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An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
LM2574
,
LM2574HV
SNVS104E JUNE 1999REVISED JULY 2018
LM2574x SIMPLE SWITCHER® 0.5-A Step-Down Voltage Regulator
1
1 Features
1 3.3-V, 5-V, 12-V, 15-V, and Adjustable Output
Versions
Adjustable Version Output Voltage Range: 1.23 V
to 37 V (57 V for HV version) ±4% Maximum Over
Line and Load Conditions
Specified 0.5-A Output Current
Wide Input Voltage Range: 40 V, up to 60 V 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
Create a Custom Design Using the LM2574 With
the WEBENCH®Power Designer
2 Applications
Simple High-Efficiency Step-Down (Buck)
Regulator
Efficient Preregulator for Linear Regulators
On-Card Switching Regulators
Positive-to-Negative Converter (Buck-Boost)
3 Description
The LM2574xx series of regulators are monolithic
integrated circuits that provide all the active functions
for a step-down (buck) switching regulator, capable of
driving a 0.5-A load with excellent line and load
regulation. These devices are available in fixed output
voltages of 3.3 V, 5 V, 12 V, 15 V, 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 LM2574xx 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 (PCB) 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 specified ±4% tolerance on
output voltage within specified input voltages and
output load conditions, and ±10% on the oscillator
frequency. External shutdown 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
conditions.
Device Information(1)
PART NUMBER PACKAGE BODY SIZE (NOM)
LM2574, LM2574HV SOIC (14) 8.992 mm × 7.498 mm
PDIP (8) 6.35 mm × 9.81 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application (Fixed Output Voltage Versions)
2
LM2574
,
LM2574HV
SNVS104E JUNE 1999REVISED JULY 2018
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Table of Contents
1 Features.................................................................. 1
2 Applications ........................................................... 1
3 Description............................................................. 1
4 Revision History..................................................... 2
5 Pin Configuration and Functions......................... 3
6 Specifications......................................................... 4
6.1 Absolute Maximum Ratings ...................................... 4
6.2 ESD Ratings.............................................................. 4
6.3 Recommended Operating Conditions....................... 4
6.4 Thermal Information.................................................. 4
6.5 Electrical Characteristics for All Output Voltage
Versions..................................................................... 5
6.6 Electrical Characteristics 3.3-V Version................. 5
6.7 Electrical Characteristics 5-V Version.................... 6
6.8 Electrical Characteristics 12-V Version.................. 6
6.9 Electrical Characteristics 15-V Version.................. 6
6.10 Electrical Characteristics Adjustable Version....... 7
6.11 Typical Characteristics............................................ 8
7 Detailed Description............................................ 11
7.1 Overview................................................................. 11
7.2 Functional Block Diagram....................................... 11
7.3 Feature Description................................................. 11
7.4 Device Functional Modes........................................ 13
8 Application and Implementation ........................ 14
8.1 Application Information............................................ 14
8.2 Typical Applications ................................................ 19
9 Power Supply Recommendations...................... 26
10 Layout................................................................... 26
10.1 Layout Guidelines ................................................. 26
10.2 Layout Example .................................................... 27
10.3 Grounding ............................................................. 27
10.4 Thermal Considerations........................................ 27
11 Device and Documentation Support................. 29
11.1 Device Support...................................................... 29
11.2 Documentation Support ........................................ 31
11.3 Receiving Notification of Documentation Updates 31
11.4 Community Resources.......................................... 31
11.5 Trademarks........................................................... 31
11.6 Electrostatic Discharge Caution............................ 31
11.7 Glossary................................................................ 31
12 Mechanical, Packaging, and Orderable
Information........................................................... 31
4 Revision History
Changes from Revision D (April 2016) to Revision E Page
Added links for WEBENCH ................................................................................................................................................... 1
maximum supply voltage in Abs Max Ratings from "4.5" to "45" to correct typo................................................................... 4
Changes from Revision C (April 2013) to Revision D Page
Added Device Information table, ESD Ratings table, Feature Description section, Device Functional Modes,
Application and Implementation section, Power Supply Recommendations section, Layout section, Device and
Documentation Support section, and Mechanical, Packaging, and Orderable Information section ..................................... 1
Changed RθJA value in SOIC column to 77.1 ........................................................................................................................ 4
Split test conditions row of the Electrical Characteristics table to include TJ= 25°C and TJ< 25°C MIN, TYP, and
MAX values............................................................................................................................................................................. 5
Split test conditions in ILrow to rearrange the MIN, TYP, and MAX values ......................................................................... 5
Changes from Revision B (November 2004) to Revision C Page
Changed layout of National Data Sheet to TI format ............................................................................................................. 1
1NC 14 NC
2NC 13 NC
3FB 12 OUTPUT
4SIG_GND 11 NC
5ON/OFF 10 VIN
6PWR_GND 9 NC
7NC 8 NC
1FB 8 NC
2SIG_GND 7 OUTPUT
3ON/OFF 6 NC
4PWR_GND 5 VIN
3
LM2574
,
LM2574HV
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SNVS104E JUNE 1999REVISED JULY 2018
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5 Pin Configuration and Functions
P Package
8-Pin PDIP
Top View NPA Package
14-Pin SOIC
Top View
Pin Functions
PIN I/O DESCRIPTION
NAME PDIP SOIC
FB 1 3 I Feedback sense input pin. Connect to the midpoint of feedback divider to set
VOUT for ADJ version or connect this pin directly to the output capacitor for a
fixed output version.
NC 8, 6 1, 2, 7, 8, 9,
11, 13, 14 No internal connection, but must be soldered to PCB for best heat transfer.
ON/OFF 3 5 I Enable input to the voltage regulator. High = OFF and low = ON. Connect to
GND to enable the voltage regulator. Do not leave this pin float.
OUTPUT 7 12 O Emitter pin of the power transistor. This is a switching node. Attached this pin
to an inductor and the cathode of the external diode.
PWR_GND 4 6 Power ground pins. Connect to system ground and SIF GND, ground pins of
CIN and COUT. Path to CIN must be as short as possible.
SIG_GND 2 4 Signal ground pin. Ground reference for internal references and logic. Connect
to system ground.
VIN 5 10 I Supply input pin to collector pin of high-side transistor. Connect to power
supply and input bypass capacitors CIN. Path from VIN pin to high frequency
bypass CIN and PWR GND must be as short as possible.
4
LM2574
,
LM2574HV
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(1) Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted)(1)
MIN MAX UNIT
Maximum supply voltage LM2574 45 V
LM2574HV 63
ON/OFF pin input voltage –0.3 VIN V
Output voltage to ground, steady-state –1 V
Power dissipation Internally limited
Lead temperature, soldering (10 s) 260 °C
Maximum junction temperature 150 °C
Storage temperature, Tstg –65 150 °C
(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
6.2 ESD Ratings VALUE UNIT
V(ESD) Electrostatic discharge Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001(1) ±2000 V
6.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted) MIN NOM MAX UNIT
Supply voltage LM2574 40 V
LM2574HV 60
TJTemperature –40 125 °C
(1) For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report.
(2) The package thermal impedance is calculated in accordance with JESD 51-7.
(3) Thermal resistances were simulated on a 4-layer, JEDEC board.
6.4 Thermal Information
THERMAL METRIC(1)(2) LM2574, LM2574HV
UNITP (PDIP) NPA (SOIC)
8 PINS 14 PINS
RθJA Junction-to-ambient thermal resistance(3) 60.4 77.1 °C/W
RθJC(top) Junction-to-case (top) thermal resistance(3) 59.9 29.2 °C/W
RθJB Junction-to-board thermal resistance(3) 37.9 33.3 °C/W
ψJT Junction-to-top characterization parameter 17.1 2 °C/W
ψJB Junction-to-board characterization parameter 37.7 32.8 °C/W
RθJC(bot) Junction-to-case (bottom) thermal resistance(3) °C/W
5
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,
LM2574HV
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(1) All limits specified at room temperature TYP and MAX. All room temperature limits are 100% production tested. All limits at temperature
extremes are specified through correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate
Average Outgoing Quality Level.
(2) 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% (see Figure 6).
(3) Output pin sourcing current. No diode, inductor or capacitor connected to output pin.
(4) Feedback pin removed from output and connected to 0 V.
(5) Feedback pin removed from output and connected to 12 V for the adjustable, 3.3-V, and 5-V versions, and 25 V for the 12-V and 15-V
versions, to force the output transistor OFF.
(6) VIN = 40 V (60 V for high voltage version).
6.5 Electrical Characteristics for All Output Voltage Versions
TJ= 25°C, and MIN and MAX apply over full operating temperature range. VIN = 12 V for the 3.3-V, 5-V, and adjustable
version, VIN = 25 V for the 12-V version, and VIN = 30 V for the 15-V version, ILOAD = 100 mA (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN(1) TYP MAX(1) UNIT
IbFeedback bias current Adjustable version
only, VOUT = 5 V TJ= 25°C 50 100 nA
–40°C < TJ< 125°C 500
fOOscillator frequency See(2) TJ= 25°C 47 52 58 kHz
–40°C < TJ< 125°C 42 63
VSAT Saturation voltage IOUT = 0.5 A(3) TJ= 25°C 0.9 1.2 V
–40°C < TJ< 125°C 1.4
DC Maximum duty cycle (ON) See(4) 93% 98%
ICL Current limit Peak current(2)(3) 0.7 1 1.6 A
0.65 1.8
ILCurrent output leakage Output = 0 V 2 mA
Output = –1 V(5)(6) 7.5 30
IQQuiescent current See(5) 5 10 mA
ISTBY Standby quiescent current ON/OFF pin = 5 V (OFF) 50 200 μA
ON/OFF CONTROL (SEE Figure 27)
VIH ON/OFF pin logic input level VOUT = 0 V TJ= 25°C 2.2 1.4 V
–40°C < TJ< 125°C 2.4
VIL VOUT = Nominal output
voltage TJ= 25°C 1.2 1 V
–40°C < TJ< 125°C 0.8
IHON/OFF pin input current ON/OFF pin = 5 V (OFF) 12 30 μA
IIL ON/OFF pin = 0 V (ON) 0 10 μA
(1) Test Circuit in Figure 22 and Figure 27.
(2) All limits specified at room temperature and at temperature extremes . All room temperature limits are 100% production tested. All limits
at temperature extremes are specified via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to
calculate Average Outgoing Quality Level.
6.6 Electrical Characteristics 3.3-V Version
TJ= 25°C, and all MIN and MAX apply over full operating temperature range (unless otherwise noted).
PARAMETER(1) TEST CONDITIONS MIN(2) TYP MAX(2) UNIT
VOUT Output voltage
VIN = 12 V, ILOAD = 100 mA 3.234 3.3 3.366
V
LM2574, 4.75 V VIN 40 V,
0.1 A ILOAD 0.5 A TJ= 25°C 3.168 3.3 3.432
40°C TJ125°C 3.135 3.465
LM2574HV, 4.75 V VIN 60 V,
0.1 A ILOAD 0.5 A TJ= 25°C 3.168 3.3 3.45
40°C TJ125°C 3.135 3.482
ηEfficiency VIN = 12 V, ILOAD = 0.5 A 72%
6
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,
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(1) Test circuit in Figure 22 and Figure 27.
(2) All limits specified at room temperature TYP and MAX. All room temperature limits are 100% production tested. All limits at temperature
extremes are specified through correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate
Average Outgoing Quality Level.
6.7 Electrical Characteristics 5-V Version
TJ= 25°C, and all MIN and MAX apply over full operating temperature range (unless otherwise noted).
PARAMETER(1) TEST CONDITIONS MIN TYP(2) MAX(2) UNIT
VOUT Output voltage
VIN = 12 V, ILOAD = 100 mA 4.9 5 5.1
V
LM2574, 7 V VIN 40 V,
0.1 A ILOAD 0.5 A TJ= 25°C 4.8 5 5.2
40°C TJ125°C 4.75 5.25
LM2574HV, 7 V VIN 60 V,
0.1 A ILOAD 0.5 A TJ= 25°C 4.8 5 5.225
40°C TJ125°C 4.75 5.275
ηEfficiency VIN = 12 V, ILOAD = 0.5 A 77%
(1) Test circuit in Figure 22 and Figure 27.
(2) All limits specified at room temperature TYP and MAX. All room temperature limits are 100% production tested. All limits at temperature
extremes are specified through correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate
Average Outgoing Quality Level.
6.8 Electrical Characteristics 12-V Version
TJ= 25°C, and all MIN and MAX apply over full operating temperature range (unless otherwise noted).
PARAMETER(1) CONDITIONS MIN TYP(2) MAX(2) UNIT
VOUT Output voltage
VIN = 25 V, ILOAD = 100 mA 11.76 12 12.24
V
LM2574, 15 V VIN 40 V,
0.1 A ILOAD 0.5 A TJ= 25°C 11.52 12 12.48
40°C TJ125°C 11.4 12.6
LM2574HV, 15 V VIN 60 V,
0.1 A ILOAD 0.5 A TJ= 25°C 11.52 12 12.54
40°C TJ125°C 11.4 12.66
ηEfficiency VIN = 15 V, ILOAD = 0.5 A 88%
(1) Test circuit in Figure 22 and Figure 27.
(2) All limits specified at room temperature TYP and MAX. All room temperature limits are 100% production tested. All limits at temperature
extremes are specified through correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate
Average Outgoing Quality Level.
6.9 Electrical Characteristics 15-V Version
TJ= 25°C, and all MIN and MAX apply over full operating temperature range (unless otherwise noted).
PARAMETER(1) TEST CONDITIONS MIN TYP(2) MAX(2) UNIT
VOUT Output voltage
VIN = 30 V, ILOAD = 100 mA 14.7 15 15.3
V
LM2574, 18 V VIN 40 V,
0.1A ILOAD 0.5 A TJ= 25°C 14.4 15 15.6
40°C TJ125°C 14.25 15.75
LM2574HV, 18 V VIN 60 V,
0.1 A ILOAD 0.5 A TJ= 25°C 14.4 15 15.68
40°C TJ125°C 14.25 15.83
ηEfficiency VIN = 18 V, ILOAD = 0.5 A 88%
7
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(1) Test circuit in Figure 22 and Figure 27.
(2) All limits specified at room temperature TYP and MAX. All room temperature limits are 100% production tested. All limits at temperature
extremes are specified through correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate
Average Outgoing Quality Level.
6.10 Electrical Characteristics Adjustable Version
TJ= 25°C, and all MIN and MAX apply over full operating temperature range. VIN = 12 V, ILOAD = 100 mA (unless otherwise
noted).
PARAMETER(1) TEST CONDITIONS MIN TYP(2) MAX(2) UNIT
VFB Feedback voltage
VIN = 12 V, ILOAD = 100 mA 1.217 1.23 1.243
V
LM2574, 7 V VIN 40 V,
0.1 A ILOAD 0.5 A,
VOUT programmed for 5 V
TJ= 25°C 1.193 1.23 1.267
40°C TJ125°C 1.18 1.28
LM2574HV, 7 V VIN 60 V,
0.1 A ILOAD 0.5 A,
VOUT programmed for 5 V
TJ= 25°C 1.193 1.23 1.273
40°C TJ125°C 1.18 1.286
ηEfficiency VIN = 12 V, VOUT = 5 V, ILOAD = 0.5 A 77%
8
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,
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6.11 Typical Characteristics
See Figure 27.
Figure 1. Normalized Output Voltage Figure 2. Line Regulation
Figure 3. Dropout Voltage Figure 4. Current Limit
Figure 5. Supply Current Figure 6. Standby Quiescent Current
9
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Typical Characteristics (continued)
See Figure 27.
Figure 7. Oscillator Frequency Figure 8. Switch Saturation Voltage
Figure 9. Efficiency Figure 10. Minimum Operating Voltage
Figure 11. Supply Current vs Duty Cycle Figure 12. Feedback Voltage vs Duty Cycle
10
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,
LM2574HV
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Typical Characteristics (continued)
See Figure 27.
Figure 13. Feedback Pin Current Figure 14. Junction-to-Ambient Thermal Resistance
Unregulated
DC Input
Copyright © 2016, Texas Instruments Incorporated
CIN
+
2
Feedback 1
5
VIN Internal
Regulator ON / OFF
Signal
GND
+
±+
±
1.23 V
BAND ± GAP
REFRENCE Reset Thermal
Shutdown Current
Limit
52 kHz
OSCILLATOR
Compatator
Fixed Gain
Error Amp
R2
R1
ON / OFF
0.5 Amp
Switch
3
7
4
DRIVER
Pwr Gnd
Output +COUT
D1
L1
L
O
A
D
VOUT
11
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,
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7 Detailed Description
7.1 Overview
The LM2574 SIMPLE SWITCHER®regulator is an easy-to-use, non-synchronous, step-down DC-DC converter
with a wide input voltage range from 40 V to up to 60 V for a HV version. It is capable of delivering up to 0.5-A
DC load current with excellent line and load regulation. These devices are available in fixed output voltages of
3.3 V, 5 V, 12 V, 15 V, and an adjustable output version. The family requires few external components and the
pin arrangement was designed for simple, optimum PCB layout.
7.2 Functional Block Diagram
Note: Pin numbers are for the 8-pin PDIP package
R1 = 1 k
3.3 V, R2 = 1.7 k
5 V, R2 = 3.1 k
12 V, R2 = 8.84 k
15 V, R2 = 11.3 k
For adjustable version,
R1 = Open, R2 = 0 Ω
7.3 Feature Description
7.3.1 Current Limit
The LM2574 device has current limiting to prevent the switch current from exceeding safe values during an
accidental overload on the output. This value (ICL) can be found in Electrical Characteristics for All Output
Voltage Versions.
7.3.2 Undervoltage Lockout
In some applications, it is desirable to keep the regulator off until the input voltage reaches a certain threshold.
An undervoltage lockout circuit which accomplishes this task is shown in Figure 15 while Figure 16 shows the
same circuit applied to a buck-boost configuration. These circuits keep the regulator off until the input voltage
reaches a predetermined level in Equation 1.
VTH VZ1 + 2 VBE (1)
12
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Feature Description (continued)
Note: Complete circuit not shown (see Figure 20).
Note: Pin numbers are for 8-pin PDIP package.
Figure 15. Undervoltage Lockout for Buck Circuit
Note: Complete circuit not shown (see Figure 20).
Note: Pin numbers are for 8-pin PDIP package.
Figure 16. Undervoltage Lockout for Buck-Boost Circuit
7.3.3 Delayed Start-Up
The ON/OFF pin can be used to provide a delayed start-up feature as shown in Figure 17. With an input voltage
of 20 V and for the part values shown, the circuit provides approximately 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.
7.3.4 Adjustable Output, Low-Ripple Power Supply
A 500-mA power supply that features an adjustable output voltage is shown in Figure 18. An additional L-C filter
that reduces the output ripple by a factor of 10 or more is included in this circuit.
13
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Feature Description (continued)
Note: Complete circuit not shown.
Note: Pin numbers are for 8-pin PDIP package.
Figure 17. Delayed Start-Up
Note: Pin numbers are for 8-pin PDIP package.
Figure 18. 1.2-V to 55-V Adjustable 500-mA Power Supply With Low-Output Ripple
7.4 Device Functional Modes
7.4.1 Shutdown Mode
The ON/OFF pin provides electrical ON and OFF control for the LM2574. When the voltage of this pin is higher
than 1.4 V, the device is shutdown mode. The typical standby current in this mode is 50 μA.
7.4.2 Active Mode
When the voltage of the ON/OFF pin is below 1.2 V, the device starts switching and the output voltage rises until
it reaches a normal regulation voltage.
OUT
ON
OUT IN
V
t
T V V
=
+
ON
LOAD
t
1.2 I
T
´ ´
14
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8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Input Capacitor (CIN)
To maintain stability, the regulator input pin must be bypassed with at least a 22-μF electrolytic capacitor. The
leads of the capacitor 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
temperatures and age. Paralleling a ceramic or solid tantalum capacitor increases the regulator stability at cold
temperatures. For maximum capacitor operating lifetime, the RMS ripple current rating of the capacitor must be
greater than Equation 2.
where
for a buck regulator
for a buck-boost regulator (2)
8.1.2 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 performance and requirements.
The LM2574 (or any of the SIMPLE SWITCHER family) can be used for both continuous and discontinuous
modes of operation.
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 continuously, especially at low output load currents.
To simplify the inductor selection process, an inductor selection guide (nomograph) was designed. This guide
assumes continuous mode operation, and selects an inductor that allows a peak-to-peak inductor ripple current
(ΔIIND) to be a certain percentage of the maximum design load current. In the LM2574 SIMPLE SWITCHER, the
peak-to-peak inductor ripple current percentage (of load current) is allowed to change as different design load
currents are selected. By allowing the percentage of inductor ripple current to increase for lower current
applications, the inductor size and value can be kept relatively low.
8.1.3 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 sawtooth 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).
IND
I212 106 mA
2 2
D
= = =
IND
LOAD
I212
I 0.4 A 506 mA
2 2
D
æ ö æ ö
= + = + =
ç ÷ ç ÷
è ø
è ø
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Application Information (continued)
If the load current drops to a low enough level, the bottom of the sawtooth current waveform reaches zero, and
the switcher changes 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) is forced to run discontinuous if the load
current is light enough.
The curve shown in Figure 19 illustrates how the peak-to-peak inductor ripple current (ΔIIND) 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).
Figure 19. Inductor Ripple Current (ΔiIND) Range
Consider the following example:
VOUT =5Vat0.4A
VIN = 10-V minimum up to 20-V maximum
The selection guide in Figure 24 shows that for a 0.4-A load current, and an input voltage range between 10 V
and 20 V, the inductance region selected by the guide is 330 μH. This value of inductance allows a peak-to-peak
inductor ripple current (ΔIIND) to flow that is a percentage of the maximum load current. For this inductor value,
the ΔIIND also varies depending on the input voltage. As the input voltage increases to 20 V, it approaches the
upper border of the inductance region, and the inductor ripple current increases. Referring to the curve in
Figure 19, it can be seen that at the 0.4-A load current level, and operating near the upper border of the 330-μH
inductance region, the ΔIIND is 53% of 0.4 A, or 212 mAp-p.
This ΔIIND 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 capacitor, the output ripple voltage can be calculated, or conversely, measuring the output ripple
voltage and knowing the ΔIIND, the ESR can be calculated.
From the previous example, the peak-to-peak inductor ripple current (ΔIIND) = 212 mAp-p. When the ΔIND 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 in Equation 3.
(3)
2. Minimum load current before the circuit becomes discontinuous in Equation 4.
(4)
3. Output ripple voltage = (ΔIIND) × (ESR of COUT)
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Application Information (continued)
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.
Inductors are available in different styles such as pot core, toroid, E-frame, bobbin core, and so forth, 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 because
the magnetic flux is not completely 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 must 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 affects the energy storage
capabilities of the inductor and could cause inductor overheating. Different inductor types have different
saturation characteristics, and consider this when selecting an inductor. The inductor manufacturers' data sheets
include current and energy limits to avoid inductor saturation.
8.1.4 Output Capacitor
An output capacitor is required to filter the output voltage and is needed for loop stability. The capacitor must be
located near the LM2574 using short PCB 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, physical size, and the type of construction. In
general, low-value or low-voltage (less than 12 V) electrolytic capacitors usually have higher ESR numbers.
The amount of output ripple voltage is primarily a function of the equivalent series resistance (ESR ) of the output
capacitor and the amplitude of the inductor ripple current, ΔIIND (see Inductor Ripple Current (ΔiIND)).
The lower capacitor values (100 μF to 330 μF) allows typically 50 mV to 150 mV of output ripple voltage, while
larger-value capacitors reduce the ripple to approximately 20 mV to 50 mV (as seen in Equation 5).
Output Ripple Voltage = (ΔIIND) (ESR of COUT) (5)
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 reduce the output ripple to 10 mV or 20 mV. However, when operating in the continuous mode,
reducing the ESR below 0.03 Ωcan cause instability in the regulator.
Tantalum capacitors can have a very low ESR, and must be carefully evaluated if it is the only output capacitor.
Because of their good low temperature characteristics, a tantalum can be used in parallel with aluminum
electrolytics, with the tantalum making up 10% or 20% of the total capacitance.
The ripple current rating of the capacitor at 52 kHz must be at least 50% higher than the peak-to-peak inductor
ripple current.
8.1.5 Catch Diode
Buck regulators require a diode to provide a return path for the inductor current when the switch is off. This diode
must be located close to the LM2574 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 5 V). fast-recovery, high-efficiency, or ultra-fast
recovery diodes are also suitable, but some types with an abrupt turnoff characteristic may cause instability and
EMI problems. A fast-recovery diode with soft recovery characteristics is a better choice. Standard 60-Hz diodes
(for example, 1N4001 or 1N5400, and so forth) are also not suitable. See Table 1 for Schottky and soft fast-
recovery diode selection guide.
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Table 1. Diode Selection Guide
VR1-A DIODES
SCHOTTKY FAST RECOVERY
20 V 1N5817
SR102
MBR120P
30 V
1N5818
SR103
11DQ03
MBR130P
10JQ030
The following diodes are all rated to 100 V
11DF1
10JF1
MUR110
HER102
40 V
1N5819
SR104
11DQ04
11JQ04
MBR140P
50 V MBR150
SR105
11DQ05
11JQ05
60 V MBR160
SR106
11DQ06
11JQ06
90 V 11DQ09
8.1.6 Output Voltage Ripple and Transients
The output voltage of a switching power supply contains 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 sawtooth ripple current multiplied by the ESR of the output
capacitor (see Inductor Selection).
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 and 100 μF) can be added to the output (as shown in Figure 18) to further
reduce the amount of output ripple and transients. A 10 × reduction in output ripple voltage and transients is
possible with this filter.
8.1.7 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 kΩbecause of the
increased chance of noise pickup.
8.1.8 ON/OFF Input
For normal operation, the ON/OFF pin must be grounded or driven with a low-level TTL voltage (typically less
than 1.6 V). 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 must not be left
open.
( )
LOAD IN OUT IN OUT
P
IN IN OUT 1 OSC
I V V V V 1
I
V V V 2 L f
´ + ´
» + ´
+ ´ ´
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8.1.9 Additional Applications
8.1.9.1 Inverting Regulator
Figure 20 shows a LM2574-12 in a buck-boost configuration to generate a negative 12-V output from a positive
input voltage. This circuit bootstraps the ground pin of the regulator to the negative output voltage, then by
grounding the feedback pin, the regulator senses the inverted output voltage and regulates it to 12 V.
Note: Pin numbers are for the 8-pin PDIP package.
Figure 20. Inverting Buck-Boost Develops, 12 V
For an input voltage of 8 V or more, the maximum available output current in this configuration is approximately
100 mA. At lighter loads, the minimum input voltage required drops to approximately 4.7 V.
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
0.6 A. Using a delayed turnon or an undervoltage lockout circuit (described in Negative Boost Regulator) 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 design
procedure 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 Equation 6.
where
fosc = 52 kHz. Under normal continuous inductor current operating conditions,
the minimum VIN represents the worst case. Select an inductor that is rated for the peak current anticipated. (6)
Also, the maximum voltage appearing across the regulator is the absolute sum of the input and output voltage.
For a 12-V output, the maximum input voltage for the LM2574 is 28 V, or 48 V for the LM2574HV.
8.1.9.2 Negative Boost Regulator
Another variation on the buck-boost topology is the negative boost configuration. The circuit in Figure 21 accepts
an input voltage ranging from 5 V to 12 V and provides a regulated 12-V output. Input voltages greater than
12 V causes the output to rise greater than 12 V, but does not damage the regulator.
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Note: Pin numbers are