LM2574,LM2574HV
LM2574/LM2574HV SIMPLE SWITCHER 0.5A Step-Down Voltage Regulator
Literature Number: SNVS104B
LM2574/LM2574HV
SIMPLE SWITCHER0.5A Step-Down Voltage Regulator
General Description
The LM2574 series of regulators are monolithic integrated
circuits that provide all the active functions for a step-down
(buck) switching regulator, capable of driving a 0.5A load
with excellent line and load regulation. These devices are
available 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 LM2574 series offers a high-efficiency replacement for
popular three-terminal linear regulators. Because of its high
efficiency, the copper traces on the printed circuit board are
normally the only heat sinking needed.
A standard series of inductors optimized for use with the
LM2574 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 0.5A 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
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)
01139401
Note: Pin numbers are for 8-pin DIP package.
Patent Pending
SIMPLE SWITCHERis a trademark of National Semiconductor Corporation
July 1999
LM2574/LM2574HV SIMPLE SWITCHER 0.5A Step-Down Voltage Regulator
© 2004 National Semiconductor Corporation DS011394 www.national.com
Connection Diagrams
8-Lead DIP
14-Lead Wide
Surface Mount (WM)
01139402
* No internal connection, but should be soldered to PC board for best heat
transfer.
Top View
Order Number LM2574-3.3HVN, LM2574HVN-5.0,
LM2574HVN-12, LM2574HVN-15, LM2574HVN-ADJ,
LM2574N-3.3, LM2574N-5.0, LM2574N-12,
LM2574N-15 or LM2574N-ADJ
See NS Package Number N08A
01139403
Top View
Order Number LM2574HVM-3.3, LM2574HVM-5.0,
LM2574HVM-12, LM2574HVM-15, LM2574HVM-ADJ,
LM2574M-3.3 LM2574M-5.0, LM2574M-12,
LM2574M-15 or LM2574M-ADJ
See NS Package Number M14B
LM2574/LM2574HV
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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
LM2574 45V
LM2574HV 63V
ON /OFF Pin Input Voltage −0.3V V+V
IN
Output Voltage to Ground
(Steady State) −1V
Minimum ESD Rating
(C = 100 pF, R = 1.5 k)2kV
Storage Temperature Range −65˚C to +150˚C
Lead Temperature
(Soldering, 10 seconds) 260˚C
Maximum Junction Temperature 150˚C
Power Dissipation Internally Limited
Operating Ratings
Temperature Range
LM2574/LM2574HV −40˚C T
J
+125˚C
Supply Voltage
LM2574 40V
LM2574HV 60V
LM2574-3.3, LM2574HV-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 LM2574-3.3 Units
(Limits)
LM2574HV-3.3
Typ Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
V
OUT
Output Voltage V
IN
= 12V, I
LOAD
= 100 mA 3.3 V
3.234 V(Min)
3.366 V(Max)
V
OUT
Output Voltage 4.75V V
IN
40V, 0.1A I
LOAD
0.5A 3.3 V
LM2574 3.168/3.135 V(Min)
3.432/3.465 V(Max)
V
OUT
Output Voltage 4.75V V
IN
60V, 0.1A I
LOAD
0.5A 3.3
LM2574HV 3.168/3.135 V(Min)
3.450/3.482 V(Max)
ηEfficiency V
IN
= 12V, I
LOAD
= 0.5A 72 %
LM2574-5.0, LM2574HV-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 LM2574-5.0 Units
(Limits)
LM2574HV-5.0
Typ Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
V
OUT
Output Voltage V
IN
= 12V, I
LOAD
= 100 mA 5 V
4.900 V(Min)
5.100 V(Max)
V
OUT
Output Voltage 7V V
IN
40V, 0.1A I
LOAD
0.5A 5 V
LM2574 4.800/4.750 V(Min)
5.200/5.250 V(Max)
V
OUT
Output Voltage 7V V
IN
60V, 0.1A I
LOAD
0.5A 5
LM2574HV 4.800/4.750 V(Min)
5.225/5.275 V(Max)
LM2574/LM2574HV
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LM2574-5.0, LM2574HV-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 LM2574-5.0 Units
(Limits)
LM2574HV-5.0
Typ Limit
(Note 2)
ηEfficiency V
IN
= 12V, I
LOAD
= 0.5A 77 %
LM2574-12, LM2574HV-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 LM2574-12 Units
(Limits)
LM2574HV-12
Typ Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
V
OUT
Output Voltage V
IN
= 25V, I
LOAD
= 100 mA 12 V
11.76 V(Min)
12.24 V(Max)
V
OUT
Output Voltage 15V V
IN
40V, 0.1A I
LOAD
0.5A 12 V
LM2574 11.52/11.40 V(Min)
12.48/12.60 V(Max)
V
OUT
Output Voltage 15V V
IN
60V, 0.1A I
LOAD
0.5A 12
LM2574HV 11.52/11.40 V(Min)
12.54/12.66 V(Max)
ηEfficiency V
IN
= 15V, I
LOAD
= 0.5A 88 %
LM2574-15, LM2574HV-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 LM2574-15 Units
(Limits)
LM2574HV-15
Typ Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
V
OUT
Output Voltage V
IN
= 30V, I
LOAD
= 100 mA 15 V
14.70 V(Min)
15.30 V(Max)
V
OUT
Output Voltage 18V V
IN
40V, 0.1A I
LOAD
0.5A 15 V
LM2574 14.40/14.25 V(Min)
15.60/15.75 V(Max)
V
OUT
Output Voltage 18V V
IN
60V, 0.1A I
LOAD
0.5A 15
LM2574HV 14.40/14.25 V(Min)
15.68/15.83 V(Max)
ηEfficiency V
IN
= 18V, I
LOAD
= 0.5A 88 %
LM2574/LM2574HV
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LM2574-ADJ, LM2574HV-ADJ
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, I
LOAD
= 100 mA.
Symbol Parameter Conditions LM2574-ADJ Units
(Limits)
LM2574HV-ADJ
Typ Limit
(Note 2)
SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2
V
FB
Feedback Voltage V
IN
= 12V, I
LOAD
= 100 mA 1.230 V
1.217 V(Min)
1.243 V(Max)
V
FB
Feedback Voltage 7V V
IN
40V, 0.1A I
LOAD
0.5A 1.230 V
LM2574 V
OUT
Programmed for 5V. Circuit of Figure 2 1.193/1.180 V(Min)
1.267/1.280 V(Max)
V
FB
Feedback Voltage 7V V
IN
60V, 0.1A I
LOAD
0.5A 1.230
LM2574HV V
OUT
Programmed for 5V. Circuit of Figure 2 1.193/1.180 V(Min)
1.273/1.286 V(Max)
ηEfficiency V
IN
= 12V, V
OUT
= 5V, I
LOAD
= 0.5A 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
= 100 mA.
Symbol Parameter Conditions LM2574-XX Units
(Limits)
LM2574HV-XX
Typ Limit
(Note 2)
DEVICE PARAMETERS
I
b
Feedback Bias
Current
Adjustable Version Only, V
OUT
= 5V 50 100/500 nA
f
O
Oscillator Frequency (see Note 10) 52 kHz
47/42 kHz(Min)
58/63 kHz(Max)
V
SAT
Saturation Voltage I
OUT
= 0.5A (Note 4) 0.9 V
1.2/1.4 V(max)
DC Max Duty Cycle (ON) (Note 5) 98 %
93 %(Min)
I
CL
Current Limit Peak Current, (Notes 4, 10) 1.0 A
0.7/0.65 A(Min)
1.6/1.8 A(Max)
I
L
Output Leakage (Notes 6, 7) Output = 0V 2 mA(Max)
Current Output = −1V 7.5 mA
Output = −1V 30 mA(Max)
I
Q
Quiescent Current (Note 6) 5 mA
10 mA(Max)
I
STBY
Standby Quiescent ON /OFF Pin= 5V (OFF) 50 µA
Current 200 µA(Max)
θ
JA
Thermal Resistance N Package, Junction to Ambient (Note 8) 92
θ
JA
N Package, Junction to Ambient (Note 9) 72 ˚C/W
θ
JA
M Package, Junction to Ambient (Note 8) 102
LM2574/LM2574HV
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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
= 100 mA.
Symbol Parameter Conditions LM2574-XX Units
(Limits)
LM2574HV-XX
Typ Limit
(Note 2)
DEVICE PARAMETERS
θ
JA
M Package, Junction to Ambient (Note 9) 78
ON /OFF CONTROL Test Circuit Figure 2
V
IH
ON /OFF Pin Logic V
OUT
= 0V 1.4 2.2/2.4 V(Min)
V
IL
Input Level V
OUT
= Nominal Output Voltage 1.2 1.0/0.8 V(Max)
I
H
ON /OFF Pin Input ON /OFF Pin = 5V (OFF) 12 µA
Current 30 µA(Max)
I
IL
ON /OFF Pin = 0V (ON) A
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 room temperature limits are 100%
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to
calculate Average Outgoing Quality Level.
Note 3: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2574
is used as shown in the Figure 2 test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.
Note 4: Output pin sourcing current. No diode, inductor or capacitor connected to output pin.
Note 5: Feedback pin removed from output and connected to 0V.
Note 6: Feedback pin 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 7: VIN = 40V (60V for high voltage version).
Note 8: Junction to ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads. Additional copper area will
lower thermal resistance further. See application hints in this data sheet and the thermal model in Switchers Made Simple software.
Note 9: Junction to ambient thermal resistance with approximately 4 square inches of 1 oz. (0.0014 in. thick) printed circuit board copper surrounding the leads.
Additional copper area will lower thermal resistance further. (See Note 8.)
Note 10: 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%.
Typical Performance Characteristics
(Circuit of Figure 2)
Normalized Output Voltage Line Regulation
01139427 01139428
LM2574/LM2574HV
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Typical Performance Characteristics (Circuit of Figure 2) (Continued)
Dropout Voltage Current Limit
01139429 01139430
Supply Current
Standby
Quiescent Current
01139431 01139432
Oscillator Frequency
Switch Saturation
Voltage
01139433 01139434
LM2574/LM2574HV
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Typical Performance Characteristics (Circuit of Figure 2) (Continued)
Efficiency Minimum Operating Voltage
01139435 01139436
Supply Current
vs Duty Cycle
Feedback Voltage
vs Duty Cycle
01139437 01139438
Feedback
Pin Current
Junction to Ambient
Thermal Resistance
01139439 01139440
LM2574/LM2574HV
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Typical Performance Characteristics (Circuit of Figure 2) (Continued)
Continuous Mode Switching Waveforms
V
OUT
= 5V, 500 mA Load Current, L = 330 µH
Discontinuous Mode Switching Waveforms
V
OUT
= 5V, 100 mA Load Current, L = 100 µH
01139406
Notes:
A: Output Pin Voltage, 10V/div
B: Inductor Current, 0.2 A/div
C: Output Ripple Voltage, 20 mV/div,
AC-Coupled
Horizontal Time Base: 5 µs/div
01139407
Notes:
A: Output Pin Voltage, 10V/div
B: Inductor Current, 0.2 A/div
C: Output Ripple Voltage, 20 mV/div,
AC-Coupled
Horizontal Time Base: 5 µs/div
500 mA Load Transient Response for Continuous
Mode Operation. L = 330 µH, C
OUT
= 300 µF
250 mA Load Transient Response for Discontinuous
Mode Operation.L=6H,C
OUT
= 470 µF
01139408
Notes:
A: Output Voltage, 50 mV/div.
AC Coupled
B: 100 mA to 500 mA Load Pulse
Horizontal Time Base: 200 µs/div
01139409
Notes:
A: Output Voltage, 50 mV/div.
AC Coupled
B: 50 mA to 250 mA Load Pulse
Horizontal Time Base: 200 µs/div
LM2574/LM2574HV
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Block Diagram
01139410
R1=1k
3.3V, R2 = 1.7k
5V, R2 = 3.1k
12V, R2 = 8.84k
15V, R2 = 11.3k
For Adj. Version
R1 = Open, R2 = 0
Note: Pin numbers are for the 8-pin DIP package.
FIGURE 1.
LM2574/LM2574HV
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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 construction should be used for best results. When
using the Adjustable version, physically locate the program-
ming resistors near the regulator, to keep the sensitive feed-
back wiring short.
Fixed Output Voltage Versions
01139411
CIN 22 µF, 75V
Aluminum Electrolytic
COUT 220 µF, 25V
Aluminum Electrolytic
D1 Schottky, 11DQ06
L1 330 µH, 52627
(for 5V in, 3.3V out, use
100 µH, RL-1284-100)
R1 2k, 0.1%
R2 6.12k, 0.1%
Adjustable Output Voltage Version
01139412
FIGURE 2.
LM2574/LM2574HV
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Test Circuit and Layout Guidelines
(Continued) U.S. Source
Note 1: Pulse Engineering, (619) 674-8100
P.O. Box 12236, San Diego, CA 92112
Note 2: Renco Electronics Inc., (516) 586-5566
60 Jeffryn Blvd. East, Deer Park, NY 11729
*Contact Manufacturer
European Source
Note 3: NPI/APC +44 (0) 634 290588
47 Riverside, Medway City Estate
Strood, Rochester, Kent ME2 4DP. UK
*Contact Manufacturer
Inductor Pulse Eng. Renco NPI
Value (Note 1) (Note 2) (Note 3)
68 µH * RL-1284-68-43 NP5915
100 µH * RL-1284-100-43 NP5916
150 µH 52625 RL-1284-150-43 NP5917
220 µH 52626 RL-1284-220-43 NP5918/5919
330 µH 52627 RL-1284-330-43 NP5920/5921
470 µH 52628 RL-1284-470-43 NP5922
680 µH 52629 RL-1283-680-43 NP5923
1000 µH 52631 RL-1283-1000-43 *
1500 µH * RL-1283-1500-43 *
2200 µH * RL-1283-2200-43 *
FIGURE 3. Inductor Selection by
Manufacturers Part Number
LM2574/LM2574HV
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LM2574 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) = 15V I
LOAD
(Max) = 0.4A
1. Inductor Selection (L1)
A. Select the correct Inductor value selection guide from
Figures 4, 5, 6,orFigure 7. (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).
C. Select an appropriate inductor from the table shown in
Figure 3. Part numbers are listed for three inductor
manufacturers. The inductor chosen must be rated for
operation at the LM2574 switching frequency (52 kHz) and
for a current rating of 1.5 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 5.
B. From the selection guide, the inductance area intersected
by the 15V line and 0.4A line is 330.
C. Inductor value required is 330 µH. From the table in
Figure 3, choose Pulse Engineering PE-52627, Renco
RL-1284-330, or NPI NP5920/5921.
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 1.5 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 LM2574. 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 20V 1N5817 or SR102 Schottky diode, or any of
the suggested fast-recovery diodes shown in Figure 9.
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 22 µF aluminum electrolytic capacitor located near the
input and ground pins provides sufficient bypassing.
LM2574/LM2574HV
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LM2574 Series Buck Regulator
Design Procedure (Continued)
INDUCTOR VALUE SELECTION GUIDES (For
Continuous Mode Operation)
01139426
FIGURE 4. LM2574HV-3.3 Inductor Selection Guide
01139413
FIGURE 5. LM2574HV-5.0 Inductor Selection Guide
01139414
FIGURE 6. LM2574HV-12 Inductor Selection Guide
01139415
FIGURE 7. LM2574HV-15 Inductor Selection Guide
LM2574/LM2574HV
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LM2574 Series Buck Regulator Design Procedure (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
= 24V V
IN
(Max) = 40V I
LOAD
(Max) = 0.4A 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 coeffi-
cient and stability with time, use 1% metal film resistors)
1. Programming Output Voltage (Selecting R1 and R2)
R
2
= 1k (19.51−1) = 18.51k, closest 1% value is 18.7k
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 8.
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
value for that region.
E. Select an appropriate inductor from the table shown in
Figure 3. Part numbers are listed for three inductor manufac-
turers. The inductor chosen must be rated for operation at the
LM2574 switching frequency (52 kHz) and for a current rating
of 1.5 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(Vµs)
B. ET = 185 V µs
C. I
LOAD
(Max) = 0.4A
D. Inductance Region = 1000
E. Inductor Value = 1000 µH Choose fromPulse Engineering
Part #PE-52631, or Renco Part #RL-1283-1000.
01139416
FIGURE 8. LM2574HV-ADJ Inductor Selection Guide
LM2574/LM2574HV
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LM2574 Series Buck Regulator Design Procedure (Continued)
PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions)
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 5 µF and
1000 µF that will satisfy the loop requirements for stable op-
eration. 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 voltage rating should be at last 1.5 times
greater than the output voltage. For a 24V regulator, a rating of
at least 35V is recommended. Higher voltage electrolytic ca-
pacitors generally have lower ESR numbers, and for this rea-
sion it may be necessary to select a capacitor rate for a higher
voltage than would normally be needed.
3. Output Capacitor Selection (C
OUT
)
However, for acceptable output ripple voltage select C
OUT
100 µF C
OUT
= 100 µF electrolytic capacitor
4. Catch Diode Selection (D1)
A. The catch-diode current rating must be at least 1.5 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 LM2574. The most stressful condition for
this diode is an overload or shorted output condition. Suitable
diodes are shown in the selection guide of Figure 9.
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 1A current rating is adequate.
B. Use a 50V MBR150 or 11DQ05 Schottky diode, or any of
the suggested fast-recovery diodes in Figure 9.
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 22 µF aluminum electrolytic capacitor located near the
input and ground pins provides sufficient bypassing. See
(Figure 9).
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.
LM2574/LM2574HV
www.national.com 16
LM2574 Series Buck Regulator Design Procedure (Continued)
Application Hints
INPUT CAPACITOR (C
IN
)
To maintain stability, the regulator input pin must be by-
passed with at least a 22 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 LM2574 (or any of the Simple Switcher family) can be
used for both continuous and discontinuous modes of opera-
tion.
In many cases the preferred mode of operation is in the
continuous mode. It offers better load regulation, lower peak
switch, inductor and diode currents, and can have lower
output ripple voltage. But it does require relatively large
inductor values to keep the inductor current flowing continu-
ously, especially at low output load currents.
To simplify the inductor selection process, an inductor selec-
tion guide (nomograph) was designed (see Figure 4 through
Figure 8). This guide assumes continuous mode operation,
and selects an inductor that will allow a peak-to-peak induc-
tor ripple current (I
IND
) to be a certain percentage of the
maximum design load current. In the LM2574 SIMPLE
SWITCHER, the peak-to-peak inductor ripple current per-
centage (of load current) is allowed to change as different
design load currents are selected. By allowing the percent-
age of inductor ripple current to increase for lower current
applications, the inductor size and value can be kept rela-
tively low.
V
R
1 Amp Diodes
Schottky Fast Recovery
20V 1N5817
SR102
MBR120P
30V 1N5818
SR103
11DQ03 The
MBR130P following
10JQ030 diodes
40V 1N5819 are all
SR104 rated to
11DQ04 100V
11JQ04
MBR140P
50V MBR150 11DF1
SR105 10JF1
11DQ05 MUR110
11JQ05 HER102
60V MBR160
SR106
11DQ06
11JQ06
90V 11DQ09
FIGURE 9. Diode Selection Guide
LM2574/LM2574HV
www.national.com17
Application Hints (Continued)
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.
The curve shown in Figure 10 illustrates how the peak-to-
peak inductor ripple current (I
IND
) is allowed to change as
different maximum load currents are selected, and also how
it changes as the operating point varies from the upper
border to the lower border within an inductance region (see
Inductor Selection guides).
Consider the following example:
V
OUT
=5V@0.4A
V
IN
= 10V minimum up to 20V maximum
The selection guide in Figure 5 shows that for a 0.4A load
current, and an input voltage range between 10V and 20V,
the inductance region selected by the guide is 330 µH. This
value of inductance will allow a peak-to-peak inductor ripple
current (I
IND
) to flow that will be a percentage of the maxi-
mum load current. For this inductor value, the I
IND
will also
vary depending on the input voltage. As the input voltage
increases to 20V, it approaches the upper border of the
inductance region, and the inductor ripple current increases.
Referring to the curve in Figure 10, it can be seen that at the
0.4A load current level, and operating near the upper border
of the 330 µH inductance region, the I
IND
will be 53% of
0.4A, or 212 mA p-p.
This I
IND
is important because from this number the peak
inductor current rating can be determined, the minimum load
current required before the circuit goes to discontinuous
operation, and also, knowing the ESR of the output capaci-
tor, the output ripple voltage can be calculated, or con-
versely, measuring the output ripple voltage and knowing the
I
IND
, the ESR can be calculated.
From the previous example, the Peak-to-peak Inductor
Ripple Current (I
IND
) = 212 mA p-p. Once the
IND
value is
known, the following three formulas can be used to calculate
additional information about the switching regulator circuit:
1. Peak Inductor or peak switch current
2. Minimum load current before the circuit becomes dis-
continuous
3. Output Ripple Voltage = (I
IND
) x (ESR of C
OUT
)
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,
toroid, 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 EMl 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 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 can cause the inductor current to rise very
rapidly and will affect the energy storage capabilities of the
inductor and could cause inductor overheating. Different
inductor types have different saturation characteristics, and
this should be kept in mind when selecting an inductor. The
inductor manufacturers’ data sheets include current and en-
ergy limits to avoid inductor saturation.
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 LM2574 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.
01139418
FIGURE 10. Inductor Ripple Current (I
IND
) Range
Based on Selection Guides from Figure 4 through
Figure 8.
LM2574/LM2574HV
www.national.com 18
Application Hints (Continued)
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 (100 µF- 330 µF) will allow
typically 50 mV to 150 mV of output ripple voltage, while
larger-value capacitors will reduce the ripple to approxi-
mately 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
0.03can 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. 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 LM2574 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 9 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 the fast
switching action of the output switch, and the parasitic induc-
tance 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 evalu-
ate 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 16 ) 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 LM2574 (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 LM2574
to avoid picking up unwanted noise. Avoid using resistors
greater than 100 kbecause 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
The 8-pin molded DIP and the 14-pin surface mount pack-
age have separate power and signal ground pins. Both
ground pins should be soldered directly to wide printed
circuit board copper traces to assure low inductance connec-
tions and good thermal properties.
THERMAL CONSIDERATIONS
The 8-pin DIP (N) package and the 14-pin Surface Mount
(M) package are molded plastic packages with solid copper
lead frames. The copper lead frame conducts the majority of
the heat from the die, through the leads, to the printed circuit
board copper, which acts as the heat sink. For best thermal
performance, wide copper traces should be used, and all
ground and unused 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 (lower thermal resistance) to the surrounding air, and
even double-sided or multilayer boards provide better heat
paths to the surrounding air. Unless the power levels are
small, using a socket for the 8-pin package is not recom-
mended because of the additional thermal resistance it in-
troduces, and the resultant higher junction temperature.
Because of the 0.5A current rating of the LM2574, the total
package power dissipation for this switcher is quite low,
ranging from approximately 0.1W up to 0.75W under varying
conditions. In a carefully engineered printed circuit board,
both the N and the M package can easily dissipate up to
0.75W, even at ambient temperatures of 60˚C, and still keep
the maximum junction temperature below 125˚C.
A curve displaying thermal resistance vs. pc board area for
the two packages is shown in the Typical Performance Char-
acteristics curves section of this data sheet.
These thermal resistance numbers are approximate, and
there can be many factors that will affect the final thermal
resistance. Some of these factors include board size, shape,
thickness, position, location, and board temperature. Other
factors are, the area of printed circuit copper, copper thick-
ness, trace width, multi-layer, single- or double-sided, and
the amount of solder on the board. The effectiveness of the
pc board to dissipate