ON Semiconductor LM2576 Technical data

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LM2576
3.0 A, 15 V, Step−Down Switching Regulator
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.
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.
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.
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
Wide Input Voltage Range
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
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
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1
5
Heatsink surface connected to Pin 3
1
5
Pin 1. V
2. Output
3. Ground
4. Feedback
5. ON/OFF
1
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.
TO−220
TV SUFFIX
CASE 314B
TO−220
T SUFFIX
CASE 314D
in
D2PAK
D2T SUFFIX
CASE 936A
© Semiconductor Components Industries, LLC, 2006
January, 2006 − Rev. 8
1 Publication Order Number:
LM2576/D
Typical Application (Fixed Output Voltage Versions)
Unregulated
DC Input
LM2576
Feedback
L1
100 mH
D1 1N5822
Driver
Thermal
Shutdown
C
out
1000 mF
1.0 Amp Switch
5.0 V Regulated Output 3.0 A Load
ON/OFF
5
Voltage Versions
For adjustable version R1 = open, R2 = 0 W
Output
2 GND
3
Output
3.3 V
5.0 V
D1
12 V 15 V
L1
R2
(W)
1.7 k
3.1 k
8.84 k
11.3 k
Regulated
Output
V
out
C
out
Load
/OFF5
4
Output
2
7.0 V − 40 V
Unregulated
DC Input
C
100 mF
+V
in
LM2576
1
in
GN
3ON
D
Representative Block Diagram and Typical Application
+V
in
1
C
in
4
Feedback
R2
Fixed Gain Error Amplifier
R1
1.0 k
Freq Shift
18 kHz
1.235 V
Band−Gap
Reference
3.1 V Internal
Comparator
52 kHz
Oscillator
Regulator
ON/OFF
Current
Limit
Latch
Reset
This device contains 162 active transistors.
Figure 1. Block Diagram and Typical Application
MAXIMUM RATINGS
Rating Symbol Value Unit
Maximum Supply Voltage V
in
ON/OFF Pin Input Voltage −0.3 V V +V Output Voltage to Ground (Steady−State) −1.0 V Power Dissipation
Case 314B and 314D (TO−220, 5−Lead) P
Thermal Resistance, Junction−to−Ambient Thermal Resistance, Junction−to−Case
Case 936A (D2PAK) P
Thermal Resistance, Junction−to−Ambient
Thermal Resistance, Junction−to−Case Storage Temperature Range T Minimum ESD Rating (Human Body Model: C = 100 pF, R = 1.5 kW)
D
R
q
JA
R
q
JC D
R
q
JA
R
q
JC
stg
2.0 kV Lead Temperature (Soldering, 10 seconds) 260 °C Maximum Junction Temperature T
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.
45 V
in
Internally Limited W
65 °C/W
5.0 °C/W
Internally Limited W
70 °C/W
5.0 °C/W
−65 to +150 °C
150 °C
V
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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 Symbol Value Unit
Operating Junction Temperature Range T Supply Voltage V
J
in
SYSTEM PARAMETERS (Note 1 Test Circuit Figure 15)
ELECTRICAL CHARACTERISTICS
for the 12 V version, and Vin = 30 V for the 15 V version. I 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 (Vin = 12 V, I Output Voltage (6.0 V ≤ Vin 40 V, 0.5 A I
TJ = 25°C 3.168 3.3 3.432 TJ = −40 to +125°C 3.135 3.465
Efficiency (V
= 12 V, I
in
LM2576−5 (Note 1 Test Circuit Figure 15)
Output Voltage (Vin = 12 V, I Output Voltage (8.0 V ≤ Vin 40 V, 0.5 A I
TJ = 25°C 4.8 5.0 5.2 TJ = −40 to +125°C 4.75 5.25
Efficiency (Vin = 12 V, I
LM2576−12 (Note 1 Test Circuit Figure 15)
Output Voltage (Vin = 25 V, I Output Voltage (15 V ≤ Vin 40 V, 0.5 A I
TJ = 25°C 11.52 12 12.48 TJ = −40 to +125°C 11.4 12.6
Efficiency (Vin = 15 V, I
LM2576−15 (Note 1 Test Circuit Figure 15)
Output Voltage (V
= 30 V, I
in
Output Voltage (18 V ≤ Vin 40 V, 0.5 A I
TJ = 25°C 14.4 15 15.6 T
= −40 to +125°C 14.25 15.75
J
Efficiency (Vin = 18 V, I
LM2576 ADJUSTABLE VERSION (Note 1 Test Circuit Figure 15)
Feedback Voltage (V Feedback Voltage (8.0 V ≤ Vin 40 V, 0.5 A I
TJ = 25°C 1.193 1.23 1.267 T
= −40 to +125°C 1.18 1.28
J
Efficiency (V
= 12 V, I
in
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
Load
= 3.0 A) η 75 %
Load
Load
= 3.0 A) η 77 %
Load
Load
= 3.0 A) η 88 %
Load
Load
= 3.0 A) η 88 %
Load
= 12 V, I
in
= 3.0 A, V
Load
Load
(Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and Adjustable version, Vin = 25 V
= 500 mA. For typical values T
Load
= 0.5 A, T
= 0.5 A, TJ = 25°C) V
= 0.5 A, TJ = 25°C) V
= 0.5 A, TJ = 25°C) V
= 0.5 A, V
= 25°C) V
J
3.0 A) V
Load
3.0 A) V
Load
3.0 A) V
Load
3.0 A) V
Load
= 5.0 V, TJ = 25°C) V
out
3.0 A, V
Load
= 5.0 V) η 77 %
out
= 5.0 V) V
out
= −40°C T
low
out out
out out
out out
out out
out out
= +125°C
high
= 25°C, for min/max values TJ is the
J
3.234 3.3 3.366 V
4.9 5.0 5.1 V
11.76 12 12.24 V
14.7 15 15.3 V
1.217 1.23 1.243 V
−40 to +125 °C 40 V
V
V
V
V
V
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LM2576
DEVICE PARAMETERS
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, V
for the 12 V version, and Vin = 30 V for the 15 V version. I operating junction temperature range that applies [Note 2], unless otherwise noted.)
Characteristics Symbol Min Typ Max Unit
ALL OUTPUT VOLTAGE VERSIONS
Feedback Bias Current (V
= 5.0 V Adjustable Version Only) I
out
TJ = 25°C 25 100 TJ = −40 to +125°C 200
Oscillator Frequency Note 3 f
TJ = 25°C 52 TJ = 0 to +125°C 47 58 TJ = −40 to +125°C 42 63
Saturation Voltage (I
= 3.0 A Note 4) V
out
TJ = 25°C 1.5 1.8 TJ = −40 to +125°C 2.0
Max Duty Cycle (“on”) Note 5 DC 94 98 % Current Limit (Peak Current Notes 3 and 4) I
TJ = 25°C 4.2 5.8 6.9 TJ = −40 to +125°C 3.5 7.5
Output Leakage Current Notes 6 and 7, TJ = 25°C I
Output = 0 V 0.8 2.0 Output = −1.0 V 6.0 20
Quiescent Current Note 6 I
TJ = 25°C 5.0 9.0 TJ = −40 to +125°C 11
Standby Quiescent Current (ON/OFF Pin = 5.0 V (“off”)) I
TJ = 25°C 80 200 TJ = −40 to +125°C 400
ON/OFF Pin Logic Input Level (Test Circuit Figure 15) V
V
= 0 V V
out
TJ = 25°C 2.2 1.4 TJ = −40 to +125°C 2.4
V
= Nominal Output Voltage V
out
TJ = 25°C 1.2 1.0 TJ = −40 to +125°C 0.8
ON/OFF Pin Input Current (Test Circuit Figure 15)
ON/OFF Pin = 5.0 V (“off”), TJ = 25°C I ON/OFF Pin = 0 V (“on”), TJ = 25°C I
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
= 500 mA. For typical values T
Load
= 12 V for the 3.3 V, 5.0 V, and Adjustable version, Vin = 25 V
in
b
osc
sat
CL
L
Q
stby
IH
IL
= 25°C, for min/max values TJ is the
J
nA
kHz
mA
mA
mA
mA
IH IL
15 30
0 5.0
V
A
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LM2576
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
1.0
Vin = 20 V
0.8 I
Load
0.6 Normalized at TJ = 25°C
0.4
0.2
0
−0.2
−0.4
−0.6
, OUTPUT VOLTAGE CHANGE (%)
out
−0.8
V
−1.0
Figure 2. Normalized Output Voltage
2.0
1.5
1.0
0.5
INPUT − OUTPUT DIFFERENTIAL (V)
0
1.4
1.2
I
= 500 mA
= 500 mA
1.0
Load
TJ = 25°C
0.8
0.6
3.3 V, 5.0 V and ADJ
0.4
0.2
0
−0.2
, OUTPUT VOLTAGE CHANGE (%)
out
−0.4
V
1251007550250−25−50 403530252015105.00
TJ, JUNCTION TEMPERATURE (°C)
−0.6
Vin, INPUT VOLTAGE (V)
Figure 3. Line Regulation
6.5
I
= 3.0 A
Load
6.0
5.5
, OUTPUT CURRENT (A) I
5.0
O
4.5
4.0
TJ, JUNCTION TEMPERATURE (°C)
I
= 500 mA
Load
TJ, JUNCTION TEMPERATURE (°C)
L1 = 150 mH R
= 0.1 W
ind
1251007550250−25−50 1251007550250−25−50
Figure 4. Dropout Voltage Figure 5. Current Limit
12 V and 15 V
Vin = 25 V
, QUIESCENT CURRENT (mA)
Q
I
8.0
6.0
4.0
20
V
= 5.0 V
18
16
out
Measured at Ground Pin TJ = 25°C
14
I
= 3.0 A
12
Load
10
I
= 200 mA
Load
403530252015105.00 1251007550250−25−50
Vin, INPUT VOLTAGE (V)
μA)
, STANDBY QUIESCENT CURRENT (
I
stby
200
180
160
140
120
100
V
= 5.0 V
ON/OFF
Vin = 40 V
80
60
Vin = 12 V
40
20
0
TJ, JUNCTION TEMPERATURE (°C)
Figure 6. Quiescent Current Figure 7. Standby Quiescent Current
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LM2576
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
, STANDBY QUIESCENT CURRENT (μA)I
NORMALIZED FREQUENCY (%)
200
180
160
140
120
100
80
60
40
20
stby
0
TJ = 25°C
Vin, INPUT VOLTAGE (V)
40302520151050 0 0.5 1.0 1.5 2.0 3.0
35 2.5
, SATURATION VOLTAGE (V)
V
sat
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Figure 8. Standby Quiescent Current
8.0
6.0
4.0
2.0
−2.0
−4.0
−6.0
−8.0
−10
Vin = 12 V Normalized at 25°C
0
1251007550250−25−50 1251007550250−25−50
TJ, JUNCTION TEMPERATURE (°C)
, INPUT VOLTAGE (V) V
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
in
1.0
0.5
0
Figure 10. Oscillator Frequency Figure 11. Minimum Operating Voltage
−40°C
25°C
125°C
SWITCH CURRENT (A)
Figure 9. Switch Saturation Voltage
Adjustable Version Only
V
' 1.23 V
out
I
= 500 mA
Load
TJ, JUNCTION TEMPERATURE (°C)
, FEEDBACK PIN CURRENT (nA)
b
I
−100
100
−20
−40
−60
−80
80
60
40
20
0
TJ, JUNCTION TEMPERATURE (°C)
Adjustable Version Only
1251007550250−25−50
Figure 12. Feedback Pin Current
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LM2576
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
50 V
A
0
4.0 A
B
2.0 A
0
4.0 A
C
2.0 A
D
0
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
Output
Voltage
Change
− 100 mV
Load
Current
100 mV
0
3.0 A
2.0 A
1.0 A
0
100 ms/DIV5 ms/DIV
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Fixed Output Voltage Versions
7.0 V − 40 V Unregulated
DC Input
7.0 V − 40 V Unregulated
DC Input
LM2576
V
in
LM2576
Fixed Output
1
53ON/OFFGN
C
in
100 mF
V
in
1
C
in
100 mF
D
Cin− 100 mF, 75 V, Aluminium Electrolytic
− 1000 mF, 25 V, Aluminium Electrolytic
C
out
D1 − Schottky, MBR360 L1 − 100 mH, Pulse Eng. PE−92108 R1 − 2.0 k, 0.1% R2 − 6.12 k, 0.1%
Adjustable Output Voltage Versions
Feedback
LM2576
Adjustable
D
4
Output
2
53ON/OFFGN
Feedback
4
Output
2
L1
100 mH
D1 MBR360
L1
100 mH
D1 MBR360
C
out
1000 mF
C
out
1000 mF
R2
R1
V
Load
out
V
out
5,000 V
Load
V
out
R2 + R1ǒ
Where V 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.
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+ V
ref
ref
R2
ǒ
1.0 )
V
out
V
ref
= 1.23 V, R1
–1.0Ǔ
R1
Ǔ
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.
8
LM2576
PIN FUNCTION DESCRIPTION
Pin Symbol Description (Refer to Figure 1)
1 V
2 Output This is the emitter of the internal switch. The saturation voltage V
3 GND Circuit ground pin. See the information about the printed circuit board layout. 4 Feedback This pin senses regulated output voltage to complete the feedback loop. The signal is divided by the internal resistor
5 ON/OFF It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the total input supply
in
Buck Converter Basics
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:
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.
in
Power Switch
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 (Cin in Figure 1).
of this output switch is typically 1.5 V. It should 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.
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.
current to approximately 80 mA. The threshold voltage is typically 1.4 V. Applying a voltage above this value (up to +Vin) 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 regulator will be in the “on” condition.
sat
DESIGN PROCEDURE
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:
t
on
d +
, where T is the period of switching.
T
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
I
L(on)
+
ǒ
Vin–V
DV
L
out
L
Ǔ
t
on
C
out
R
Load
of the catch diode voltage and the inductor current.
V
on(SW)
Power Switch
Off
Diode VoltageInductor Current
VD(FWD)
Power
Switch
On
Power Switch
Off
Power Switch
On
Time
Figure 16. Basic Buck Converter
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
L(off)
+
ǒ
V
out
–V
L
Ǔ
t
D
off
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I
pk
I
min
Diode Diode
Power Switch
Power Switch
Figure 17. Buck Converter Idealized Waveforms
9
I
Load
(AV)
Time
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:
V
= Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V)
out
V
= Maximum Input Voltage
in(max)
I
Load(max)
= Maximum Load Current
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.
2. Input Capacitor Selection (Cin)
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 input pin +Vin and ground pin GND. This capacitor should be located close to the IC using short leads. This capacitor should have a low ESR (Equivalent Series Resistance) value.
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.
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.
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.
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:
I
p(max)
+ I
Load(max)
ǒ
Vin–V
)
2L
out
Ǔ
t
on
where ton is the “on” time of the power switch and
V
out
t
+
on
1.0
x
V
f
osc
in For additional information about the inductor, see the inductor section in the “Application Hints” section of
this data sheet.
Given Parameters:
V
= 5.0 V
out
V
= 15 V
in(max)
I
Load(max)
= 3.0 A
1. Controller IC Selection
According to the required input voltage, output voltage, current polarity and current value, use the LM2576−5 controller IC
2. Input Capacitor Selection (Cin)
A 100 mF, 25 V aluminium electrolytic capacitor located near to the input and ground pins provides sufficient bypassing.
3. Catch Diode Selection (D1)
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)
A.Use the inductor selection guide shown in Figures 19.
B.From the selection guide, the inductance area intersected
by the 15 V line and 3.0 A line is L100.
C.Inductor value required is 100 mH. From Table 2, choose
an inductor from any of the listed manufacturers.
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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
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. 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.
Procedure (Adjustable Output Version: LM2576−ADJ)
Procedure Example
Given Parameters:
V
= Regulated Output Voltage
out
V
= Maximum DC Input Voltage
in(max)
I
Load(max)
1. Programming Output Voltage
To select the right programming resistor R1 and R2 value (see Figure 2) use the following formula:
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).
= Maximum Load Current
+ V
ǒ
1.0 )
ref
R2 + R1
V
out
)
out
R2
Ǔ
where V
R1
V
out
ǒ
–1.0
V
ref
Ǔ
= 1.23 V
ref
5. Output Capacitor Selection (C
A.C
= 680 mF to 2000 mF standard aluminium electrolytic.
out
B.Capacitor voltage rating = 20 V.
Given Parameters:
V
= 8.0 V
out
V
= 25 V
in(max)
I
Load(max)
1. Programming Output Voltage (selecting R1 and R2) Select R1 and R2:
= 2.5 A
V
+ 1.23ǒ1.0 )
out
V
out
R2 + R1
R2 = 9.91 kW, choose a 9.88 k metal film resistor.
ǒ
V
ref
* 1.0Ǔ+ 1.8 k
)
out
R2
Ǔ
Select R1 = 1.8 kW
R1
8.0 V
ǒ
1.23 V
* 1.0
Ǔ
2. Input Capacitor Selection (Cin)
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 input pin +V 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)
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.
and ground pin GND This capacitor should be
in
2. Input Capacitor Selection (Cin)
A 100 mF, 150 V aluminium electrolytic capacitor located near
the input and ground pin provides sufficient bypassing.
3. Catch Diode Selection (D1) 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
4. Inductor Selection (L1)
A.Use the following formula to calculate the inductor Volt x
microsecond [V x ms] constant:
V
out
ExT+ǒVin–V
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.
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.
D.From the inductor code, identify the inductor value. Then
select an appropriate 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:
I
p(max)
where ton is the “on” time of the power switch and
For additional information about the inductor, see the inductor section in the “External Components” section of this data sheet.
5. Output Capacitor Selection (C
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.
For stable operation, the capacitor must satisfy the following requirement:
C
w 13,300
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.
out
+ I
Load(max)
t
on
+
Ǔ
V
in
V
out
V
in
V
V
out
x
F[Hz]
ǒ
Vin–V
)
1.0
x
f
osc
)
out
in(max)
xL[μH]
10
6
2L
[V x ms]
Ǔ
out
[μF]
t
on
Load
4. Inductor Selection (L1) A.Calculate E x T [V x ms] constant:
B.E x T = 80 [V x ms]
C.I
D.Proper inductor value = 150 mH
.
5. Output Capacitor Selection (C A.
8.0
ExT+(25–8.0)x
Load(max)
Inductance Region = H150
Choose the inductor from Table 2.
To achieve an acceptable ripple voltage, select C
= 2.5 A
C
w 13,300 x
out
= 680 mF electrolytic capacitor.
out
x
25
25
8 x 150
1000
+ 80 [V x ms]
52
)
out
+ 332.5 μF
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LM2576
LM2576 Series Buck Regulator Design Procedures (continued)
Indicator Value Selection Guide (For Continuous Mode Operation)
MAXIMUM INPUT VOLTAGE (V)
MAXIMUM INPUT VOLTAGE (V)
60
L680
8.0
7.0
6.0
5.0
40 20 15
10
L470
L330
0.4 0.6 1.0 2.0
L220
L150
L100
L68
L47
3.02.51.50.80.50.3 0.3
IL, MAXIMUM LOAD CURRENT (A)
MAXIMUM INPUT VOLTAGE (V)MAXIMUM INPUT VOLTAGE (V)
60
40
20 15
12
10
9.0
8.0
7.0
Figure 18. LM2576−3.3
60
40 35
H1500
30 25
20
18
16
15
14
L680
H1000
H680
H470
L470
L330
IL, MAXIMUM LOAD CURRENT (A)
L220
1.0 1.5 2.0 2.5
H330
L150
H220
H150
L100
L68
3.00.80.60.50.40.3
60
40 35
30
25
22
20
19
18
17
H1500
L680
L680
H470H1000 H680 H220H330 H150
L470
L330
L220
IL, MAXIMUM LOAD CURRENT (A)
Figure 19. LM2576−5
H1000
H680
H470
L470
L330
L220
IL, MAXIMUM LOAD CURRENT (A)
L150
1.2
H330
L150
L100
H220
L100
L68
L47
3.02.51.50.80.50.4 0.6 1.0 2.0
H150
L68
3.00.80.60.50.40.3 1.0 1.5 2.0 2.5
Figure 20. LM2576−12 Figure 21. LM2576−15
300 250
H2000
200
150
100
90 80 70 60 50 45 40
ET, VOLTAGE TIME (V s)μ
35 30
25 20
H1500
H1000
H680
H470
H330
H220
L680
L470
L330
L220
L150
L100
L68
0.4 0.6 0.8 1.0 2.5
IL, MAXIMUM LOAD CURRENT (A)
H150
L47
3.02.01.50.50.3
Figure 22. LM2576−ADJ
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Table 1. Diode Selection Guide
3.0 A 4.0 − 6.0 A 3.0 A 4.0 − 6.0 A
Through
V
R
20 V 1N5820
Hole
MBR320P
SR302
Surface
Mount
SK32 1N5823
LM2576
Schottky Fast Recovery
Through
Hole
SR502 SB520
Surface
Mount
Through
Hole
Surface
Mount
Through
Hole
Surface
Mount
30 V 1N5821
MBR330
SR303
31DQ03
40 V 1N5822
MBR340
SR304
31DQ04
50 V MBR350
31DQ05
SR305
60 V MBR360
DQ06
SR306
NOTE: Diodes listed in bold are available from ON Semiconductor.
SK33
30WQ03
SK34
30WQ04
MBRS340T3
MBRD340
SK35
30WQ05
MBRS360T3
MBRD360
1N5824
SR503 SB530
1N5825
SR504 SB540
SB550 50WQ05
50SQ080 MBRD660CT
50WQ03
MBRD640CT
50WQ04
Table 2. Inductor Selection by Manufacturer’s Part Number
Inductor
Code
L47 L68
L100
Inductor
Value
47 mH 68 mH
100 mH
Tech 39 Schott Corp. Pulse Eng. Renco
77 212 671 26980 PE−53112 RL2442 77 262 671 26990 PE−92114 RL2443 77 312 671 27000 PE−92108 RL2444
MUR320
31DF1
HER302
(all diodes
rated
to at least
100 V)
MURS320T3
MURD320
30WF10
(all diodes
rated
to at least
100 V)
MUR420
HER602
(all diodes
rated
to at least
100 V)
MURD620CT
50WF10
(all diodes
rated
to at least
100 V)
L150 L220 L330 L470
L680 H150 H220 H330 H470 H680
H1000 H1500 H2200
NOTE: *Contact Manufacturer
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
77 360 671 27010 PE−53113 RL1954 77 408 671 27020 PE−52626 RL1953 77 456 671 27030 PE−52627 RL1952
* 671 27040 PE−53114 RL1951 77 506 671 27050 PE−52629 RL1950 77 362 671 27060 PE−53115 RL2445 77 412 671 27070 PE−53116 RL2446 77 462 671 27080 PE−53117 RL2447
* 671 27090 PE−53118 RL1961 77 508 671 27100 PE−53119 RL1960 77 556 671 27110 PE−53120 RL1959
* 671 27120 PE−53121 RL1958
* 671 27130 PE−53122 RL2448
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LM2576
V
a
out
r.
Table 3. Example of Several Inductor Manufacturers Phone/Fax Numbers
Pulse Engineering, Inc.
Phone Fax
+ 1−619−674−8100 + 1−619−674−8262
Pulse Engineering, Inc. Europe
Renco Electronics, Inc.
Tech 39
Schott Corporation
EXTERNAL COMPONENTS
Input Capacitor (Cin)
The Input Capacitor Should Have a Low ESR
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:
I
> 1.2 x d x I
rms
Load
where d is the duty cycle, for a buck regulator
t
on
out
+
T
V
in
for a buck*boost regulato
nd d +
d +
|V
t
on
+
T
|V
|
out
| ) V
Phone Fax
Phone Fax
Phone Fax
Phone Fax
Output Capacitor (C
+ 353−9324−107 + 353−9324−459
+ 1−516−645−5828 + 1−516−586−5562
+ 33−1−4115−1681 + 33−1−4709−5051
+ 1−612−475−1173 + 1−612−475−1786
)
out
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.
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 t h e 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.
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LM2576
Catch Diode
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.
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 p rovide t he b est efficiency e specially i n low output voltage applications (5.0 V and lower). Another choice could be F ast−Recovery , or U ltra− Fa st Recovery d iodes. 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 f ulfill s ome q uality , l ow noise design r equirements. 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.
Inductor
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.
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.
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
ripple voltage. On the other hand it does require larger 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.
2.0 A
Inductor
Current
Waveform
0 A
2.0 A
Power
Switch
Current
Waveform
0 A
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 dif ferent styles o f 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.
VERTRICAL RESOLUTION 1.0 A/DIV
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LM2576
When multiple switching regulators are located on the 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
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
GENERAL RECOMMENDATIONS
Output Voltage Ripple and Transients
Source of the Output Ripple
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.
Voltage spikes caused by
Filtered
Output
Voltage
Unfiltered
Output
Voltage
HORIZONTAL TIME BASE: 5.0 ms/DIV
Figure 25. Output Ripple Voltage Waveforms
switching action of the output switch and the parasitic inductance of the output capacitor
20 mV/DIV
VERTRICAL
RESOLUTION
inductor and/or the LM2576. Different inductor types have different saturation characteristics, and this should be kept in mind when selecting an inductor.
0.4 A
Inductor
Current
Waveform
Waveform
Minimizing the Output Ripple
0 A
0.4 A
Power
Switch
Current
0 A
HORIZONTAL TIME BASE: 5.0 ms/DIV
Figure 24. Discontinuous Mode Switching Current
Waveforms
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 lar ger 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.
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 TO−220(T, TV) and a 5−pin surface mount D2PAK(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.
The Surface Mount Package D2PAK and its Heatsinking
The other type of package, the surface mount D2PAK, is 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 package is soldered to should be at least 0.4 in2 (or 260 mm2) and ideally should have 2 or more square inches (1300 mm2) of 0.0028 inch copper. Additional increases of copper area
VERTICAL RESOLUTION 200 mA/DIV
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LM2576
beyond approximately 6.0 in2 (4000 mm2) 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’).
Thermal Analysis and Design
The following procedure must be performed to determine
whether or not a heatsink will be required. First determine:
1. P
maximum regulator power dissipation in the
D(max)
application.
2. T
) maximum ambient temperature in the
A(max
application.
3. T
J(max)
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.
4. R
5. R
qJC qJA
package thermal resistance junction−case. package thermal resistance junction−ambient.
(Refer to Maximum Ratings on page 2 of this data sheet or R
qJC
and R
qJA
values).
The following formula is to calculate the approximate
total power dissipated by the LM2576:
PD = (Vin x IQ) + d x I
Load
x V
sat
where d is the duty cycle and for buck converter
V
t
I
(quiescent current) and V
Q
d +
on
O
+
T
,
V
in
can be found in the
sat
LM2576 data sheet, Vinis minimum input voltage applied, VOis the regulator output voltage, I
is the load current.
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:
where (R
TJ = (R
)(PD) represents the junction temperature rise
qJA
) (PD) + T
q
JA
A
caused by the dissipated power and TA is the maximum ambient temperature.
Packages on a Heatsink
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:
where R
TJ = PD (R
is the thermal resistance junction−case,
qJC
R
is the thermal resistance case−heatsink,
qCS
R
is the thermal resistance heatsink−ambient.
qSA
+ R
q
JA
+ R
q
CS
) + T
q
SA
A
If the actual operating temperature is greater than the selected safe operating junction temperature, then a larger heatsink is required.
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.
The size, quantity and spacing o f o ther c omponents o n t he board can a lso i nfluence i t s e f fectiveness t o d issipate t he heat.
12 to 40 V Unregulated DC Input
100 mF
Figure 26. Inverting Buck−Boost Develops −12 V
+V
in
LM2576−12
1
C
in
D
Feedback
4
Output
2
53ON/OFFGN
L1
68 mH
D1 1N5822
C
out
2200 mF
−12 V @ 0.7 A Regulated
Output
ADDITIONAL APPLICATIONS
Inverting Regulator
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.
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LM2576
n
t
5
This circuit configuration is able to deliver approximately
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.
The high input current needed for startup is now partially supplied by the input capacitor Cin.
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.
Design Recommendations:
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 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).
The recommended range of inductor values for the inverting converter design is between 68 mH and 220 mH. T o 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:
where t
I
peak
on
I
[
+
Vin) |VO|
Load(Vin
|VO|
) |VO|)
V
in
x
1.0
f
osc
)
, and f
Vinxt
2L
osc
on
1
+ 52 kHz.
Under normal continuous inductor current operating
conditions, the worst case occurs when Vin is minimal.
12 V to 25 V Unregulated DC Input
C
in
100 mF
/50 V
0.1 mF
C1
+V
in
1
R1
47 k
LM2576−12
R2 47 k
Figure 27. Inverting Buck−Boost Regulator
with Delayed startup
+V
in
C
in
Shutdown
On
Off
Input
R3
470
.0 V
0
NOTE: This picture does not show the complete circuit.
100 mF
Figure 28. Inverting Buck−Boost Regulator Shutdow
Circuit Using an Optocoupler
35ON/OFF GN
D
R1
47 k
MOC8101
Feedback
4
Output
2
+V
in
1
L1
68 mH
D1 1N5822
LM2576−XX
ON/OFF
R2 47 k
C
out
2200 mF /16 V
−12 V @ 700 m A Regulated
Output
35GN
D
−V
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.
ou
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19
LM2576
n
R2
5.6 k
Shutdown Input
+V
Q1 2N3906
in
1
LM2576−XX
35GN
ON/OFF
R1 12 k
D
−V
out
+V
+V
in
NOTE: This picture does not show the complete circuit.
Off
0
On
C
in
100 mF
Figure 29. Inverting Buck−Boost Regulator Shutdow
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.
Another important point is that these negative boost 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.
Delayed Startup
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.
+V
in
+V
in
1
LM2576−XX
C
out
2200 mF Low Esr
V
out
= −12 V
C
in
100 mF
V
in
−5.0 V to −12 V
V
in
1
LM2576−12
53
100 mH
ON/OFFGND
4
Feedback
Output
2
1N5820
Typical Load Current 400 mA for Vin = −5.2 V 750 mA for Vin = −7.0 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.
C1
0.1 mF
C
in
100 mF
NOTE: This picture does not show the complete circuit.
R1
47 k
35GN
ON/OFF
R2 47 k
D
Figure 31. Delayed Startup Circuitry
Undervoltage Lockout
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:
R2
Vth[ V
Z1
)
ǒ
1.0 ) R1
Ǔ
(Q1)
V
BE
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20
+V
in
R2
10 k
Z1
1N5242B
R1
10 k
NOTE: This picture does not show the complete circuit.
R3
47 k
Q1 2N3904
+V
in
1
C
in
100 mF
LM2576−XX
35GN
ON/OFF
Vth 13 V
D
Figure 32. Undervoltage Lockout Circuit for
Buck Converter
The following formula is used to obtain the peak inductor
current:
where t
I
peak
on
I
[
+
Vin) |VO|
Load(Vin
|VO|
) |VO|)
V
in
x
1.0
f
osc
)
, and f
Vinxt
2L
osc
on
1
+ 52 kHz.
LM2576
Under normal continuous inductor current operating
conditions, the worst case occurs when Vin is minimal.
+V
in
R2
15 k
Z1
1N5242B
R1
15 k
NOTE: This picture does not show the complete circuit.
R3
47 k
Q1 2N3904
Figure 33. Undervoltage Lockout Circuit for
Buck−Boost Converter
Adjustable Output, Low−Ripple Power Supply
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
in
1
C
in
100 mF
LM2576−XX
ON/OFF
35GN
D
Vth 13 V
V
out
40 V Max Unregulated DC Input
C
100 mF
Feedback
+V
in
LM2574−Adj
1
in
D
53ON/OFFGN
4
Output
2
150 mH
D1 1N5822
L1
R2 50 k
C
out
2200 mF
R1
1.21 k
Figure 34. 1.2 to 35 V Adjustable 3.0 A Power Supply with Low Output Ripple
L2
20 mH
C1
100 mF
Optional Output
Ripple Filter
Output
Voltage
1.2 to 35 V @ 3.0 A
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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
Unregulated
= 7.0 to 40 V
+V
in
+V
in
DC Input
1
LM2576−5
D
C1
100 mF
ON/OFF
/50 V
GND
in
C1 − 100 mF, 50 V, Aluminium Electrolytic C2 − 1000 mF, 16 V, Aluminium Electrolytic D1 − 3.0 A, 40 V, Schottky Rectifier, 1N5822 L1 − 150 mH, RL2444, Renco Electronics
Figure 35. Schematic Diagram of the LM2576−5 Step−Down Converter
4
Output
2
/OFFGN
53ON
150 mH
D1 1N5822
L1
Regulated Output V
= 5.0 V @ 3.0 A
out1
C
out
1000 mF /16 V
GND
out
LM2576
U1
D1
+
C2
C1
+
+V
in
L1
GND-
in
NOTE: Not to scale. NOTE: Not to scale.
Figure 36. Printed Circuit Board Layout
Component Side
V
ou t
ON/OFF
GND
out
00060_00
Figure 37. Printed Circuit Board Layout
Copper Side
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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
+Vin = 10 V to 40 V
100 mF
/50 V
C1 − 100 mF, 50 V, Aluminium Electrolytic C2 − 1000 mF, 16 V, Aluminium Electrolytic D1 − 3.0 A, 40 V, Schottky Rectifier, 1N5822 L1 − 150 mH, RL2444, Renco Electronics R1 − 1.8 kW, 0.25 W R2 − 10 kW, 0.25 W
C1
+V
in
1
LM2576−ADJ
53ON/OFFGN
D
ON/OFF
Output
2
150 mH
D1 1N5822
L1
R2 10 k
C2 1000 mF /16 V
R1
1.8 k
V
+ V
out
V
= 1.23 V
ref
R1 is between 1.0 k and 5.0 k
)ǒ1.0 )
ref
Regulated Output Filtered
V
= 8.0 V @ 3.0 A
out2
R2 R1
Figure 38. Schematic Diagram of the 8.0 V @ 3.0 A Step−Down Converter Using the LM2576−ADJ
LM2576
00059_00
U1
Ǔ
D1 R1
R2
C1
+
C2
+V
in
L1
GND
in
NOTE: Not to scale. NOTE: Not to scale.
Figure 39. Printed Circuit Board Layout
Component Side
ON/OFF
+
V
out
GND
out
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
Device
Output Voltage
LM2576TV−ADJ LM2576TV−ADJG
LM2576T−ADJ LM2576T−ADJG
LM2576D2T−ADJ
1.23 V to 37 V T
LM2576D2T−ADJG
LM2576D2T−ADJR4 LM2576D2T−ADJR4G
LM2576TV−3.3 LM2576TV−3.3G
LM2576T−3.3 LM2576T−3.3G
LM2576D2T−3.3
3.3 V T
LM2576D2T−3.3G
LM2576D2TR4−3.3 LM2576D2TR4−3.3G
LM2576TV−005 LM2576TV−5G
LM2576T−005 LM2576T−005G
LM2576D2T−005
5.0 V T
LM2576D2T−005G
LM2576D2TR4−005 LM2576D2TR4−5G
LM2576TV−012 LM2576TV−012G
LM2576T−012 LM2576T−012G
LM2576D2T−012
12 V T
LM2576D2T−012G
LM2576D2TR4−012 LM2576D2TR4−012G
†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.
Operating
Temperature Range
= −40° to +125°C
J
= −40° to +125°C
J
= −40° to +125°C
J
= −40° to +125°C
J
Package Shipping
TO−220 (Vertical Mount) TO−220 (Vertical Mount)
(Pb−Free) TO−220 (Straight Lead) TO−220 (Straight Lead)
(Pb−Free) D2PAK (Surface Mount) D2PAK (Surface Mount)
(Pb−Free) D2PAK (Surface Mount) D2PAK (Surface Mount)
(Pb−Free)
TO−220 (Vertical Mount) TO−220 (Vertical Mount)
(Pb−Free) TO−220 (Straight Lead) TO−220 (Straight Lead)
(Pb−Free) D2PAK (Surface Mount) D2PAK (Surface Mount)
(Pb−Free) D2PAK (Surface Mount) D2PAK (Surface Mount)
(Pb−Free)
TO−220 (Vertical Mount) TO−220 (Vertical Mount)
(Pb−Free) TO−220 (Straight Lead) TO−220 (Straight Lead)
(Pb−Free) D2PAK (Surface Mount) D2PAK (Surface Mount)
(Pb−Free) D2PAK (Surface Mount) D2PAK (Surface Mount)
(Pb−Free)
TO−220 (Vertical Mount) TO−220 (Vertical Mount)
(Pb−Free) TO−220 (Straight Lead) TO−220 (Straight Lead)
(Pb−Free) D2PAK (Surface Mount) D2PAK (Surface Mount)
(Pb−Free) D2PAK (Surface Mount) D2PAK (Surface Mount)
(Pb−Free)
50 Units/Rail
2500 Tape & Reel
50 Units/Rail
2500 Tape & Reel
50 Units/Rail
2500 Tape & Reel
50 Units/Rail
2500 Tape & Reel
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LM2576
ORDERING INFORMATION
Nominal
Device Shipping
LM2576TV−015 LM2576TV−015G
LM2576T−015 LM2576T−15G
LM2576D2T−015 LM2576D2T−15G
†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.
Output Voltage
15 V T
Operating
Temperature Range
= −40° to +125°C
J
MARKING DIAGRAMS
Package
TO−220 (Vertical Mount) TO−220 (Vertical Mount)
(Pb−Free) TO−220 (Straight Lead) TO−220 (Straight Lead)
(Pb−Free) D2PAK (Surface Mount) D2PAK (Surface Mount)
(Pb−Free)
50 Units/Rail
TO−220
TV SUFFIX
CASE 314B
LM
2576T−xxx
AWLYWWG
1
TO−220
T SUFFIX
CASE 314D
LM
2576T−xxx
AWLYWWG
5
15
2576−xxx
AWLYWWG
15
D2PAK
D2T SUFFIX
CASE 936A
LM
2576D2T−xxx
LM
AWLYWWG
15
xxx = 3.3, 5.0, 12, 15, or ADJ A = Assembly Location WL = Wafer Lot Y = Year WW = Work Week G = Pb−Free Package
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25
LM2576
PACKAGE DIMENSIONS
TO−220
TV SUFFIX
CASE 314B−05
ISSUE L
Q
U
F
K
5X D
0.10 (0.254) PMT
B
−P−
OPTIONAL CHAMFER
C
E
A
L
S
5X J
G
M
0.24 (0.610) T
M
H
N
−T−
W
SEATING PLANE
V
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.
DIM MIN MAX MIN MAX
A 0.572 0.613 14.529 15.570 B 0.390 0.415 9.906 10.541 C 0.170 0.180 4.318 4.572 D 0.025 0.038 0.635 0.965 E 0.048 0.055 1.219 1.397 F 0.850 0.935 21.590 23.749 G 0.067 BSC 1.702 BSC H 0.166 BSC 4.216 BSC
J 0.015 0.025 0.381 0.635 K 0.900 1.100 22.860 27.940 L 0.320 0.365 8.128 9.271 N 0.320 BSC 8.128 BSC Q 0.140 0.153 3.556 3.886 S −−− 0.620 −−− 15.748 U 0.468 0.505 11.888 12.827 V −−− 0.735 −−− 18.669 W 0.090 0.110 2.286 2.794
MILLIMETERSINCHES
TO−220
T SUFFIX
CASE 314D−04
ISSUE F
−Q−
U
K
D
5 PL
B
B1
12345
0.356 (0.014) T
M
G
DETAIL A−A
A
M
Q
−T−
C
E
L
J H
B
B1
DETAIL A−A
SEATING PLANE
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 10.92 (0.043) MAXIMUM.
DIM MIN MAX MIN MAX
A 0.572 0.613 14.529 15.570 B 0.390 0.415 9.906 10.541
B1 0.375 0.415 9.525 10.541
C 0.170 0.180 4.318 4.572 D 0.025 0.038 0.635 0.965 E 0.048 0.055 1.219 1.397 G 0.067 BSC 1.702 BSC H 0.087 0.112 2.210 2.845 J 0.015 0.025 0.381 0.635 K 0.977 1.045 24.810 26.543 L 0.320 0.365 8.128 9.271 Q 0.140 0.153 3.556 3.886 U 0.105 0.117 2.667 2.972
MILLIMETERSINCHES
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26
LM2576
PACKAGE DIMENSIONS
D2PAK
D2T SUFFIX
CASE 936A−02
ISSUE C
K
B
D
0.010 (0.254) T
M
C
A
123
45
G
S
H
OPTIONAL CHAMFER
−T−
TERMINAL 6
E
V
M
L
N
P
R
SOLDERING FOOTPRINT*
8.38
0.33
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI
U
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.
DIMAMIN MAX MIN MAX
INCHES
0.386 0.403 9.804 10.236
B 0.356 0.368 9.042 9.347 C 0.170 0.180 4.318 4.572 D 0.026 0.036 0.660 0.914
E 0.045 0.055 1.143 1.397 G 0.067 BSC 1.702 BSC H 0.539 0.579 13.691 14.707 K 0.050 REF 1.270 REF
L 0.000 0.010 0.000 0.254 M 0.088 0.102 2.235 2.591 N 0.018 0.026 0.457 0.660
P 0.058 0.078 1.473 1.981 R 5 REF
S 0.116 REF 2.946 REF U 0.200 MIN 5.080 MIN
V 0.250 MIN 6.350 MIN
__
MILLIMETERS
5 REF
1.702
0.067
10.66
0.42
1.016
3.05
0.04
0.12
16.02
0.63
mm
ǒ
SCALE 3:1
inches
Ǔ
*For additional information on our Pb−Free strategy and soldering
details, please download the ON Semiconductor Soldering and Mounting Techniques Reference Manual, SOLDERRM/D.
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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, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
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For additional information, please contact your local Sales Representative.
LM2576/D
28
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