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LM2576 LM2576HV Series SIMPLE SWITCHER 3A Step-Down Voltage Regulator
September 1996
LM2576 LM2576HV Series SIMPLE SWITCHER 3A Step-Down Voltage Regulator
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 g4% tolerance on output voltage within specified input voltages and output load conditions and g10% on the oscillator frequency External shutdown is included featuring 50 mA (typical) standby current The output switch includes cycle-by-cycle current limiting as well as thermal shutdown for full protection under fault conditions
Features
Y Y
Y Y
Y Y Y Y Y Y Y
3 3V 5V 12V 15V and adjustable output versions Adjustable version output voltage range 1 23V to 37V (57V for HV version) g4% max over line and load conditions Guaranteed 3A output current Wide input voltage range 40V up to 60V for HV version Requires only 4 external components 52 kHz fixed frequency internal oscillator TTL shutdown capability low power standby mode High efficiency Uses readily available standard inductors Thermal shutdown and current limit protection P a Product Enhancement tested
Applications
Y Y Y Y
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 (Fixed Output Voltage Versions)
TL H 11476 - 1
FIGURE 1
Block Diagram
3 3V R2 e 1 7k 5V R2 e 3 1k 12V R2 e 8 84k 15V R2 e 11 3k For ADJ Version R1 e Open R2 e 0X Patent Pending
SIMPLE SWITCHER is a registered trademark of National Semiconductor Corporation C1996 National Semiconductor Corporation
TL H 11476 - 2
TL H 11476
RRD-B30M106 Printed in U S A
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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 45V 63V
b 0 3V s V s a VIN b 1V
Minimum ESD Rating (C e 100 pF R e 1 5 kX) Lead Temperature (Soldering 10 Seconds) Maximum Junction Temperature
2 kV 260 C 150 C
Operating Ratings
Temperature Range LM2576 LM2576HV Supply Voltage LM2576 LM2576HV
b 40 C s TJ s a 125 C
Internally Limited b 65 C to a 150 C
40V 60V
LM2576-3 3 LM2576HV-3 3 Electrical Characteristics Specifications with standard type face are for TJ e 25 C
type apply over full Operating Temperature Range Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN e 12V ILOAD e 0 5A Circuit of Figure 2 6V s VIN s 40V 0 5A s ILOAD s 3A Circuit of Figure 2 6V s VIN s 60V 0 5A s ILOAD s 3A Circuit of Figure 2 VIN e 12V ILOAD e 3A 33
and those with boldface
LM2576-3 3 LM2576HV-3 3 Limit (Note 2)
Units (Limits)
3 234 3 366 33 3 168 3 135 3 432 3 465 33 3 168 3 135 3 450 3 482 75
V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) %
VOUT
Output Voltage LM2576 Output Voltage LM2576HV Efficiency
VOUT
h
LM2576-5 0 LM2576HV-5 0 Electrical Characteristics Specifications with standard type face are for TJ e 25 C
type apply over full Operating Temperature Range Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN e 12V ILOAD e 0 5A Circuit of Figure 2 0 5A s ILOAD s 3A 8V s VIN s 40V Circuit of Figure 2 0 5A s ILOAD s 3A 8V s VIN s 60V Circuit of Figure 2 VIN e 12V ILOAD e 3A 50
and those with boldface
LM2576-5 0 LM2576HV-5 0 Limit (Note 2)
Units (Limits)
4 900 5 100 50 4 800 4 750 5 200 5 250 50 4 800 4 750 5 225 5 275 77
V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) %
VOUT
Output Voltage LM2576 Output Voltage LM2576HV Efficiency
VOUT
h
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LM2576-12 LM2576HV-12 Electrical Characteristics Specifications with standard type face are for TJ e 25 C
type apply over full Operating Temperature Range Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN e 25V ILOAD e 0 5A Circuit of Figure 2 0 5A s ILOAD s 3A 15V s VIN s 40V Circuit of Figure 2 0 5A s ILOAD s 3A 15V s VIN s 60V Circuit of Figure 2 VIN e 15V ILOAD e 3A 12
and those with boldface
LM2576-12 LM2576HV-12 Limit (Note 2)
Units (Limits)
11 76 12 24 12 11 52 11 40 12 48 12 60 12 11 52 11 40 12 54 12 66 88
V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) %
VOUT
Output Voltage LM2576 Output Voltage LM2576HV Efficiency
VOUT
h
LM2576-15 LM2576HV-15 Electrical Characteristics Specifications with standard type face are for TJ e 25 C
type apply over full Operating Temperature Range Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN e 25V ILOAD e 0 5A Circuit of Figure 2 0 5A s ILOAD s 3A 18V s VIN s 40V Circuit of Figure 2 0 5A s ILOAD s 3A 18V s VIN s 60V Circuit of Figure 2 VIN e 18V ILOAD e 3A 15
and those with boldface
LM2576-15 LM2576HV-15 Limit (Note 2)
Units (Limits)
14 70 15 30 15 14 40 14 25 15 60 15 75 15 14 40 14 25 15 68 15 83 88
V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) %
VOUT
Output Voltage LM2576 Output Voltage LM2576HV Efficiency
VOUT
h
LM2576-ADJ LM2576HV-ADJ Electrical Characteristics Specifications with standard type face are for TJ e 25 C
type apply over full Operating Temperature Range Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Feedback Voltage VIN e 12V ILOAD e 0 5A VOUT e 5V Circuit of Figure 2 0 5A s ILOAD s 3A 8V s VIN s 40V VOUT e 5V Circuit of Figure 2 0 5A s ILOAD s 3A 8V s VIN s 60V VOUT e 5V Circuit of Figure 2 VIN e 12V ILOAD e 3A VOUT e 5V 1 230
and those with boldface
LM2576-ADJ LM2576HV-ADJ Limit (Note 2)
Units (Limits)
1 217 1 243 1 230 1 193 1 180 1 267 1 280 1 230 1 193 1 180 1 273 1 286 77
V V(Min) V(Max) V V(Min) V(Max) V V(Min) V(Max) %
VOUT
Feedback Voltage LM2576 Feedback Voltage LM2576HV Efficiency
VOUT
h
3
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and those with boldface type apply over full Operating Temperature Range Unless otherwise specified VIN e 12V for the 3 3V 5V and Adjustable version VIN e 25V for the 12V version and VIN e 30V for the 15V version ILOAD e 500 mA LM2576-XX LM2576HV-XX Typ DEVICE PARAMETERS Ib fO Feedback Bias Current Oscillator Frequency VOUT e 5V (Adjustable Version Only) (Note 11) 50 52 47 42 58 63 VSAT DC ICL Saturation Voltage Max Duty Cycle (ON) Current Limit IOUT e 3A (Note 4) (Note 5) (Notes 4 and 11) 14 18 20 98 93 58 42 35 69 75 IL Output Leakage Current (Notes 6 and 7) Output e 0V Output e b1V Output e b1V 2 75 30 5 10 ISTBY iJA iJA iJC iJA VIH VIL IIH IIL Standby Quiescent Current Thermal Resistance ON OFF Pin e 5V (OFF) T Package T Package T Package S Package Junction to Ambient (Note 8) Junction to Ambient (Note 9) Junction to Case Junction to Ambient (Note 10) 50 200 65 45 2 50 14 12 12 30 ON OFF Pin e 0V (ON) 0 10 22 24 10 08 100 500 nA kHz kHz (Min) kHz (Max) V V(Max) % %(Min) A A(Min) A(Max) mA(Max) mA mA(Max) mA mA(Max) mA mA(Max) CW Limit (Note 2) Units (Limits)
All Output Voltage Versions Electrical Characteristics Specifications with standard type face are for TJ e 25 C
Symbol
Parameter
Conditions
IQ
Quiescent Current
(Note 6)
ON OFF CONTROL Test Circuit Figure 2 ON OFF Pin Logic Input Level ON OFF Pin Input Current VOUT e 0V VOUT e Nominal Output Voltage ON OFF Pin e 5V (OFF) V(Min) V(Max) mA mA(Max) mA mA(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 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 a 12V for the Adjustable 3 3V and 5V versions and a 25V for the 12V and 15V versions to force the output transistor OFF Note 7 VIN e 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 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 containing approximately 4 square inches of copper area surrounding the leads inch leads in a socket or on a PC inch leads soldered to a PC board
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 iJA is 50 C W with 1 square inch of copper area iJA is 37 C W and with 1 6 or more square inches of copper area iJA 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%
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Typical Performance Characteristics (Circuit of Figure 2 )
Normalized Output Voltage
Line Regulation
Dropout Voltage
Current Limit
Quiescent Current
Standby Quiescent Current
Oscillator Frequency
Switch Saturation Voltage
Efficiency
Minimum Operating Voltage
Quiescent Current vs Duty Cycle
Feedback Voltage vs Duty Cycle
TL H 11476 - 3
5
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Typical Performance Characteristics (Circuit of Figure 2 ) (Continued)
Maximum Power Dissipation (TO-263) (See Note 10)
TL H 11476 - 24
Feedback Pin Current
Switching Waveforms
TL H 11476 - 6 TL H 11476-4
VOUT e 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 ms div
Load Transient Response
TL H 11476-5
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
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Test Circuit and Layout Guidelines
Fixed Output Voltage Versions
CIN COUT D1 L1 R1 R2
TL H 11476 - 7
100 mF 75V Aluminum Electrolytic 1000 mF 25V Aluminum Electrolytic Schottky MBR360 100 mH Pulse Eng PE-92108 2k 0 1% 6 12k 0 1%
Adjustable Output Voltage Version
VOUT e VREF
1
a
R2 R1
J J
R2 e R1
VOUT b1 VREF
where VREF e 1 23V R1 between 1k and 5k
TL H 11476 - 8
FIGURE 2
7
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LM2576 Series Buck Regulator Design Procedure
PROCEDURE (Fixed Output Voltage Versions) Given VOUT e Regulated Output Voltage (3 3V 5V 12V or 15V) VIN(Max) e Maximum Input Voltage ILOAD(Max) e Maximum Load Current 1 Inductor Selection (L1) A Select the correct Inductor value selection guide from Figures 3 4 5 or 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 c 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 mF and 470 mF 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 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 e 5V VIN(Max) e 15V ILOAD(Max) e 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 mH From the table in Figure 3 Choose AIE 415-0930 Pulse Engineering PE92108 or Renco RL2444
2
Output Capacitor Selection (COUT) A COUT e 680 mF to 2000 mF standard aluminum electrolytic B Capacitor voltage rating e 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 mF 25V aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing
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LM2576 Series Buck Regulator Design Procedure (Continued)
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
TL H 11476 - 9
TL H 11476 - 10
FIGURE 3 LM2576(HV)-3 3
FIGURE 4 LM2576(HV)-5 0
TL H 11476 - 11
TL H 11476 - 12
FIGURE 5 LM2576(HV)-12
FIGURE 6 LM2576(HV)-15
TL H 11476 - 13
FIGURE 7 LM2576(HV)-ADJ 9
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LM2576 Series Buck Regulator Design Procedure (Continued)
PROCEDURE (Adjustable Output Voltage Versions) Given VOUT e Regulated Output Voltage VIN(Max) e Maximum Input Voltage ILOAD(Max) e Maximum Load Current F e 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 R2 VOUT e VREF 1 a where VREF e 1 23V R1 R1 can be between 1k and 5k (For best temperature coefficient and stability with time use 1% metal film resistors) VOUT b1 R2 e R1 VREF EXAMPLE (Adjustable Output Voltage Versions) Given VOUT e 10V VIN(Max) e 25V ILOAD(Max) e 3A F e 52 kHz 1 Programming Output Voltage (Selecting R1 and R2) VOUT e 1 23 1 a R2 e R1
J
V
VOUT
REF
R2 R1
b1
J
J
Select R1 e 1k
e 1k
1 23V 1 J
10V
b
R2 e 1k (8 13 b 1) e 7 13k closest 1% value is 7 15k
J
2
Inductor Selection (L1) A Calculate the inductor Volt microsecond constant E T (V ms) from the following formula VOUT 1000 (V ms) E T e (VIN b VOUT) VIN F (in kHz) 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 c ILOAD For additional inductor information see the inductor section in the application hints section of this data sheet 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 VIN(Max) (mF) COUT t 13 300 VOUT L(mH) The above formula yields capacitor values between 10 mF and 2200 mF 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
2
Inductor Selection (L1) A Calculate E T (V ms) E T e (25 b 10) 10 1000 e 115 V ms 25 52
B E T e 115 V ms C ILOAD(Max) e 3A D Inductance Region e H150 E Inductor Value e 150 mH Choose from AIE part 415-0936 Pulse Engineering part PE-531115 or Renco part RL2445
3
3
Output Capacitor Selection (COUT) 25 e 22 2 mF A COUT l 13 300 10 150 However for acceptable output ripple voltage select COUT t 680 mF COUT e 680 mF electrolytic capacitor
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LM2576 Series Buck Regulator Design Procedure (Continued)
PROCEDURE (Adjustable Output Voltage Versions) 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 Input Capacitor (CIN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation VR 20V Schottky 3A 1N5820 MBR320P SR302 1N5821 MBR330 31DQ03 SR303 1N5822 MBR340 31DQ04 SR304 MBR350 31DQ05 SR305 MBR360 DQ06 SR306 4A - 6A 1N5823 3A 4 EXAMPLE (Adjustable Output Voltage Versions) 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
5
Input Capacitor (CIN) A 100 mF aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing
Fast Recovery 4A -6A
30V
50WQ03 1N5824 The following diodes are all rated to 100V 31DF1 HER302 The following diodes are all rated to 100V 50WF10 MUR410 HER602
40V
MBR340 50WQ04 1N5825 50WQ05
50V
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 (3 ) diskette for IBM compatible computers from a National Semiconductor sales office in your area
60V
50WR06 50SQ060 FIGURE 8 Diode Selection Guide
Inductor Code L47 L68 L100 L150 L220 L330 L470 L680 H150 H220 H330 H470 H680 H1000 H1500 H2200
Inductor Value 47 mH 68 mH 100 mH 150 mH 220 mH 330 mH 470 mH 680 mH 150 mH 220 mH 330 mH 470 mH 680 mH 1000 mH 1500 mH 2200 mH
Schott (Note 1) 671 26980 671 26990 671 27000 671 27010 671 27020 671 27030 671 27040 671 27050 671 27060 671 27070 671 27080 671 27090 671 27100 671 27110 671 27120 671 27130
Pulse Eng (Note 2) PE-53112 PE-92114 PE-92108 PE-53113 PE-52626 PE-52627 PE-53114 PE-52629 PE-53115 PE-53116 PE-53117 PE-53118 PE-53119 PE-53120 PE-53121 PE-53122
Renco (Note 3) RL2442 RL2443 RL2444 RL1954 RL1953 RL1952 RL1951 RL1950 RL2445 RL2446 RL2447 RL1961 RL1960 RL1959 RL1958 RL2448
Note 1 Schott Corporation (612) 475-1173 1000 Parkers Lake Road Wayzata MN 55391 Note 2 Pulse Engineering (619) 674-8100 P O Box 12235 San Diego CA 92112 Note 3 Renco Electronics Incorporated (516) 586-5566 60 Jeffryn Blvd East Deer Park NY 11729
FIGURE 9 Inductor Selection by Manufacturer's Part Number 11
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Application Hints
INPUT CAPACITOR (CIN) To maintain stability the regulator input pin must be bypassed with at least a 100 mF electrolytic capacitor The capacitor's leads must be kept short and located near the regulator If the operating temperature range includes temperatures below b25 C the input capacitor value may need to be larger With most electrolytic capacitors the capacitance value decreases and the ESR increases with lower temperatures and age Paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold temperatures For maximum capacitor operating lifetime the capacitor's RMS ripple current rating should be greater than 12c 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 rapidly Different inductor types have different saturation characteristics and this should be kept in mind when selecting 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 peakto-peak amplitude of this inductor current waveform remains constant As the load current rises or falls the entire sawtooth 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 physical 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 capacitor and the amplitude of the inductor ripple current (DIIND) See the section on inductor ripple current in Application Hints The lower capacitor values (220 mF - 1000 mF) will allow typically 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 e (DIIND) (ESR of COUT)
TJ
tON
c ILOAD
where
tON V e OUT for a buck regulator T VIN
and
tON lVOUTl for a buck-boost regulator e T lVOUTl a VIN
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
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Application Hints (Continued)
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 03X 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 Because of their good low temperature characteristics a tantalum can be used in parallel with aluminum electrolytics with the tantalum making up 10% or 20% of the total capacitance The capacitor's ripple current rating at 52 kHz should be at least 50% higher than the peak-to-peak inductor ripple current 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 turnoff 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 mH 100 mF) can be added to the output (as shown in Figure 15 ) to further reduce the amount of output ripple and transients A 10 c 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 kX 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 a 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) 2 Maximum regulator power dissipation (in application) 3 Maximum allowed junction temperature (125 C for the LM2576) For a safe conservative design a temperature approximately 15 C cooler than the maximum temperatures should be selected 4 LM2576 package thermal resistances iJA and iJC Total power dissipated by the LM2576 can be estimated as follows PD e (VIN)(IQ) a (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 DTJ e (PD) (iJA) To arrive at the actual operating junction temperature add the junction temperature rise to the maximum ambient temperature TJ e DTJ a 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 DTJ e (PD) (iJC a iinterface a iHeat sink) The operating junction temperature will be TJ e TA a DTJ 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 13
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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 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 b12V 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 mH and 220 mH 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 VV 1 ILOAD (VIN a lVOl) a IN l Ol c Ip VIN VIN a lVOl 2L1 fosc Where fosc e 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 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 b5V to b12V and provides a regulated b12V output Input voltages greater than b12V will cause the output to rise above b12V but will not damage the regulator
Typical Load Current 400 mA for VIN e b 5 2V 750 mA for VIN e b 7V Note Heat sink may be required
TL H 11476 - 15
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 a 2VBE (Q1)
TL H 11476-14
FIGURE 10 Inverting Buck-Boost Develops b12V Also the maximum voltage appearing across the regulator is the absolute sum of the input and output voltage For a b 12V output the maximum input voltage for the LM2576 is a 28V or a 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
TL H 11476 - 16
Note Complete circuit not shown
FIGURE 12 Undervoltage Lockout for Buck Circuit
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Additional Applications (Continued)
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
TL H 11476 - 17
Note Complete circuit not shown (see Figure 10)
FIGURE 13 Undervoltage Lockout for Buck-Boost Circuit
TL H 11476 - 18
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 switching 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
Note Complete circuit not shown
FIGURE 14 Delayed Startup
TL H 11476 - 19
FIGURE 15 1 2V to 55V Adjustable 3A Power Supply with Low Output Ripple
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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 switch's on-time to the oscillator period 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 capacitor's 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 regulator's output voltage It is usually dominated by the output capacitor's ESR multiplied by the inductor's ripple current (DIIND) 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 (DIIND) 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 OPERATING VOLT MICROSECOND CONSTANT (ETop) The product (in VoItms) of the voltage applied to the inductor and the time the voltage is applied This ETop 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
for buck regulator
tON V e OUT De T VIN tON lVOl e De T lVOl a VIN
for buck-boost regulator
CATCH DIODE OR CURRENT STEERING DIODE The diode which provides a return path for the load current when the LM2576 switch is OFF EFFICIENCY (h) The proportion of input power actually delivered to the load POUT POUT e he PIN POUT a PLOSS CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR) The purely resistive component of a real capacitor's impedance (see Figure 16 ) It causes power loss resulting in capacitor heating which directly affects the capacitor's operating lifetime When used as a switching regulator output filter higher ESR values result in higher output ripple voltages
TL H 11476-20
FIGURE 16 Simple Model of a Real Capacitor Most standard aluminum electrolytic capacitors in the 100 mF - 1000 mF range have 0 5X to 0 1X ESR Highergrade capacitors (``low-ESR'' ``high-frequency'' or ``low-inductance''') in the 100 mF-1000 mF range generally have ESR of less than 0 15X
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Connection Diagrams
(XX indicates output voltage option See ordering information table for complete part number ) Straight Leads 5-Lead TO-220 (T) Top View Bent Staggered Leads 5-Lead TO-220 (T) Top View
TL H 11476 - 21
TL H 11476 - 22
LM2576T-XX or LM2576HVT-XX NS Package Number T05A TO-263 (S) 5-Lead Surface-Mount Package Top View
Side View
TL H 11476 - 23
LM2576T-XX Flow LB03 or LM2576HVT-XX Flow LB03 NS Package Number T05D
TL H 11476 - 25
Side View
TL H 11476 - 26
LM2576S-XX or LM2576HVS-XX NS Package Number TS5B LM2576SX-XX or LM2576HVSX-XX NS Package Number TS5B Tape and Reel
Ordering Information
Temperature Range Output Voltage 33 LM2576HVS-3 3 LM2576S-3 3 50 LM2576HVS-5 0 LM2576S-5 0 12 LM2576HVS-12 LM2576S-12 15 LM2576HVS-15 LM2576S-15 ADJ LM2576HVS-ADJ LM2576S-ADJ TS5B Tape Reel T05A TO-220 T05D NS Package Package Number TS5B TO-263 Type
LM2576HVSX-3 3 LM2576HVSX-5 0 LM2576HVSX-12 LM2576HVSX-15 LM2576HVSX-ADJ LM2576SX-3 3
b 40 C s TA LM2576HVT-3 3
s 125 C
LM2576SX-5 0 LM2576HVT-5 0 LM2576T-5 0 LM2576HVT-5 0 Flow LB03 LM2576T-5 0 Flow LB03
LM2576SX-12 LM2576HVT-12 LM2576T-12 LM2576HVT-12 Flow LB03 LM2576T-12 Flow LB03
LM2576SX-15 LM2576HVT-15 LM2576T-15 LM2576HVT-15 Flow LB03 LM2576T-15 Flow LB03
LM2576SX-ADJ LM2576HVT-ADJ LM2576T-ADJ LM2576HVT-ADJ Flow LB03 LM2576T-ADJ Flow LB03
LM2576T-3 3 LM2576HVT-3 3 Flow LB03 LM2576T-3 3 Flow LB03
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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 NS Package Number T05A
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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 NS Package Number T05D
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LM2576 LM2576HV Series SIMPLE SWITCHER 3A Step-Down Voltage Regulator
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 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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