LM2576/LM2576HV Series SIMPLE SWITCHER 3A Step-Down Voltage Regulator

August 2004

General Description
The LM2576 series of regulators are monolithic integrated circuits that provide all the active functions for a step-down (buck) switching regulator, capable of driving 3A load with excellent line and load regulation. These devices are available in fixed output voltages of 3.3V, 5V, 12V, 15V, and an adjustable output version. Requiring a minimum number of external components, these regulators are simple to use and include internal frequency compensation and a fixed-frequency oscillator. The LM2576 series offers a high-efficiency replacement for popular three-terminal linear regulators. It substantially reduces the size of the heat sink, and in some cases no heat sink is required. A standard series of inductors optimized for use with the LM2576 are available from several different manufacturers. This feature greatly simplifies the design of switch-mode power supplies. Other features include a guaranteed 4% tolerance on output voltage within specified input voltages and output load conditions, and 10% on the oscillator frequency. External shutdown is included, featuring 50 A (typical) standby current. The output switch includes cycle-by-cycle current limiting, as well as thermal shutdown for full protection under fault conditions.

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 n n n Simple high-efficiency step-down (buck) regulator Efficient pre-regulator for linear regulators On-card switching regulators Positive to negative converter (Buck-Boost)

Typical Application

Versions)

(Fixed Output Voltage

01147601

FIGURE 1.

SIMPLE SWITCHER is a registered trademark of National Semiconductor Corporation.
2004 National Semiconductor Corporation

DS011476

www.national.com

LM2576/LM2576HV

Block Diagram

01147602

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

Ordering Information

Temperature Range Output Voltage 3.3 5.0 LM2576HVS-5.0 LM2576S-5.0 LM2576SX-5.0 LM2576HVT-5.0 LM2576T-5.0 LM2576HVT-5.0 Flow LB03 LM2576T-5.0 Flow LBLM2576S-12 LM2576SX-12 LM2576T-12 Flow LB03 LM2576T-12 Flow LBLM2576S-15 LM2576SX-15 LM2576T-15 Flow LB03 LM2576T-15 Flow LB03 ADJ LM2576S-ADJ LM2576SX-ADJ LM2576T-ADJ T05D Flow LB03 LM2576T-ADJ Flow LB03 TS5B Tape & Reel T05A TO-220 LM2576HVS-12 LM2576HVS-15 LM2576HVS-ADJ NS Package Package Type Number TS5B TO-263
40C TA LM2576HVS-3.3 125C LM2576S-3.3 LM2576SX-3.3 LM2576HVT-3.3 LM2576T-3.3 LM2576HVT-3.3 Flow LB03 LM2576T-3.3 Flow LB03
LM2576HVSX-3.3 LM2576HVSX-5.0 LM2576HVSX-12 LM2576HVSX-15 LM2576HVSX-ADJ LM2576HVT-12 LM2576HVT-15 LM2576HVT-ADJ LM2576HVT-12 LM2576HVT-15 LM2576HVT-ADJ
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 LM2576HV ON /OFF Pin Input Voltage Output Voltage to Ground (Steady State) Power Dissipation Storage Temperature Range Maximum Junction Temperature 1V Internally Limited 65C to +150C 150C 45V 63V 0.3V V +VIN
Minimum ESD Rating (C = 100 pF, R = 1.5 k) Lead Temperature (Soldering, 10 Seconds) 260C 2 kV

Operating Ratings

Temperature Range LM2576/LM2576HV Supply Voltage LM2576 LM2576HV 40V 60V 40C TJ +125C
LM2576-3.3, LM2576HV-3.3 Electrical Characteristics
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN = 12V, ILOAD = 0.5A Circuit of Figure 2 VOUT Output Voltage LM2576 VOUT Output Voltage LM2576HV Efficiency 6V VIN 40V, 0.5A ILOAD 3A Circuit of Figure 2 6V VIN 60V, 0.5A ILOAD 3A Circuit of Figure 2 VIN = 12V, ILOAD = 3A 75 3.3 3.168/3.135 3.450/3.482 3.3 3.168/3.135 3.432/3.465 3.3 3.234 3.366 V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) % LM2576-3.3 LM2576HV-3.3 Limit (Note 2) Units (Limits)
LM2576-5.0, LM2576HV-5.0 Electrical Characteristics

Specifications with standard type face are for TJ = 25C, and those with Figure 2 boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN = 12V, ILOAD = 0.5A Circuit of Figure 2 VOUT Output Voltage LM2576 VOUT Output Voltage LM2576HV 0.5A ILOAD 3A, 8V VIN 40V Circuit of Figure 2 0.5A ILOAD 3A, 8V VIN 60V Circuit of Figure 2 5.0 4.800/4.750 5.225/5.275 5.0 4.800/4.750 5.200/5.250 5.0 4.900 5.100 V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) LM2576-5.0 LM2576HV-5.0 Limit (Note 2) Units (Limits)
LM2576-5.0, LM2576HV-5.0 Electrical Characteristics (Continued)
Specifications with standard type face are for TJ = 25C, and those with Figure 2 boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 Efficiency VIN = 12V, ILOAD = 3A 77 % LM2576-5.0 LM2576HV-5.0 Limit (Note 2) Units (Limits)
LM2576-12, LM2576HV-12 Electrical Characteristics
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN = 25V, ILOAD = 0.5A Circuit of Figure 2 VOUT Output Voltage LM2576 VOUT Output Voltage LM2576HV Efficiency 0.5A ILOAD 3A, 15V VIN 40V Circuit of Figure 2 0.5A ILOAD 3A, 15V VIN 60V Circuit of Figure 2 VIN = 15V, ILOAD = 3A 11.52/11.40 12.54/12.11.52/11.40 12.48/12.11.76 12.24 V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) % LM2576-12 LM2576HV-12 Limit (Note 2) Units (Limits)
LM2576-15, LM2576HV-15 Electrical Characteristics
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN = 25V, ILOAD = 0.5A Circuit of Figure 2 VOUT Output Voltage LM2576 VOUT Output Voltage LM2576HV Efficiency 0.5A ILOAD 3A, 18V VIN 40V Circuit of Figure 2 0.5A ILOAD 3A, 18V VIN 60V Circuit of Figure 2 VIN = 18V, ILOAD = 3A 14.40/14.25 15.68/15.14.40/14.25 15.60/15.14.70 15.30 V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) % LM2576-15 LM2576HV-15 Limit (Note 2) Units (Limits)
LM2576-ADJ, LM2576HV-ADJ Electrical Characteristics
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Feedback Voltage VIN = 12V, ILOAD = 0.5A VOUT = 5V, Circuit of Figure 2 VOUT Feedback Voltage LM2576 VOUT Feedback Voltage LM2576HV Efficiency 0.5A ILOAD 3A, 8V VIN 40V VOUT = 5V, Circuit of Figure 2 0.5A ILOAD 3A, 8V VIN 60V VOUT = 5V, Circuit of Figure 2 VIN = 12V, ILOAD = 3A, VOUT = 5V 77 1.230 1.193/1.180 1.273/1.286 1.230 1.193/1.180 1.267/1.280 1.230 1.217 1.243 V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) % LM2576-ADJ LM2576HV-ADJ Limit (Note 2) Units (Limits)

All Output Voltage Versions Electrical Characteristics
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version, VIN = 25V for the 12V version, and VIN = 30V for the 15V version. ILOAD = 500 mA. Symbol Parameter Conditions LM2576-XX LM2576HV-XX Typ DEVICE PARAMETERS Ib fO Feedback Bias Current Oscillator Frequency VOUT = 5V (Adjustable Version Only) (Note 11) 47/42 58/63 VSAT DC ICL Saturation Voltage Max Duty Cycle (ON) Current Limit IOUT = 3A (Note 4) (Note 5) (Notes 4, 11) 1.4 1.8/2.93 5.8 4.2/3.5 6.9/7.5 IL Output Leakage Current (Notes 6, 7): Output = 0V Output = 1V Output = 1V IQ ISTBY Quiescent Current Standby Quiescent Current (Note 6) ON /OFF Pin = 5V (OFF) 7.2 100/500 nA kHz kHz (Min) kHz (Max) V V(Max) % %(Min) A A(Min) A(Max) mA(Max) mA mA(Max) mA mA(Max) A A(Max) Limit (Note 2) Units (Limits)
All Output Voltage Versions Electrical Characteristics (Continued)
Specifications with standard type face are for TJ = 25C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version, VIN = 25V for the 12V version, and VIN = 30V for the 15V version. ILOAD = 500 mA. Symbol Parameter Conditions LM2576-XX LM2576HV-XX Typ DEVICE PARAMETERS JA JA JC JA VIH VIL IIH IIL ON /OFF Pin Logic Input Level ON /OFF Pin Input Current ON /OFF Pin = 0V (ON) Thermal Resistance T Package, Junction to Ambient (Note 8) T Package, Junction to Ambient (Note 9) T Package, Junction to Case S Package, Junction to Ambient (Note 10) VOUT = 0V VOUT = Nominal Output Voltage ON /OFF Pin = 5V (OFF) 1.4 1.30 2.2/2.4 1.0/0.8 V(Min) V(Max) A A(Max) A A(Max) C/W Limit (Note 2) Units (Limits)
ON /OFF CONTROL Test Circuit Figure 2
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 2 test 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 12 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, with 14 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, JA is 50C/W, with 1 square inch of copper area, JA is 37C/W, and with 1.6 or more square inches of copper area, JA is 32C/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
Typical Performance Characteristics (Circuit of Figure 2)

Dropout Voltage

(Continued) Current Limit

01147629

01147630

Quiescent Current

Standby Quiescent Current

01147631

01147632

Oscillator Frequency

Switch Saturation Voltage

01147633

01147634

Efficiency

(Continued)
Minimum Operating Voltage

01147635

01147636
Quiescent Current vs Duty Cycle
Feedback Voltage vs Duty Cycle

01147637

01147638

Feedback Pin Current

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 s/div

Load Transient Response

01147605
Test Circuit and Layout Guidelines
As in any switching regulator, layout is very important. Rapidly 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 construction should be used for best results. When using the Adjustable version, physically locate the programming resistors near the regulator, to keep the sensitive feedback wiring short.
Fixed Output Voltage Versions

01147607

CIN 100 F, 75V, Aluminum Electrolytic COUT 1000 F, 25V, Aluminum Electrolytic D1 Schottky, MBR360 LH, 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 Series Buck Regulator Design Procedure
PROCEDURE (Fixed Output Voltage Versions) Given: VOUT = Regulated Output Voltage (3.3V, 5V, 12V, or 15V) VIN(Max) = Maximum Input Voltage ILOAD(Max) = Maximum Load Current 1. Inductor Selection (L1) A. Select the correct Inductor value selection guide from Figures 3, 4, 5 or 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 VIN(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 ILOAD. For additional inductor information, see the inductor section in the Application Hints section of this data sheet. 2. Output Capacitor Selection (COUT) 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 F and 470 F is recommended. B. The capacitors 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. 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. 4. Input Capacitor (CIN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation. EXAMPLE (Fixed Output Voltage Versions) Given: VOUT = 5V VIN(Max) = 15V ILOAD(Max) = 3A

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 H. From the table in Figure 3. Choose AIE 415-0930, Pulse Engineering PE92108, or Renco RL2444.
2. Output Capacitor Selection (COUT) A. COUT = 680 F to 2000 F standard aluminum electrolytic. B.Capacitor voltage rating = 20V.
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 (CIN) A 100 F, 25V aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing.
LM2576 Series Buck Regulator Design Procedure (Continued)
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)

01147611

FIGURE 5. LM2576(HV)-12

01147609

FIGURE 3. LM2576(HV)-3.3

01147612

01147610

FIGURE 6. LM2576(HV)-15

FIGURE 4. LM2576(HV)-5.0

01147613

FIGURE 7. LM2576(HV)-ADJ
PROCEDURE (Adjustable Output Voltage Versions) Given: VOUT = Regulated Output Voltage VIN(Max) = Maximum Input Voltage ILOAD(Max) = Maximum Load Current F = Switching Frequency (Fixed at 52 kHz) 1. Programming Output Voltage (Selecting R1 and R2, as shown in Figure 2) Use the following formula to select the appropriate resistor values.
EXAMPLE (Adjustable Output Voltage Versions) Given: VOUT = 10V VIN(Max) = 25V ILOAD(Max) = 3A F = 52 kHz 1. Programming Output Voltage (Selecting R1 and R2)
R1 can be between 1k and 5k. (For best temperature coefficient and stability with time, use 1% metal film resistors)
R2 = 1k (8.13 1) = 7.13k, closest 1% value is 7.15k
PROCEDURE (Adjustable Output Voltage Versions) 2. Inductor Selection (L1) A. Calculate the inductor Volt microsecond constant, E T (V s), from the following formula:
EXAMPLE (Adjustable Output Voltage Versions) 2. Inductor Selection (L1) A. Calculate E T (V s)
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 frequency (52 kHz) and for a current rating of 1.15 x ILOAD. For additional inductor information, see the inductor section in the application hints section of this data sheet. 3. Output Capacitor Selection (COUT) 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:

B. E T = 115 V s C. ILOAD(Max) = 3A D. Inductance Region = H150 E. Inductor Value = 150 H Choose from AIE part #415-0936 Pulse Engineering part #PE-531115, or Renco part #RL2445.
3. Output Capacitor Selection (COUT)
However, for acceptable output ripple voltage select COUT 680 F COUT = 680 F electrolytic capacitor
The above formula yields capacitor values between 10 F and 2200 F 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 capacitors 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. 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. 5. Input Capacitor (CIN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation. 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 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 (CIN) A 100 F aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing. switching regulators. Switchers Made Simple (Version 3.3) is available on a (312") diskette for IBM compatible computers from a National Semiconductor sales office in your area.
VR 3A 20V 1N5820 MBR320P SR302 30V 1N5821 MBR330 31DQ03 SR303 40V 1N5822 MBR340 31DQ04 SR304 50V MBR350 31DQ05 SR305 60V MBR360 DQ06 SR306

Schottky 4A6A 1N5823 3A

Fast Recovery 4A6A
50WQ03 1N5824 The following diodes are all rated to 100V 31DF1 HER302 The following diodes are all rated to 100V 50WF10 MUR410 HER602

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 performance 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 values 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 currents, 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 discontinuous 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 Simple will 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 completely contained within the core, it generates more electromagnetic 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

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 voltage drop, Schottky diodes provide the best efficiency, especially 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 problems. 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 8 for Schottky 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 sawtooth 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 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 H & 100 F) 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 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 +VIN 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 connections 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). Maximum regulator power dissipation (in application). Maximum allowed junction temperature (125C for the LM2576). For a safe, conservative design, a temperature approximately 15C cooler than the maximum temperatures should be selected. 4. LM2576 package thermal resistances JA and JC. Total power dissipated by the LM2576 can be estimated as follows: PD = (VIN)(IQ) + (VO/VIN)(ILOAD)(VSAT) where IQ (quiescent current) and VSAT can be found in the Characteristic Curves shown previously, VIN is the applied minimum input voltage, VO is the regulated output voltage, and ILOAD 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: TJ = (PD) (JA) To arrive at the actual operating junction temperature, add the junction temperature rise to the maximum ambient temperature. TJ = TJ + TA 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: TJ = (PD) (JC + interface + Heat sink) The operating junction temperature will be: TJ = TA + TJ As above, if the actual operating junction temperature is greater than the selected safe operating junction temperature, 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 calculate the heat sink thermal resistance required to maintain the regulators junction temperature below the maximum operating temperature. 2. 3.

Additional Applications

INVERTING REGULATOR Figure 10 shows a LM2576-12 in a buck-boost configuration to generate a negative 12V output from a positive input voltage. This circuit bootstraps the regulators 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.
The switch currents in this buck-boost configuration are higher than in the standard buck-mode design, thus lowering the available output current. Also, the start-up input current of the buck-boost converter is higher than the standard buck-mode regulator, and this may overload an input power source with a current limit less than 5A. Using a delayed turn-on or an undervoltage lockout circuit (described in the next section) would allow the input voltage to rise to a high enough level before the switcher would be allowed to turn on. Because of the structural differences between the buck and the buck-boost regulator topologies, the buck regulator design procedure section can not be used to to select the inductor or the output capacitor. The recommended range of inductor values for the buck-boost design is between 68 H and 220 H, and the output capacitor values must be larger than what is normally required for buck designs. Low input voltages or high output currents require a large value output capacitor (in the thousands of micro Farads). The peak inductor current, which is the same as the peak switch current, can be calculated from the following formula:

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Typical Load Current 400 mA for VIN = 5.2V 750 mA for VIN = 7V Note: Heat sink may be required.
FIGURE 11. Negative Boost Because of the boosting function of this type of regulator, the switch current is relatively high, especially at low input voltages. Output load current limitations are a result of the maximum current rating of the switch. Also, boost regulators can not provide current limiting load protection in the event of a shorted load, so some other means (such as a fuse) may be necessary. UNDERVOLTAGE LOCKOUT In some applications it is desirable to keep the regulator off until the input voltage reaches a certain threshold. An undervoltage lockout circuit which accomplishes this task is shown in Figure 12, while Figure 13 shows the same circuit applied to a buck-boost configuration. These circuits keep the regulator off until the input voltage reaches a predetermined level. VTH VZ1 + 2VBE(Q1)
Where fosc = 52 kHz. Under normal continuous inductor current operating conditions, the minimum VIN represents the worst case. Select an inductor that is rated for the peak current anticipated.

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FIGURE 10. Inverting Buck-Boost Develops 12V Also, the maximum voltage appearing across the regulator is the absolute sum of the input and output voltage. For a 12V output, the maximum input voltage for the LM2576 is +28V, or +48V for the LM2576HV. The Switchers Made Simple (version 3.0) design software can be used to determine the feasibility of regulator designs using different topologies, different input-output parameters, different components, etc.
Note: Complete circuit not shown.

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NEGATIVE BOOST REGULATOR Another variation on the buck-boost topology is the negative boost configuration. The circuit in Figure 11 accepts an input voltage ranging from 5V to 12V and provides a regulated 12V output. Input voltages greater than 12V will cause the output to rise above 12V, but will not damage the regulator.
FIGURE 12. Undervoltage Lockout for Buck Circuit
ing. Increasing the RC time constant can provide longer delay times. But excessively large RC time constants can cause problems with input voltages that are high in 60 Hz or 120 Hz ripple, by coupling the ripple into the ON /OFF pin. ADJUSTABLE OUTPUT, LOW-RIPPLE POWER SUPPLY A 3A power supply that features an adjustable output voltage is shown in Figure 15. An additional L-C filter that reduces the output ripple by a factor of 10 or more is included in this circuit.

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Note: Complete circuit not shown (see Figure 10).
FIGURE 13. Undervoltage Lockout for Buck-Boost Circuit

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DELAYED STARTUP The ON /OFF pin can be used to provide a delayed startup feature as shown in Figure 14. With an input voltage of 20V and for the part values shown, the circuit provides approximately 10 ms of delay time before the circuit begins switch-

FIGURE 14. Delayed Startup

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FIGURE 15. 1.2V to 55V Adjustable 3A Power Supply with Low Output Ripple

Definition of Terms

BUCK REGULATOR A switching regulator topology in which a higher voltage is converted to a lower voltage. Also known as a step-down switching regulator. BUCK-BOOST REGULATOR A switching regulator topology in which a positive voltage is converted to a negative voltage without a transformer. DUTY CYCLE (D) Ratio of the output switchs on-time to the oscillator period. CATCH DIODE OR CURRENT STEERING DIODE The diode which provides a return path for the load current when the LM2576 switch is OFF. EFFICIENCY () The proportion of input power actually delivered to the load.
OPERATING VOLT MICROSECOND CONSTANT (E Top) The product (in VoIt s) 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.
CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR) The purely resistive component of a real capacitors impedance (see Figure 16). It causes power loss resulting in capacitor heating, which directly affects the capacitors operating lifetime. When used as a switching regulator output filter, higher ESR values result in higher output ripple voltages.
Connection Diagrams (Note 15)
Straight Leads 5-Lead TO-220 (T) Top View

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FIGURE 16. Simple Model of a Real Capacitor Most standard aluminum electrolytic capacitors in the 100 F1000 F range have 0.5 to 0.1 ESR. Highergrade capacitors (low-ESR, high-frequency, or lowinductance) in the 100 F1000 F range generally have ESR of less than 0.15. EQUIVALENT SERIES INDUCTANCE (ESL) The pure inductance component of a capacitor (see Figure 16). The amount of inductance is determined to a large extent on the capacitors construction. In a buck regulator, this unwanted inductance causes voltage spikes to appear on the output. OUTPUT RIPPLE VOLTAGE The AC component of the switching regulators output voltage. It is usually dominated by the output capacitors ESR multiplied by the inductors ripple current (IIND). The peakto-peak value of this sawtooth ripple current can be determined by reading the Inductor Ripple Current section of the Application hints. CAPACITOR RIPPLE CURRENT RMS value of the maximum allowable alternating current at which a capacitor can be operated continuously at a specified temperature. STANDBY QUIESCENT CURRENT (ISTBY) Supply current required by the LM2576 when in the standby mode (ON /OFF pin is driven to TTL-high voltage, thus turning the output switch OFF). INDUCTOR RIPPLE CURRENT (IIND) The peak-to-peak value of the inductor current waveform, typically a sawtooth waveform when the regulator is operating in the continuous mode (vs. discontinuous mode). 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 inductor appears less inductive and the resistive component dominates. Inductor current is then limited only by the DC resistance of the wire and the available source current.

www.national.com 20

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LM2576T-XX or LM2576HVT-XX NS Package Number T05A TO-263 (S) 5-Lead Surface-Mount Package Top View

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LM2576S-XX or LM2576HVS-XX NS Package Number TS5B LM2576SX-XX or LM2576HVSX-XX NS Package Number TS5B, Tape and Reel Bent, Staggered Leads 5-Lead TO-220 (T) Top View

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LM2576T-XX Flow LB03 or LM2576HVT-XX Flow LB03 NS Package Number T05D
Note 15: (XX indicates output voltage option. See ordering information table for complete part number.)

Physical Dimensions

unless otherwise noted

inches (millimeters)

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 NS Package Number T05A
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 NS Package Number T05D
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 NS Package Number TS5B 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 NS Package Number TS5B
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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

Buck-converter Charger also Provides System Power
Robert Hanrahan - National Semiconductor 11/16/96 Many systems require long time operation during periods of power loss. Often a Lead Acid battery (gel or wet-cell) is found to be the best solution because of the high capacity and relative low cost. The battery is charged during normal operation, and used to power the system during power loss. These systems require a circuit to charge the battery as well as regulate voltage for the system Vcc. In these systems one must provide a current limited voltage to the battery for charging, while developing system Vcc in both the charge or discharge condition. Many older designs would utilize inefficient linear regulators to provide these functions. These designs require a large heat sink for regulating the battery voltage to the system Vcc (typically 5V). One may utilize switching regulator technology to provide a much more efficient design at about the same relative cost as a linear regulator design. Many of these designs utilize low voltage AC power that is usually provided by a low cost wall transformer. Because of the switching technology utilized in this design, one could accommodate a wide input voltage range and thus may be used for power line voltages from 100V to 240V without any circuit changes. One of the best approaches to the design is a current limited voltage source that sources current into the battery until the battery voltage reaches a voltage setpoint. The charger then operates in a constant voltage mode, supplying the current required to maintain the voltage. Most lead acid batteries have a voltage setpoint of 13.8V at 25oC. The current limit is set depending on the exact battery and charge time requirement. The design shown in Figure 1 employs two Simple Switcher Buck converters from National Semiconductor. The first regulator U1 is an LM2576 or LM2596 Simple Switcher used to efficiently step down the unregulated input voltage from the output of the rectifier. This buck converter generates the input voltage for the battery while also providing voltage to the second regulator. Both buck regulators may utilize either a slower 52 kHz converter or a higher frequency device marked respectively. The higher frequency devices employ added features such as sync input and soft-start. The second regulator U2, is a small DIP or SO LM2574 or LM2594 capable of providing up to 0.5A system Vcc. One must consider the system current requirements when setting the current limit value of the charger. The current limit value set by the gain of U3 must be increased by the current required to supply power to the system.

Vbatt 12-13.8V

470uH MRB350

R5 0.1

MRB350

0.01uF

D5 330uH

Vout 5VDC @0.5A

330uF + C3 R3 100K R6 Vbatt 10K D2 LMC7101 U3+ 10K 1N914 0V R7 100K R4 C2 0.001uF R2 22K
LM2574-5 or LM2594-5 on/off

1N5817 D4 330uF

U1 BR1

Vin 16-40VAC

C1 330uF
LM2576-ADJ or LM2596-ADJ on/off

R1 2.1K

The first regulator provides the charge voltage setpoint with current regulation while the second regulator provides the system Vcc FIGURE 1. U1 is regulated at the Battery charge voltage with the feedback network R1/R2. These resistors are chosen by Vout=1.23(1+R2/R1). The diode D3 provides current switching between U1 and the battery during power loss. The shaded area is used to measure and regulate the current flow into the battery during battery charge. The circuit utilizes a shunt resistor to measure the current to the battery, and amplifies it via the amplifier U3. The LMC7101 shown for U3 is a National Semiconductor CMOS OP-Amp that provides an output voltage inversely proportional to current. The LMC7101 provides a BW of 1 MHz and is available in a very small SOT23 package. Other Op-Amps such as the National LMC6482 will also do the job, and are available in standard DIP and SO packages. With the gain of 10 provided by the Op-Amp, the diode D2 will forward bias and pull up the feedback voltage when the output current is about 1.6A (Vref+diode drop). During normal voltage regulation the diode is reverse biased. The second regulator U2 is used to provide 5V to the system. This buck regulator efficiently provides system power when the input is at its highest voltage of about 13.8V or at the lower voltage that will be generated when U1 is current limiting.

330uF + C3 R3 100K R6 Vbatt 10K D2 LMC7101 U3+ 10K 1N914 0V R7 100K Current Limiter R4 C2 0.001uF Vbatt 10K R8

R2 22K R1 2.1K

R10 100K

- U4LMC7211 + 0V

1M R11

LM4041 10K R9 -1.2

OPTIONAL Charge Detector
An additional SOT23 comparator and voltage reference provides a battery charge indication. FIGURE 2. Some systems may need an indication of charge complete. In a system that utilizes a microcontroller with on chip A/D (such as the National COP8ACC), one could connect the output of U3 into the input of the A/D, and read the charge current directly. Depending of the voltage reference used for the A/D, and accuracy needed, one may need to add another Op-Amp stage prior to the A/D. Figure 2 shows the circuit with an added SOT23 voltage comparator. The output of the current amplifier U2 is compared to the voltage set by the potentiometer R9. This voltage can be set to represent the current flow that takes place at the end of charge. Simple Switcher designs can be analyzed and verified by utilizing a software package called Switchers Made Simple from National Semiconductor. By splitting the design into two buck voltage regulators, one may utilize the software to obtain component values with vendor part numbers, junction temperatures, stability, and lots more design information. The software package is available free of charge by calling National at 1-800-272-9959 or through the Web site at www.national.com.