LM2576,LM2576HV
LM2576/LM2576HV Series SIMPLE SWITCHER 3A Step-Down Voltage Regulator
Literature Number: SNVS107B
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 avail-
able 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 re-
duces 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 out-
put voltage within specified input voltages and output load
conditions, and ±10% on the oscillator frequency. External
shutdown is included, featuring 50 µA (typical) standby cur-
rent. The output switch includes cycle-by-cycle current limit-
ing, as well as thermal shutdown for full protection under
fault conditions.
Features
n3.3V, 5V, 12V, 15V, and adjustable output versions
nAdjustable version output voltage range,
1.23V to 37V (57V for HV version) ±4% max over
line and load conditions
nGuaranteed 3A output current
nWide input voltage range, 40V up to 60V for
HV version
nRequires only 4 external components
n52 kHz fixed frequency internal oscillator
nTTL shutdown capability, low power standby mode
nHigh efficiency
nUses readily available standard inductors
nThermal shutdown and current limit protection
nP+ Product Enhancement tested
Applications
nSimple high-efficiency step-down (buck) regulator
nEfficient pre-regulator for linear regulators
nOn-card switching regulators
nPositive to negative converter (Buck-Boost)
Typical Application (Fixed Output Voltage
Versions)
SIMPLE SWITCHER®is a registered trademark of National Semiconductor Corporation.
01147601
FIGURE 1.
August 2004
LM2576/LM2576HV Series SIMPLE SWITCHER 3A Step-Down Voltage Regulator
© 2004 National Semiconductor Corporation DS011476 www.national.com
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 = 0Ω
Patent Pending
Ordering Information
Temperature
Range
Output Voltage NS Package Package
Type
3.3 5.0 12 15 ADJ Number
−40˚C ≤T
A
≤125˚C
LM2576HVS-3.3 LM2576HVS-5.0 LM2576HVS-12 LM2576HVS-15 LM2576HVS-ADJ TS5B TO-263
LM2576S-3.3 LM2576S-5.0 LM2576S-12 LM2576S-15 LM2576S-ADJ
LM2576HVSX-3.3 LM2576HVSX-5.0 LM2576HVSX-12 LM2576HVSX-15 LM2576HVSX-ADJ TS5B
Tape & Reel
LM2576SX-3.3 LM2576SX-5.0 LM2576SX-12 LM2576SX-15 LM2576SX-ADJ
LM2576HVT-3.3 LM2576HVT-5.0 LM2576HVT-12 LM2576HVT-15 LM2576HVT-ADJ T05A TO-220
LM2576T-3.3 LM2576T-5.0 LM2576T-12 LM2576T-15 LM2576T-ADJ
LM2576HVT-3.3 LM2576HVT-5.0 LM2576HVT-12 LM2576HVT-15 LM2576HVT-ADJ T05D
Flow LB03 Flow LB03 Flow LB03 Flow LB03 Flow LB03
LM2576T-3.3 LM2576T-5.0 LM2576T-12 LM2576T-15 LM2576T-ADJ
Flow LB03 Flow LB03 Flow LB03 Flow LB03 Flow LB03
LM2576/LM2576HV
www.national.com 2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Supply Voltage
LM2576 45V
LM2576HV 63V
ON /OFF Pin Input Voltage −0.3V ≤V≤+V
IN
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 kΩ)2kV
Lead Temperature
(Soldering, 10 Seconds) 260˚C
Operating Ratings
Temperature Range
LM2576/LM2576HV −40˚C ≤T
J
≤+125˚C
Supply Voltage
LM2576 40V
LM2576HV 60V
LM2576-3.3, LM2576HV-3.3
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Temperature
Range.
Symbol Parameter Conditions LM2576-3.3 Units
(Limits)
LM2576HV-3.3
Typ Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
V
OUT
Output Voltage V
IN
= 12V, I
LOAD
= 0.5A 3.3 V
Circuit of Figure 2 3.234 V(Min)
3.366 V(Max)
V
OUT
Output Voltage 6V ≤V
IN
≤40V, 0.5A ≤I
LOAD
≤3A 3.3 V
LM2576 Circuit of Figure 2 3.168/3.135 V(Min)
3.432/3.465 V(Max)
V
OUT
Output Voltage 6V ≤V
IN
≤60V, 0.5A ≤I
LOAD
≤3A 3.3 V
LM2576HV Circuit of Figure 2 3.168/3.135 V(Min)
3.450/3.482 V(Max)
ηEfficiency V
IN
= 12V, I
LOAD
=3A 75 %
LM2576-5.0, LM2576HV-5.0
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with Figure 2 boldface type apply over full Operating Tem-
perature Range.
Symbol Parameter Conditions LM2576-5.0 Units
(Limits)
LM2576HV-5.0
Typ Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
V
OUT
Output Voltage V
IN
= 12V, I
LOAD
= 0.5A 5.0 V
Circuit of Figure 2 4.900 V(Min)
5.100 V(Max)
V
OUT
Output Voltage 0.5A ≤I
LOAD
≤3A, 5.0 V
LM2576 8V ≤V
IN
≤40V 4.800/4.750 V(Min)
Circuit of Figure 2 5.200/5.250 V(Max)
V
OUT
Output Voltage 0.5A ≤I
LOAD
≤3A, 5.0 V
LM2576HV 8V ≤V
IN
≤60V 4.800/4.750 V(Min)
Circuit of Figure 2 5.225/5.275 V(Max)
LM2576/LM2576HV
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LM2576-5.0, LM2576HV-5.0
Electrical Characteristics (Continued)
Specifications with standard type face are for T
J
= 25˚C, and those with Figure 2 boldface type apply over full Operating Tem-
perature Range.
Symbol Parameter Conditions LM2576-5.0 Units
(Limits)
LM2576HV-5.0
Typ Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
ηEfficiency V
IN
= 12V, I
LOAD
=3A 77 %
LM2576-12, LM2576HV-12
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Temperature
Range.
Symbol Parameter Conditions LM2576-12 Units
(Limits)
LM2576HV-12
Typ Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
V
OUT
Output Voltage V
IN
= 25V, I
LOAD
= 0.5A 12 V
Circuit of Figure 2 11.76 V(Min)
12.24 V(Max)
V
OUT
Output Voltage 0.5A ≤I
LOAD
≤3A, 12 V
LM2576 15V ≤V
IN
≤40V 11.52/11.40 V(Min)
Circuit of Figure 2 12.48/12.60 V(Max)
V
OUT
Output Voltage 0.5A ≤I
LOAD
≤3A, 12 V
LM2576HV 15V ≤V
IN
≤60V 11.52/11.40 V(Min)
Circuit of Figure 2 12.54/12.66 V(Max)
ηEfficiency V
IN
= 15V, I
LOAD
=3A 88 %
LM2576-15, LM2576HV-15
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Temperature
Range.
Symbol Parameter Conditions LM2576-15 Units
(Limits)
LM2576HV-15
Typ Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
V
OUT
Output Voltage V
IN
= 25V, I
LOAD
= 0.5A 15 V
Circuit of Figure 2 14.70 V(Min)
15.30 V(Max)
V
OUT
Output Voltage 0.5A ≤I
LOAD
≤3A, 15 V
LM2576 18V ≤V
IN
≤40V 14.40/14.25 V(Min)
Circuit of Figure 2 15.60/15.75 V(Max)
V
OUT
Output Voltage 0.5A ≤I
LOAD
≤3A, 15 V
LM2576HV 18V ≤V
IN
≤60V 14.40/14.25 V(Min)
Circuit of Figure 2 15.68/15.83 V(Max)
ηEfficiency V
IN
= 18V, I
LOAD
=3A 88 %
LM2576/LM2576HV
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LM2576-ADJ, LM2576HV-ADJ
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Temperature
Range.
Symbol Parameter Conditions LM2576-ADJ Units
(Limits)
LM2576HV-ADJ
Typ Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
V
OUT
Feedback Voltage V
IN
= 12V, I
LOAD
= 0.5A 1.230 V
V
OUT
= 5V, 1.217 V(Min)
Circuit of Figure 2 1.243 V(Max)
V
OUT
Feedback Voltage 0.5A ≤I
LOAD
≤3A, 1.230 V
LM2576 8V ≤V
IN
≤40V 1.193/1.180 V(Min)
V
OUT
= 5V, Circuit of Figure 2 1.267/1.280 V(Max)
V
OUT
Feedback Voltage 0.5A ≤I
LOAD
≤3A, 1.230 V
LM2576HV 8V ≤V
IN
≤60V 1.193/1.180 V(Min)
V
OUT
= 5V, Circuit of Figure 2 1.273/1.286 V(Max)
ηEfficiency V
IN
= 12V, I
LOAD
= 3A, V
OUT
=5V 77 %
All Output Voltage Versions
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Temperature
Range. Unless otherwise specified, V
IN
= 12V for the 3.3V, 5V, and Adjustable version, V
IN
= 25V for the 12V version, and V
IN
= 30V for the 15V version. I
LOAD
= 500 mA.
Symbol Parameter Conditions LM2576-XX Units
(Limits)
LM2576HV-XX
Typ Limit
(Note 2)
DEVICE PARAMETERS
I
b
Feedback Bias Current V
OUT
= 5V (Adjustable Version Only) 50 100/500 nA
f
O
Oscillator Frequency (Note 11) 52 kHz
47/42 kHz
(Min)
58/63 kHz
(Max)
V
SAT
Saturation Voltage I
OUT
= 3A (Note 4) 1.4 V
1.8/2.0 V(Max)
DC Max Duty Cycle (ON) (Note 5) 98 %
93 %(Min)
I
CL
Current Limit (Notes 4, 11) 5.8 A
4.2/3.5 A(Min)
6.9/7.5 A(Max)
I
L
Output Leakage Current (Notes 6, 7): Output = 0V 2 mA(Max)
Output = −1V 7.5 mA
Output = −1V 30 mA(Max)
I
Q
Quiescent Current (Note 6) 5 mA
10 mA(Max)
I
STBY
Standby Quiescent ON /OFF Pin = 5V (OFF) 50 µA
Current 200 µA(Max)
LM2576/LM2576HV
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All Output Voltage Versions
Electrical Characteristics (Continued)
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Temperature
Range. Unless otherwise specified, V
IN
= 12V for the 3.3V, 5V, and Adjustable version, V
IN
= 25V for the 12V version, and V
IN
= 30V for the 15V version. I
LOAD
= 500 mA.
Symbol Parameter Conditions LM2576-XX Units
(Limits)
LM2576HV-XX
Typ Limit
(Note 2)
DEVICE PARAMETERS
θ
JA
Thermal Resistance T Package, Junction to Ambient (Note 8) 65
θ
JA
T Package, Junction to Ambient (Note 9) 45 ˚C/W
θ
JC
T Package, Junction to Case 2
θ
JA
S Package, Junction to Ambient (Note 10) 50
ON /OFF CONTROL Test Circuit Figure 2
V
IH
ON /OFF Pin V
OUT
= 0V 1.4 2.2/2.4 V(Min)
V
IL
Logic Input Level V
OUT
= Nominal Output Voltage 1.2 1.0/0.8 V(Max)
I
IH
ON /OFF Pin Input ON /OFF Pin = 5V (OFF) 12 µA
Current 30 µA(Max)
I
IL
ON /OFF Pin = 0V (ON) 0µA
10 µA(Max)
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods.
Note 3: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the
LM2576/LM2576HV is used as shown in the Figure 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
1
⁄
2
inch leads in a socket, or on a PC
board with minimum copper area.
Note 9: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with
1
⁄
4
inch leads soldered to a PC board
containing approximately 4 square inches of copper area surrounding the leads.
Note 10: If the TO-263 package is used, the thermal resistance can be reduced by increasing the PC board copper area thermally connected to the package. Using
0.5 square inches of copper area, θJA is 50˚C/W, with 1 square inch of copper area, θJA is 37˚C/W, and with 1.6 or more square inches of copper area, θJA is 32˚C/W.
Note 11: The oscillator frequency reduces to approximately 11 kHz in the event of an output short or an overload which causes the regulated output voltage to drop
approximately 40% from the nominal output voltage. This self protection feature lowers the average power dissipation of the IC by lowering the minimum duty cycle
from 5% down to approximately 2%.
Typical Performance Characteristics
(Circuit of Figure 2)
Normalized Output Voltage Line Regulation
01147627 01147628
LM2576/LM2576HV
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Typical Performance Characteristics (Circuit of Figure 2) (Continued)
Efficiency Minimum Operating Voltage
01147635 01147636
Quiescent Current
vs Duty Cycle
Feedback Voltage
vs Duty Cycle
01147637 01147638
Minimum Operating Voltage
Quiescent Current
vs Duty Cycle
01147636 01147637
LM2576/LM2576HV
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Typical Performance Characteristics (Circuit of Figure 2) (Continued)
Feedback Voltage
vs Duty Cycle Feedback Pin Current
01147638
01147604
Maximum Power Dissipation
(TO-263) (See Note 10) Switching Waveforms
01147624 01147606
VOUT = 15V
A: Output Pin Voltage, 50V/div
B: Output Pin Current, 2A/div
C: Inductor Current, 2A/div
D: Output Ripple Voltage, 50 mV/div,
AC-Coupled
Horizontal Time Base: 5 µs/div
Load Transient Response
01147605
LM2576/LM2576HV
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Test Circuit and Layout Guidelines
As in any switching regulator, layout is very important. Rap-
idly switching currents associated with wiring inductance
generate voltage transients which can cause problems. For
minimal inductance and ground loops, the length of the leads
indicated by heavy lines should be kept as short as possible.
Single-point grounding (as indicated) or ground plane con-
struction should be used for best results. When using the
Adjustable version, physically locate the programming resis-
tors near the regulator, to keep the sensitive feedback wiring
short.
Fixed Output Voltage Versions
01147607
CIN — 100 µF, 75V, Aluminum Electrolytic
COUT — 1000 µF, 25V, Aluminum Electrolytic
D1— Schottky, MBR360
L1— 100 µH, Pulse Eng. PE-92108
R1— 2k, 0.1%
R2— 6.12k, 0.1%
Adjustable Output Voltage Version
01147608
where VREF = 1.23V, R1 between 1k and 5k.
FIGURE 2.
LM2576/LM2576HV
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LM2576 Series Buck Regulator
Design Procedure
PROCEDURE (Fixed Output Voltage Versions) EXAMPLE (Fixed Output Voltage Versions)
Given: V
OUT
= Regulated Output Voltage (3.3V, 5V, 12V,
or 15V) V
IN
(Max) = Maximum Input Voltage I
LOAD
(Max) =
Maximum Load Current
Given: V
OUT
=5VV
IN
(Max) = 15V I
LOAD
(Max) = 3A
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 V
IN
(Max) and
I
LOAD
(Max), and note the inductor code for that region.
C. Identify the inductor value from the inductor code, and
select an appropriate inductor from the table shown in
Figure 3. Part numbers are listed for three inductor
manufacturers. The inductor chosen must be rated for
operation at the LM2576 switching frequency (52 kHz)
and for a current rating of 1.15 x I
LOAD
. For additional
inductor information, see the inductor section in the
Application Hints section of this data sheet.
1. Inductor Selection (L1)
A. Use the selection guide shown in Figure 4.
B. From the selection guide, the inductance area
intersected by the 15V line and 3A line is L100.
C. Inductor value required is 100 µH. From the table in
Figure 3. Choose AIE 415-0930, Pulse Engineering
PE92108, or Renco RL2444.
2. Output Capacitor Selection (C
OUT
)
A. The value of the output capacitor together with the
inductor defines the dominate pole-pair of the switching
regulator loop. For stable operation and an acceptable
output ripple voltage, (approximately 1% of the output
voltage) a value between 100 µF and 470 µF is
recommended.
B. The capacitor’s voltage rating should be at least 1.5
times greater than the output voltage. For a 5V regulator,
a rating of at least 8V is appropriate, and a 10V or 15V
rating is recommended.
Higher voltage electrolytic capacitors generally have
lower ESR numbers, and for this reason it may be
necessary to select a capacitor rated for a higher voltage
than would normally be needed.
2. Output Capacitor Selection (C
OUT
)
A. C
OUT
= 680 µF to 2000 µF standard aluminum
electrolytic.
B.Capacitor voltage rating = 20V.
3. Catch Diode Selection (D1)
A.The catch-diode current rating must be at least 1.2
times greater than the maximum load current. Also, if the
power supply design must withstand a continuous output
short, the diode should have a current rating equal to the
maximum current limit of the LM2576. The most stressful
condition for this diode is an overload or shorted output
condition.
B. The reverse voltage rating of the diode should be at
least 1.25 times the maximum input voltage.
3. Catch Diode Selection (D1)
A.For this example, a 3A current rating is adequate.
B. Use a 20V 1N5823 or SR302 Schottky diode, or any
of the suggested fast-recovery diodes shown in Figure 8.
4. Input Capacitor (C
IN
)
An aluminum or tantalum electrolytic bypass capacitor
located close to the regulator is needed for stable
operation.
4. Input Capacitor (C
IN
)
A 100 µF, 25V aluminum electrolytic capacitor located
near the input and ground pins provides sufficient
bypassing.
LM2576/LM2576HV
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LM2576 Series Buck Regulator
Design Procedure (Continued)
INDUCTOR VALUE SELECTION GUIDES (For
Continuous Mode Operation)
01147609
FIGURE 3. LM2576(HV)-3.3
01147610
FIGURE 4. LM2576(HV)-5.0
01147611
FIGURE 5. LM2576(HV)-12
01147612
FIGURE 6. LM2576(HV)-15
LM2576/LM2576HV
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LM2576 Series Buck Regulator Design Procedure (Continued)
PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions)
Given: V
OUT
= Regulated Output Voltage V
IN
(Max) =
Maximum Input Voltage I
LOAD
(Max) = Maximum Load
Current F = Switching Frequency (Fixed at 52 kHz)
Given: V
OUT
= 10V V
IN
(Max) = 25V I
LOAD
(Max) = 3A F =
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.
R
1
can be between 1k and 5k. (For best temperature
coefficient and stability with time, use 1% metal film resis-
tors)
1. Programming Output Voltage (Selecting R1 and R2)
R
2
= 1k (8.13 − 1) = 7.13k, closest 1% value is 7.15k
01147613
FIGURE 7. LM2576(HV)-ADJ
LM2576/LM2576HV
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LM2576 Series Buck Regulator Design Procedure (Continued)
PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions)
2. Inductor Selection (L1)
A. Calculate the inductor Volt •microsecond constant,
E•T(V•µs), from the following formula:
B. Use the E •T value from the previous formula and
match it with the E •T number on the vertical axis of the
Inductor Value Selection Guide shown in Figure 7.
C. On the horizontal axis, select the maximum load current.
D. Identify the inductance region intersected by the E •T
value and the maximum load current value, and note the
inductor code for that region.
E. Identify the inductor value from the inductor code, and
select an appropriate inductor from the table shown in
Figure 9. Part numbers are listed for three inductor manu-
facturers. The inductor chosen must be rated for operation
at the LM2576 switching frequency (52 kHz) and for a
current rating of 1.15 x I
LOAD
. For additional inductor infor-
mation, see the inductor section in the application hints
section of this data sheet.
2. Inductor Selection (L1)
A. Calculate E •T(V•µs)
B. E•T = 115 V •µs
C. I
LOAD
(Max) = 3A
D. Inductance Region = H150
E. Inductor Value = 150 µH Choose from AIE part #415-
0936 Pulse Engineering part #PE-531115, or Renco part
#RL2445.
3. Output Capacitor Selection (C
OUT
)
A. The value of the output capacitor together with the
inductor defines the dominate pole-pair of the switching
regulator loop. For stable operation, the capacitor must
satisfy the following requirement:
The above formula yields capacitor values between 10 µF
and 2200 µF that will satisfy the loop requirements for
stable operation. But to achieve an acceptable output
ripple voltage, (approximately 1% of the output voltage)
and transient response, the output capacitor may need to
be several times larger than the above formula yields.
B. The capacitor’s voltage rating should be at last 1.5 times
greater than the output voltage. For a 10V regulator, a
rating of at least 15V or more is recommended. Higher
voltage electrolytic capacitors generally have lower ESR
numbers, and for this reason it may be necessary to select
a capacitor rate for a higher voltage than would normally be
needed.
3. Output Capacitor Selection (C
OUT
)
However, for acceptable output ripple voltage select C
OUT
≥680 µF C
OUT
= 680 µF electrolytic capacitor
4. Catch Diode Selection (D1)
A. The catch-diode current rating must be at least 1.2
times greater than the maximum load current. Also, if the
power supply design must withstand a continuous output
short, the diode should have a current rating equal to the
maximum current limit of the LM2576. The most stressful
condition for this diode is an overload or shorted output.
See diode selection guide in Figure 8.
B. The reverse voltage rating of the diode should be at
least 1.25 times the maximum input voltage.
4. Catch Diode Selection (D1)
A. For this example, a 3.3A current rating is adequate.
B. Use a 30V 31DQ03 Schottky diode, or any of the
suggested fast-recovery diodes in Figure 8.
5. Input Capacitor (C
IN
)
An aluminum or tantalum electrolytic bypass capacitor
located close to the regulator is needed for stable
operation.
5. Input Capacitor (C
IN
)
A 100 µF aluminum electrolytic capacitor located near
the input and ground pins provides sufficient bypassing.
To further simplify the buck regulator design procedure, Na-
tional Semiconductor is making available computer design
software to be used with the SIMPLE SWITCHER line of
switching regulators. Switchers Made Simple (Version 3.3)
is available on a (3
1
⁄
2
") diskette for IBM compatible comput-
ers from a National Semiconductor sales office in your area.
LM2576/LM2576HV
www.national.com 14
Application Hints
INPUT CAPACITOR (C
IN
)
To maintain stability, the regulator input pin must be by-
passed with at least a 100 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
V
R
Schottky Fast Recovery
3A 4A–6A 3A 4A–6A
20V 1N5820 1N5823
The following
diodes are all
rated to 100V
31DF1
HER302
The following
diodes are all
rated to 100V
50WF10
MUR410
HER602
MBR320P
SR302
30V 1N5821 50WQ03
MBR330 1N5824
31DQ03
SR303
40V 1N5822 MBR340
MBR340 50WQ04
31DQ04 1N5825
SR304
50V MBR350 50WQ05
31DQ05
SR305
60V MBR360 50WR06
DQ06 50SQ060
SR306
FIGURE 8. Diode Selection Guide
Inductor Inductor Schott Pulse Eng. Renco
Code Value (Note 12) (Note 13) (Note 14)
L47 47 µH 671 26980 PE-53112 RL2442
L68 68 µH 671 26990 PE-92114 RL2443
L100 100 µH 671 27000 PE-92108 RL2444
L150 150 µH 671 27010 PE-53113 RL1954
L220 220 µH 671 27020 PE-52626 RL1953
L330 330 µH 671 27030 PE-52627 RL1952
L470 470 µH 671 27040 PE-53114 RL1951
L680 680 µH 671 27050 PE-52629 RL1950
H150 150 µH 671 27060 PE-53115 RL2445
H220 220 µH 671 27070 PE-53116 RL2446
H330 330 µH 671 27080 PE-53117 RL2447
H470 470 µH 671 27090 PE-53118 RL1961
H680 680 µH 671 27100 PE-53119 RL1960
H1000 1000 µH 671 27110 PE-53120 RL1959
H1500 1500 µH 671 27120 PE-53121 RL1958
H2200 2200 µH 671 27130 PE-53122 RL2448
Note 12: Schott Corporation, (612) 475-1173, 1000 Parkers Lake Road, Wayzata, MN 55391.
Note 13: Pulse Engineering, (619) 674-8100, P.O. Box 12235, San Diego, CA 92112.
Note 14: Renco Electronics Incorporated, (516) 586-5566, 60 Jeffryn Blvd. East, Deer Park, NY 11729.
FIGURE 9. Inductor Selection by Manufacturer’s Part Number
LM2576/LM2576HV
www.national.com15
Application Hints (Continued)
INDUCTOR SELECTION
All switching regulators have two basic modes of operation:
continuous and discontinuous. The difference between the
two types relates to the inductor current, whether it is flowing
continuously, or if it drops to zero for a period of time in the
normal switching cycle. Each mode has distinctively different
operating characteristics, which can affect the regulator per-
formance and requirements.
The LM2576 (or any of the SIMPLE SWITCHER family) can
be used for both continuous and discontinuous modes of
operation.
The inductor value selection guides in Figure 3 through
Figure 7 were designed for buck regulator designs of the
continuous inductor current type. When using inductor val-
ues shown in the inductor selection guide, the peak-to-peak
inductor ripple current will be approximately 20% to 30% of
the maximum DC current. With relatively heavy load cur-
rents, the circuit operates in the continuous mode (inductor
current always flowing), but under light load conditions, the
circuit will be forced to the discontinuous mode (inductor
current falls to zero for a period of time). This discontinuous
mode of operation is perfectly acceptable. For light loads
(less than approximately 300 mA) it may be desirable to
operate the regulator in the discontinuous mode, primarily
because of the lower inductor values required for the discon-
tinuous mode.
The selection guide chooses inductor values suitable for
continuous mode operation, but if the inductor value chosen
is prohibitively high, the designer should investigate the
possibility of discontinuous operation. The computer design
software Switchers Made 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 com-
pletely contained within the core, it generates more electro-
magnetic interference (EMI). This EMI can cause problems
in sensitive circuits, or can give incorrect scope readings
because of induced voltages in the scope probe.
The inductors listed in the selection chart include ferrite pot
core construction for AIE, powdered iron toroid for Pulse
Engineering, and ferrite bobbin core for Renco.
An inductor should not be operated beyond its maximum
rated current because it may saturate. When an inductor
begins to saturate, the inductance decreases rapidly and the
inductor begins to look mainly resistive (the DC resistance of
the winding). This will cause the switch current to rise very
rapidly. Different inductor types have different saturation
characteristics, and this should be kept in mind when select-
ing an inductor.
The inductor manufacturer’s data sheets include current and
energy limits to avoid inductor saturation.
INDUCTOR RIPPLE CURRENT
When the switcher is operating in the continuous mode, the
inductor current waveform ranges from a triangular to a
sawtooth type of waveform (depending on the input voltage).
For a given input voltage and output voltage, the peak-to-
peak amplitude of this inductor current waveform remains
constant. As the load current rises or falls, the entire saw-
tooth current waveform also rises or falls. The average DC
value of this waveform is equal to the DC load current (in the
buck regulator configuration).
If the load current drops to a low enough level, the bottom of
the sawtooth current waveform will reach zero, and the
switcher will change to a discontinuous mode of operation.
This is a perfectly acceptable mode of operation. Any buck
switching regulator (no matter how large the inductor value
is) will be forced to run discontinuous if the load current is
light enough.
OUTPUT CAPACITOR
An output capacitor is required to filter the output voltage and
is needed for loop stability. The capacitor should be located
near the LM2576 using short pc board traces. Standard
aluminum electrolytics are usually adequate, but low ESR
types are recommended for low output ripple voltage and
good stability. The ESR of a capacitor depends on many
factors, some which are: the value, the voltage rating, physi-
cal size and the type of construction. In general, low value or
low voltage (less than 12V) electrolytic capacitors usually
have higher ESR numbers.
The amount of output ripple voltage is primarily a function of
the ESR (Equivalent Series Resistance) of the output ca-
pacitor and the amplitude of the inductor ripple current
(∆I
IND
). See the section on inductor ripple current in Applica-
tion Hints.
The lower capacitor values (220 µF–1000 µF) will allow
typically 50 mV to 150 mV of output ripple voltage, while
larger-value capacitors will reduce the ripple to approxi-
mately 20 mV to 50 mV.
Output Ripple Voltage = (∆I
IND
) (ESR of C
OUT
)
To further reduce the output ripple voltage, several standard
electrolytic capacitors may be paralleled, or a higher-grade
capacitor may be used. Such capacitors are often called
“high-frequency,” “low-inductance,” or “low-ESR.” These will
reduce the output ripple to 10 mV or 20 mV. However, when
operating in the continuous mode, reducing the ESR below
0.03Ωcan cause instability in the regulator.
Tantalum capacitors can have a very low ESR, and should
be carefully evaluated if it is the only output capacitor. Be-
cause of their good low temperature characteristics, a tan-
talum can be used in parallel with aluminum electrolytics,
with the tantalum making up 10% or 20% of the total capaci-
tance.
The capacitor’s ripple current rating at 52 kHz should be at
least 50% higher than the peak-to-peak inductor ripple cur-
rent.
LM2576/LM2576HV
www.national.com 16
Application Hints (Continued)
CATCH DIODE
Buck regulators require a diode to provide a return path for
the inductor current when the switch is off. This diode should
be located close to the LM2576 using short leads and short
printed circuit traces.
Because of their fast switching speed and low forward volt-
age drop, Schottky diodes provide the best efficiency, espe-
cially in low output voltage switching regulators (less than
5V). Fast-Recovery, High-Efficiency, or Ultra-Fast Recovery
diodes are also suitable, but some types with an abrupt
turn-off characteristic may cause instability and EMI prob-
lems. A fast-recovery diode with soft recovery characteristics
is a better choice. Standard 60 Hz diodes (e.g., 1N4001 or
1N5400, etc.) are also not suitable. See Figure 8 for Schot-
tky and “soft” fast-recovery diode selection guide.
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
The output voltage of a switching power supply will contain a
sawtooth ripple voltage at the switcher frequency, typically
about 1% of the output voltage, and may also contain short
voltage spikes at the peaks of the sawtooth waveform.
The output ripple voltage is due mainly to the inductor saw-
tooth ripple current multiplied by the ESR of the output
capacitor. (See the inductor selection in the application
hints.)
The voltage spikes are present because of the the fast
switching action of the output switch, and the parasitic induc-
tance of the output filter capacitor. To minimize these voltage
spikes, special low inductance capacitors can be used, and
their lead lengths must be kept short. Wiring inductance,
stray capacitance, as well as the scope probe used to evalu-
ate these transients, all contribute to the amplitude of these
spikes.
An additional small LC filter (20 µH & 100 µF) can be added
to the output (as shown in Figure 15) to further reduce the
amount of output ripple and transients. A 10 x reduction in
output ripple voltage and transients is possible with this filter.
FEEDBACK CONNECTION
The LM2576 (fixed voltage versions) feedback pin must be
wired to the output voltage point of the switching power
supply. When using the adjustable version, physically locate
both output voltage programming resistors near the LM2576
to avoid picking up unwanted noise. Avoid using resistors
greater than 100 kΩbecause of the increased chance of
noise pickup.
ON /OFF INPUT
For normal operation, the ON /OFF pin should be grounded
or driven with a low-level TTL voltage (typically below 1.6V).
To put the regulator into standby mode, drive this pin with a
high-level TTL or CMOS signal. The ON /OFF pin can be
safely pulled up to +V
IN
without a resistor in series with it.
The ON /OFF pin should not be left open.
GROUNDING
To maintain output voltage stability, the power ground con-
nections must be low-impedance (see Figure 2). For the
5-lead TO-220 and TO-263 style package, both the tab and
pin 3 are ground and either connection may be used, as they
are both part of the same copper lead frame.
HEAT SINK/THERMAL CONSIDERATIONS
In many cases, only a small heat sink is required to keep the
LM2576 junction temperature within the allowed operating
range. For each application, to determine whether or not a
heat sink will be required, the following must be identified:
1. Maximum ambient temperature (in the application).
2. Maximum regulator power dissipation (in application).
3. Maximum allowed junction temperature (125˚C for the
LM2576). For a safe, conservative design, a tempera-
ture approximately 15˚C cooler than the maximum tem-
peratures should be selected.
4. LM2576 package thermal resistances θ
JA
and θ
JC
.
Total power dissipated by the LM2576 can be estimated as
follows:
P
D
=(V
IN
)(I
Q
)+(V
O
/V
IN
)(I
LOAD
)(V
SAT
)
where I
Q
(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:
∆T
J
=(P
D
)(θ
JA
)
To arrive at the actual operating junction temperature, add
the junction temperature rise to the maximum ambient tem-
perature.
T
J
=∆T
J
+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:
∆T
J
=(P
D
)(θ
JC
+θ
interface
+θ
Heat sink
)
The operating junction temperature will be:
T
J
=T
A
+∆T
J
As above, if the actual operating junction temperature is
greater than the selected safe operating junction tempera-
ture, then a larger heat sink is required (one that has a lower
thermal resistance).
Included on the Switcher Made Simple design software is a
more precise (non-linear) thermal model that can be used to
determine junction temperature with different input-output
parameters or different component values. It can also calcu-
late the heat sink thermal resistance required to maintain the
regulators junction temperature below the maximum operat-
ing temperature.
Additional Applications
INVERTING REGULATOR
Figure 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 −12V.
For an input voltage of 12V or more, the maximum available
output current in this configuration is approximately 700 mA.
At lighter loads, the minimum input voltage required drops to
approximately 4.7V.
LM2576/LM2576HV
www.national.com17