LM1575,LM2575,LM2575HV
LM1575/LM2575/LM2575HV SIMPLE SWITCHER 1A Step-Down Voltage Regulator
Literature Number: SNVS106D
April 2007
LM1575/LM2575/LM2575HV
SIMPLE SWITCHER® 1A Step-Down Voltage Regulator
General Description
The LM2575 series of regulators are monolithic integrated
circuits that provide all the active functions for a step-down
(buck) switching regulator, capable of driving a 1A load with
excellent line and load regulation. These devices are avail-
able in fixed output voltages of 3.3V, 5V, 12V, 15V, and an
adjustable output version.
Requiring a minimum number of external components, these
regulators are simple to use and include internal frequency
compensation and a fixed-frequency oscillator.
The LM2575 series offers a high-efficiency replacement for
popular three-terminal linear regulators. It substantially re-
duces the size of the heat sink, and in many cases no heat
sink is required.
A standard series of inductors optimized for use with the
LM2575 are available from several different manufacturers.
This feature greatly simplifies the design of switch-mode pow-
er supplies.
Other features include a guaranteed ±4% tolerance on output
voltage within specified input voltages and output load con-
ditions, and ±10% on the oscillator frequency. External shut-
down is included, featuring 50 μA (typical) standby current.
The output switch includes cycle-by-cycle current limiting, as
well as thermal shutdown for full protection under fault con-
ditions.
Features
3.3V, 5V, 12V, 15V, and adjustable output versions
Adjustable version output voltage range,
1.23V to 37V (57V for HV version) ±4% max over
line and load conditions
Guaranteed 1A output current
Wide input voltage range, 40V up to 60V for HV version
Requires only 4 external components
52 kHz fixed frequency internal oscillator
TTL shutdown capability, low power standby mode
High efficiency
Uses readily available standard inductors
Thermal shutdown and current limit protection
P+ Product Enhancement tested
Applications
Simple high-efficiency step-down (buck) regulator
Efficient pre-regulator for linear regulators
On-card switching regulators
Positive to negative converter (Buck-Boost)
Typical Application
(Fixed Output Voltage Versions)
1147501
Note: Pin numbers are for the TO-220 package.
SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation
© 2007 National Semiconductor Corporation 11475 www.national.com
LM1575/LM2575/LM2575HV Series SIMPLE SWITCHER 1A Step-Down Voltage Regulator
Block Diagram and Typical Application
1147502
3.3V, R2 = 1.7k
5V, R2 = 3.1k
12V, R2 = 8.84k
15V, R2 = 11.3k
For ADJ. Version
R1 = Open, R2 = 0Ω
Note: Pin numbers are for the TO-220 package.
FIGURE 1.
www.national.com 2
LM1575/LM2575/LM2575HV
Connection Diagrams
(XX indicates output voltage option. See Ordering Information table for complete part number.)
Straight Leads
5–Lead TO-220 (T)
1147522
Top View
LM2575T-XX or LM2575HVT-XX
See NS Package Number T05A
Bent, Staggered Leads
5-Lead TO-220 (T)
1147523
Top View
1147524
Side View
LM2575T-XX Flow LB03 or
LM2575HVT-XX Flow LB03
See NS Package Number T05D
16–Lead DIP (N or J)
1147525
*No Internal Connection
Top View
LM2575N-XX or LM2575HVN-XX
See NS Package Number N16A
LM1575J-XX-QML
See NS Package Number J16A
24-Lead Surface Mount (M)
1147526
*No Internal Connection
Top View
LM2575M-XX or LM2575HVM-XX
See NS Package Number M24B
TO-263(S)
5-Lead Surface-Mount Package
1147529
Top View
1147530
Side View
LM2575S-XX or LM2575HVS-XX
See NS Package Number TS5B
3 www.national.com
LM1575/LM2575/LM2575HV
Ordering Information
Package NSC Standard High Temperature
Type Package Voltage Rating Voltage Rating Range
Number (40V) (60V)
5-Lead TO-220 T05A LM2575T-3.3 LM2575HVT-3.3
Straight Leads LM2575T-5.0 LM2575HVT-5.0
LM2575T-12 LM2575HVT-12
LM2575T-15 LM2575HVT-15
LM2575T-ADJ LM2575HVT-ADJ
5-Lead TO-220 T05D LM2575T-3.3 Flow LB03 LM2575HVT-3.3 Flow LB03
Bent and LM2575T-5.0 Flow LB03 LM2575HVT-5.0 Flow LB03
Staggered Leads LM2575T-12 Flow LB03 LM2575HVT-12 Flow LB03
LM2575T-15 Flow LB03 LM2575HVT-15 Flow LB03
LM2575T-ADJ Flow LB03 LM2575HVT-ADJ Flow LB03
16-Pin Molded N16A LM2575N-5.0 LM2575HVN-5.0 −40°C TJ +125°C
DIP LM2575N-12 LM2575HVN-12
LM2575N-15 LM2575HVN-15
LM2575N-ADJ LM2575HVN-ADJ
24-Pin M24B LM2575M-5.0 LM2575HVM-5.0
Surface Mount LM2575M-12 LM2575HVM-12
LM2575M-15 LM2575HVM-15
LM2575M-ADJ LM2575HVM-ADJ
5-Lead TO-263 TS5B LM2575S-3.3 LM2575HVS-3.3
Surface Mount LM2575S-5.0 LM2575HVS-5.0
LM2575S-12 LM2575HVS-12
LM2575S-15 LM2575HVS-15
LM2575S-ADJ LM2575HVS-ADJ
16-Pin Ceramic J16A LM1575J-3.3-QML
DIP LM1575J-5.0-QML
LM1575J-12-QML −55°C TJ +150°C
LM1575J-15-QML
LM1575J-ADJ-QML
www.national.com 4
LM1575/LM2575/LM2575HV
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Supply Voltage
LM1575/LM2575 45V
LM2575HV 63V
ON /OFF Pin Input Voltage −0.3V V +VIN
Output Voltage to Ground
(Steady State) −1V
Power Dissipation Internally Limited
Storage Temperature Range −65°C to +150°C
Maximum Junction Temperature 150°C
Minimum ESD Rating
(C = 100 pF, R = 1.5 kΩ) 2 kV
Lead Temperature
(Soldering, 10 sec.) 260°C
Operating Ratings
Temperature Range
LM1575 −55°C TJ +150°C
LM2575/LM2575HV −40°C TJ +125°C
Supply Voltage
LM1575/LM2575 40V
LM2575HV 60V
LM1575-3.3, LM2575-3.3, LM2575HV-3.3
Electrical Characteristics
Specifications with standard type face are for TJ = 25°C, and those with boldface type apply over full Operating Temperature
Range .
Symbol Parameter Conditions Typ LM1575-3.3 LM2575-3.3 Units
(Limits)
LM2575HV-3.3
Limit Limit
(Note 2) (Note 3)
SYSTEM PARAMETERS (Note 4) Test Circuit Figure 2
VOUT Output Voltage VIN = 12V, ILOAD = 0.2A 3.3 V
Circuit of Figure 2 3.267 3.234 V(Min)
3.333 3.366 V(Max)
VOUT Output Voltage 4.75V VIN 40V, 0.2A ILOAD 1A 3.3 V
LM1575/LM2575 Circuit of Figure 2 3.200/3.168 3.168/3.135 V(Min)
3.400/3.432 3.432/3.465 V(Max)
VOUT Output Voltage 4.75V VIN 60V, 0.2A ILOAD 1A 3.3 V
LM2575HV Circuit of Figure 2 3.200/3.168 3.168/3.135 V(Min)
3.416/3.450 3.450/3.482 V(Max)
ηEfficiency VIN = 12V, ILOAD = 1A 75 %
LM1575-5.0, LM2575-5.0, LM2575HV-5.0
Electrical Characteristics
Specifications with standard type face are for TJ = 25°C, and those with boldface type apply over full Operating Temperature
Range.
Symbol Parameter Conditions Typ LM1575-5.0 LM2575-5.0 Units
(Limits)
LM2575HV-5.0
Limit Limit
(Note 2) (Note 3)
SYSTEM PARAMETERS (Note 4) Test Circuit Figure 2
VOUT Output Voltage VIN = 12V, ILOAD = 0.2A 5.0 V
Circuit of Figure 2 4.950 4.900 V(Min)
5.050 5.100 V(Max)
VOUT Output Voltage 0.2A ILOAD 1A, 5.0 V
LM1575/LM2575 8V VIN 40V 4.850/4.800 4.800/4.750 V(Min)
Circuit of Figure 2 5.150/5.200 5.200/5.250 V(Max)
5 www.national.com
LM1575/LM2575/LM2575HV
Symbol Parameter Conditions Typ LM1575-5.0 LM2575-5.0 Units
(Limits)
LM2575HV-5.0
Limit Limit
(Note 2) (Note 3)
VOUT Output Voltage 0.2A ILOAD 1A, 5.0 V
LM2575HV 8V VIN 60V 4.850/4.800 4.800/4.750 V(Min)
Circuit of Figure 2 5.175/5.225 5.225/5.275 V(Max)
ηEfficiency VIN = 12V, ILOAD = 1A 77 %
LM1575-12, LM2575-12, LM2575HV-12
Electrical Characteristics
Specifications with standard type face are for TJ = 25°C, and those with boldface type apply over full Operating Temperature
Range .
Symbol Parameter Conditions Typ LM1575-12 LM2575-12 Units
(Limits)
LM2575HV-12
Limit Limit
(Note 2) (Note 3)
SYSTEM PARAMETERS (Note 4) Test Circuit Figure 2
VOUT Output Voltage VIN = 25V, ILOAD = 0.2A 12 V
Circuit of Figure 2 11.88 11.76 V(Min)
12.12 12.24 V(Max)
VOUT Output Voltage 0.2A ILOAD 1A, 12 V
LM1575/LM2575 15V VIN 40V 11.64/11.52 11.52/11.40 V(Min)
Circuit of Figure 2 12.36/12.48 12.48/12.60 V(Max)
VOUT Output Voltage 0.2A ILOAD 1A, 12 V
LM2575HV 15V VIN 60V 11.64/11.52 11.52/11.40 V(Min)
Circuit of Figure 2 12.42/12.54 12.54/12.66 V(Max)
ηEfficiency VIN = 15V, ILOAD = 1A 88 %
LM1575-15, LM2575-15, LM2575HV-15
Electrical Characteristics
Specifications with standard type face are for TJ = 25°C, and those with boldface type apply over full Operating Temperature
Range .
Symbol Parameter Conditions Typ LM1575-15 LM2575-15 Units
(Limits)
LM2575HV-15
Limit Limit
(Note 2) (Note 3)
SYSTEM PARAMETERS (Note 4) Test Circuit Figure 2
VOUT Output Voltage VIN = 30V, ILOAD = 0.2A 15 V
Circuit of Figure 2 14.85 14.70 V(Min)
15.15 15.30 V(Max)
VOUT Output Voltage 0.2A ILOAD 1A, 15 V
LM1575/LM2575 18V VIN 40V 14.55/14.40 14.40/14.25 V(Min)
Circuit of Figure 2 15.45/15.60 15.60/15.75 V(Max)
VOUT Output Voltage 0.2A ILOAD 1A, 15 V
LM2575HV 18V VIN 60V 14.55/14.40 14.40/14.25 V(Min)
Circuit of Figure 2 15.525/15.675 15.68/15.83 V(Max)
ηEfficiency VIN = 18V, ILOAD = 1A 88 %
www.national.com 6
LM1575/LM2575/LM2575HV
LM1575-ADJ, LM2575-ADJ, LM2575HV-ADJ
Electrical Characteristics
Specifications with standard type face are for TJ= 25°C, and those with boldface type apply over full Operating Temperature
Range.
Symbol Parameter Conditions Typ LM1575-ADJ LM2575-ADJ Units
(Limits)
LM2575HV-ADJ
Limit Limit
(Note 2) (Note 3)
SYSTEM PARAMETERS (Note 4) Test Circuit Figure 2
VOUT Feedback Voltage VIN = 12V, ILOAD = 0.2A 1.230 V
VOUT = 5V 1.217 1.217 V(Min)
Circuit of Figure 2 1.243 1.243 V(Max)
VOUT Feedback Voltage 0.2A ILOAD 1A, 1.230 V
LM1575/LM2575 8V VIN 40V 1.205/1.193 1.193/1.180 V(Min)
VOUT = 5V, Circuit of Figure 2 1.255/1.267 1.267/1.280 V(Max)
VOUT Feedback Voltage 0.2A ILOAD 1A, 1.230 V
LM2575HV 8V VIN 60V 1.205/1.193 1.193/1.180 V(Min)
VOUT = 5V, Circuit of Figure 2 1.261/1.273 1.273/1.286 V(Max)
ηEfficiency VIN = 12V, ILOAD = 1A, VOUT = 5V 77 %
All Output Voltage Versions
Electrical Characteristics
Specifications with standard type face are for TJ = 25°C, and those with boldface type apply over full Operating Temperature
Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version, VIN = 25V for the 12V version, and VIN =
30V for the 15V version. ILOAD = 200 mA.
Symbol Parameter Conditions Typ LM1575-XX LM2575-XX Units
(Limits)
LM2575HV-XX
Limit Limit
(Note 2) (Note 3)
DEVICE PARAMETERS
IbFeedback Bias Current VOUT = 5V (Adjustable Version Only) 50 100/500 100/500 nA
fOOscillator Frequency (Note 13) 52 kHz
47/43 47/42 kHz(Min)
58/62 58/63 kHz(Max)
VSAT Saturation Voltage IOUT = 1A (Note 5) 0.9 V
1.2/1.4 1.2/1.4 V(Max)
DC Max Duty Cycle (ON) (Note 6) 98 %
93 93 %(Min)
ICL Current Limit Peak Current (Notes 5, 13) 2.2 A
1.7/1.3 1.7/1.3 A(Min)
3.0/3.2 3.0/3.2 A(Max)
ILOutput Leakage (Notes 7, 8) Output = 0V 2 2 mA(Max)
Current          Output = −1V 7.5 mA
         Output = −1V 30 30 mA(Max)
IQQuiescent Current (Note 7) 5 mA
10/12 10 mA(Max)
ISTBY Standby Quiescent ON /OFF Pin = 5V (OFF) 50 μA
Current 200/500 200 μA(Max)
7 www.national.com
LM1575/LM2575/LM2575HV
Symbol Parameter Conditions Typ LM1575-XX LM2575-XX Units
(Limits)
LM2575HV-XX
Limit Limit
(Note 2) (Note 3)
θJA Thermal Resistance T Package, Junction to Ambient (Note 9) 65
θJA T Package, Junction to Ambient (Note 10) 45 °C/W
θJC T Package, Junction to Case 2
θJA N Package, Junction to Ambient (Note 11) 85
θJA M Package, Junction to Ambient (Note 11) 100
θJA S Package, Junction to Ambient (Note 12) 37
ON /OFF CONTROL Test Circuit Figure 2
VIH ON /OFF Pin Logic VOUT = 0V 1.4 2.2/2.4 2.2/2.4 V(Min)
VIL Input Level VOUT = Nominal Output Voltage 1.2 1.0/0.8 1.0/0.8 V(Max)
IIH ON /OFF Pin Input ON /OFF Pin = 5V (OFF) 12 μA
Current 30 30 μA(Max)
IIL ON /OFF Pin = 0V (ON) 0 μA
10 10 μA(Max)
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All limits are used to calculate Average
Outgoing Quality Level, and all are 100% production tested.
Note 3: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods.
Note 4: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM1575/
LM2575 is used as shown in the Figure 2 test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.
Note 5: Output (pin 2) sourcing current. No diode, inductor or capacitor connected to output pin.
Note 6: Feedback (pin 4) removed from output and connected to 0V.
Note 7: Feedback (pin 4) removed from output and connected to +12V for the Adjustable, 3.3V, and 5V versions, and +25V for the 12V and 15V versions, to
force the output transistor OFF.
Note 8: VIN = 40V (60V for the high voltage version).
Note 9: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with ½ inch leads in a socket, or on a
PC board with minimum copper area.
Note 10: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with ½ inch leads soldered to a PC
board containing approximately 4 square inches of copper area surrounding the leads.
Note 11: Junction to ambient thermal resistance with approximately 1 square inch of pc board copper surrounding the leads. Additional copper area will lower
thermal resistance further. See thermal model in Switchers made Simple software.
Note 12: If the TO-263 package is used, the thermal resistance can be reduced by increasing the PC board copper area thermally connected to the package:
Using 0.5 square inches of copper area, θJA is 50°C/W; with 1 square inch of copper area, θJA is 37°C/W; and with 1.6 or more square inches of copper area,
θJA is 32°C/W.
Note 13: 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 power dissipation of the IC by lowering the minimum
duty cycle from 5% down to approximately 2%.
Note 14: Refer to RETS LM1575J for current revision of military RETS/SMD.
www.national.com 8
LM1575/LM2575/LM2575HV
Typical Performance Characteristics (Circuit of Figure 2)
Normalized Output Voltage
1147532
Line Regulation
1147533
Dropout Voltage
1147534
Current Limit
1147535
Quiescent Current
1147536
Standby
Quiescent Current
1147537
9 www.national.com
LM1575/LM2575/LM2575HV
Oscillator Frequency
1147538
Switch Saturation
Voltage
1147539
Efficiency
1147540
Minimum Operating Voltage
1147541
Quiescent Current
vs Duty Cycle
1147542
Feedback Voltage
vs Duty Cycle
1147543
www.national.com 10
LM1575/LM2575/LM2575HV
Feedback Pin Current
1147505
Maximum Power Dissipation
(TO-263) (See (Note 12))
1147528
Switching Waveforms
1147506
VOUT = 5V
A: Output Pin Voltage, 10V/div
B: Output Pin Current, 1A/div
C: Inductor Current, 0.5A/div
D: Output Ripple Voltage, 20 mV/div,
AC-Coupled
Horizontal Time Base: 5 μs/div
Load Transient Response
1147507
11 www.national.com
LM1575/LM2575/LM2575HV
Test Circuit and Layout Guidelines
As in any switching regulator, layout is very important. Rapidly
switching currents associated with wiring inductance gener-
ate voltage transients which can cause problems. For minimal
inductance and ground loops, the length of the leads indicated
by heavy lines should be kept as short as possible. Single-
point grounding (as indicated) or ground plane construction
should be used for best results. When using the Adjustable
version, physically locate the programming resistors near the
regulator, to keep the sensitive feedback wiring short.
Fixed Output Voltage Versions
1147508
CIN — 100 μF, 75V, Aluminum Electrolytic
COUT — 330 μF, 25V, Aluminum Electrolytic
D1 — Schottky, 11DQ06
L1 — 330 μH, PE-52627 (for 5V in, 3.3V out, use 100 μH, PE-92108)
Adjustable Output Voltage Version
1147509
where VREF = 1.23V, R1 between 1k and 5k.
R1 — 2k, 0.1%
R2 — 6.12k, 0.1%
Note: Pin numbers are for the TO-220 package.
FIGURE 2.
www.national.com 12
LM1575/LM2575/LM2575HV
LM2575 Series Buck Regulator Design Procedure
PROCEDURE (Fixed Output Voltage Versions) EXAMPLE (Fixed Output Voltage Versions)
Given:
VOUT = Regulated Output Voltage (3.3V, 5V, 12V, or 15V)
VIN(Max) = Maximum Input Voltage
ILOAD(Max) = Maximum Load Current
Given:
VOUT = 5V
VIN(Max) = 20V
ILOAD(Max) = 0.8A
1. Inductor Selection (L1)
A. Select the correct Inductor value selection guide from Figures
3, 4, 5, 6 (Output voltages of 3.3V, 5V, 12V or 15V respectively).
For other output voltages, see the design procedure for the ad-
justable version.
B. From the inductor value selection guide, identify the inductance
region intersected by VIN(Max) and ILOAD(Max), and note the in-
ductor code for that region.
C. Identify the inductor value from the inductor code, and select an
appropriate inductor from the table shown in Figure 9. Part numbers
are listed for three inductor manufacturers. The inductor chosen
must be rated for operation at the LM2575 switching frequency (52
kHz) and for a current rating of 1.15 × ILOAD. For additional inductor
information, see the inductor section in the Application Hints section
of this data sheet.
1. Inductor Selection (L1)
A. Use the selection guide shown in Figure 4.
B. From the selection guide, the inductance area intersected by
the 20V line and 0.8A line is L330.
C. Inductor value required is 330 μH. From the table in Figure 9,
choose AIE 415-0926, Pulse Engineering PE-52627, or RL1952.
2. Output Capacitor Selection (COUT)
A. The value of the output capacitor together with the inductor de-
fines the dominate pole-pair of the switching regulator loop. For
stable operation and an acceptable output ripple voltage, (approx-
imately 1% of the output voltage) a value between 100 μF and 470
μF is recommended.
B. The capacitor's voltage rating should be at least 1.5 times
greater than the output voltage. For a 5V regulator, a rating of at
least 8V is appropriate, and a 10V or 15V rating is recommended.
Higher voltage electrolytic capacitors generally have lower ESR
numbers, and for this reason it may be necessary to select a ca-
pacitor rated for a higher voltage than would normally be needed.
2. Output Capacitor Selection (COUT)
A. COUT = 100 μF to 470 μF standard aluminum electrolytic.
B. Capacitor voltage rating = 20V.
3. Catch Diode Selection (D1)
A. The catch-diode current rating must be at least 1.2 times greater
than the maximum load current. Also, if the power supply design
must withstand a continuous output short, the diode should have a
current rating equal to the maximum current limit of the LM2575.
The most stressful condition for this diode is an overload or shorted
output condition.
B. The reverse voltage rating of the diode should be at least 1.25
times the maximum input voltage.
3. Catch Diode Selection (D1)
A. For this example, a 1A current rating is adequate.
B. Use a 30V 1N5818 or SR103 Schottky diode, or any of the
suggested fast-recovery diodes shown in Figure 8.
4. Input Capacitor (CIN)
An aluminum or tantalum electrolytic bypass capacitor located
close to the regulator is needed for stable operation.
4. Input Capacitor (CIN)
A 47 μF, 25V aluminum electrolytic capacitor located near the input
and ground pins provides sufficient bypassing.
13 www.national.com
LM1575/LM2575/LM2575HV
Inductor Value Selection Guides
(For Continuous Mode Operation)
1147510
FIGURE 3. LM2575(HV)-3.3
1147511
FIGURE 4. LM2575(HV)-5.0
1147512
FIGURE 5. LM2575(HV)-12
1147513
FIGURE 6. LM2575(HV)-15
1147514
FIGURE 7. LM2575(HV)-ADJ
www.national.com 14
LM1575/LM2575/LM2575HV
PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions)
Given:
VOUT = Regulated Output Voltage
VIN(Max) = Maximum Input Voltage
ILOAD(Max) = Maximum Load Current
F = Switching Frequency (Fixed at 52 kHz)
Given:
VOUT = 10V
VIN(Max) = 25V
ILOAD(Max) = 1A
F = 52 kHz
1. Programming Output Voltage (Selecting R1 and R2, as shown
in Figure 2 )
Use the following formula to select the appropriate resistor values.
R1 can be between 1k and 5k. (For best temperature coefficient and
stability with time, use 1% metal film resistors)
1.Programming Output Voltage (Selecting R1 and R2)
R2 = 1k (8.13 − 1) = 7.13k, closest 1% value is 7.15k
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant,
E • T (V • μs), from the following formula:
B. Use the E • T value from the previous formula and match it with
the E • T number on the vertical axis of the Inductor Value Selec-
tion Guide shown in Figure 7.
C. On the horizontal axis, select the maximum load current.
D. Identify the inductance region intersected by the E • T value and
the maximum load current value, and note the inductor code for that
region.
E. Identify the inductor value from the inductor code, and select an
appropriate inductor from the table shown in Figure 9. Part numbers
are listed for three inductor manufacturers. The inductor chosen
must be rated for operation at the LM2575 switching frequency (52
kHz) and for a current rating of 1.15 × ILOAD. For additional inductor
information, see the inductor section in the application hints section
of this data sheet.
2. Inductor Selection (L1)
A. Calculate E • T (V • μs)
B. E • T = 115 V • μs
C. ILOAD(Max) = 1A
D. Inductance Region = H470
E. Inductor Value = 470 μH Choose from AIE part #430-0634,
Pulse Engineering part #PE-53118, or Renco part #RL-1961.
3. Output Capacitor Selection (COUT)
A. The value of the output capacitor together with the inductor de-
fines the dominate pole-pair of the switching regulator loop. For
stable operation, the capacitor must satisfy the following require-
ment:
The above formula yields capacitor values between 10 μF and 2000
μF that will satisfy the loop requirements for stable operation. But
to achieve an acceptable output ripple voltage, (approximately 1%
of the output voltage) and transient response, the output capacitor
may need to be several times larger than the above formula yields.
B. The capacitor's voltage rating should be at last 1.5 times greater
than the output voltage. For a 10V regulator, a rating of at least 15V
or more is recommended.
Higher voltage electrolytic capacitors generally have lower ESR
numbers, and for this reason it may be necessary to select a ca-
pacitor rate for a higher voltage than would normally be needed.
3. Output Capacitor Selection (COUT)
A.
However, for acceptable output ripple voltage select
COUT 220 μF
COUT = 220 μF electrolytic capacitor
(Continued) (Continued)
15 www.national.com
LM1575/LM2575/LM2575HV
PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions)
4. Catch Diode Selection (D1)
A. The catch-diode current rating must be at least 1.2 times greater
than the maximum load current. Also, if the power supply design
must withstand a continuous output short, the diode should have a
current rating equal to the maximum current limit of the LM2575.
The most stressful condition for this diode is an overload or shorted
output. See diode selection guide in Figure 8.
B. The reverse voltage rating of the diode should be at least 1.25
times the maximum input voltage.
4. Catch Diode Selection (D1)
A. For this example, a 3A current rating is adequate.
B. Use a 40V MBR340 or 31DQ04 Schottky diode, or any of the
suggested fast-recovery diodes in Figure 8.
5. Input Capacitor (CIN)
An aluminum or tantalum electrolytic bypass capacitor located
close to the regulator is needed for stable operation.
5. Input Capacitor (CIN)
A 100 μF aluminum electrolytic capacitor located near the input and
ground pins provides sufficient bypassing.
To further simplify the buck regulator design procedure, National Semiconductor is making available computer design software to
be used with the Simple Switcher line of switching regulators. Switchers Made Simple (version 3.3) is available on a (3½″) diskette
for IBM compatible computers from a National Semiconductor sales office in your area.
www.national.com 16
LM1575/LM2575/LM2575HV
VRSchottky Fast Recovery
1A 3A 1A 3A
20V 1N5817 1N5820
MBR120P MBR320
SR102 SR302
30V 1N5818 1N5821
MBR130P MBR330 The following
diodes are all
rated to 100V
  
11DF1
MUR110
HER102
The following
diodes are all
rated to 100V
  
31DF1
MURD310
HER302
11DQ03 31DQ03
SR103 SR303
40V 1N5819 IN5822
MBR140P MBR340
11DQ04 31DQ04
SR104 SR304
50V MBR150 MBR350
11DQ05 31DQ05
SR105 SR305
60V MBR160 MBR360
11DQ06 31DQ06
SR106 SR306
FIGURE 8. Diode Selection Guide
Inductor Inductor Schott Pulse Eng. Renco
Code Value (Note 15) (Note 16) (Note 17)
L100 100 μH67127000 PE-92108 RL2444
L150 150 μH67127010 PE-53113 RL1954
L220 220 μH67127020 PE-52626 RL1953
L330 330 μH67127030 PE-52627 RL1952
L470 470 μH67127040 PE-53114 RL1951
L680 680 μH67127050 PE-52629 RL1950
H150 150 μH67127060 PE-53115 RL2445
H220 220 μH67127070 PE-53116 RL2446
H330 330 μH67127080 PE-53117 RL2447
H470 470 μH67127090 PE-53118 RL1961
H680 680 μH67127100 PE-53119 RL1960
H1000 1000 μH67127110 PE-53120 RL1959
H1500 1500 μH67127120 PE-53121 RL1958
H2200 2200 μH67127130 PE-53122 RL2448
Note 15: Schott Corp., (612) 475-1173, 1000 Parkers Lake Rd., Wayzata, MN 55391.
Note 16: Pulse Engineering, (619) 674-8100, P.O. Box 12236, San Diego, CA 92112.
Note 17: Renco Electronics Inc., (516) 586-5566, 60 Jeffryn Blvd. East, Deer Park, NY 11729.
FIGURE 9. Inductor Selection by Manufacturer's Part Number
17 www.national.com
LM1575/LM2575/LM2575HV
Application Hints
INPUT CAPACITOR (CIN)
To maintain stability, the regulator input pin must be bypassed
with at least a 47 μF electrolytic capacitor. The capacitor's
leads must be kept short, and located near the regulator.
If the operating temperature range includes temperatures be-
low −25°C, the input capacitor value may need to be larger.
With most electrolytic capacitors, the capacitance value de-
creases and the ESR increases with lower temperatures and
age. Paralleling a ceramic or solid tantalum capacitor will in-
crease the regulator stability at cold temperatures. For maxi-
mum capacitor operating lifetime, the capacitor's RMS ripple
current rating should be greater than
INDUCTOR SELECTION
All switching regulators have two basic modes of operation:
continuous and discontinuous. The difference between the
two types relates to the inductor current, whether it is flowing
continuously, or if it drops to zero for a period of time in the
normal switching cycle. Each mode has distinctively different
operating characteristics, which can affect the regulator per-
formance and requirements.
The LM2575 (or any of the Simple Switcher family) can be
used for both continuous and discontinuous modes of oper-
ation.
The inductor value selection guides in Figure 3 through Figure
7 were designed for buck regulator designs of the continuous
inductor current type. When using inductor values shown in
the inductor selection guide, the peak-to-peak inductor ripple
current will be approximately 20% to 30% of the maximum DC
current. With relatively heavy load currents, the circuit oper-
ates in the continuous mode (inductor current always flowing),
but under light load conditions, the circuit will be forced to the
discontinuous mode (inductor current falls to zero for a period
of time). This discontinuous mode of operation is perfectly
acceptable. For light loads (less than approximately 200 mA)
it may be desirable to operate the regulator in the discontin-
uous mode, primarily because of the lower inductor values
required for the discontinuous mode.
The selection guide chooses inductor values suitable for con-
tinuous mode operation, but if the inductor value chosen is
prohibitively high, the designer should investigate the possi-
bility of discontinuous operation. The computer design soft-
ware Switchers Made Simple will provide all component
values for discontinuous (as well as continuous) mode of op-
eration.
Inductors are available in different styles such as pot core,
toriod, E-frame, bobbin core, etc., as well as different core
materials, such as ferrites and powdered iron. The least ex-
pensive, the bobbin core type, consists of wire wrapped on a
ferrite rod core. This type of construction makes for an inex-
pensive inductor, but since the magnetic flux is not completely
contained within the core, it generates more electromagnetic
interference (EMI). This EMI can cause problems in sensitive
circuits, or can give incorrect scope readings because of in-
duced voltages in the scope probe.
The inductors listed in the selection chart include ferrite pot
core construction for AIE, powdered iron toroid for Pulse En-
gineering, and ferrite bobbin core for Renco.
An inductor should not be operated beyond its maximum rat-
ed current because it may saturate. When an inductor begins
to saturate, the inductance decreases rapidly and the inductor
begins to look mainly resistive (the DC resistance of the wind-
ing). This will cause the switch current to rise very rapidly.
Different inductor types have different saturation characteris-
tics, and this should be kept in mind when selecting an in-
ductor.
The inductor manufacturer's data sheets include current and
energy limits to avoid inductor saturation.
INDUCTOR RIPPLE CURRENT
When the switcher is operating in the continuous mode, the
inductor current waveform ranges from a triangular to a saw-
tooth type of waveform (depending on the input voltage). For
a given input voltage and output voltage, the peak-to-peak
amplitude of this inductor current waveform remains constant.
As the load current rises or falls, the entire sawtooth current
waveform also rises or falls. The average DC value of this
waveform is equal to the DC load current (in the buck regu-
lator configuration).
If the load current drops to a low enough level, the bottom of
the sawtooth current waveform will reach zero, and the
switcher will change to a discontinuous mode of operation.
This is a perfectly acceptable mode of operation. Any buck
switching regulator (no matter how large the inductor value is)
will be forced to run discontinuous if the load current is light
enough.
OUTPUT CAPACITOR
An output capacitor is required to filter the output voltage and
is needed for loop stability. The capacitor should be located
near the LM2575 using short pc board traces. Standard alu-
minum electrolytics are usually adequate, but low ESR types
are recommended for low output ripple voltage and good sta-
bility. The ESR of a capacitor depends on many factors, some
which are: the value, the voltage rating, physical size and the
type of construction. In general, low value or low voltage (less
than 12V) electrolytic capacitors usually have higher ESR
numbers.
The amount of output ripple voltage is primarily a function of
the ESR (Equivalent Series Resistance) of the output capac-
itor and the amplitude of the inductor ripple current (ΔIIND).
See the section on inductor ripple current in Application Hints.
The lower capacitor values (220 μF–680 μF) will allow typi-
cally 50 mV to 150 mV of output ripple voltage, while larger-
value capacitors will reduce the ripple to approximately 20 mV
to 50 mV.
Output Ripple Voltage = (ΔIIND) (ESR of COUT)
To further reduce the output ripple voltage, several standard
electrolytic capacitors may be paralleled, or a higher-grade
capacitor may be used. Such capacitors are often called
“high-frequency,” “low-inductance,” or “low-ESR.” These will
reduce the output ripple to 10 mV or 20 mV. However, when
operating in the continuous mode, reducing the ESR below
0.05Ω can cause instability in the regulator.
Tantalum capacitors can have a very low ESR, and should be
carefully evaluated if it is the only output capacitor. Because
of their good low temperature characteristics, a tantalum can
www.national.com 18
LM1575/LM2575/LM2575HV
be used in parallel with aluminum electrolytics, with the tan-
talum making up 10% or 20% of the total capacitance.
The capacitor's ripple current rating at 52 kHz should be at
least 50% higher than the peak-to-peak inductor ripple cur-
rent.
CATCH DIODE
Buck regulators require a diode to provide a return path for
the inductor current when the switch is off. This diode should
be located close to the LM2575 using short leads and short
printed circuit traces.
Because of their fast switching speed and low forward voltage
drop, Schottky diodes provide the best efficiency, especially
in low output voltage switching regulators (less than 5V). Fast-
Recovery, High-Efficiency, or Ultra-Fast Recovery diodes are
also suitable, but some types with an abrupt turn-off charac-
teristic may cause instability and EMI problems. A fast-recov-
ery diode with soft recovery characteristics is a better choice.
Standard 60 Hz diodes (e.g., 1N4001 or 1N5400, etc.) are
also not suitable. See Figure 8 for Schottky and “soft” fast-
recovery diode selection guide.
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
The output voltage of a switching power supply will contain a
sawtooth ripple voltage at the switcher frequency, typically
about 1% of the output voltage, and may also contain short
voltage spikes at the peaks of the sawtooth waveform.
The output ripple voltage is due mainly to the inductor saw-
tooth ripple current multiplied by the ESR of the output ca-
pacitor. (See the inductor selection in the application hints.)
The voltage spikes are present because of the fast switching
action of the output switch, and the parasitic inductance of the
output filter capacitor. To minimize these voltage spikes, spe-
cial low inductance capacitors can be used, and their lead
lengths must be kept short. Wiring inductance, stray capaci-
tance, as well as the scope probe used to evaluate these
transients, all contribute to the amplitude of these spikes.
An additional small LC filter (20 μH & 100 μF) can be added
to the output (as shown in Figure 15) to further reduce the
amount of output ripple and transients. A 10 × reduction in
output ripple voltage and transients is possible with this filter.
FEEDBACK CONNECTION
The LM2575 (fixed voltage versions) feedback pin must be
wired to the output voltage point of the switching power sup-
ply. When using the adjustable version, physically locate both
output voltage programming resistors near the LM2575 to
avoid picking up unwanted noise. Avoid using resistors
greater than 100 kΩ because of the increased chance of noise
pickup.
ON /OFF INPUT
For normal operation, the ON /OFF pin should be grounded
or driven with a low-level TTL voltage (typically below 1.6V).
To put the regulator into standby mode, drive this pin with a
high-level TTL or CMOS signal. The ON /OFF pin can be
safely pulled up to +VIN without a resistor in series with it. The
ON /OFF pin should not be left open.
GROUNDING
To maintain output voltage stability, the power ground con-
nections must be low-impedance (see Figure 2). For the TO-3
style package, the case is ground. For the 5-lead TO-220 style
package, both the tab and pin 3 are ground and either con-
nection may be used, as they are both part of the same copper
lead frame.
With the N or M packages, all the pins labeled ground, power
ground, or signal ground should be soldered directly to wide
printed circuit board copper traces. This assures both low in-
ductance connections and good thermal properties.
HEAT SINK/THERMAL CONSIDERATIONS
In many cases, no heat sink is required to keep the LM2575
junction temperature within the allowed operating range. For
each application, to determine whether or not a heat sink will
be required, the following must be identified:
1. Maximum ambient temperature (in the application).
2. Maximum regulator power dissipation (in application).
3. Maximum allowed junction temperature (150°C for the
LM1575 or 125°C for the LM2575). For a safe,
conservative design, a temperature approximately 15°C
cooler than the maximum temperature should be
selected.
4. LM2575 package thermal resistances θJA and θJC.
Total power dissipated by the LM2575 can be estimated as
follows:
PD = (VIN) (IQ) + (VO/VIN) (ILOAD) (VSAT)
where IQ (quiescent current) and VSAT can be found in the
Characteristic Curves shown previously, VIN is the applied
minimum input voltage, VO is the regulated output voltage,
and ILOAD is the load current. The dynamic losses during turn-
on and turn-off are negligible if a Schottky type catch diode is
used.
When no heat sink is used, the junction temperature rise can
be determined by the following:
ΔTJ = (PD) (θJA)
To arrive at the actual operating junction temperature, add the
junction temperature rise to the maximum ambient tempera-
ture.
TJ = ΔTJ + TA
If the actual operating junction temperature is greater than the
selected safe operating junction temperature determined in
step 3, then a heat sink is required.
When using a heat sink, the junction temperature rise can be
determined by the following:
ΔTJ = (PD) (θJC + θinterface + θHeat sink)
The operating junction temperature will be:
TJ = TA + ΔTJ
As above, if the actual operating junction temperature is
greater than the selected safe operating junction tempera-
ture, then a larger heat sink is required (one that has a lower
thermal resistance).
When using the LM2575 in the plastic DIP (N) or surface
mount (M) packages, several items about the thermal prop-
erties of the packages should be understood. The majority of
the heat is conducted out of the package through the leads,
with a minor portion through the plastic parts of the package.
Since the lead frame is solid copper, heat from the die is
readily conducted through the leads to the printed circuit
board copper, which is acting as a heat sink.
For best thermal performance, the ground pins and all the
unconnected pins should be soldered to generous amounts
of printed circuit board copper, such as a ground plane. Large
areas of copper provide the best transfer of heat to the sur-
rounding air. Copper on both sides of the board is also helpful
in getting the heat away from the package, even if there is no
direct copper contact between the two sides. Thermal resis-
19 www.national.com
LM1575/LM2575/LM2575HV
tance numbers as low as 40°C/W for the SO package, and
30°C/W for the N package can be realized with a carefully
engineered pc board.
Included on the Switchers Made Simple design software is
a more precise (non-linear) thermal model that can be used
to determine junction temperature with different input-output
parameters or different component values. It can also calcu-
late the heat sink thermal resistance required to maintain the
regulators junction temperature below the maximum operat-
ing temperature.
Additional Applications
INVERTING REGULATOR
Figure 10 shows a LM2575-12 in a buck-boost configuration
to generate a negative 12V output from a positive input volt-
age. This circuit bootstraps the regulator's ground pin to the
negative output voltage, then by grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it to −12V.
For an input voltage of 12V or more, the maximum available
output current in this configuration is approximately 0.35A. At
lighter loads, the minimum input voltage required drops to
approximately 4.7V.
The switch currents in this buck-boost configuration are high-
er than in the standard buck-mode design, thus lowering the
available output current. Also, the start-up input current of the
buck-boost converter is higher than the standard buck-mode
regulator, and this may overload an input power source with
a current limit less than 1.5A. Using a delayed turn-on or an
undervoltage lockout circuit (described in the next section)
would allow the input voltage to rise to a high enough level
before the switcher would be allowed to turn on.
Because of the structural differences between the buck and
the buck-boost regulator topologies, the buck regulator de-
sign procedure section can not be used to select the inductor
or the output capacitor. The recommended range of inductor
values for the buck-boost design is between 68 μH and 220
μH, and the output capacitor values must be larger than what
is normally required for buck designs. Low input voltages or
high output currents require a large value output capacitor (in
the thousands of micro Farads).
The peak inductor current, which is the same as the peak
switch current, can be calculated from the following formula:
Where fosc = 52 kHz. Under normal continuous inductor cur-
rent operating conditions, the minimum VIN represents the
worst case. Select an inductor that is rated for the peak cur-
rent anticipated.
Also, the maximum voltage appearing across the regulator is
the absolute sum of the input and output voltage. For a −12V
output, the maximum input voltage for the LM2575 is +28V,
or +48V for the LM2575HV.
The Switchers Made Simple (version 3.3) design software
can be used to determine the feasibility of regulator designs
using different topologies, different input-output parameters,
different components, etc.
1147515
FIGURE 10. Inverting Buck-Boost Develops −12V
www.national.com 20
LM1575/LM2575/LM2575HV
NEGATIVE BOOST REGULATOR
Another variation on the buck-boost topology is the negative
boost configuration. The circuit in Figure 11 accepts an input
voltage ranging from −5V to −12V and provides a regulated
−12V output. Input voltages greater than −12V will cause the
output to rise above −12V, but will not damage the regulator.
Because of the boosting function of this type of regulator, the
switch current is relatively high, especially at low input volt-
ages. Output load current limitations are a result of the max-
imum current rating of the switch. Also, boost regulators can
not provide current limiting load protection in the event of a
shorted load, so some other means (such as a fuse) may be
necessary.
1147516
Typical Load Current
200 mA for VIN = −5.2V
500 mA for VIN = −7V
Note: Pin numbers are for TO-220 package.
FIGURE 11. Negative Boost
UNDERVOLTAGE LOCKOUT
In some applications it is desirable to keep the regulator off
until the input voltage reaches a certain threshold. An under-
voltage lockout circuit which accomplishes this task is shown
in Figure 12, while Figure 13 shows the same circuit applied
to a buck-boost configuration. These circuits keep the regu-
lator off until the input voltage reaches a predetermined level.
VTH ≈ VZ1 + 2VBE (Q1)
DELAYED STARTUP
The ON /OFF pin can be used to provide a delayed startup
feature as shown in Figure 14. With an input voltage of 20V
and for the part values shown, the circuit provides approxi-
mately 10 ms of delay time before the circuit begins switching.
Increasing the RC time constant can provide longer delay
times. But excessively large RC time constants can cause
problems with input voltages that are high in 60 Hz or 120 Hz
ripple, by coupling the ripple into the ON /OFF pin.
ADJUSTABLE OUTPUT, LOW-RIPPLE
POWER SUPPLY
A 1A power supply that features an adjustable output voltage
is shown in Figure 15. An additional L-C filter that reduces the
output ripple by a factor of 10 or more is included in this circuit.
1147517
Note: Complete circuit not shown.
Note: Pin numbers are for the TO-220 package.
FIGURE 12. Undervoltage Lockout for Buck Circuit
1147518
Note: Complete circuit not shown (see Figure 10).
Note: Pin numbers are for the TO-220 package.
FIGURE 13. Undervoltage Lockout
for Buck-Boost Circuit
1147519
Note: Complete circuit not shown.
Note: Pin numbers are for the TO-220 package.
FIGURE 14. Delayed Startup
21 www.national.com
LM1575/LM2575/LM2575HV