National Semiconductor LM1575 Series User manual

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
power supplies.
Other features include a guaranteed ±4% tolerance on out-
put voltage within specified input voltages and output load
conditions, and ±10% on the oscillator frequency. External
shutdown is included, featuring 50 µA (typical) standby cur-
rent. The output switch includes cycle-by-cycle current limit-
ing, as well as thermal shutdown for full protection under
fault conditions.
Features
n3.3V, 5V, 12V, 15V, and adjustable output versions
nAdjustable version output voltage range,
1.23V to 37V (57V for HV version) ±4% max over
line and load conditions
nGuaranteed 1A output current
nWide input voltage range, 40V up to 60V for HV version
nRequires only 4 external components
n52 kHz fixed frequency internal oscillator
nTTL shutdown capability, low power standby mode
nHigh efficiency
nUses readily available standard inductors
nThermal shutdown and current limit protection
nP
+
Product Enhancement tested
Applications
nSimple high-efficiency step-down (buck) regulator
nEfficient pre-regulator for linear regulators
nOn-card switching regulators
nPositive to negative converter (Buck-Boost)
Typical Application (Fixed Output Voltage
Versions)
01147501
Note: Pin numbers are for the TO-220 package.
SIMPLE SWITCHER®is a registered trademark of National Semiconductor Corporation.
August 2004
LM1575/LM2575/LM2575HV Series SIMPLE SWITCHER 1A Step-Down Voltage Regulator
© 2004 National Semiconductor Corporation DS011475 www.national.com

Block Diagram and Typical Application
Connection Diagrams (XX indicates output voltage option. See Ordering Information table for complete part
number.)
Straight Leads
5–Lead TO-220 (T)
Bent, Staggered Leads
5-Lead TO-220 (T)
01147522
Top View
LM2575T-XX or LM2575HVT-XX
See NS Package Number T05A
01147523
Top View
01147524
Side View
LM2575T-XX Flow LB03 or
LM2575HVT-XX Flow LB03
See NS Package Number T05D
01147502
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.
LM1575/LM2575/LM2575HV
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Connection Diagrams (XX indicates output voltage option. See Ordering Information table for complete part
number.) (Continued)
16–Lead DIP (N or J) 24-Lead Surface Mount (M)
01147525
*No Internal Connection
Top View
LM2575N-XX or LM2575HVN-XX
See NS Package Number N16A
LM1575J-XX-QML
See NS Package Number J16A
01147526
*No Internal Connection
Top View
LM2575M-XX or LM2575HVM-XX
See NS Package Number M24B
TO-263(S)
5-Lead Surface-Mount Package
01147529
Top View
01147530
Side View
LM2575S-XX or LM2575HVS-XX
See NS Package Number TS5B
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
LM1575/LM2575/LM2575HV
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Ordering Information (Continued)
Package NSC Standard High Temperature
Type Package Voltage Rating Voltage Rating Range
Number (40V) (60V)
16-Pin Molded N16A LM2575N-5.0 LM2575HVN-5.0 −40˚C ≤T
J
≤+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 ≤T
J
≤+150˚C
LM1575J-15-QML
LM1575J-ADJ-QML
LM1575/LM2575/LM2575HV
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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≤+V
IN
Output Voltage to Ground
(Steady State) −1V
Power Dissipation Internally Limited
Storage Temperature Range −65˚C to +150˚C
Maximum Junction Temperature 150˚C
Minimum ESD Rating
(C = 100 pF, R = 1.5 kΩ)2kV
Lead Temperature
(Soldering, 10 sec.) 260˚C
Operating Ratings
Temperature Range
LM1575 −55˚C ≤T
J
≤+150˚C
LM2575/LM2575HV −40˚C ≤T
J
≤+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 T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture 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
V
OUT
Output Voltage V
IN
= 12V, I
LOAD
= 0.2A 3.3 V
Circuit of Figure 2 3.267 3.234 V(Min)
3.333 3.366 V(Max)
V
OUT
Output Voltage 4.75V ≤V
IN
≤40V, 0.2A ≤I
LOAD
≤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)
V
OUT
Output Voltage 4.75V ≤V
IN
≤60V, 0.2A ≤I
LOAD
≤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 V
IN
= 12V, I
LOAD
=1A 75 %
LM1575-5.0, LM2575-5.0, LM2575HV-5.0
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture 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
V
OUT
Output Voltage V
IN
= 12V, I
LOAD
= 0.2A 5.0 V
Circuit of Figure 2 4.950 4.900 V(Min)
5.050 5.100 V(Max)
V
OUT
Output Voltage 0.2A ≤I
LOAD
≤1A, 5.0 V
LM1575/LM2575 8V ≤V
IN
≤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)
V
OUT
Output Voltage 0.2A ≤I
LOAD
≤1A, 5.0 V
LM2575HV 8V ≤V
IN
≤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)
LM1575/LM2575/LM2575HV
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LM1575-5.0, LM2575-5.0, LM2575HV-5.0
Electrical Characteristics (Continued)
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions Typ LM1575-5.0 LM2575-5.0 Units
(Limits)
LM2575HV-5.0
Limit Limit
(Note 2) (Note 3)
ηEfficiency V
IN
= 12V, I
LOAD
=1A 77 %
LM1575-12, LM2575-12, LM2575HV-12
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture 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
V
OUT
Output Voltage V
IN
= 25V, I
LOAD
= 0.2A 12 V
Circuit of Figure 2 11.88 11.76 V(Min)
12.12 12.24 V(Max)
V
OUT
Output Voltage 0.2A ≤I
LOAD
≤1A, 12 V
LM1575/LM2575 15V ≤V
IN
≤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)
V
OUT
Output Voltage 0.2A ≤I
LOAD
≤1A, 12 V
LM2575HV 15V ≤V
IN
≤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 V
IN
= 15V, I
LOAD
=1A 88 %
LM1575-15, LM2575-15, LM2575HV-15
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture 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
V
OUT
Output Voltage V
IN
= 30V, I
LOAD
= 0.2A 15 V
Circuit of Figure 2 14.85 14.70 V(Min)
15.15 15.30 V(Max)
V
OUT
Output Voltage 0.2A ≤I
LOAD
≤1A, 15 V
LM1575/LM2575 18V ≤V
IN
≤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)
V
OUT
Output Voltage 0.2A ≤I
LOAD
≤1A, 15 V
LM2575HV 18V ≤V
IN
≤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 V
IN
= 18V, I
LOAD
=1A 88 %
LM1575/LM2575/LM2575HV
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LM1575-ADJ, LM2575-ADJ, LM2575HV-ADJ
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Temperature
Range.
Symbol Parameter Conditions Typ LM1575-ADJ LM2575-ADJ Units
(Limits)
LM2575HV-ADJ
Limit Limit
(Note 2) (Note 3)
SYSTEM PARAMETERS (Note 4) Test Circuit Figure 2
V
OUT
Feedback Voltage V
IN
= 12V, I
LOAD
= 0.2A 1.230 V
V
OUT
= 5V 1.217 1.217 V(Min)
Circuit of Figure 2 1.243 1.243 V(Max)
V
OUT
Feedback Voltage 0.2A ≤I
LOAD
≤1A, 1.230 V
LM1575/LM2575 8V ≤V
IN
≤40V 1.205/1.193 1.193/1.180 V(Min)
V
OUT
= 5V, Circuit of Figure 2 1.255/1.267 1.267/1.280 V(Max)
V
OUT
Feedback Voltage 0.2A ≤I
LOAD
≤1A, 1.230 V
LM2575HV 8V ≤V
IN
≤60V 1.205/1.193 1.193/1.180 V(Min)
V
OUT
= 5V, Circuit of Figure 2 1.261/1.273 1.273/1.286 V(Max)
ηEfficiency V
IN
= 12V, I
LOAD
= 1A, V
OUT
=5V 77 %
All Output Voltage Versions
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range. Unless otherwise specified, V
IN
= 12V for the 3.3V, 5V, and Adjustable version, V
IN
= 25V for the 12V version,
and V
IN
= 30V for the 15V version. I
LOAD
= 200 mA.
Symbol Parameter Conditions Typ LM1575-XX LM2575-XX Units
(Limits)
LM2575HV-XX
Limit Limit
(Note 2) (Note 3)
DEVICE PARAMETERS
I
b
Feedback Bias Current V
OUT
= 5V (Adjustable Version Only) 50 100/500 100/500 nA
f
O
Oscillator Frequency (Note 13) 52 kHz
47/43 47/42 kHz(Min)
58/62 58/63 kHz(Max)
V
SAT
Saturation Voltage I
OUT
= 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)
I
CL
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)
I
L
Output Leakage (Notes 7, 8) Output = 0V 2 2 mA(Max)
Current Output = −1V 7.5 mA
Output = −1V 30 30 mA(Max)
I
Q
Quiescent Current (Note 7) 5 mA
10/12 10 mA(Max)
I
STBY
Standby Quiescent ON /OFF Pin = 5V (OFF) 50 µA
Current 200/500 200 µA(Max)
LM1575/LM2575/LM2575HV
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All Output Voltage Versions
Electrical Characteristics (Continued)
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range. Unless otherwise specified, V
IN
= 12V for the 3.3V, 5V, and Adjustable version, V
IN
= 25V for the 12V version,
and V
IN
= 30V for the 15V version. I
LOAD
= 200 mA.
Symbol Parameter Conditions Typ LM1575-XX LM2575-XX Units
(Limits)
LM2575HV-XX
Limit Limit
(Note 2) (Note 3)
DEVICE PARAMETERS
θ
JA
Thermal Resistance T Package, Junction to Ambient (Note 9) 65
θ
JA
T Package, Junction to Ambient (Note 10) 45 ˚C/W
θ
JC
T Package, Junction to Case 2
θ
JA
N Package, Junction to Ambient (Note 11) 85
θ
JA
M Package, Junction to Ambient (Note 11) 100
θ
JA
S Package, Junction to Ambient (Note 12) 37
ON /OFF CONTROL Test Circuit Figure 2
V
IH
ON /OFF Pin Logic V
OUT
= 0V 1.4 2.2/2.4 2.2/2.4 V(Min)
V
IL
Input Level V
OUT
= Nominal Output Voltage 1.2 1.0/0.8 1.0/0.8 V(Max)
I
IH
ON /OFF Pin Input ON /OFF Pin = 5V (OFF) 12 µA
Current 30 30 µA(Max)
I
IL
ON /OFF Pin = 0V (ON) 0µA
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
1
⁄ 2 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
1
⁄ 2 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.
LM1575/LM2575/LM2575HV
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Typical Performance Characteristics (Circuit of Figure 2)
Normalized Output Voltage Line Regulation
01147532 01147533
Dropout Voltage Current Limit
01147534 01147535
Quiescent Current
Standby
Quiescent Current
01147536 01147537
LM1575/LM2575/LM2575HV
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Typical Performance Characteristics (Circuit of Figure 2) (Continued)
Oscillator Frequency
Switch Saturation
Voltage
01147538 01147539
Efficiency Minimum Operating Voltage
01147540 01147541
Quiescent Current
vs Duty Cycle
Feedback Voltage
vs Duty Cycle
01147542 01147543
LM1575/LM2575/LM2575HV
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Typical Performance Characteristics (Circuit of Figure 2) (Continued)
Feedback Pin Current
Maximum Power Dissipation
(TO-263) (See (Note 12))
01147505 01147528
Switching Waveforms Load Transient Response
01147506
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
01147507
Test Circuit and Layout Guidelines
As in any switching regulator, layout is very important. Rap-
idly switching currents associated with wiring inductance
generate voltage transients which can cause problems. For
minimal inductance and ground loops, the length of the leads
indicated by heavy lines should be kept as short as possible.
Single-point grounding (as indicated) or ground plane con-
struction should be used for best results. When using the
Adjustable version, physically locate the programming resis-
tors near the regulator, to keep the sensitive feedback wiring
short.
LM1575/LM2575/LM2575HV
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Test Circuit and Layout Guidelines (Continued)
Fixed Output Voltage Versions
01147508
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
01147509
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.
LM1575/LM2575/LM2575HV
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LM2575 Series Buck Regulator Design Procedure
PROCEDURE (Fixed Output Voltage Versions) EXAMPLE (Fixed Output Voltage Versions)
Given: V
OUT
= Regulated Output Voltage (3.3V, 5V, 12V, or
15V) V
IN
(Max) = Maximum Input Voltage I
LOAD
(Max) =
Maximum Load Current
Given: V
OUT
=5VV
IN
(Max) = 20V I
LOAD
(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 adjustable
version. B. From the inductor value selection guide, identify
the inductance region intersected by V
IN
(Max) and
I
LOAD
(Max), and note the inductor code for that region. C.
Identify the inductor value from the inductor code, and
select an appropriate inductor from the table shown in
Figure 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 x I
LOAD
. For additional inductor
information, see the inductor section in the Application Hints
section of this data sheet.
1. Inductor Selection (L1) A. Use the selection guide
shown in Figure 4.B. From the selection guide, the
inductance area intersected by the 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 (C
OUT
)A.The value of the
output capacitor together with the inductor defines the
dominate pole-pair of the switching regulator loop. For
stable operation and an acceptable output ripple voltage,
(approximately 1% of the output voltage) a value between
100 µF and 470 µF is recommended. B. The capacitor’s
voltage rating should be at least 1.5 times greater than the
output voltage. For a 5V regulator, a rating of at least 8V is
appropriate, and a 10V or 15V rating is recommended.
Higher voltage electrolytic capacitors generally have lower
ESR numbers, and for this reason it may be necessary to
select a capacitor rated for a higher voltage than would
normally be needed.
2. Output Capacitor Selection (C
OUT
)A.C
OUT
= 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 (C
IN
)An aluminum or tantalum
electrolytic bypass capacitor located close to the regulator is
needed for stable operation.
4. Input Capacitor (C
IN
)A 47 µF, 25V aluminum electrolytic
capacitor located near the input and ground pins provides
sufficient bypassing.
LM1575/LM2575/LM2575HV
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Inductor Value Selection Guides (For Continuous Mode Operation)
01147510
FIGURE 3. LM2575(HV)-3.3
01147511
FIGURE 4. LM2575(HV)-5.0
01147512
FIGURE 5. LM2575(HV)-12
01147513
FIGURE 6. LM2575(HV)-15
01147514
FIGURE 7. LM2575(HV)-ADJ
LM1575/LM2575/LM2575HV
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Inductor Value Selection Guides (For Continuous Mode Operation) (Continued)
PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions)
Given: V
OUT
= Regulated Output Voltage V
IN
(Max) =
Maximum Input Voltage I
LOAD
(Max) = Maximum Load
Current F = Switching Frequency (Fixed at 52 kHz)
Given: V
OUT
= 10V V
IN
(Max) = 25V I
LOAD
(Max) = 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.
R
1
can be between 1k and 5k. (For best temperature coef-
ficient 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 Selection Guide shown in Figure 7.C. On the hori-
zontal axis, select the maximum load current. D. Identify the
inductance region intersected by the E •T value and the
maximum load current value, and note the inductor code for
that region. E. Identify the inductor value from the inductor
code, and select an appropriate inductor from the table shown
in Figure 9. Part numbers are listed for three inductor manu-
facturers. The inductor chosen must be rated for operation at
the LM2575 switching frequency (52 kHz) and for a current
rating of 1.15 x I
LOAD
. For additional inductor information, see
the inductor section in the application hints section of this data
sheet.
2. Inductor Selection (L1) A. Calculate E •T(V•µs)
B. E•T = 115 V •µs C. I
LOAD
(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 (C
OUT
)A.The value of the
output capacitor together with the inductor defines the
dominate pole-pair of the switching regulator loop. For
stable operation, the capacitor must satisfy the following
requirement:
The above formula yields capacitor values between 10 µF
and 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 re-
sponse, the output capacitor may need to be several times
larger than the above formula yields. B. The capacitor’s volt-
age 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 gen-
erally have lower ESR numbers, and for this reason it may be
necessary to select a capacitor rate for a higher voltage than
would normally be needed.
3. Output Capacitor Selection (C
OUT
)A.
However, for acceptable output ripple voltage select C
OUT
≥
220 µF C
OUT
= 220 µF electrolytic capacitor
4. Catch Diode Selection (D1) A. The catch-diode current
rating must be at least 1.2 times greater than the maximum
load current. Also, if the power supply design must
withstand a continuous output short, the diode should have
a current rating equal to the maximum current limit of the
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.
LM1575/LM2575/LM2575HV
www.national.com15

Inductor Value Selection Guides (For Continuous Mode Operation) (Continued)
PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions)
5. Input Capacitor (C
IN
)An aluminum or tantalum
electrolytic bypass capacitor located close to the regulator is
needed for stable operation.
5. Input Capacitor (C
IN
)A 100 µF aluminum electrolytic
capacitor located near the input and ground pins provides
sufficient bypassing.
To further simplify the buck regulator design procedure, 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 (3
1
⁄ 2 ")
diskette for IBM compatible computers from a National Semiconductor sales office in your area.
LM1575/LM2575/LM2575HV
www.national.com 16

Inductor Value Selection Guides (For Continuous Mode Operation) (Continued)
V
R
Schottky 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 µH 67127000 PE-92108 RL2444
L150 150 µH 67127010 PE-53113 RL1954
L220 220 µH 67127020 PE-52626 RL1953
L330 330 µH 67127030 PE-52627 RL1952
L470 470 µH 67127040 PE-53114 RL1951
L680 680 µH 67127050 PE-52629 RL1950
H150 150 µH 67127060 PE-53115 RL2445
H220 220 µH 67127070 PE-53116 RL2446
H330 330 µH 67127080 PE-53117 RL2447
H470 470 µH 67127090 PE-53118 RL1961
H680 680 µH 67127100 PE-53119 RL1960
H1000 1000 µH 67127110 PE-53120 RL1959
H1500 1500 µH 67127120 PE-53121 RL1958
H2200 2200 µH 67127130 PE-53122 RL2448
Note 15: Schott Corp., [phone], 1000 Parkers Lake Rd., Wayzata, MN 55391.
Note 16: Pulse Engineering, [phone], P.O. Box 12236, San Diego, CA 92112.
Note 17: Renco Electronics Inc., [phone], 60 Jeffryn Blvd. East, Deer Park, NY 11729.
FIGURE 9. Inductor Selection by Manufacturer’s Part Number
LM1575/LM2575/LM2575HV
www.national.com17

Application Hints
INPUT CAPACITOR (C
IN
)
To maintain stability, the regulator input pin must be by-
passed 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
below −25˚C, the input capacitor value may need to be
larger. With most electrolytic capacitors, the capacitance
value decreases and the ESR increases with lower tempera-
tures and age. Paralleling a ceramic or solid tantalum ca-
pacitor will increase the regulator stability at cold tempera-
tures. For maximum capacitor operating lifetime, the
capacitor’s RMS ripple current rating should be greater than
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 opera-
tion.
The inductor value selection guides in Figure 3 through
Figure 7 were designed for buck regulator designs of the
continuous inductor current type. When using inductor val-
ues shown in the inductor selection guide, the peak-to-peak
inductor ripple current will be approximately 20% to 30% of
the maximum DC current. With relatively heavy load cur-
rents, the circuit operates in the continuous mode (inductor
current always flowing), but under light load conditions, the
circuit will be forced to the discontinuous mode (inductor
current falls to zero for a period of time). This discontinuous
mode of operation is perfectly acceptable. For light loads
(less than approximately 200 mA) it may be desirable to
operate the regulator in the discontinuous mode, primarily
because of the lower inductor values required for the discon-
tinuous mode.
The selection guide chooses inductor values suitable for
continuous mode operation, but if the inductor value chosen
is prohibitively high, the designer should investigate the
possibility of discontinuous operation. The computer design
software Switchers Made Simple will provide all component
values for discontinuous (as well as continuous) mode of
operation.
Inductors are available in different styles such as pot core,
toriod, E-frame, bobbin core, etc., as well as different core
materials, such as ferrites and powdered iron. The least
expensive, the bobbin core type, consists of wire wrapped
on a ferrite rod core. This type of construction makes for an
inexpensive inductor, but since the magnetic flux is not com-
pletely contained within the core, it generates more electro-
magnetic interference (EMI). This EMI can cause problems
in sensitive circuits, or can give incorrect scope readings
because of induced voltages in the scope probe.
The inductors listed in the selection chart include ferrite pot
core construction for AIE, powdered iron toroid for Pulse
Engineering, and ferrite bobbin core for Renco.
An inductor should not be operated beyond its maximum
rated current because it may saturate. When an inductor
begins to saturate, the inductance decreases rapidly and the
inductor begins to look mainly resistive (the DC resistance of
the winding). This will cause the switch current to rise very
rapidly. Different inductor types have different saturation
characteristics, and this should be kept in mind when select-
ing an inductor.
The inductor manufacturer’s data sheets include current and
energy limits to avoid inductor saturation.
INDUCTOR RIPPLE CURRENT
When the switcher is operating in the continuous mode, the
inductor current waveform ranges from a triangular to a
sawtooth type of waveform (depending on the input voltage).
For a given input voltage and output voltage, the peak-to-
peak amplitude of this inductor current waveform remains
constant. As the load current rises or falls, the entire saw-
tooth current waveform also rises or falls. The average DC
value of this waveform is equal to the DC load current (in the
buck regulator configuration).
If the load current drops to a low enough level, the bottom of
the sawtooth current waveform will reach zero, and the
switcher will change to a discontinuous mode of operation.
This is a perfectly acceptable mode of operation. Any buck
switching regulator (no matter how large the inductor value
is) will be forced to run discontinuous if the load current is
light enough.
OUTPUT CAPACITOR
An output capacitor is required to filter the output voltage and
is needed for loop stability. The capacitor should be located
near the LM2575 using short pc board traces. Standard
aluminum electrolytics are usually adequate, but low ESR
types are recommended for low output ripple voltage and
good stability. The ESR of a capacitor depends on many
factors, some which are: the value, the voltage rating, physi-
cal size and the type of construction. In general, low value or
low voltage (less than 12V) electrolytic capacitors usually
have higher ESR numbers.
The amount of output ripple voltage is primarily a function of
the ESR (Equivalent Series Resistance) of the output ca-
pacitor and the amplitude of the inductor ripple current
(∆I
IND
). See the section on inductor ripple current in Applica-
tion Hints.
The lower capacitor values (220 µF–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 = (∆I
IND
) (ESR of C
OUT
)
To further reduce the output ripple voltage, several standard
electrolytic capacitors may be paralleled, or a higher-grade
capacitor may be used. Such capacitors are often called
“high-frequency,” “low-inductance,” or “low-ESR.” These will
reduce the output ripple to 10 mV or 20 mV. However, when
operating in the continuous mode, reducing the ESR below
0.05Ωcan cause instability in the regulator.
LM1575/LM2575/LM2575HV
www.national.com 18

Application Hints (Continued)
Tantalum capacitors can have a very low ESR, and should
be carefully evaluated if it is the only output capacitor. Be-
cause of their good low temperature characteristics, a tan-
talum can be used in parallel with aluminum electrolytics,
with the tantalum making up 10% or 20% of the total capaci-
tance.
The capacitor’s ripple current rating at 52 kHz should be at
least 50% higher than the peak-to-peak inductor ripple cur-
rent.
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 volt-
age drop, Schottky diodes provide the best efficiency, espe-
cially in low output voltage switching regulators (less than
5V). Fast-Recovery, High-Efficiency, or Ultra-Fast Recovery
diodes are also suitable, but some types with an abrupt
turn-off characteristic may cause instability and EMI prob-
lems. A fast-recovery diode with soft recovery characteristics
is a better choice. Standard 60 Hz diodes (e.g., 1N4001 or
1N5400, etc.) are also not suitable. See Figure 8 for Schot-
tky and “soft” fast-recovery diode selection guide.
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
The output voltage of a switching power supply will contain a
sawtooth ripple voltage at the switcher frequency, typically
about 1% of the output voltage, and may also contain short
voltage spikes at the peaks of the sawtooth waveform.
The output ripple voltage is due mainly to the inductor saw-
tooth ripple current multiplied by the ESR of the output
capacitor. (See the inductor selection in the application
hints.)
The voltage spikes are present because of the fast switching
action of the output switch, and the parasitic inductance of
the output filter capacitor. To minimize these voltage spikes,
special low inductance capacitors can be used, and their
lead lengths must be kept short. Wiring inductance, stray
capacitance, as well as the scope probe used to 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 x 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
supply. 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 kΩbecause of the increased chance of
noise pickup.
ON /OFF INPUT
For normal operation, the ON /OFF pin should be grounded
or driven with a low-level TTL voltage (typically below 1.6V).
To put the regulator into standby mode, drive this pin with a
high-level TTL or CMOS signal. The ON /OFF pin can be
safely pulled up to +V
IN
without a resistor in series with it.
The ON /OFF pin should not be left open.
GROUNDING
To maintain output voltage stability, the power ground con-
nections must be low-impedance (see Figure 2). For the
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 connection 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
inductance 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, conser-
vative design, a temperature approximately 15˚C cooler
than the maximum temperature should be selected.
4. LM2575 package thermal resistances θ
JA
and θ
JC
.
Total power dissipated by the LM2575 can be estimated as
follows:
P
D
=(V
IN
)(I
Q
)+(V
O
/V
IN
)(I
LOAD
)(V
SAT
)
where I
Q
(quiescent current) and V
SAT
can be found in the
Characteristic Curves shown previously, V
IN
is the applied
minimum input voltage, V
O
is the regulated output voltage,
and I
LOAD
is the load current. The dynamic losses during
turn-on and turn-off are negligible if a Schottky type catch
diode is used.
When no heat sink is used, the junction temperature rise can
be determined by the following:
∆T
J
=(P
D
)(θ
JA
)
To arrive at the actual operating junction temperature, add
the junction temperature rise to the maximum ambient tem-
perature.
T
J
=∆T
J
+T
A
If the actual operating junction temperature is greater than
the selected safe operating junction temperature determined
in step 3, then a heat sink is required.
When using a heat sink, the junction temperature rise can be
determined by the following:
∆T
J
=(P
D
)(θ
JC
+θ
interface
+θ
Heat sink
)
The operating junction temperature will be:
T
J
=T
A
+∆T
J
As above, if the actual operating junction temperature is
greater than the selected safe operating junction tempera-
ture, then a larger heat sink is required (one that has a lower
thermal resistance).
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.
LM1575/LM2575/LM2575HV
www.national.com19

Application Hints (Continued)
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
surrounding 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 resistance 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
voltage. This circuit bootstraps the regulator’s ground pin to
the negative output voltage, then by grounding the feedback
pin, the regulator senses the inverted output voltage and
regulates it to −12V.
For an input voltage of 12V or more, the maximum available
output current in this configuration is approximately 0.35A. At
lighter loads, the minimum input voltage required drops to
approximately 4.7V.
The switch currents in this buck-boost configuration are
higher 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 f
osc
= 52 kHz. Under normal continuous inductor
current operating conditions, the minimum V
IN
represents
the worst case. Select an inductor that is rated for the peak
current 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.
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
maximum 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.
01147515
FIGURE 10. Inverting Buck-Boost Develops −12V
LM1575/LM2575/LM2575HV
www.national.com 20