© Semiconductor Components Industries, LLC, 2005
September, 2005 − Rev. 6 1Publication Order Number:
LM2576/D
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.
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
See detailed ordering and shipping information in the package
dimensions section on page 24 of this data sheet.
ORDERING INFORMATION
1
5
TO−220
TV SUFFIX
CASE 314B
1
5
Heatsink surface connected to Pin 3
TO−220
T SUFFIX
CASE 314D
Pin 1. Vin
2. Output
3. Ground
4. Feedback
5. ON/OFF
D2PAK
D2T SUFFIX
CASE 936A
Heatsink surface (shown as terminal 6 in
case outline drawing) is connected to Pin 3
1
5
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See general marking information in the device marking
section on page 25 of this data sheet.
DEVICE MARKING INFORMATION
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Figure 1. Block Diagram and Typical Application
7.0 V − 40 V
Unregulated
DC Input
L1
100 mH
GN
D
+Vin
1
Cin
100 mF3ON
/OFF5
Output
2
Feedback
4
D1
1N5822 Cout
1000 mF
Typical Application (Fixed Output Voltage Versions)
Representative Block Diagram and Typical Application
Unregulated
DC Input
+Vin
1
Cout
Feedback
4
Cin
L1
D1
R2
R1
1.0 k Output
2
GND
3
ON/OFF
5
Reset
Latch
Thermal
Shutdown
52 kHz
Oscillator
1.235 V
Band−Gap
Reference
Freq
Shift
18 kHz
Comparator
Fixed Gain
Error Amplifier
Current
Limit
Driver
1.0 Amp
Switch
ON/OFF
3.1 V Internal
Regulator
Regulated
Output
Vout
Load
Output
Voltage Versions
3.3 V
5.0 V
12 V
15 V
R2
(W)
1.7 k
3.1 k
8.84 k
11.3 k
For adjustable version
R1 = open, R2 = 0 W
LM2576
5.0 V Regulated
Output 3.0 A Load
This device contains 162 active transistors.
MAXIMUM RATINGS
Rating Symbol Value Unit
Maximum Supply Voltage Vin 45 V
ON/OFF Pin Input Voltage −0.3 V V +Vin V
Output Voltage to Ground (Steady−State) −1.0 V
Power Dissipation
Case 314B and 314D (TO−220, 5−Lead) PDInternally Limited W
Thermal Resistance, Junction−to−Ambient RqJA 65 °C/W
Thermal Resistance, Junction−to−Case RqJC 5.0 °C/W
Case 936A (D2PAK) PDInternally Limited W
Thermal Resistance, Junction−to−Ambient RqJA 70 °C/W
Thermal Resistance, Junction−to−Case RqJC 5.0 °C/W
Storage Temperature Range Tstg −65 to +150 °C
Minimum ESD Rating (Human Body Model: C = 100 pF, R = 1.5 kW) 2.0 kV
Lead Temperature (Soldering, 10 seconds) 260 °C
Maximum Junction Temperature TJ150 °C
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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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 TJ−40 to +125 °C
Supply Voltage Vin 40 V
SYSTEM PARAMETERS (Note 1 Test Circuit Figure 15)
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and Adjustable version, Vin = 25 V
for the 12 V version, and Vin = 30 V for the 15 V version. ILoad = 500 mA. For typical values TJ = 25°C, for min/max values TJ is the
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, ILoad = 0.5 A, TJ = 25°C) Vout 3.234 3.3 3.366 V
Output Voltage (6.0 V Vin 40 V, 0.5 A ILoad 3.0 A) Vout V
TJ = 25°C 3.168 3.3 3.432
TJ = −40 to +125°C 3.135 3.465
Efficiency (Vin = 12 V, ILoad = 3.0 A) η 75 %
LM2576−5 (Note 1 Test Circuit Figure 15)
Output Voltage (Vin = 12 V, ILoad = 0.5 A, TJ = 25°C) Vout 4.9 5.0 5.1 V
Output Voltage (8.0 V Vin 40 V, 0.5 A ILoad 3.0 A) Vout V
TJ = 25°C 4.8 5.0 5.2
TJ = −40 to +125°C 4.75 5.25
Efficiency (Vin = 12 V, ILoad = 3.0 A) η 77 %
LM2576−12 (Note 1 Test Circuit Figure 15)
Output Voltage (Vin = 25 V, ILoad = 0.5 A, TJ = 25°C) Vout 11.76 12 12.24 V
Output Voltage (15 V Vin 40 V, 0.5 A ILoad 3.0 A) Vout V
TJ = 25°C 11.52 12 12.48
TJ = −40 to +125°C 11.4 12.6
Efficiency (Vin = 15 V, ILoad = 3.0 A) η 88 %
LM2576−15 (Note 1 Test Circuit Figure 15)
Output Voltage (Vin = 30 V, ILoad = 0.5 A, TJ = 25°C) Vout 14.7 15 15.3 V
Output Voltage (18 V Vin 40 V, 0.5 A ILoad 3.0 A) Vout V
TJ = 25°C 14.4 15 15.6
TJ = −40 to +125°C 14.25 15.75
Efficiency (Vin = 18 V, ILoad = 3.0 A) η 88 %
LM2576 ADJUSTABLE VERSION (Note 1 Test Circuit Figure 15)
Feedback Voltage (Vin = 12 V, ILoad = 0.5 A, Vout = 5.0 V, TJ = 25°C) Vout 1.217 1.23 1.243 V
Feedback Voltage (8.0 V Vin 40 V, 0.5 A ILoad 3.0 A, Vout = 5.0 V) Vout V
TJ = 25°C 1.193 1.23 1.267
TJ = −40 to +125°C 1.18 1.28
Efficiency (Vin = 12 V, ILoad = 3.0 A, Vout = 5.0 V) η 77 %
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: Tlow = −40°C Thigh = +125°C
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DEVICE PARAMETERS
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and Adjustable version, Vin = 25 V
for the 12 V version, and Vin = 30 V for the 15 V version. ILoad = 500 mA. For typical values TJ = 25°C, for min/max values TJ is the
operating junction temperature range that applies [Note 2], unless otherwise noted.)
Characteristics Symbol Min Typ Max Unit
ALL OUTPUT VOLTAGE VERSIONS
Feedback Bias Current (Vout = 5.0 V Adjustable Version Only) IbnA
TJ = 25°C 25 100
TJ = −40 to +125°C 200
Oscillator Frequency Note 3 fosc kHz
TJ = 25°C 52
TJ = 0 to +125°C 47 58
TJ = −40 to +125°C 42 63
Saturation Voltage (Iout = 3.0 A Note 4) Vsat V
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) ICL A
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 ILmA
Output = 0 V 0.8 2.0
Output = −1.0 V 6.0 20
Quiescent Current Note 6 IQmA
TJ = 25°C 5.0 9.0
TJ = −40 to +125°C 11
Standby Quiescent Current (ON/OFF Pin = 5.0 V (“off”)) Istby mA
TJ = 25°C 80 200
TJ = −40 to +125°C 400
ON/OFF Pin Logic Input Level (Test Circuit Figure 15) V
Vout = 0 V VIH
TJ = 25°C 2.2 1.4
TJ = −40 to +125°C 2.4
Vout = Nominal Output Voltage VIL
TJ = 25°C 1.2 1.0
TJ = −40 to +125°C 0.8
ON/OFF Pin Input Current (Test Circuit Figure 15) mA
ON/OFF Pin = 5.0 V (“off”), TJ = 25°C IIH 15 30
ON/OFF Pin = 0 V (“on”), TJ = 25°C IIL 0 5.0
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. Vin = 40 V.
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IQ, QUIESCENT CURRENT (mA)
40
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
V
out, OUTPUT VOLTAGE CHANGE (%)
V
out, OUTPUT VOLTAGE CHANGE (%)
, STANDBY QUIESCENT CURRENT (
TJ, JUNCTION TEMPERATURE (°C)
IO, OUTPUT CURRENT (A)
TJ, JUNCTION TEMPERATURE (°C)
Vin, INPUT VOLTAGE (V)
Vin, INPUT VOLTAGE (V)
INPUT − OUTPUT DIFFERENTIAL (V)
TJ, JUNCTION TEMPERATURE (°C)
Figure 2. Normalized Output Voltage
TJ, JUNCTION TEMPERATURE (°C)
Figure 3. Line Regulation
Figure 4. Dropout Voltage Figure 5. Current Limit
Figure 6. Quiescent Current Figure 7. Standby Quiescent Current
ILoad = 200 mA
ILoad = 3.0 A
Vin = 12 V
Vin = 40 V
L1 = 150 mH
Rind = 0.1 W
ILoad = 500 mA
ILoad = 3.0 A
Vout = 5.0 V
Measured at
Ground Pin
TJ = 25°C
VON/OFF = 5.0 V
μA)
1.0
0.6
0.2
0
−0.2
−0.4
−1.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
−0.2
−0.4
−0.6
2.0
1.5
1.0
0.5
0
6.5
6.0
5.5
5.0
4.5
4.0
20
18
16
14
12
10
8.0
6.0
4.0
200
180
160
140
120
100
80
60
20
0
1251007550250−25−50 403530252015105.00
1251007550250−25−50 1251007550250−25−50
403530252015105.00 1251007550250−25−50
−0.8
−0.6
0.4
0.8 Vin = 20 V
ILoad = 500 mA
Normalized at TJ = 25°C
ILoad = 500 mA
TJ = 25°C
3.3 V, 5.0 V and ADJ
12 V and 15 V
Vin = 25 V
Istby
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Vsat , SATURATION VOLTAGE (V)
2.0
2.5
3.0
4.0
Ib, FEEDBACK PIN CURRENT (nA)
, STANDBY QUIESCENT CURRENT (μA)I stby
, INPUT VOLTAGE (V)
TJ, JUNCTION TEMPERATURE (°C)
SWITCH CURRENT (A)
NORMALIZED FREQUENCY (%)
TJ, JUNCTION TEMPERATURE (°C)
Figure 8. Standby Quiescent Current
Vin, INPUT VOLTAGE (V)
Figure 9. Switch Saturation Voltage
Figure 10. Oscillator Frequency Figure 11. Minimum Operating Voltage
Figure 12. Feedback Pin Current
Vin = 12 V
Normalized at
25°C
TJ = 25°C
Adjustable Version Only
200
180
140
120
100
80
60
40
20
0
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
8.0
6.0
4.0
2.0
0
−2.0
−4.0
−6.0
−8.0
−10
5.0
4.5
3.5
1.5
1.0
0.5
0
40302520151050 0 0.5 1.0 1.5 2.0 3.0
1251007550250−25−50 1251007550250−25−50
TJ, JUNCTION TEMPERATURE (°C)
Adjustable Version Only
100
80
60
40
20
0
−20
−40
−60
−80
−100 1251007550250−25−50
160
35 2.5
−40°C
25°C
125°C
Vout ' 1.23 V
ILoad = 500 mA
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
Vin
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2.0 A
0
0
A
B
C
100 ms/DIV5 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
50 V
0
4.0 A
2.0 A
100 mV
Output
Voltage
Change
0
3.0 A
2.0 A
1.0 A
0
4.0 A
− 100 mV
Load
Current
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)
D
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Figure 15. Typical Test Circuit
D1
MBR360
L1
100 mH
Output
2
4
Feedback
Cout
1000 mF
Cin
100 mF
LM2576
Fixed Output
1
53ON/OFFGN
D
Vin
Load
Vout
D1
MBR360
L1
100 mH
Output
2
4
Feedback
Cout
1000 mF
Cin
100 mF
LM2576
Adjustable
1
53ON/OFFGN
D
Vin
Load
Vout
5,000 V
Fixed Output Voltage Versions
Adjustable Output Voltage Versions
Vout +Vrefǒ1.0 )R2
R1Ǔ
R2 +R1ǒVout
Vref
–1.0Ǔ
Where Vref = 1.23 V, R1
between 1.0 k and 5.0 k
R2
R1
Cin 100 mF, 75 V, Aluminium Electrolytic
Cout 1000 mF, 25 V, Aluminium Electrolytic
D1 Schottky, MBR360
L1 100 mH, Pulse Eng. PE−92108
R1 2.0 k, 0.1%
R2 6.12 k, 0.1%
7.0 V − 40 V
Unregulated
DC Input
7.0 V − 40 V
Unregulated
DC Input
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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PIN FUNCTION DESCRIPTION
Pin Symbol Description (Refer to Figure 1)
1 Vin 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).
2 Output This is the emitter of the internal switch. The saturation voltage Vsat 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.
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
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
+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.
DESIGN PROCEDURE
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:
IL(on) +ǒVin –V
outǓton
L
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.
Figure 16. Basic Buck Converter
DVin RLoad
L
Cout
Power
Switch
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:
IL(off) +ǒVout –V
DǓtoff
L
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:
d+ton
T, where T is the period of switching.
For the buck converter with ideal components, the duty
cycle can also be described as:
d+Vout
Vin
Figure 17 shows the buck converter, idealized waveforms
of the catch diode voltage and the inductor current.
Power
Switch
Figure 17. Buck Converter Idealized Waveforms
Power
Switch
Off
Power
Switch
Off
Power
Switch
On
Power
Switch
On
Von(SW)
VD(FWD)
Time
Time
ILoad(AV)
Imin
Ipk
Diode Diode
Power
Switch
Diode VoltageInductor Current
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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:
Vout = Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V)
Vin(max) = Maximum Input Voltage
ILoad(max) = Maximum Load Current
Given Parameters:
Vout = 5.0 V
Vin(max) = 15 V
ILoad(max) = 3.0 A
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.
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)
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.
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.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.
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.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:
where ton is the “on” time of the power switch and
For additional information about the inductor, see the
inductor section in the “Application Hints” section of
this data sheet.
Ip(max) +ILoad(max))ǒVin–VoutǓton
2L
ton +Vout
Vin x1.0
fosc
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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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 (Cout)
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.
5. Output Capacitor Selection (Cout)
A.Cout = 680 mF to 2000 mF standard aluminium electrolytic.
B.Capacitor voltage rating = 20 V.
Procedure (Adjustable Output Version: LM2576−ADJ)
Procedure Example
Given Parameters:
Vout = Regulated Output Voltage
Vin(max) = Maximum DC Input Voltage
ILoad(max) = Maximum Load Current
Given Parameters:
Vout = 8.0 V
Vin(max) = 25 V
ILoad(max) = 2.5 A
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).
Vout +Vref ǒ1.0 )R2
R1Ǔ
R2 +R1ǒVout
Vref –1.0
Ǔ
where Vref = 1.23 V
1. Programming Output Voltage (selecting R1 and R2)
Select R1 and R2:
R2 = 9.91 kW, choose a 9.88 k metal film resistor.
R2 +R1ǒVout
Vref *1.0Ǔ+1.8 kǒ8.0 V
1.23 V *1.0Ǔ
Vout +1.23ǒ1.0 )R2
R1ǓSelect R1 = 1.8 kW
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.
For additional information see input capacitor section in the
“Application Hints” section of this data sheet.
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.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.
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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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:
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 ILoad.
The inductor current rating can also be determined by
calculating the inductor peak current:
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.
ExT+ǒVin –V
outǓVout
Vin x106
F[Hz] [V x ms]
Ip(max) +ILoad(max))ǒVin –V
outǓton
2L
ton +Vout
Vin x1.0
fosc
4. Inductor Selection (L1)
A.Calculate E x T [V x ms] constant:
B.E x T = 80 [V x ms]
C.ILoad(max) = 2.5 A
Inductance Region = H150
D.Proper inductor value = 150 mH
Choose the inductor from Table 2.
ExT+(25–8.0
)x8.0
25 x1000
52 +80 [V x ms]
5. Output Capacitor Selection (Cout)
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:
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.
Cout w13,300 Vin(max)
Vout xL[μH] [μF]
5. Output Capacitor Selection (Cout)
A.
To achieve an acceptable ripple voltage, select
Cout = 680 mF electrolytic capacitor.
Cout w13,300 x 25
8 x 150 +332.5 μF
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ET, VOLTAGE TIME (V s)μ
100
80
90
3.02.51.50.80.50.4 0.6 1.0 2.0
5.0
60
40
20
15
10
8.0
7.0
6.0
MAXIMUM INPUT VOLTAGE (V)MAXIMUM INPUT VOLTAGE (V)
IL, MAXIMUM LOAD CURRENT (A)
IL, MAXIMUM LOAD CURRENT (A)
IL, MAXIMUM LOAD CURRENT (A)
MAXIMUM INPUT VOLTAGE (V)
IL, MAXIMUM LOAD CURRENT (A)
MAXIMUM INPUT VOLTAGE (V)
Figure 18. LM2576−3.3
IL, MAXIMUM LOAD CURRENT (A)
Figure 19. LM2576−5
L680
Figure 20. LM2576−12 Figure 21. LM2576−15
Figure 22. LM2576−ADJ
LM2576 Series Buck Regulator Design Procedures (continued)
Indicator Value Selection Guide (For Continuous Mode Operation)
60
40
20
15
12
10
9.0
8.0
7.0
60
40
35
25
20
18
30
16
15
14
60
40
30
25
22
20
19
18
17
300
70
60
50
45
40
35
30
25
20
3.02.51.50.80.50.3 0.3
3.00.80.60.50.40.3
3.02.01.50.50.3
L330
L470
L150
L220
0.4 0.6 1.0 2.0
L100
L68
L47
H470H1000 H680 H220H330 H150
L680
L330
L470
L150
L220
L100
L68
L47
1.2
H470
H1000 H680
H220
H330 H150
L680
L330
L470
L150
L220
L100
L68
H1500
1.0 1.5 2.0 2.5
H470
H1000 H680
H220
H330 H150
L680
L330
L150
L220
L100
L68
H1500
3.00.80.60.50.40.3 1.0 1.5 2.0 2.5
35
L470
H680
H220
H330
H150
L680
L330
L150
L220
L100 L68
H1500
L470
H2000
L47
H470
H1000
150
200
250
0.4 0.6 0.8 1.0 2.5
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Table 1. Diode Selection Guide
VR
Schottky Fast Recovery
3.0 A 4.0 − 6.0 A 3.0 A 4.0 − 6.0 A
Through
Hole Surface
Mount Through
Hole Surface
Mount Through
Hole Surface
Mount Through
Hole Surface
Mount
20 V 1N5820
MBR320P
SR302
SK32 1N5823
SR502
SB520
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)
30 V 1N5821
MBR330
SR303
31DQ03
SK33
30WQ03 1N5824
SR503
SB530
50WQ03
40 V 1N5822
MBR340
SR304
31DQ04
SK34
30WQ04
MBRS340T3
MBRD340
1N5825
SR504
SB540
MBRD640CT
50WQ04
50 V MBR350
31DQ05
SR305
SK35
30WQ05 SB550 50WQ05
60 V MBR360
DQ06
SR306
MBRS360T3
MBRD360 50SQ080 MBRD660CT
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 47 mH77 212 671 26980 PE−53112 RL2442
L68 68 mH77 262 671 26990 PE−92114 RL2443
L100 100 mH77 312 671 27000 PE−92108 RL2444
L150 150 mH77 360 671 27010 PE−53113 RL1954
L220 220 mH77 408 671 27020 PE−52626 RL1953
L330 330 mH77 456 671 27030 PE−52627 RL1952
L470 470 mH*671 27040 PE−53114 RL1951
L680 680 mH77 506 671 27050 PE−52629 RL1950
H150 150 mH77 362 671 27060 PE−53115 RL2445
H220 220 mH77 412 671 27070 PE−53116 RL2446
H330 330 mH77 462 671 27080 PE−53117 RL2447
H470 470 mH*671 27090 PE−53118 RL1961
H680 680 mH77 508 671 27100 PE−53119 RL1960
H1000 1000 mH77 556 671 27110 PE−53120 RL1959
H1500 1500 mH*671 27120 PE−53121 RL1958
H2200 2200 mH*671 27130 PE−53122 RL2448
NOTE: *Contact Manufacturer
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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 Phone
Fax + 353−9324−107
+ 353−9324−459
Renco Electronics, Inc. Phone
Fax + 1−516−645−5828
+ 1−516−586−5562
Tech 39 Phone
Fax + 33−1−4115−1681
+ 33−1−4709−5051
Schott Corporation Phone
Fax + 1−612−475−1173
+ 1−612−475−1786
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 Cin
The important parameter of the input capacitor i s 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 capacitors RMS ripple current rating
should be:
Irms > 1.2 x d x ILoad
where d is the duty cycle, for a buck regulator
d+ton
T+
V
out
Vin
a
nd d +ton
T+|Vout|
|V
out
|)V
in
for a buck*boost regulato
r.
Output Capacitor (Cout)
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 capacitors 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. Bu t i f t h e selected capacitors 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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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 provide t he best e f ficiency e specially in l ow output
voltage applications (5.0 V and lower). Another choice
could b e F ast−Recovery , o r Ultra−Fast R ecovery diodes. I t
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 b etter f ulfill s ome q uality, l ow n oise d esign 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 o f fers greater output power, lower peak current s 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.
HORIZONTAL TIME BASE: 5.0 ms/DIV
Figure 23. Continuous Mode Switching Current
Waveforms
VERTRICAL RESOLUTION 1.0 A/DIV
2.0 A
0 A
2.0 A
0 A
Inductor
Current
Waveform
Power
Switch
Current
Waveform
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 di fferent 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.
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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 inductors maximum current rating may
cause the inductor to overheat because of the copper wire
losses, o r 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
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
0 A
0.4 A
0 A
Inductor
Current
Waveform
Power
Switch
Current
Waveform
Figure 24. Discontinuous Mode Switching Current
Waveforms
VERTICAL RESOLUTION 200 mA/DIV
HORIZONTAL TIME BASE: 5.0 ms/DIV
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.
Unfiltered
Output
Voltage
Filtered
Output
Voltage
HORIZONTAL TIME BASE: 5.0 ms/DIV
Figure 25. Output Ripple Voltage Waveforms
VERTRICAL
Voltage spikes
caused by
switching action
of the output
switch and the
parasitic
inductance of the
output capacitor
RESOLUTION
20 mV/DIV
Minimizing 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 la r ger value output capacitor. There is also another wa y
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 n o
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 D 2PAK 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
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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. PD(max) maximum regulator power dissipation in the
application.
2. TA(max) maximum ambient temperature in the
application.
3. TJ(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. RqJC package thermal resistance junction−case.
5. RqJA package thermal resistance junction−ambient.
(Refer to Maximum Ratings on page 2 of this data sheet or
RqJC and RqJA values).
The following formula is to calculate the approximate
total power dissipated by the LM2576:
PD = (Vin x IQ) + d x ILoad x Vsat
where d is the duty cycle and for buck converter
d+ton
T+VO
Vin,
IQ(quiescent current) and Vsat can be found in the
LM2576 data sheet,
Vin is minimum input voltage applied,
VOis the regulator output voltage,
ILoad is the load current.
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 (RqJA)(PD) represents the junction temperature rise
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:
TJ = PD (RqJA + RqCS + RqSA) + TA
where RqJC is the thermal resistance junction−case,
RqCS is the thermal resistance case−heatsink,
RqSA 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.
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 si ze, q uantity and s pacing o f o ther c omponents on t he
board c an a lso i nfluence i ts e ffectiveness t o d issi pate t he h eat.
Figure 26. Inverting Buck−Boost Develops −12 V
D1
1N5822
L1
68 mH
Output
2
4
Feedback
12 to 40 V
Unregulated
DC Input
Cin
100 mF
1
53ON/OFFGN
D
+Vin
−12 V @ 0.7 A
Regulated
Output
Cout
2200 mF
LM2576−12
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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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 start−up 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 start−up 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 start−up currents required
by this inverting regulator topology, the use of a delayed
start−up or a n undervoltage lockout circuit is recommended.
Using a delayed start−up 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 start−up is now partially
supplied by the input capacitor Cin.
It has been already mentioned above, that in some
situations, the delayed start−up or the undervoltage lockout
features could be very useful. A delayed start−up 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 Cout.
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. 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:
where ton +|VO|
Vin )|VO|x1.0
fosc, and fosc +52 kHz.
Ipeak [ILoad (Vin )|VO|)
Vin )Vin xt
on
2L1
Under normal continuous inductor current operating
conditions, the worst case occurs when Vin is minimal.
Figure 27. Inverting Buck−Boost Regulator
with Delayed start−up
D1
1N5822
L1
68 mH
Output
2
4
Feedback
12 V to 25 V
Unregulated
DC Input
Cin
100 mF
/50 V
1
35ON/OFF GN
D
+Vin
−12 V @ 700 m A
Regulated
Output
Cout
2200 mF
/16 V
LM2576−12
C1
0.1 mF
R1
47 k R2
47 k
Figure 28. Inverting Buck−Boost Regulator Shutdow
n
Circuit Using an Optocoupler
LM2576−XX
1
35GN
D
ON/OFF
+Vin
R2
47 k
Cin
100 mF
NOTE: This picture does not show the complete circuit.
R1
47 k
R3
470
Shutdown
Input
MOC8101
−Vou
t
Off
On
5
.0 V
0
+Vin
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.
LM2576
http://onsemi.com
20
Figure 29. Inverting Buck−Boost Regulator Shutdow
n
Circuit Using a PNP Transistor
NOTE: This picture does not show the complete circuit.
R2
5.6 k
Q1
2N3906
LM2576−XX
1
35GN
D
ON/OFF
R1
12 k −Vout
+Vin
Shutdown
Input
Off
On
+V
0
+Vin
Cin
100 mF
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.
Figure 30. Negative Boost Regulator
1N5820
100 mH
Output
2
4
Feedback
Vout = −12 V
Typical Load Current
400 mA for Vin = −5.2 V
750 mA for Vin = −7.0 V
−5.0 V to −12 V
Cout
2200 mF
Low Esr
Cin
100 mF
LM2576−12
1
53 ON/OFFGND
Vin
Vin
Design Recommendations:
The same design rules as for the previous inverting
buck−boost converter can be applied. The output capacitor
Cout must be chosen larger than would be required for a what
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.
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 Start−up
There are some applications, like the inverting regulator
already mentioned above, which require a higher amount of
start−up current. In such cases, if the input power source is
limited, this delayed start−up 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.
Figure 31. Delayed Start−up Circuitry
R1
47 k
LM2576−XX
1
35GN
D
ON/OFF
R2
47 k
+Vin
+Vin
C1
0.1 mF
Cin
100 mF
NOTE: This picture does not show the complete circuit.
Undervoltage Lockout
Some applications require the regulator to remain of f 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:
Vth [VZ1 )ǒ1.0 )R2
R1ǓVBE (Q1)