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PDF下载LM2576
3.0 A, 15 V, Step−Down
Switching Regulator
The LM2576 series of regulators are monolithic integrated circuits
ideally suited for easy and convenient design of a step−down
switching regulator (buck converter). All circuits of this series are
capable of driving a 3.0 A load with excellent line and load regulation.
These devices are available in fixed output voltages of 3.3 V, 5.0 V,
12 V, 15 V, and an adjustable output version.
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These regulators were designed to minimize the number of external
components to simplify the power supply design. Standard series of
inductors optimized for use with the LM2576 are offered by several
different inductor manufacturers.
TO−220
TV SUFFIX
CASE 314B
1
5
Since the LM2576 converter is a switch−mode power supply, its
efficiency is significantly higher in comparison with popular
three−terminal linear regulators, especially with higher input voltages.
In many cases, the power dissipated is so low that no heatsink is
required or its size could be reduced dramatically.
Heatsink surface connected to Pin 3
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.
The LM2576 features include a guaranteed 4% tolerance on output
voltage within specified input voltages and output load conditions, and
10% on the oscillator frequency ( 2% over 0°C to 125°C). External
shutdown is included, featuring 80 mA (typical) standby current. The
output switch includes cycle−by−cycle current limiting, as well as
thermal shutdown for full protection under fault conditions.
TO−220
T SUFFIX
CASE 314D
1
5
Pin 1.
V
in
2. Output
3. Ground
4. Feedback
5. ON/OFF
Features
• 3.3 V, 5.0 V, 12 V, 15 V, and Adjustable Output Versions
• Adjustable Version Output Voltage Range, 1.23 to 37 V 4%
Maximum Over Line and Load Conditions
• Guaranteed 3.0 A Output Current
2
D PAK
D2T SUFFIX
CASE 936A
• Wide Input Voltage Range
1
• 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
• Moisture Sensitivity Level (MSL) Equals 1
• Pb−Free Packages are Available
5
Heatsink surface (shown as terminal 6 in
case outline drawing) is connected to Pin 3
ORDERING INFORMATION
See detailed ordering and shipping information in the package
dimensions section on page 24 of this data sheet.
DEVICE MARKING INFORMATION
See general marking information in the device marking
section on page 25 of this data sheet.
Applications
• Simple High−Efficiency Step−Down (Buck) Regulator
• Efficient Pre−Regulator for Linear Regulators
• On−Card Switching Regulators
• Positive to Negative Converter (Buck−Boost)
• Negative Step−Up Converters
• Power Supply for Battery Chargers
©
Semiconductor Components Industries, LLC, 2006
1
Publication Order Number:
January, 2006 − Rev. 8
LM2576/D
LM2576
Typical Application (Fixed Output Voltage Versions)
Feedback
4
L1
100 mH
7.0 V − 40 V
Unregulated
DC Input
+V
in
LM2576
Output
2
1
5.0 V Regulated
Output 3.0 A Load
C
100 mF
in
D1
1N5822
C
out
1000 mF
3
GN
D
5
ON/OFF
Representative Block Diagram and Typical Application
+V
in
ON/OFF
Unregulated
DC Input
3.1 V Internal
Regulator
Output
Voltage Versions
R2
(W)
ON/OFF
1
5
C
3.3 V
5.0 V
12 V
15 V
1.7 k
3.1 k
8.84 k
11.3 k
in
4
Feedback
Current
Limit
For adjustable version
R1 = open, R2 = 0 W
R2
Fixed Gain
Error Amplifier
Comparator
Driver
Regulated
Output
R1
1.0 k
Latch
Freq
Shift
L1
V
out
Output
18 kHz
1.0 Amp
Switch
2
GND
1.235 V
Band−Gap
Reference
C
D1
out
Thermal
Shutdown
52 kHz
Oscillator
3
Reset
Load
This device contains 162 active transistors.
Figure 1. Block Diagram and Typical Application
MAXIMUM RATINGS
Rating
Symbol
Value
45
Unit
Maximum Supply Voltage
ON/OFF Pin Input Voltage
V
in
V
V
V
−
−0.3 V ≤ V ≤ +V
−1.0
in
Output Voltage to Ground (Steady−State)
−
Power Dissipation
Case 314B and 314D (TO−220, 5−Lead)
Thermal Resistance, Junction−to−Ambient
Thermal Resistance, Junction−to−Case
P
Internally Limited
W
D
R
R
65
5.0
°C/W
°C/W
W
q
JA
JC
D
q
2
Case 936A (D PAK)
P
Internally Limited
Thermal Resistance, Junction−to−Ambient
Thermal Resistance, Junction−to−Case
R
70
5.0
°C/W
°C/W
q
JA
JC
R
q
Storage Temperature Range
T
−65 to +150
2.0
°C
kV
°C
°C
stg
Minimum ESD Rating (Human Body Model: C = 100 pF, R = 1.5 kW)
Lead Temperature (Soldering, 10 seconds)
Maximum Junction Temperature
−
−
260
T
150
J
Maximum ratings are those values beyond which device damage can occur. Maximum ratings applied to the device are individual stress limit
values (not normal operating conditions) and are not valid simultaneously. If these limits are exceeded, device functional operation is not implied,
damage may occur and reliability may be affected.
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2
LM2576
OPERATING RATINGS (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.)
Rating
Operating Junction Temperature Range
Symbol
Value
−40 to +125
40
Unit
°C
T
J
Supply Voltage
V
in
V
SYSTEM PARAMETERS (Note 1 Test Circuit Figure 15)
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, V = 12 V for the 3.3 V, 5.0 V, and Adjustable version, V = 25 V
in
in
for the 12 V version, and V = 30 V for the 15 V version. I
= 500 mA. For typical values T = 25°C, for min/max values T is the
in
Load
J
J
operating junction temperature range that applies Note 2, unless otherwise noted.)
Characteristics
Symbol
Min
Typ
Max
Unit
LM2576−3.3 (Note 1 Test Circuit Figure 15)
Output Voltage (V = 12 V, I
= 0.5 A, T = 25°C)
V
out
3.234
3.3
3.366
V
V
in
Load
J
Output Voltage (6.0 V ≤ V ≤ 40 V, 0.5 A ≤ I
≤ 3.0 A)
V
out
in
Load
T = 25°C
T = −40 to +125°C
J
3.168
3.135
3.3
−
3.432
3.465
J
Efficiency (V = 12 V, I
= 3.0 A)
η
−
75
−
%
in
Load
LM2576−5 (Note 1 Test Circuit Figure 15)
Output Voltage (V = 12 V, I = 0.5 A, T = 25°C)
V
out
4.9
5.0
5.1
V
V
in
Load
J
Output Voltage (8.0 V ≤ V ≤ 40 V, 0.5 A ≤ I
≤ 3.0 A)
V
out
in
Load
T = 25°C
T = −40 to +125°C
J
4.8
4.75
5.0
−
5.2
5.25
J
Efficiency (V = 12 V, I
= 3.0 A)
Load
η
−
77
−
%
in
LM2576−12 (Note 1 Test Circuit Figure 15)
Output Voltage (V = 25 V, I = 0.5 A, T = 25°C)
V
out
11.76
12
12.24
V
V
in
Load
J
Output Voltage (15 V ≤ V ≤ 40 V, 0.5 A ≤ I
≤ 3.0 A)
V
out
in
Load
T = 25°C
T = −40 to +125°C
J
11.52
11.4
12
−
12.48
12.6
J
Efficiency (V = 15 V, I
= 3.0 A)
Load
η
−
88
−
%
in
LM2576−15 (Note 1 Test Circuit Figure 15)
Output Voltage (V = 30 V, I = 0.5 A, T = 25°C)
V
out
14.7
15
15.3
V
V
in
Load
J
Output Voltage (18 V ≤ V ≤ 40 V, 0.5 A ≤ I
≤ 3.0 A)
V
out
in
Load
T = 25°C
T = −40 to +125°C
J
14.4
14.25
15
−
15.6
15.75
J
Efficiency (V = 18 V, I
= 3.0 A)
Load
η
−
88
−
%
in
LM2576 ADJUSTABLE VERSION (Note 1 Test Circuit Figure 15)
Feedback Voltage (V = 12 V, I
= 0.5 A, V = 5.0 V, T = 25°C)
V
out
1.217
1.23
1.243
V
V
in
Load
out
J
Feedback Voltage (8.0 V ≤ V ≤ 40 V, 0.5 A ≤ I
≤ 3.0 A, V = 5.0 V)
V
out
in
Load
out
T = 25°C
T = −40 to +125°C
J
1.193
1.18
1.23
−
1.267
1.28
J
Efficiency (V = 12 V, I
= 3.0 A, V = 5.0 V)
η
−
77
−
%
in
Load
out
1. External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2576 is used as shown in the Figure 15 test circuit, system performance will be as shown in system parameters section.
2. Tested junction temperature range for the LM2576:
T
= −40°C
T
= +125°C
low
high
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3
LM2576
DEVICE PARAMETERS
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, V = 12 V for the 3.3 V, 5.0 V, and Adjustable version, V = 25 V
in
in
for the 12 V version, and V = 30 V for the 15 V version. I
= 500 mA. For typical values T = 25°C, for min/max values T is the
in
Load
J
J
operating junction temperature range that applies [Note 2], unless otherwise noted.)
Characteristics Symbol
ALL OUTPUT VOLTAGE VERSIONS
Min
Typ
Max
Unit
Feedback Bias Current (V = 5.0 V Adjustable Version Only)
I
nA
out
b
T = 25°C
T = −40 to +125°C
J
−
−
25
−
100
200
J
Oscillator Frequency Note 3
f
kHz
V
osc
T = 25°C
−
47
42
52
−
−
−
58
63
J
T = 0 to +125°C
J
T = −40 to +125°C
J
Saturation Voltage (I = 3.0 A Note 4)
V
sat
out
T = 25°C
T = −40 to +125°C
J
−
−
1.5
−
1.8
2.0
J
Max Duty Cycle (“on”) Note 5
DC
94
98
−
%
A
Current Limit (Peak Current Notes 3 and 4)
I
CL
T = 25°C
T = −40 to +125°C
J
4.2
3.5
5.8
−
6.9
7.5
J
Output Leakage Current Notes 6 and 7, T = 25°C
Output = 0 V
Output = −1.0 V
I
mA
mA
mA
V
J
L
−
−
0.8
6.0
2.0
20
Quiescent Current Note 6
I
Q
T = 25°C
−
−
5.0
−
9.0
11
J
T = −40 to +125°C
J
Standby Quiescent Current (ON/OFF Pin = 5.0 V (“off”))
I
stby
T = 25°C
−
−
80
−
200
400
J
T = −40 to +125°C
J
ON/OFF Pin Logic Input Level (Test Circuit Figure 15)
V
out
= 0 V
V
IH
T = 25°C
T = −40 to +125°C
J
2.2
2.4
1.4
−
−
−
J
V
out
= Nominal Output Voltage
V
IL
T = 25°C
T = −40 to +125°C
J
−
−
1.2
−
1.0
0.8
J
ON/OFF Pin Input Current (Test Circuit Figure 15)
mA
ON/OFF Pin = 5.0 V (“off”), T = 25°C
I
I
−
−
15
0
30
5.0
J
IH
ON/OFF Pin = 0 V (“on”), T = 25°C
J
IL
3. The oscillator frequency reduces to approximately 18 kHz in the event of an output short or an overload which causes the regulated output
voltage to drop approximately 40% from the nominal output voltage. This self protection feature lowers the average dissipation of the IC by
lowering the minimum duty cycle from 5% down to approximately 2%.
4. Output (Pin 2) sourcing current. No diode, inductor or capacitor connected to output pin.
5. Feedback (Pin 4) removed from output and connected to 0 V.
6. Feedback (Pin 4) removed from output and connected to +12 V for the Adjustable, 3.3 V, and 5.0 V versions, and +25 V for the 12 V and
15 V versions, to force the output transistor “off”.
7. V = 40 V.
in
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4
LM2576
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
1.0
0.8
0.6
1.4
1.2
V
= 20 V
= 500 mA
in
I
= 500 mA
Load
I
Load
T = 25°C
J
1.0
0.8
0.6
0.4
Normalized at T = 25°C
J
0.4
0.2
3.3 V, 5.0 V and ADJ
0
−0.2
−0.4
0.2
0
12 V and 15 V
−0.2
−0.4
−0.6
−0.6
−0.8
−1.0
−50
−25
0
25
50
75
100
125
0
5.0
10
15
20
25
30
35
40
T , JUNCTION TEMPERATURE (°C)
J
V , INPUT VOLTAGE (V)
in
Figure 2. Normalized Output Voltage
Figure 3. Line Regulation
2.0
1.5
1.0
0.5
0
6.5
6.0
5.5
V
in
= 25 V
I
= 3.0 A
Load
5.0
4.5
4.0
I
= 500 mA
Load
L1 = 150 mH
= 0.1 W
R
ind
−50
−25
0
25
50
75
100
125
−50
−25
0
25
50
75
100
125
T , JUNCTION TEMPERATURE (°C)
J
T , JUNCTION TEMPERATURE (°C)
J
Figure 4. Dropout Voltage
Figure 5. Current Limit
20
18
200
V
= 5.0 V
V
= 5.0 V
out
180
160
140
120
100
80
ON/OFF
Measured at
Ground Pin
T = 25°C
J
16
14
V
in
= 40 V
I
= 3.0 A
Load
12
10
V
in
= 12 V
60
I
= 200 mA
Load
8.0
6.0
4.0
40
20
0
−50
0
5.0
10
15
20
25
30
35
40
−25
0
25
50
75
100
125
V , INPUT VOLTAGE (V)
in
T , JUNCTION TEMPERATURE (°C)
J
Figure 6. Quiescent Current
Figure 7. Standby Quiescent Current
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5
LM2576
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
200
180
160
140
120
1.6
1.4
1.2
T = 25°C
J
−40°C
1.0
100
80
0.8 25°C
0.6
125°C
60
0.4
0.2
40
20
0
0
0
0
5
10
15
20
25
30
35
40
0.5
1.0
1.5
2.0
2.5
3.0
V , INPUT VOLTAGE (V)
in
SWITCH CURRENT (A)
Figure 8. Standby Quiescent Current
Figure 9. Switch Saturation Voltage
8.0
6.0
4.0
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
Adjustable Version Only
V
= 12 V
Normalized at
in
25°C
2.0
0
−2.0
V
' 1.23 V
= 500 mA
−4.0
−6.0
−8.0
−10
out
I
Load
0
−50
−50
−25
0
25
50
75
100
125
−25
0
25
50
75
100
125
T , JUNCTION TEMPERATURE (°C)
J
T , JUNCTION TEMPERATURE (°C)
J
Figure 10. Oscillator Frequency
Figure 11. Minimum Operating Voltage
100
80
Adjustable Version Only
60
40
20
0
−20
−40
−60
−80
−100
−50
−25
0
25
50
75
100
125
T , JUNCTION TEMPERATURE (°C)
J
Figure 12. Feedback Pin Current
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6
LM2576
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
50 V
0
A
B
100 mV
Output
0
4.0 A
Voltage
Change
2.0 A
0
− 100 mV
3.0 A
4.0 A
2.0 A
0
C
D
Load
2.0 A
Current
1.0 A
0
5 ms/DIV
100 ms/DIV
Figure 13. Switching Waveforms
Figure 14. Load Transient Response
Vout = 15 V
A: Output Pin Voltage, 10 V/DIV
B: Inductor Current, 2.0 A/DIV
C: Inductor Current, 2.0 A/DIV, AC−Coupled
D: Output Ripple Voltage, 50 mV/dDIV, AC−Coupled
Horizontal Time Base: 5.0 ms/DIV
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7
LM2576
Fixed Output Voltage Versions
Feedback
4
V
in
LM2576
L1
100 mH
Fixed Output
1
V
out
Output
2
3
GN
D
5
ON/OFF
7.0 V − 40 V
Unregulated
DC Input
C
100 mF
in
C
out
1000 mF
D1
MBR360
Load
C
C
D1
L1
R1
R2
−
−
−
−
−
−
100 mF, 75 V, Aluminium Electrolytic
1000 mF, 25 V, Aluminium Electrolytic
Schottky, MBR360
100 mH, Pulse Eng. PE−92108
2.0 k, 0.1%
in
out
6.12 k, 0.1%
Adjustable Output Voltage Versions
Feedback
4
V
in
LM2576
L1
100 mH
V
out
5,000 V
Adjustable
1
Output
2
ON/OFF
3
GN
D
5
7.0 V − 40 V
Unregulated
DC Input
R2
C
100 mF
in
C
out
1000 mF
D1
MBR360
Load
R1
R2
Ǔ
R1
ǒ1.0 )ꢀ
V
+ V
out
refꢀ
V
out
R2 + R1
ǒ
ꢀ–ꢀ1.0
Ǔ
V
ref
Where V = 1.23 V, R1
ref
between 1.0 k and 5.0 k
Figure 15. Typical Test Circuit
PCB LAYOUT GUIDELINES
As in any switching regulator, the layout of the printed
circuit board is very important. Rapidly switching currents
associated with wiring inductance, stray capacitance and
parasitic inductance of the printed circuit board traces can
generate voltage transients which can generate
electromagnetic interferences (EMI) and affect the desired
operation. As indicated in the Figure 15, to minimize
inductance and ground loops, the length of the leads
indicated by heavy lines should be kept as short as possible.
For best results, single−point grounding (as indicated) or
ground plane construction should be used.
On the other hand, the PCB area connected to the Pin 2
(emitter of the internal switch) of the LM2576 should be
kept to a minimum in order to minimize coupling to sensitive
circuitry.
Another sensitive part of the circuit is the feedback. It is
important to keep the sensitive feedback wiring short. To
assure this, physically locate the programming resistors near
to the regulator, when using the adjustable version of the
LM2576 regulator.
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LM2576
PIN FUNCTION DESCRIPTION
Pin
Symbol
Description (Refer to Figure 1)
1
V
in
This pin is the positive input supply for the LM2576 step−down switching regulator. In order to minimize voltage
transients and to supply the switching currents needed by the regulator, a suitable input bypass capacitor must be
present (C in Figure 1).
in
2
Output
This is the emitter of the internal switch. The saturation voltage V of this output switch is typically 1.5 V. It should
sat
be kept in mind that the PCB area connected to this pin should be kept to a minimum in order to minimize coupling
to sensitive circuitry.
3
4
GND
Circuit ground pin. See the information about the printed circuit board layout.
Feedback
This pin senses regulated output voltage to complete the feedback loop. The signal is divided by the internal resistor
divider network R2, R1 and applied to the non−inverting input of the internal error amplifier. In the Adjustable version
of the LM2576 switching regulator this pin is the direct input of the error amplifier and the resistor network R2, R1 is
connected externally to allow programming of the output voltage.
5
ON/OFF
It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the total input supply
current to approximately 80 mA. The threshold voltage is typically 1.4 V. Applying a voltage above this value (up to
+V ) shuts the regulator off. If the voltage applied to this pin is lower than 1.4 V or if this pin is left open, the
in
regulator will be in the “on” condition.
DESIGN PROCEDURE
Buck Converter Basics
This period ends when the power switch is once again
turned on. Regulation of the converter is accomplished by
varying the duty cycle of the power switch. It is possible to
describe the duty cycle as follows:
The LM2576 is a “Buck” or Step−Down Converter which
is the most elementary forward−mode converter. Its basic
schematic can be seen in Figure 16.
The operation of this regulator topology has two distinct
time periods. The first one occurs when the series switch is
on, the input voltage is connected to the input of the inductor.
The output of the inductor is the output voltage, and the
rectifier (or catch diode) is reverse biased. During this
period, since there is a constant voltage source connected
across the inductor, the inductor current begins to linearly
ramp upwards, as described by the following equation:
t
on
T
d +
, where T is the period of switching.
For the buck converter with ideal components, the duty
cycle can also be described as:
V
out
d +
V
in
Figure 17 shows the buck converter, idealized waveforms
of the catch diode voltage and the inductor current.
ǒVin outǓ ton
– V
I
+
L(on)
L
V
on(SW)
During this “on” period, energy is stored within the core
material in the form of magnetic flux. If the inductor is
properly designed, there is sufficient energy stored to carry
the requirements of the load during the “off” period.
Power
Switch
Power
Switch
Off
Power
Switch
On
Power
Switch
Off
Power
Switch
On
L
V (FWD)
D
C
V
in
D
out
R
Load
Time
Figure 16. Basic Buck Converter
I
pk
The next period is the “off” period of the power switch.
When the power switch turns off, the voltage across the
inductor reverses its polarity and is clamped at one diode
voltage drop below ground by the catch diode. The current
now flows through the catch diode thus maintaining the load
current loop. This removes the stored energy from the
inductor. The inductor current during this time is:
I
(AV)
Load
I
min
Power
Switch
Power
Switch
Diode
Diode
Time
Figure 17. Buck Converter Idealized Waveforms
ǒVout DǓ toff
– V
I
+
L(off)
L
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9
LM2576
Procedure (Fixed Output Voltage Version) In order to simplify the switching regulator design, a step−by−step
design procedure and some examples are provided.
Procedure
Example
Given Parameters:
= Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V)
Given Parameters:
= 5.0 V
V
out
V
out
V
= Maximum Input Voltage
V
= 15 V
in(max)
in(max)
I
= Maximum Load Current
I
= 3.0 A
Load(max)
Load(max)
1. Controller IC Selection
1. Controller IC Selection
According to the required input voltage, output voltage and
current, select the appropriate type of the controller IC output
voltage version.
According to the required input voltage, output voltage,
current polarity and current value, use the LM2576−5
controller IC
2. Input Capacitor Selection (C )
2. Input Capacitor Selection (C )
in
in
To prevent large voltage transients from appearing at the input
and for stable operation of the converter, an aluminium or
tantalum electrolytic bypass capacitor is needed between the
A 100 mF, 25 V aluminium electrolytic capacitor located near
to the input and ground pins provides sufficient bypassing.
input pin +V and ground pin GND. This capacitor should be
in
located close to the IC using short leads. This capacitor should
have a low ESR (Equivalent Series Resistance) value.
3. Catch Diode Selection (D1)
3. Catch Diode Selection (D1)
A. Since the diode maximum peak current exceeds the
regulator maximum load current the catch diode current
rating must be at least 1.2 times greater than the maximum
load current. For a robust design the diode should have a
current rating equal to the maximum current limit of the
LM2576 to be able to withstand a continuous output short
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
A. For this example the current rating of the diode is 3.0 A.
B. Use a 20 V 1N5820 Schottky diode, or any of the
suggested fast recovery diodes shown in Table 1.
4. Inductor Selection (L1)
4. Inductor Selection (L1)
A. According to the required working conditions, select the
correct inductor value using the selection guide from
Figures 18 to 22.
A. Use the inductor selection guide shown in Figures 19.
B. From the appropriate inductor selection guide, identify the
inductance region intersected by the Maximum Input
Voltage line and the Maximum Load Current line. Each
region is identified by an inductance value and an inductor
code.
B. From the selection guide, the inductance area intersected
by the 15 V line and 3.0 A line is L100.
C. Select an appropriate inductor from the several different
manufacturers part numbers listed in Table 2.
The designer must realize that the inductor current rating
must be higher than the maximum peak current flowing
through the inductor. This maximum peak current can be
calculated as follows:
C. Inductor value required is 100 mH. From Table 2, choose
an inductor from any of the listed manufacturers.
ǒVin outǓ ton
–V
p(max) + I
I
)
Load(max)
2L
where t is the “on” time of the power switch and
on
V
out
1.0
t
+
x
on
V
f
osc
in
For additional information about the inductor, see the
inductor section in the “Application Hints” section of
this data sheet.
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LM2576
Procedure (Fixed Output Voltage Version) (continued)In order to simplify the switching regulator design, a step−by−step
design procedure and some examples are provided.
Procedure
Example
5. Output Capacitor Selection (C )
out
5. Output Capacitor Selection (C
)
out
A. Since the LM2576 is a forward−mode switching regulator
A. C = 680 mF to 2000 mF standard aluminium electrolytic.
out
with voltage mode control, its open loop 2−pole−1−zero
frequency characteristic has the dominant pole−pair
determined by the output capacitor and inductor values. For
stable operation and an acceptable ripple voltage,
(approximately 1% of the output voltage) a value between
680 mF and 2000 mF is recommended.
B. Due to the fact that the higher voltage electrolytic capacitors
generally have lower ESR (Equivalent Series Resistance)
numbers, the output capacitor’s voltage rating should be at
least 1.5 times greater than the output voltage. For a 5.0 V
regulator, a rating at least 8.0 V is appropriate, and a 10 V or
16 V rating is recommended.
B. Capacitor voltage rating = 20 V.
Procedure (Adjustable Output Version: LM2576−ADJ)
Procedure
Example
Given Parameters:
Given Parameters:
V
out
= Regulated Output Voltage
V
out
= 8.0 V
V
= Maximum DC Input Voltage
V
= 25 V
in(max)
in(max)
I
= Maximum Load Current
I
= 2.5 A
Load(max)
Load(max)
1. Programming Output Voltage
1. Programming Output Voltage (selecting R1 and R2)
To select the right programming resistor R1 and R2 value (see
Figure 2) use the following formula:
Select R1 and R2:
R2
R1
+ 1.23ǒ1.0 )
Ǔ
V
Select R1 = 1.8 kW
out
R2
R1
ref ǒ1.0 )
Ǔ
V
+ V
where V = 1.23 V
ref
out
V
out
8.0 V
+ 1.8 kǒ
* 1.0Ǔ
1.23 V
R2 + R1ǒ Ǔ
* 1.0
Resistor R1 can be between 1.0 k and 5.0 kW. (For best
temperature coefficient and stability with time, use 1% metal
film resistors).
V
ref
R2 = 9.91 kW, choose a 9.88 k metal film resistor.
V
out
R2 + R1ǒ Ǔ
– 1.0
V
ref
2. Input Capacitor Selection (C )
2. Input Capacitor Selection (C )
in
in
To prevent large voltage transients from appearing at the input
and for stable operation of the converter, an aluminium or
tantalum electrolytic bypass capacitor is needed between the
A 100 mF, 150 V aluminium electrolytic capacitor located near
the input and ground pin provides sufficient bypassing.
input pin +V and ground pin GND This capacitor should be
in
located close to the IC using short leads. This capacitor should
have a low ESR (Equivalent Series Resistance) value.
For additional information see input capacitor section in the
“Application Hints” section of this data sheet.
3. Catch Diode Selection (D1)
3. Catch Diode Selection (D1)
A. Since the diode maximum peak current exceeds the
regulator maximum load current the catch diode current
rating must be at least 1.2 times greater than the maximum
load current. For a robust design, the diode should have a
current rating equal to the maximum current limit of the
LM2576 to be able to withstand a continuous output short.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
A. For this example, a 3.0 A current rating is adequate.
B. Use a 30 V 1N5821 Schottky diode or any suggested fast
recovery diode in the Table 1.
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LM2576
Procedure (Adjustable Output Version: LM2576−ADJ) (continued)
Procedure
Example
A. Calculate E x T [V x ms] constant:
8.0 1000
4. Inductor Selection (L1)
4. Inductor Selection (L1)
A. Use the following formula to calculate the inductor Volt x
microsecond [V x ms] constant:
E x T + ǒVin outǓ Vout
E x T + 25 – 8.0 x
x
+ 80 [V x ms]
6
10
(
)
– V
x
[V x ms]
25
52
V
F[Hz]
in
B. Match the calculated E x T value with the corresponding
number on the vertical axis of the Inductor Value Selection
Guide shown in Figure 22. This E x T 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.
B. E x T = 80 [V x ms]
C. Next step is to identify the inductance region intersected by
the E x T value and the maximum load current value on the
horizontal axis shown in Figure 25.
C. I
= 2.5 A
Load(max)
Inductance Region = H150
D. From the inductor code, identify the inductor value. Then
select an appropriate inductor from Table 2.
D. Proper inductor value = 150 mH
Choose the inductor from Table 2.
The inductor chosen must be rated for a switching
frequency of 52 kHz and for a current rating of 1.15 x I
The inductor current rating can also be determined by
calculating the inductor peak current:
.
Load
ǒVin outǓton
– V
p(max) + I
I
)
Load(max)
2L
where t is the “on” time of the power switch and
on
V
out
1.0
t
+
x
on
V
f
osc
in
For additional information about the inductor, see the
inductor section in the “External Components” section of
this data sheet.
5. Output Capacitor Selection (C
)
out
5. Output Capacitor Selection (C
)
out
A. Since the LM2576 is a forward−mode switching regulator
with voltage mode control, its open loop 2−pole−1−zero
frequency characteristic has the dominant pole−pair
determined by the output capacitor and inductor values.
A.
25
8 x 150
C
w 13,300 x
+ 332.5 μF
out
To achieve an acceptable ripple voltage, select
C
out
= 680 mF electrolytic capacitor.
For stable operation, the capacitor must satisfy the
following requirement:
V
in(max)
C
w 13,300
[μF]
out
V
x L [μH]
out
B. Capacitor values between 10 mF and 2000 mF will satisfy
the loop requirements for stable operation. To achieve an
acceptable output ripple voltage and transient response, the
output capacitor may need to be several times larger than
the above formula yields.
C. Due to the fact that the higher voltage electrolytic capacitors
generally have lower ESR (Equivalent Series Resistance)
numbers, the output capacitor’s voltage rating should be at
least 1.5 times greater than the output voltage. For a 5.0 V
regulator, a rating of at least 8.0 V is appropriate, and a 10 V
or 16 V rating is recommended.
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LM2576
LM2576 Series Buck Regulator Design Procedures (continued)
Indicator Value Selection Guide (For Continuous Mode Operation)
60
60
L680
40
20
15
H1000
H680
H470
L330
H330
H220
H150
L470
40
L330
20
15
L680
10
L220
L470
8.0
12
L150
7.0
L220
10
L100
L150
L68
9.0
6.0
L100
8.0
L47
L68
L47
5.0
0.3
7.0
0.3
0.4 0.5 0.6
0.8 1.0
1.5
2.0 2.5 3.0
0.4 0.5 0.6
0.8 1.0 1.2 1.5
2.0
2.5 3.0
I , MAXIMUM LOAD CURRENT (A)
L
I , MAXIMUM LOAD CURRENT (A)
L
Figure 18. LM2576−3.3
Figure 19. LM2576−5
60
60
40
35
30
40
35
30
H1500
H1500
25
H1000
L470
H1000
H680
H470
25
H680
H470
H330
20
18
H220
H150
H330
H220
22
H150
20
19
L680
L680
L470
16
15
L330
L330
L220
L220
L150
L150
18
L100
L100
L68
L68
14
0.3
17
0.3
0.4 0.5 0.6
0.8 1.0
1.5
2.0 2.5 3.0
0.4 0.5 0.6
0.8 1.0
1.5
2.0 2.5 3.0
I , MAXIMUM LOAD CURRENT (A)
L
I , MAXIMUM LOAD CURRENT (A)
L
Figure 20. LM2576−12
Figure 21. LM2576−15
300
250
H2000
200
150
H1500
H1000
H680
L220
H470
L150
H330
H220
100
90
H150
80
70
L680
60
50
45
40
L470
L330
L100
1.5
35
L68
30
L47
25
20
0.3
0.4 0.5 0.6
0.8 1.0
2.0 2.5 3.0
I , MAXIMUM LOAD CURRENT (A)
L
Figure 22. LM2576−ADJ
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LM2576
Table 1. Diode Selection Guide
Schottky
Fast Recovery
3.0 A
4.0 − 6.0 A
Through Surface
3.0 A
4.0 − 6.0 A
Through
Hole
Surface
Mount
Through
Hole
Surface
Mount
Through
Surface
Mount
Hole
Mount
Hole
V
R
20 V
1N5820
MBR320P
SR302
SK32
1N5823
SR502
SB520
30 V
1N5821
MBR330
SR303
SK33
30WQ03
1N5824
SR503
SB530
50WQ03
MUR320
31DF1
HER302
MURS320T3
MURD320
30WF10
MUR420
HER602
MURD620CT
50WF10
31DQ03
40 V
1N5822
MBR340
SR304
SK34
30WQ04
MBRS340T3
MBRD340
1N5825
SR504
SB540
MBRD640CT
50WQ04
(all diodes
rated
(all diodes
rated
(all diodes
rated
(all diodes
rated
31DQ04
to at least
100 V)
to at least
100 V)
to at least
100 V)
to at least
100 V)
50 V
60 V
MBR350
31DQ05
SR305
SK35
30WQ05
SB550
50WQ05
MBR360
DQ06
MBRS360T3
MBRD360
50SQ080
MBRD660CT
SR306
NOTE: Diodes listed in bold are available from ON Semiconductor.
Table 2. Inductor Selection by Manufacturer’s Part Number
Inductor
Code
Inductor
Value
Tech 39
Schott Corp.
Pulse Eng.
Renco
L47
L68
47 mH
68 mH
77 212
671 26980
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
RL2442
77 262
77 312
77 360
77 408
77 456
*
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
RL2443
RL2444
RL1954
RL1953
RL1952
RL1951
RL1950
RL2445
RL2446
RL2447
RL1961
RL1960
RL1959
RL1958
RL2448
L100
L150
L220
L330
L470
L680
H150
H220
H330
H470
H680
H1000
H1500
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
77 506
77 362
77 412
77 462
*
77 508
77 556
*
H2200
*
NOTE: *Contact Manufacturer
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LM2576
Table 3. Example of Several Inductor Manufacturers Phone/Fax Numbers
Phone
Fax
+ 1−619−674−8100
+ 1−619−674−8262
Pulse Engineering, Inc.
Pulse Engineering, Inc. Europe
Renco Electronics, Inc.
Tech 39
Phone
Fax
+ 353−9324−107
+ 353−9324−459
Phone
Fax
+ 1−516−645−5828
+ 1−516−586−5562
Phone
Fax
+ 33−1−4115−1681
+ 33−1−4709−5051
Phone
Fax
+ 1−612−475−1173
+ 1−612−475−1786
Schott Corporation
EXTERNAL COMPONENTS
Input Capacitor (Cin)
Output Capacitor (Cout
)
The Input Capacitor Should Have a Low ESR
For low output ripple voltage and good stability, low ESR
output capacitors are recommended. An output capacitor
has two main functions: it filters the output and provides
regulator loop stability. The ESR of the output capacitor and
the peak−to−peak value of the inductor ripple current are the
main factors contributing to the output ripple voltage value.
Standard aluminium electrolytics could be adequate for
some applications but for quality design, low ESR types are
recommended.
An aluminium electrolytic capacitor’s ESR value is
related to many factors such as the capacitance value, the
voltage rating, the physical size and the type of construction.
In most cases, the higher voltage electrolytic capacitors have
lower ESR value. Often capacitors with much higher
voltage ratings may be needed to provide low ESR values
that, are required for low output ripple voltage.
For stable operation of the switch mode converter a low
ESR (Equivalent Series Resistance) aluminium or solid
tantalum bypass capacitor is needed between the input pin
and the ground pin, to prevent large voltage transients from
appearing at the input. It must be located near the regulator
and use short leads. With most electrolytic capacitors, the
capacitance value decreases and the ESR increases with
lower temperatures. For reliable operation in temperatures
below −25°C larger values of the input capacitor may be
needed. Also paralleling a ceramic or solid tantalum
capacitor will increase the regulator stability at cold
temperatures.
RMS Current Rating of C
in
The important parameter of the input capacitor is the RMS
current rating. Capacitors that are physically large and have
large surface area will typically have higher RMS current
ratings. For a given capacitor value, a higher voltage
electrolytic capacitor will be physically larger than a lower
voltage capacitor, and thus be able to dissipate more heat to
the surrounding air, and therefore will have a higher RMS
current rating. The consequence of operating an electrolytic
capacitor beyond the RMS current rating is a shortened
operating life. In order to assure maximum capacitor
operating lifetime, the capacitor’s RMS ripple current rating
should be:
The Output Capacitor Requires an ESR Value
That Has an Upper and Lower Limit
As mentioned above, a low ESR value is needed for low
output ripple voltage, typically 1% to 2% of the output
voltage. But if the selected capacitor’s ESR is extremely low
(below 0.05 W), there is a possibility of an unstable feedback
loop, resulting in oscillation at the output. This situation can
occur when a tantalum capacitor, that can have a very low
ESR, is used as the only output capacitor.
At Low Temperatures, Put in Parallel Aluminium
Electrolytic Capacitors with Tantalum Capacitors
Electrolytic capacitors are not recommended for
temperatures below −25°C. The ESR rises dramatically at
cold temperatures and typically rises 3 times at −25°C and
as much as 10 times at −40°C. Solid tantalum capacitors
have much better ESR spec at cold temperatures and are
recommended for temperatures below −25°C. They can be
also used in parallel with aluminium electrolytics. The value
of the tantalum capacitor should be about 10% or 20% of the
total capacitance. The output capacitor should have at least
50% higher RMS ripple current rating at 52 kHz than the
peak−to−peak inductor ripple current.
Irms > 1.2 x d x ILoad
where d is the duty cycle, for a buck regulator
V
t
on
T
out
d +
|V
+
V
in
|
t
on
T
out
|V | ) V
and d +
+
for a buck*boost regulator.
out
in
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LM2576
Catch Diode
ripple voltage. On the other hand it does require larger
Locate the Catch Diode Close to the LM2576
The LM2576 is a step−down buck converter; it requires a
fast diode to provide a return path for the inductor current
when the switch turns off. This diode must be located close
to the LM2576 using short leads and short printed circuit
traces to avoid EMI problems.
inductor values to keep the inductor current flowing
continuously, especially at low output load currents and/or
high input voltages.
To simplify the inductor selection process, an inductor
selection guide for the LM2576 regulator was added to this
data sheet (Figures 18 through 22). This guide assumes that
the regulator is operating in the continuous mode, and
selects an inductor that will allow a peak−to−peak inductor
ripple current to be a certain percentage of the maximum
design load current. This percentage is allowed to change as
different design load currents are selected. For light loads
(less than approximately 300 mA) it may be desirable to
operate the regulator in the discontinuous mode, because the
inductor value and size can be kept relatively low.
Consequently, the percentage of inductor peak−to−peak
current increases. This discontinuous mode of operation is
perfectly acceptable for this type of switching converter.
Any buck regulator will be forced to enter discontinuous
mode if the load current is light enough.
Use a Schottky or a Soft Switching
Ultra−Fast Recovery Diode
Since the rectifier diodes are very significant sources of
losses within switching power supplies, choosing the
rectifier that best fits into the converter design is an
important process. Schottky diodes provide the best
performance because of their fast switching speed and low
forward voltage drop.
They provide the best efficiency especially in low output
voltage applications (5.0 V and lower). Another choice
could be Fast−Recovery, or Ultra−Fast Recovery diodes. It
has to be noted, that some types of these diodes with an
abrupt turnoff characteristic may cause instability or
EMI troubles.
A fast−recovery diode with soft recovery characteristics
can better fulfill some quality, low noise design requirements.
Table 1 provides a list of suitable diodes for the LM2576
regulator. Standard 50/60 Hz rectifier diodes, such as the
1N4001 series or 1N5400 series are NOT suitable.
2.0 A
Inductor
Current
Waveform
0 A
Inductor
2.0 A
Power
Switch
The magnetic components are the cornerstone of all
switching power supply designs. The style of the core and
the winding technique used in the magnetic component’s
design has a great influence on the reliability of the overall
power supply.
Current
Waveform
0 A
Using an improper or poorly designed inductor can cause
high voltage spikes generated by the rate of transitions in
current within the switching power supply, and the
possibility of core saturation can arise during an abnormal
operational mode. Voltage spikes can cause the
semiconductors to enter avalanche breakdown and the part
can instantly fail if enough energy is applied. It can also
cause significant RFI (Radio Frequency Interference) and
EMI (Electro−Magnetic Interference) problems.
HORIZONTAL TIME BASE: 5.0 ms/DIV
Figure 23. Continuous Mode Switching Current
Waveforms
Selecting the Right Inductor Style
Some important considerations when selecting a core type
are core material, cost, the output power of the power supply,
the physical volume the inductor must fit within, and the
amount of EMI (Electro−Magnetic Interference) shielding
that the core must provide. The inductor selection guide
covers different styles of inductors, such as pot core, E−core,
toroid and bobbin core, as well as different core materials
such as ferrites and powdered iron from different
manufacturers.
For high quality design regulators the toroid core seems to
be the best choice. Since the magnetic flux is contained
within the core, it generates less EMI, reducing noise
problems in sensitive circuits. The least expensive is the
bobbin core type, which consists of wire wound on a ferrite
rod core. This type of inductor generates more EMI due to
the fact that its core is open, and the magnetic flux is not
contained within the core.
Continuous and Discontinuous Mode of Operation
The LM2576 step−down converter can operate in both the
continuous and the discontinuous modes of operation. The
regulator works in the continuous mode when loads are
relatively heavy, the current flows through the inductor
continuously and never falls to zero. Under light load
conditions, the circuit will be forced to the discontinuous
mode when inductor current falls to zero for certain period
of time (see Figure 23 and Figure 24). Each mode has
distinctively different operating characteristics, which can
affect the regulator performance and requirements. In many
cases the preferred mode of operation is the continuous
mode. It offers greater output power, lower peak currents in
the switch, inductor and diode, and can have a lower output
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LM2576
When multiple switching regulators are located on the
inductor and/or the LM2576. Different inductor types have
different saturation characteristics, and this should be kept
in mind when selecting an inductor.
same printed circuit board, open core magnetics can cause
interference between two or more of the regulator circuits,
especially at high currents due to mutual coupling. A toroid,
pot core or E−core (closed magnetic structure) should be
used in such applications.
Do Not Operate an Inductor Beyond its
Maximum Rated Current
0.4 A
Inductor
Current
Waveform
0 A
Exceeding an inductor’s maximum current rating may
cause the inductor to overheat because of the copper wire
losses, or the core may saturate. Core saturation occurs when
the flux density is too high and consequently the cross
sectional area of the core can no longer support additional
lines of magnetic flux.
This causes the permeability of the core to drop, the
inductance value decreases rapidly and the inductor begins
to look mainly resistive. It has only the DC resistance of the
winding. This can cause the switch current to rise very
rapidly and force the LM2576 internal switch into
cycle−by−cycle current limit, thus reducing the DC output
load current. This can also result in overheating of the
0.4 A
Power
Switch
Current
Waveform
0 A
HORIZONTAL TIME BASE: 5.0 ms/DIV
Figure 24. Discontinuous Mode Switching Current
Waveforms
GENERAL RECOMMENDATIONS
Output Voltage Ripple and Transients
Minimizing the Output Ripple
Source of the Output Ripple
In order to minimize the output ripple voltage it is possible
to enlarge the inductance value of the inductor L1 and/or to
use a larger value output capacitor. There is also another way
to smooth the output by means of an additional LC filter (20
mH, 100 mF), that can be added to the output (see Figure 34)
to further reduce the amount of output ripple and transients.
With such a filter it is possible to reduce the output ripple
voltage transients 10 times or more. Figure 25 shows the
difference between filtered and unfiltered output waveforms
of the regulator shown in Figure 34.
Since the LM2576 is a switch mode power supply
regulator, its output voltage, if left unfiltered, will contain a
sawtooth ripple voltage at the switching frequency. The
output ripple voltage value ranges from 0.5% to 3% of the
output voltage. It is caused mainly by the inductor sawtooth
ripple current multiplied by the ESR of the output capacitor.
Short Voltage Spikes and How to Reduce Them
The regulator output voltage may also contain short
voltage spikes at the peaks of the sawtooth waveform (see
Figure 25). These voltage spikes are present because of the
fast switching action of the output switch, and the parasitic
inductance of the output filter capacitor. There are some
other important factors such as wiring inductance, stray
capacitance, as well as the scope probe used to evaluate these
transients, all these contribute to the amplitude of these
spikes. To minimize these voltage spikes, low inductance
capacitors should be used, and their lead lengths must be
kept short. The importance of quality printed circuit board
layout design should also be highlighted.
The lower waveform is from the normal unfiltered output
of the converter, while the upper waveform shows the output
ripple voltage filtered by an additional LC filter.
Heatsinking and Thermal Considerations
The Through−Hole Package TO−220
The LM2576 is available in two packages, a 5−pin
2
TO−220(T, TV) and a 5−pin surface mount D PAK(D2T).
Although the TO−220(T) package needs a heatsink under
most conditions, there are some applications that require no
heatsink to keep the LM2576 junction temperature within
the allowed operating range. Higher ambient temperatures
require some heat sinking, either to the printed circuit (PC)
board or an external heatsink.
Voltage spikes
caused by
switching action
of the output
switch and the
parasitic
inductance of the
output capacitor
Filtered
Output
Voltage
The Surface Mount Package D2PAK and its
Heatsinking
The other type of package, the surface mount D PAK, is
2
designed to be soldered to the copper on the PC board. The
copper and the board are the heatsink for this package and
the other heat producing components, such as the catch
diode and inductor. The PC board copper area that the
Unfiltered
Output
Voltage
2
2
package is soldered to should be at least 0.4 in (or 260 mm )
HORIZONTAL TIME BASE: 5.0 ms/DIV
2
and ideally should have 2 or more square inches (1300 mm )
Figure 25. Output Ripple Voltage Waveforms
of 0.0028 inch copper. Additional increases of copper area
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17
LM2576
2
2
Packages on a Heatsink
beyond approximately 6.0 in (4000 mm ) will not improve
heat dissipation significantly. If further thermal
improvements are needed, double sided or multilayer PC
boards with large copper areas should be considered. In
order to achieve the best thermal performance, it is highly
recommended to use wide copper traces as well as large
areas of copper in the printed circuit board layout. The only
exception to this is the OUTPUT (switch) pin, which should
not have large areas of copper (see page 8 ‘PCB Layout
Guideline’).
If the actual operating junction temperature is greater than
the selected safe operating junction temperature determined
in step 3, than a heatsink is required. The junction
temperature will be calculated as follows:
TJ = PD (Rq + Rq + RqSA) + TA
JA
CS
where
R
qJC
R
qCS
R
qSA
is the thermal resistance junction−case,
is the thermal resistance case−heatsink,
is the thermal resistance heatsink−ambient.
If the actual operating temperature is greater than the
selected safe operating junction temperature, then a larger
heatsink is required.
Thermal Analysis and Design
The following procedure must be performed to determine
whether or not a heatsink will be required. First determine:
Some Aspects That can Influence Thermal Design
It should be noted that the package thermal resistance and
the junction temperature rise numbers are all approximate,
and there are many factors that will affect these numbers,
such as PC board size, shape, thickness, physical position,
location, board temperature, as well as whether the
surrounding air is moving or still.
Other factors are trace width, total printed circuit copper
area, copper thickness, single− or double−sided, multilayer
board, the amount of solder on the board or even color of the
traces.
1. P
2. T
3. T
maximum regulator power dissipation in the
application.
maximum ambient temperature in the
application.
maximum allowed junction temperature
(125°C for the LM2576). For a conservative
design, the maximum junction temperature
should not exceed 110°C to assure safe
operation. For every additional +10°C
temperature rise that the junction must
withstand, the estimated operating lifetime
of the component is halved.
D(max)
)
A(max
J(max)
The size, quantity and spacing of other components on the
board can also influence its effectiveness to dissipate the heat.
4. R
5. R
package thermal resistance junction−case.
package thermal resistance junction−ambient.
qJC
qJA
12 to 40 V
Feedback
Unregulated
(Refer to Maximum Ratings on page 2 of this data sheet or
and R values).
DC Input
L1
68 mH
+V
4
in
R
qJC
qJA
LM2576−12
Output
1
The following formula is to calculate the approximate
total power dissipated by the LM2576:
C
100 mF
in
2
ON/OFF
D1
1N5822
C
out
2200 mF
3
GN
D
5
PD = (Vin x IQ) + d x ILoad x Vsat
where d is the duty cycle and for buck converter
−12 V @ 0.7 A
Regulated
Output
V
V
t
on
T
O
in
d +
+
,
Figure 26. Inverting Buck−Boost Develops −12 V
ADDITIONAL APPLICATIONS
I
(quiescent current) and V can be found in the
Q
sat
LM2576 data sheet,
V
is minimum input voltage applied,
is the regulator output voltage,
is the load current.
in
Inverting Regulator
V
O
An inverting buck−boost regulator using the LM2576−12
is shown in Figure 26. This circuit converts a positive input
voltage to a negative output voltage with a common ground
by bootstrapping the regulators ground to the negative
output voltage. By grounding the feedback pin, the regulator
senses the inverted output voltage and regulates it.
In this example the LM2576−12 is used to generate a
−12 V output. The maximum input voltage in this case
cannot exceed +28 V because the maximum voltage
appearing across the regulator is the absolute sum of the
input and output voltages and this must be limited to a
maximum of 40 V.
I
Load
The dynamic switching losses during turn−on and
turn−off can be neglected if proper type catch diode is used.
Packages Not on a Heatsink (Free−Standing)
For a free−standing application when no heatsink is used,
the junction temperature can be determined by the following
expression:
TJ = (RqJA) (PD) + TA
where (R )(P ) represents the junction temperature rise
qJA
D
caused by the dissipated power and T is the maximum
A
ambient temperature.
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18
LM2576
This circuit configuration is able to deliver approximately
I
(V ) |V |)
Load in
V
x t
on
O
in
2L
I
[
)
0.7 A to the output when the input voltage is 12 V or higher.
At lighter loads the minimum input voltage required drops
to approximately 4.7 V, because the buck−boost regulator
topology can produce an output voltage that, in its absolute
value, is either greater or less than the input voltage.
Since the switch currents in this buck−boost configuration
are higher than in the standard buck converter topology, the
available output current is lower.
This type of buck−boost inverting regulator can also
require a larger amount of startup input current, even for
light loads. This may overload an input power source with
a current limit less than 5.0 A.
Such an amount of input startup current is needed for at
least 2.0 ms or more. The actual time depends on the output
voltage and size of the output capacitor.
Because of the relatively high startup currents required by
this inverting regulator topology, the use of a delayed startup
or an undervoltage lockout circuit is recommended.
Using a delayed startup arrangement, the input capacitor
can charge up to a higher voltage before the switch−mode
regulator begins to operate.
peak
V
in
x
1
|V |
O
1.0
where t
+
, and f
+ 52 kHz.
osc
on
V
) |V |
f
osc
in
O
Under normal continuous inductor current operating
conditions, the worst case occurs when V is minimal.
in
12 V to 25 V
Unregulated
DC Input
Feedback
4
Output
+V
in
L1
68 mH
LM2576−12
1
C
100 mF
/50 V
in
C1
0.1 mF
2
5
ON/OFF 3 GN
D
C
out
2200 mF
/16 V
D1
1N5822
R1
47 k
R2
47 k
−12 V @ 700 m A
Regulated
Output
Figure 27. Inverting Buck−Boost Regulator
with Delayed startup
The high input current needed for startup is now partially
supplied by the input capacitor C .
in
It has been already mentioned above, that in some
situations, the delayed startup or the undervoltage lockout
features could be very useful. A delayed startup circuit
applied to a buck−boost converter is shown in Figure 27,
Figure 33 in the “Undervoltage Lockout” section describes
an undervoltage lockout feature for the same converter
topology.
+V
+V
in
in
LM2576−XX
1
C
R1
100 mF 47 k
in
5
ON/OFF 3 GN
D
Shutdown
Input
5.0 V
Off
R3
470
0
On
Design Recommendations:
R2
47 k
The inverting regulator operates in a different manner
than the buck converter and so a different design procedure
has to be used to select the inductor L1 or the output
−V
out
MOC8101
capacitor C
.
out
The output capacitor values must be larger than what is
normally required for buck converter designs. Low input
voltages or high output currents require a large value output
capacitor (in the range of thousands of mF).
NOTE: This picture does not show the complete circuit.
Figure 28. Inverting Buck−Boost Regulator Shutdown
Circuit Using an Optocoupler
The recommended range of inductor values for the
inverting converter design is between 68 mH and 220 mH. To
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.
The following formula is used to obtain the peak inductor
current:
With the inverting configuration, the use of the ON/OFF
pin requires some level shifting techniques. This is caused
by the fact, that the ground pin of the converter IC is no
longer at ground. Now, the ON/OFF pin threshold voltage
(1.3 V approximately) has to be related to the negative
output voltage level. There are many different possible shut
down methods, two of them are shown in Figures 28 and 29.
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19
LM2576
Shutdown
Input
Another important point is that these negative boost
+V
0
Off
converters cannot provide current limiting load protection in
the event of a short in the output so some other means, such
as a fuse, may be necessary to provide the load protection.
On
R2
5.6 k
+V
in
+V
in
Delayed Startup
1
There are some applications, like the inverting regulator
already mentioned above, which require a higher amount of
startup current. In such cases, if the input power source is
limited, this delayed startup feature becomes very useful.
To provide a time delay between the time when the input
voltage is applied and the time when the output voltage
comes up, the circuit in Figure 31 can be used. As the input
voltage is applied, the capacitor C1 charges up, and the
voltage across the resistor R2 falls down. When the voltage
on the ON/OFF pin falls below the threshold value 1.3 V, the
regulator starts up. Resistor R1 is included to limit the
maximum voltage applied to the ON/OFF pin. It reduces the
power supply noise sensitivity, and also limits the capacitor
C1 discharge current, but its use is not mandatory.
When a high 50 Hz or 60 Hz (100 Hz or 120 Hz
respectively) ripple voltage exists, a long delay time can
cause some problems by coupling the ripple into the
ON/OFF pin, the regulator could be switched periodically
on and off with the line (or double) frequency.
LM2576−XX
C
100 mF
in
Q1
2N3906
5
ON/OFF 3 GN
D
R1
12 k
−V
out
NOTE: This picture does not show the complete circuit.
Figure 29. Inverting Buck−Boost Regulator Shutdown
Circuit Using a PNP Transistor
Negative Boost Regulator
This example is a variation of the buck−boost topology
and it is called negative boost regulator. This regulator
experiences relatively high switch current, especially at low
input voltages. The internal switch current limiting results in
lower output load current capability.
The circuit in Figure 30 shows the negative boost
configuration. The input voltage in this application ranges
from −5.0 V to −12 V and provides a regulated −12 V output.
If the input voltage is greater than −12 V, the output will rise
above −12 V accordingly, but will not damage the regulator.
+V
in
+V
in
LM2576−XX
1
C1
0.1 mF
5
ON/OFF 3 GN
D
C
100 mF
in
C
out
2200 mF
Low Esr
R1
47 k
4
R2
47 k
V
in
Feedback
Output
2
LM2576−12
1
C
100 mF
in
1N5820
3
5
GND
ON/OFF
NOTE: This picture does not show the complete circuit.
V
out
= −12 V
Figure 31. Delayed Startup Circuitry
Typical Load Current
400 mA for V = −5.2 V
750 mA for V = −7.0 V
100 mH
V
in
Undervoltage Lockout
in
in
Some applications require the regulator to remain off until
the input voltage reaches a certain threshold level. Figure 32
shows an undervoltage lockout circuit applied to a buck
regulator. A version of this circuit for buck−boost converter
is shown in Figure 33. Resistor R3 pulls the ON/OFF pin
high and keeps the regulator off until the input voltage
reaches a predetermined threshold level with respect to the
ground Pin 3, which is determined by the following
expression:
−5.0 V to −12 V
Figure 30. Negative Boost Regulator
Design Recommendations:
The same design rules as for the previous inverting
buck−boost converter can be applied. The output capacitor
C
must be chosen larger than would be required for a what
out
standard buck converter. Low input voltages or high output
currents require a large value output capacitor (in the range
of thousands of mF). The recommended range of inductor
values for the negative boost regulator is the same as for
inverting converter design.
R2
R1
) ǒ1.0 ) Ǔ VBE
Z1
( )
Q1
V
[ V
th
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20
LM2576
Under normal continuous inductor current operating
conditions, the worst case occurs when V is minimal.
in
+V
in
+V
in
LM2576−XX
1
C
100 mF
in
R2
10 k
R3
47 k
+V
in
+V
in
5
ON/OFF 3 GN
D
LM2576−XX
1
C
100 mF
in
R2
15 k
R3
47 k
Z1
1N5242B
5
ON/OFF 3 GN
D
Q1
2N3904
Z1
1N5242B
R1
10 k
V
≈ 13 V
th
V
≈ 13 V
th
Q1
2N3904
R1
15 k
NOTE: This picture does not show the complete circuit.
V
out
Figure 32. Undervoltage Lockout Circuit for
Buck Converter
NOTE: This picture does not show the complete circuit.
The following formula is used to obtain the peak inductor
current:
Figure 33. Undervoltage Lockout Circuit for
Buck−Boost Converter
I
(V ) |V |)
Load in
V
x t
on
O
in
2L
I
[
)
Adjustable Output, Low−Ripple Power Supply
peak
V
in
x
1
A 3.0 A output current capability power supply that
features an adjustable output voltage is shown in Figure 34.
This regulator delivers 3.0 A into 1.2 V to 35 V output.
The input voltage ranges from roughly 3.0 V to 40 V. In order
to achieve a 10 or more times reduction of output ripple, an
additional L−C filter is included in this circuit.
|V |
O
1.0
where t
+
, and f
+ 52 kHz.
osc
on
V
) |V |
f
osc
in
O
Feedback
40 V Max
Unregulated
DC Input
4
+V
in
L1
150 mH
L2
20 mH
LM2574−Adj
Output
Voltage
1
Output
2
ON/OFF
1.2 to 35 V @ 3.0 A
R2
50 k
C
100 mF
in
3
GN
D
5
C
out
2200 mF
D1
1N5822
C1
100 mF
R1
1.21 k
Optional Output
Ripple Filter
Figure 34. 1.2 to 35 V Adjustable 3.0 A Power Supply with Low Output Ripple
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21
LM2576
THE LM2576−5 STEP−DOWN VOLTAGE REGULATOR WITH 5.0 V @ 3.0 A OUTPUT POWER CAPABILITY.
TYPICAL APPLICATION WITH THROUGH−HOLE PC BOARD LAYOUT
Feedback
4
+V
in
Unregulated
DC Input
+V = 7.0 to 40 V
L1
150 mH
LM2576−5
1
Output
2
Regulated Output
= 5.0 V @ 3.0 A
in
V
out1
3
GN
D
5
ON/OFF
C1
100 mF
/50 V
ON/OFF
C
out
1000 mF
/16 V
D1
1N5822
GND
in
GND
out
C1
C2
D1
L1
−
−
−
−
100 mF, 50 V, Aluminium Electrolytic
1000 mF, 16 V, Aluminium Electrolytic
3.0 A, 40 V, Schottky Rectifier, 1N5822
150 mH, RL2444, Renco Electronics
Figure 35. Schematic Diagram of the LM2576−5 Step−Down Converter
LM2576
U1
D1
+
C2
C1
V
ou
t
+
ON/OFF
L1
+V
in
GND-
GND
out
in
NOTE: Not to scale.
NOTE: Not to scale.
Figure 36. Printed Circuit Board Layout
Component Side
Figure 37. Printed Circuit Board Layout
Copper Side
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22
LM2576
THE LM2576−ADJ STEP−DOWN VOLTAGE REGULATOR WITH 8.0 V @ 1.0 A OUTPUT POWER
CAPABILITY. TYPICAL APPLICATION WITH THROUGH−HOLE PC BOARD LAYOUT
4
Feedback
Unregulated
DC Input
+V
in
L1
150 mH
LM2576−ADJ
Regulated
Output Filtered
1
+V = 10 V to 40 V
in
Output
2
ON/OFF
V
out2
= 8.0 V @ 3.0 A
R2
10 k
3
GN
D
5
C1
100 mF
/50 V
C2
1000 mF
/16 V
D1
1N5822
R1
1.8 k
ON/OFF
R2
R1
) ǒ1.0 )
Ǔ
V
+ V
out
ref
C1
C2
D1
L1
R1
R2
−
−
−
−
−
−
100 mF, 50 V, Aluminium Electrolytic
1000 mF, 16 V, Aluminium Electrolytic
3.0 A, 40 V, Schottky Rectifier, 1N5822
150 mH, RL2444, Renco Electronics
1.8 kW, 0.25 W
V
ref
= 1.23 V
R1 is between 1.0 k and 5.0 k
10 kW, 0.25 W
Figure 38. Schematic Diagram of the 8.0 V @ 3.0 A Step−Down Converter Using the LM2576−ADJ
LM2576
U1
D1
R1
R2
ON/OFF
C1
+
+
C2
V
out
+V
in
L1
GND
GND
out
in
NOTE: Not to scale.
NOTE: Not to scale.
Figure 39. Printed Circuit Board Layout
Component Side
Figure 40. Printed Circuit Board Layout
Copper Side
References
• National Semiconductor LM2576 Data Sheet and Application Note
• National Semiconductor LM2595 Data Sheet and Application Note
• Marty Brown “Practical Switching Power Supply Design”, Academic Press, Inc., San Diego 1990
• Ray Ridley “High Frequency Magnetics Design”, Ridley Engineering, Inc. 1995
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23
LM2576
ORDERING INFORMATION
Nominal
Output Voltage
Operating
Temperature Range
†
Device
LM2576TV−ADJ
LM2576TV−ADJG
Package
Shipping
TO−220 (Vertical Mount)
TO−220 (Vertical Mount)
(Pb−Free)
TO−220 (Straight Lead)
LM2576T−ADJ
50 Units/Rail
2500 Tape & Reel
50 Units/Rail
TO−220 (Straight Lead)
(Pb−Free)
LM2576T−ADJG
1.23 V to 37 V
T = −40° to +125°C
J
2
D PAK (Surface Mount)
LM2576D2T−ADJ
LM2576D2T−ADJG
2
D PAK (Surface Mount)
(Pb−Free)
2
D PAK (Surface Mount)
LM2576D2T−ADJR4
LM2576D2T−ADJR4G
2
D PAK (Surface Mount)
(Pb−Free)
TO−220 (Vertical Mount)
LM2576TV−3.3
TO−220 (Vertical Mount)
(Pb−Free)
LM2576TV−3.3G
TO−220 (Straight Lead)
LM2576T−3.3
TO−220 (Straight Lead)
(Pb−Free)
LM2576T−3.3G
3.3 V
T = −40° to +125°C
J
2
D PAK (Surface Mount)
LM2576D2T−3.3
2
D PAK (Surface Mount)
LM2576D2T−3.3G
(Pb−Free)
2
D PAK (Surface Mount)
LM2576D2TR4−3.3
LM2576D2TR4−3.3G
2
2500 Tape & Reel
D PAK (Surface Mount)
(Pb−Free)
TO−220 (Vertical Mount)
LM2576TV−005
LM2576TV−5G
TO−220 (Vertical Mount)
(Pb−Free)
TO−220 (Straight Lead)
LM2576T−005
50 Units/Rail
TO−220 (Straight Lead)
(Pb−Free)
LM2576T−005G
5.0 V
T = −40° to +125°C
J
2
D PAK (Surface Mount)
LM2576D2T−005
2
D PAK (Surface Mount)
LM2576D2T−005G
(Pb−Free)
2
D PAK (Surface Mount)
LM2576D2TR4−005
LM2576D2TR4−5G
2
2500 Tape & Reel
D PAK (Surface Mount)
(Pb−Free)
TO−220 (Vertical Mount)
LM2576TV−012
TO−220 (Vertical Mount)
(Pb−Free)
LM2576TV−012G
TO−220 (Straight Lead)
LM2576T−012
50 Units/Rail
TO−220 (Straight Lead)
(Pb−Free)
LM2576T−012G
12 V
T = −40° to +125°C
J
2
D PAK (Surface Mount)
LM2576D2T−012
2
D PAK (Surface Mount)
LM2576D2T−012G
(Pb−Free)
2
D PAK (Surface Mount)
LM2576D2TR4−012
LM2576D2TR4−012G
2
2500 Tape & Reel
D PAK (Surface Mount)
(Pb−Free)
†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging
Specifications Brochure, BRD8011/D.
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24
LM2576
ORDERING INFORMATION
Nominal
Output Voltage
Operating
Temperature Range
†
Device
LM2576TV−015
LM2576TV−015G
Package
Shipping
TO−220 (Vertical Mount)
TO−220 (Vertical Mount)
(Pb−Free)
TO−220 (Straight Lead)
LM2576T−015
LM2576T−15G
15 V
T = −40° to +125°C
J
50 Units/Rail
TO−220 (Straight Lead)
(Pb−Free)
2
D PAK (Surface Mount)
LM2576D2T−015
LM2576D2T−15G
2
D PAK (Surface Mount)
(Pb−Free)
†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging
Specifications Brochure, BRD8011/D.
MARKING DIAGRAMS
2
TO−220
TV SUFFIX
CASE 314B
TO−220
T SUFFIX
CASE 314D
D PAK
D2T SUFFIX
CASE 936A
LM
LM
2576−xxx
AWLYWWG
2576D2T−xxx
AWLYWWG
LM
2576T−xxx
AWLYWWG
LM
2576T−xxx
AWLYWWG
1
5
1
5
1
5
1
5
xxx = 3.3, 5.0, 12, 15, or ADJ
A
= Assembly Location
WL = Wafer Lot
= Year
WW = Work Week
= Pb−Free Package
Y
G
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25
LM2576
PACKAGE DIMENSIONS
TO−220
TV SUFFIX
CASE 314B−05
ISSUE L
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.
2. CONTROLLING DIMENSION: INCH.
3. DIMENSION D DOES NOT INCLUDE
INTERCONNECT BAR (DAMBAR) PROTRUSION.
DIMENSION D INCLUDING PROTRUSION SHALL
NOT EXCEED 0.043 (1.092) MAXIMUM.
C
B
−P−
OPTIONAL
CHAMFER
Q
F
E
A
U
INCHES
DIM MIN MAX
0.613 14.529 15.570
MILLIMETERS
L
S
MIN MAX
V
W
A
B
C
D
E
F
0.572
0.390
0.170
0.025
0.048
0.850
0.067 BSC
0.166 BSC
0.015
0.900
0.320
0.320 BSC
0.140
−−−
0.468
−−−
0.090
0.415
0.180
0.038
0.055
9.906 10.541
K
4.318
0.635
1.219
4.572
0.965
1.397
0.935 21.590 23.749
1.702 BSC
4.216 BSC
G
H
J
0.025
0.381
1.100 22.860 27.940
0.635
5X J
K
L
G
0.365
8.128
8.128 BSC
3.556
9.271
3.886
M
0.24 (0.610)
T
H
N
Q
S
U
V
W
5X D
0.153
0.620
N
−−− 15.748
M
M
0.10 (0.254)
T
P
0.505 11.888 12.827
0.735
0.110
SEATING
PLANE
−−− 18.669
2.286 2.794
−T−
TO−220
T SUFFIX
CASE 314D−04
ISSUE F
NOTES:
SEATING
−T−
PLANE
1. DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.
B
C
2. CONTROLLING DIMENSION: INCH.
3. DIMENSION D DOES NOT INCLUDE
INTERCONNECT BAR (DAMBAR) PROTRUSION.
DIMENSION D INCLUDING PROTRUSION SHALL
NOT EXCEED 10.92 (0.043) MAXIMUM.
−Q−
DETAIL A−A
B1
E
A
U
K
INCHES
DIM MIN MAX
MILLIMETERS
MIN MAX
L
A
0.572
0.390
0.613 14.529 15.570
0.415 9.906 10.541
0.415 9.525 10.541
1 2 3 4 5
B
B1 0.375
C
D
E
G
H
J
0.170
0.025
0.048
0.180 4.318
0.038 0.635
0.055 1.219
1.702 BSC
0.112 2.210
0.025 0.381
4.572
0.965
1.397
0.067 BSC
0.087
0.015
0.977
0.320
0.140
0.105
2.845
0.635
J
H
G
K
L
1.045 24.810 26.543
D 5 PL
0.365 8.128
0.153 3.556
0.117 2.667
9.271
3.886
2.972
Q
U
M
M
0.356 (0.014)
T
Q
B
B1
DETAIL A−A
http://onsemi.com
26
LM2576
PACKAGE DIMENSIONS
D2PAK
D2T SUFFIX
CASE 936A−02
ISSUE C
NOTES:
−T−
TERMINAL 6
1. DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.
2. CONTROLLING DIMENSION: INCH.
3. TAB CONTOUR OPTIONAL WITHIN DIMENSIONS A
AND K.
4. DIMENSIONS U AND V ESTABLISH A MINIMUM
MOUNTING SURFACE FOR TERMINAL 6.
5. DIMENSIONS A AND B DO NOT INCLUDE MOLD
FLASH OR GATE PROTRUSIONS. MOLD FLASH
AND GATE PROTRUSIONS NOT TO EXCEED 0.025
(0.635) MAXIMUM.
OPTIONAL
CHAMFER
A
E
U
S
K
V
B
H
1
2
3
4 5
M
L
INCHES
MILLIMETERS
DIM
A
B
C
D
E
MIN
MAX
0.403
0.368
0.180
0.036
0.055
MIN
9.804
9.042
4.318
0.660
1.143
MAX
10.236
9.347
4.572
0.914
1.397
D
P
N
0.386
0.356
0.170
0.026
0.045
M
0.010 (0.254)
T
G
R
G
H
K
L
M
N
P
0.067 BSC
1.702 BSC
14.707
1.270 REF
0.539
0.579 13.691
0.050 REF
0.000
0.088
0.018
0.058
0.010
0.102
0.026
0.078
0.000
2.235
0.457
1.473
0.254
2.591
0.660
1.981
C
R
S
U
V
5_ REF
5_ REF
0.116 REF
0.200 MIN
0.250 MIN
2.946 REF
5.080 MIN
6.350 MIN
SOLDERING FOOTPRINT*
8.38
0.33
1.702
0.067
10.66
0.42
1.016
0.04
3.05
0.12
16.02
0.63
mm
inches
ǒ
Ǔ
SCALE 3:1
*For additional information on our Pb−Free strategy and soldering
details, please download the ON Semiconductor Soldering and
Mounting Techniques Reference Manual, SOLDERRM/D.
http://onsemi.com
27
LM2576
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice
to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All
operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights
nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications
intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should
Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates,
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LITERATURE FULFILLMENT:
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For additional information, please contact your
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LM2576/D
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