) Straight Leads 5–Lead TO-220 (T) Bent, Staggered Leads 5-Lead TO-220 (T) 01147522 Top View LM2575T-XX or LM2575HVT-XX See NS Package Number T05A 01147523 Top View 01147524 Side View LM2575T-XX Flow LB03 or LM2575HVT-XX Flow LB03 See NS Package Number T05D 01147502 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 Note: Pin numbers are for the TO-220 package. FIGURE 1. LM1575/LM2575/LM2575HV www.national.com 2
Connection Diagrams (XX indicates output voltage option. See Ordering Information table for complete part number.) (Continued) 16–Lead DIP (N or J) 24-Lead Surface Mount (M) 01147525 *No Internal Connection Top View LM2575N-XX or LM2575HVN-XX See NS Package Number N16A LM1575J-XX-QML See NS Package Number J16A 01147526 *No Internal Connection Top View LM2575M-XX or LM2575HVM-XX See NS Package Number M24B TO-263(S) 5-Lead Surface-Mount Package 01147529 Top View 01147530 Side View LM2575S-XX or LM2575HVS-XX See NS Package Number TS5B
Ordering Information
Package NSC Standard High Temperature Type Package Voltage Rating Voltage Rating Range Number (40V) (60V) 5-Lead TO-220 T05A LM2575T-3.3 LM2575HVT-3.3 Straight Leads LM2575T-5.0 LM2575HVT-5.0 LM2575T-12 LM2575HVT-12 LM2575T-15 LM2575HVT-15 LM2575T-ADJ LM2575HVT-ADJ 5-Lead TO-220 T05D LM2575T-3.3 Flow LB03 LM2575HVT-3.3 Flow LB03 Bent and LM2575T-5.0 Flow LB03 LM2575HVT-5.0 Flow LB03 Staggered Leads LM2575T-12 Flow LB03 LM2575HVT-12 Flow LB03 LM2575T-15 Flow LB03 LM2575HVT-15 Flow LB03 LM2575T-ADJ Flow LB03 LM2575HVT-ADJ Flow LB03 LM1575/LM2575/LM2575HV www.national.com3
Ordering Information (Continued) Package NSC Standard High Temperature Type Package Voltage Rating Voltage Rating Range Number (40V) (60V) 16-Pin Molded N16A LM2575N-5.0 LM2575HVN-5.0 −40˚C ≤ TJ ≤ +125˚C DIP LM2575N-12 LM2575HVN-12 LM2575N-15 LM2575HVN-15 LM2575N-ADJ LM2575HVN-ADJ 24-Pin M24B LM2575M-5.0 LM2575HVM-5.0 Surface Mount LM2575M-12 LM2575HVM-12 LM2575M-15 LM2575HVM-15 LM2575M-ADJ LM2575HVM-ADJ 5-Lead TO-263 TS5B LM2575S-3.3 LM2575HVS-3.3 Surface Mount LM2575S-5.0 LM2575HVS-5.0 LM2575S-12 LM2575HVS-12 LM2575S-15 LM2575HVS-15 LM2575S-ADJ LM2575HVS-ADJ 16-Pin Ceramic J16A LM1575J-3.3-QML DIP LM1575J-5.0-QML LM1575J-12-QML −55˚C ≤ T J ≤ +150˚C LM1575J-15-QML LM1575J-ADJ-QML LM1575/LM2575/LM2575HV www.national.com 4
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 LM1575/LM2575 45V LM2575HV 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 sec.) 260˚C Operating Ratings Temperature Range LM1575 −55˚C ≤ TJ ≤ +150˚C LM2575/LM2575HV −40˚C ≤ TJ ≤ +125˚C Supply Voltage LM1575/LM2575 40V LM2575HV 60V LM1575-3.3, LM2575-3.3, LM2575HV-3.3
Electrical Characteristics
Specifications with standard type face are for T J = 25˚C, and those with boldface type apply over full Operating Tempera- ture Range . Symbol Parameter Conditions Typ LM1575-3.3 LM2575-3.3 Units (Limits)LM2575HV-3.3 Limit Limit (Note 2) (Note 3) SYSTEM PARAMETERS (Note 4) Test Circuit Figure 2 VOUT Output Voltage V IN = 12V, ILOAD = 0.2A 3.3 V Circuit of Figure 2 3.267 3.234 V(Min) 3.333 3.366 V(Max) VOUT Output Voltage 4.75V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 1A 3.3 V LM1575/LM2575 Circuit of Figure 2 3.200/3.168 3.168/3.135 V(Min) VOUT Output Voltage 4.75V ≤ VIN ≤ 60V, 0.2A ≤ ILOAD ≤ 1A 3.3 V LM2575HV Circuit of Figure 2 3.200/3.168 3.168/3.135 V(Min) η Efficiency V IN = 12V, ILOAD =1 A 7 5 % LM1575-5.0, LM2575-5.0, LM2575HV-5.0 Specifications with standard type face are for T J = 25˚C, and those with boldface type apply over full Operating Tempera- ture Range. Symbol Parameter Conditions Typ LM1575-5.0 LM2575-5.0 Units (Limits)LM2575HV-5.0 Limit Limit (Note 2) (Note 3) SYSTEM PARAMETERS (Note 4) Test Circuit Figure 2 VOUT Output Voltage V IN = 12V, ILOAD = 0.2A 5.0 V Circuit of Figure 2 4.950 4.900 V(Min) 5.050 5.100 V(Max) VOUT Output Voltage 0.2A ≤ ILOAD ≤ 1A, 5.0 V LM1575/LM2575 8V ≤ VIN ≤ 40V 4.850/ 4.800 4.800/4.750 V(Min) Circuit of Figure 2 5.150/5.200 5.200/5.250 V(Max) VOUT Output Voltage 0.2A ≤ ILOAD ≤ 1A, 5.0 V LM2575HV 8V ≤ VIN ≤ 60V 4.850/ 4.800 4.800/4.750 V(Min) Circuit of Figure 2 5.175/5.225 5.225/5.275 V(Max) LM1575/LM2575/LM2575HV www.national.com5
LM1575-5.0, LM2575-5.0, LM2575HV-5.0 Electrical Characteristics (Continued) Specifications with standard type face are for T J = 25˚C, and those with boldface type apply over full Operating Tempera- ture Range. Symbol Parameter Conditions Typ LM1575-5.0 LM2575-5.0 Units (Limits)LM2575HV-5.0 Limit Limit (Note 2) (Note 3) η Efficiency V IN = 12V, ILOAD =1 A 7 7 % LM1575-12, LM2575-12, LM2575HV-12 Specifications with standard type face are for T J = 25˚C, and those with boldface type apply over full Operating Tempera- ture Range . Symbol Parameter Conditions Typ LM1575-12 LM2575-12 Units (Limits)LM2575HV-12 Limit Limit (Note 2) (Note 3) SYSTEM PARAMETERS (Note 4) Test Circuit Figure 2 VOUT Output Voltage V IN = 25V, ILOAD = 0.2A 12 V Circuit of Figure 2 11.88 11.76 V(Min) 12.12 12.24 V(Max) VOUT Output Voltage 0.2A ≤ ILOAD ≤ 1A, 12 V LM1575/LM2575 15V ≤ VIN ≤ 40V 11.64/ 11.52 11.52/11.40 V(Min) Circuit of Figure 2 12.36/12.48 12.48/12.60 V(Max) VOUT Output Voltage 0.2A ≤ ILOAD ≤ 1A, 12 V LM2575HV 15V ≤ VIN ≤ 60V 11.64/ 11.52 11.52/11.40 V(Min) Circuit of Figure 2 12.42/12.54 12.54/12.66 V(Max) η Efficiency V IN = 15V, ILOAD =1 A 8 8 % LM1575-15, LM2575-15, LM2575HV-15 Specifications with standard type face are for T J = 25˚C, and those with boldface type apply over full Operating Tempera- ture Range . Symbol Parameter Conditions Typ LM1575-15 LM2575-15 Units (Limits)LM2575HV-15 Limit Limit (Note 2) (Note 3) SYSTEM PARAMETERS (Note 4) Test Circuit Figure 2 VOUT Output Voltage V IN = 30V, ILOAD = 0.2A 15 V Circuit of Figure 2 14.85 14.70 V(Min) 15.15 15.30 V(Max) VOUT Output Voltage 0.2A ≤ ILOAD ≤ 1A, 15 V LM1575/LM2575 18V ≤ VIN ≤ 40V 14.55/ 14.40 14.40/14.25 V(Min) Circuit of Figure 2 15.45/15.60 15.60/15.75 V(Max) VOUT Output Voltage 0.2A ≤ ILOAD ≤ 1A, 15 V LM2575HV 18V ≤ VIN ≤ 60V 14.55/ 14.40 14.40/14.25 V(Min) Circuit of Figure 2 15.525/15.675 15.68/15.83 V(Max) η Efficiency V IN = 18V, ILOAD =1 A 8 8 % LM1575/LM2575/LM2575HV www.national.com 6
LM1575-ADJ, LM2575-ADJ, LM2575HV-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 Typ LM1575-ADJ LM2575-ADJ Units (Limits)LM2575HV-ADJ Limit Limit (Note 2) (Note 3) SYSTEM PARAMETERS (Note 4) Test Circuit Figure 2 VOUT Feedback Voltage V IN = 12V, ILOAD = 0.2A 1.230 V VOUT = 5V 1.217 1.217 V(Min) Circuit of Figure 2 1.243 1.243 V(Max) VOUT Feedback Voltage 0.2A ≤ ILOAD ≤ 1A, 1.230 V LM1575/LM2575 8V ≤ VIN ≤ 40V 1.205/ 1.193 1.193/1.180 V(Min) VOUT = 5V, Circuit of Figure 2 1.255/1.267 1.267/1.280 V(Max) VOUT Feedback Voltage 0.2A ≤ ILOAD ≤ 1A, 1.230 V LM2575HV 8V ≤ VIN ≤ 60V 1.205/ 1.193 1.193/1.180 V(Min) VOUT = 5V, Circuit of Figure 2 1.261/1.273 1.273/1.286 V(Max) η Efficiency V IN = 12V, ILOAD = 1A, 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 Tempera- ture Range. Unless otherwise specified, V IN = 12V for the 3.3V, 5V, and Adjustable version, V IN = 25V for the 12V version, and VIN = 30V for the 15V version. I LOAD = 200 mA. Symbol Parameter Conditions Typ LM1575-XX LM2575-XX Units (Limits)LM2575HV-XX Limit Limit (Note 2) (Note 3) DEVICE PARAMETERS I b Feedback Bias Current V OUT = 5V (Adjustable Version Only) 50 100/ 500 100/500 nA fO Oscillator Frequency (Note 13) 52 kHz 47/43 47/42 kHz(Min) 58/62 58/63 kHz(Max) VSAT Saturation Voltage I OUT = 1A (Note 5) 0.9 V DC Max Duty Cycle (ON) (Note 6) 98 % 93 93 %(Min) ICL Current Limit Peak Current (Notes 5, 13) 2.2 A IL Output Leakage (Notes 7, 8) Output = 0V 2 2 mA(Max) Current Output = −1V 7.5 mA Output = −1V 30 30 mA(Max) I Q Quiescent Current (Note 7) 5 mA 10/12 10 mA(Max) ISTBY Standby Quiescent ON /OFF Pin = 5V (OFF) 50 µA Current 200/500 200 µA(Max) LM1575/LM2575/LM2575HV www.national.com7
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 Tempera- ture Range. Unless otherwise specified, V IN = 12V for the 3.3V, 5V, and Adjustable version, V IN = 25V for the 12V version, and VIN = 30V for the 15V version. I LOAD = 200 mA. Symbol Parameter Conditions Typ LM1575-XX LM2575-XX Units (Limits)LM2575HV-XX Limit Limit (Note 2) (Note 3) DEVICE PARAMETERS θ JA Thermal Resistance T Package, Junction to Ambient (Note 9) 65 θJA T Package, Junction to Ambient (Note 10) 45 ˚C/W θJC T Package, Junction to Case 2 θJA N Package, Junction to Ambient (Note 11) 85 θJA M Package, Junction to Ambient (Note 11) 100 θJA S Package, Junction to Ambient (Note 12) 37 ON /OFF CONTROL Test Circuit Figure 2 IIH ON /OFF Pin Input ON /OFF Pin = 5V (OFF) 12 µA Current 30 30 µA(Max) IIL ON /OFF Pin = 0V (ON) 0µ A 10 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 limits are used to calculate Average Outgoing Quality Level, and all are 100% production tested. Note 3: 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 4: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM1575/LM2575 is used as shown in the Figure 2 test circuit, system performance will be as shown in system parameters section of Electrical Characteristics. Note 5: Output (pin 2) sourcing current. No diode, inductor or capacitor connected to output pin. Note 6: Feedback (pin 4) removed from output and connected to 0V. Note 7: Feedback (pin 4) 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 8: VIN = 40V (60V for the high voltage version). Note 9: 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 10: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with1⁄2 inch leads soldered to a PC board containing approximately 4 square inches of copper area surrounding the leads. Note 11: Junction to ambient thermal resistance with approximately 1 square inch of pc board copper surrounding the leads. Additional copper area will lower thermal resistance further. See thermal model in Switchers made Simple software. Note 12: 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 13: The oscillator frequency reduces to approximately 18 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%. Note 14: Refer to RETS LM1575J for current revision of military RETS/SMD. LM1575/LM2575/LM2575HV www.national.com 8
Typical Performance Characteristics (Circuit of Figure 2) Normalized Output Voltage Line Regulation 01147532 01147533 Dropout Voltage Current Limit 01147534 01147535 Quiescent Current Standby Quiescent Current 01147536 01147537 LM1575/LM2575/LM2575HV www.national.com9
Typical Performance Characteristics (Circuit of Figure 2) (Continued) Oscillator Frequency Switch Saturation Voltage 01147538 01147539 Efficiency Minimum Operating Voltage 01147540 01147541 Quiescent Current vs Duty Cycle Feedback Voltage vs Duty Cycle 01147542 01147543 LM1575/LM2575/LM2575HV www.national.com 10
Typical Performance Characteristics (Circuit of Figure 2) (Continued) Feedback Pin Current Maximum Power Dissipation (TO-263) (See (Note 12)) 01147505 01147528 Switching Waveforms Load Transient Response 01147506 VOUT =5 V A: Output Pin Voltage, 10V/div B: Output Pin Current, 1A/div C: Inductor Current, 0.5A/div D: Output Ripple Voltage, 20 mV/div, AC-Coupled Horizontal Time Base: 5 µs/div 01147507 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. LM1575/LM2575/LM2575HV www.national.com11
Test Circuit and Layout Guidelines (Continued) Fixed Output Voltage Versions 01147508 CIN — 100 µF, 75V, Aluminum Electrolytic COUT — 330 µF, 25V, Aluminum Electrolytic D1 — Schottky, 11DQ06 L1 — 330 µH, PE-52627 (for 5V in, 3.3V out, use 100 µH, PE-92108) Adjustable Output Voltage Version 01147509 where VREF = 1.23V, R1 between 1k and 5k. R1 — 2k, 0.1% R2 — 6.12k, 0.1% Note: Pin numbers are for the TO-220 package. FIGURE 2. LM1575/LM2575/LM2575HV www.national.com 12
- Inductor Selection (L1) A. Select the correct Inductor
ILOAD(Max), and note the inductor code for that region. C. Figure 9. Part numbers are listed for three inductor
- Inductor Selection (L1) A. Use the selection guide
- Output Capacitor Selection (C
appropriate, and a 10V or 15V rating is recommended.
- Output Capacitor Selection (C OUT)A . COUT = 100 µF
- Catch Diode Selection (D1) A. The catch-diode current
- Catch Diode Selection (D1) A. For this example, a 1A
needed for stable operation.
- Input Capacitor (C IN) A 47 µF, 25V aluminum electrolytic
- Programming Output Voltage (Selecting R1 and R2, as
appropriate resistor values.
- Inductor Selection (L1) A. Calculate the inductor Volt
- Inductor Selection (L1) A. Calculate E
- Output Capacitor Selection (C OUT)A . The value of the
- Output Capacitor Selection (C
- Catch Diode Selection (D1) A. The catch-diode current
Figure 8. B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage.
- Catch Diode Selection (D1) A. For this example, a 3A
fast-recovery diodes in Figure 8.
Inductor Value Selection Guides (For Continuous Mode Operation) (Continued) PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions) 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, National 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 (31⁄2") diskette for IBM compatible computers from a National Semiconductor sales office in your area. LM1575/LM2575/LM2575HV www.national.com 16
INPUT CAPACITOR (CIN) To maintain stability, the regulator input pin must be by- passed with at least a 47 µF electrolytic capacitor. The capacitor’s leads must be kept short, and located near the regulator. If the operating temperature range includes temperatures below −25˚C, the input capacitor value may need to be larger. With most electrolytic capacitors, the capacitance value decreases and the ESR increases with lower tempera- tures and age. Paralleling a ceramic or solid tantalum ca- pacitor will increase the regulator stability at cold tempera- tures. For maximum capacitor operating lifetime, the capacitor’s RMS ripple current rating should be greater than 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 LM2575 (or any of the Simple Switcher family) can be used for both continuous and discontinuous modes of opera- tion. 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 200 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 LM2575 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–680 µF) will allow typi- cally 50 mV to 150 mV of output ripple voltage, while larger- value capacitors will reduce the ripple to approximately 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 0.05Ω can cause instability in the regulator. LM1575/LM2575/LM2575HV www.national.com 18
Application Hints (Continued) 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. 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 LM2575 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 fast switching action of the output switch, and the parasitic inductance 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 evaluate 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 LM2575 (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 LM2575 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 TO-3 style package, the case is ground. For the 5-lead TO-220 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. With the N or M packages, all the pins labeled ground, power ground, or signal ground should be soldered directly to wide printed circuit board copper traces. This assures both low inductance connections and good thermal properties. HEAT SINK/THERMAL CONSIDERATIONS In many cases, no heat sink is required to keep the LM2575 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 (150˚C for the LM1575 or 125˚C for the LM2575). For a safe, conser- vative design, a temperature approximately 15˚C cooler than the maximum temperature should be selected. 4. LM2575 package thermal resistances θ JA and θJC. Total power dissipated by the LM2575 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: TJ =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). When using the LM2575 in the plastic DIP (N) or surface mount (M) packages, several items about the thermal prop- erties of the packages should be understood. The majority of the heat is conducted out of the package through the leads, with a minor portion through the plastic parts of the package. LM1575/LM2575/LM2575HV www.national.com19
Definition of Terms (Continued) CONTINUOUS/DISCONTINUOUS MODE OPERATION Relates to the inductor current. In the continuous mode, the inductor current is always flowing and never drops to zero, vs. the discontinuous mode, where the inductor current drops to zero for a period of time in the normal switching cycle. INDUCTOR SATURATION The condition which exists when an inductor cannot hold any more magnetic flux. When an inductor saturates, the induc- tor appears less inductive and the resistive component domi- nates. Inductor current is then limited only by the DC resis- tance of the wire and the available source current. OPERATING VOLT MICROSECOND CONSTANT (E Top) The product (in VoItµs) of the voltage applied to the inductor and the time the voltage is applied. This E Top 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. LM1575/LM2575/LM2575HV www.national.com23
Physical Dimensions inches (millimeters) unless otherwise noted 16-Lead Ceramic Dual-in-Line (J) Order Number LM1575J-3.3/883, LM1575J-5.0/883, LM1575J-12/883, LM1575J-15/883, or LM1575J-ADJ/883 14-Lead Wide Surface Mount (WM) Order Number LM2575M-5.0, LM2575HVM-5.0, LM2575M-12, LM2575HVM-12, LM2575M-15, LM2575HVM-15, LM2575M-ADJ or LM2575HVM-ADJ LM1575/LM2575/LM2575HV www.national.com 24
Physical Dimensions inches (millimeters) unless otherwise noted (Continued) 16-Lead Molded DIP (N) Order Number LM2575N-5.0, LM2575HVN-5.0, LM2575N-12, LM2575HVN-12, LM2575N-15, LM2575HVN-15, LM2575N-ADJ or LM2575HVN-ADJ 5-Lead TO-220 (T) Order Number LM2575T-3.3, LM2575HVT-3.3, LM2575T-5.0, LM2575HVT-5.0, LM2575T-12, LM2575HVT-12, LM2575T-15, LM2575HVT-15, LM2575T-ADJ or LM2575HVT-ADJ LM1575/LM2575/LM2575HV www.national.com25
Physical Dimensions inches (millimeters) unless otherwise noted (Continued) TO-263, Molded, 5-Lead Surface Mount Order Number LM2575S-3.3, LM2575HVS-3.3, LM2575S-5.0, LM2575HVS-5.0, LM2575S-12, LM2575HVS-12, LM2575S-15, LM2575HVS-15, LM2575S-ADJ or LM2575HVS-ADJ LM1575/LM2575/LM2575HV www.national.com 26
Physical Dimensions inches (millimeters) unless otherwise noted (Continued) Bent, Staggered 5-Lead TO-220 (T) Order Number LM2575T-3.3 Flow LB03, LM2575HVT-3.3 Flow LB03, LM2575T-5.0 Flow LB03, LM2575HVT-5.0 Flow LB03, LM2575T-12 Flow LB03, LM2575HVT-12 Flow LB03, LM2575T-15 Flow LB03, LM2575HVT-15 Flow LB03, LM2575T-ADJ Flow LB03 or LM2575HVT-ADJ Flow LB03 LIFE SUPPORT POLICY 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 LM1575/LM2575/LM2575HV Series SIMPLE SWITCHER 1A 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.