ON LM2574 0.5 a, adjustable output voltage, step-down switching regulator Datasheet

LM2574
0.5 A, Adjustable Output
Voltage, Step-Down
Switching Regulator
The LM2574 series of regulators are monolithic integrated circuits
ideally suited for easy and convenient design of a step–down
switching regulator (buck converter). All circuits of this series are
capable of driving a 0.5 A load with excellent line and load regulation.
These devices are available in fixed output voltages of 3.3 V, 5.0 V,
12 V, 15 V, and an adjustable output version.
These regulators were designed to minimize the number of external
components to simplify the power supply design. Standard series of
inductors optimized for use with the LM2574 are offered by several
different inductor manufacturers.
Since the LM2574 converter is a switch–mode power supply, its
efficiency is significantly higher in comparison with popular
three–terminal linear regulators, especially with higher input voltages.
In most cases, the power dissipated by the LM2574 regulator is so low,
that the copper traces on the printed circuit board are normally the only
heatsink needed and no additional heatsinking is required.
The LM2574 features include a guaranteed ±4% tolerance on output
voltage within specified input voltages and output load conditions, and
±10% on the oscillator frequency (±2% over 0°C to +125°C). External
shutdown is included, featuring 60 µA (typical) standby current. The
output switch includes cycle–by–cycle current limiting, as well as
thermal shutdown for full protection under fault conditions.
Features
• 3.3 V, 5.0 V, 12 V, 15 V, and Adjustable Output Versions
• Adjustable Version Output Voltage Range, 1.23 to 37 V ±4% max
•
•
•
•
•
•
•
•
over Line and Load Conditions
Guaranteed 0.5 A Output Current
Wide Input Voltage Range: 4.75 to 40 V
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
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SO–16W
DW SUFFIX
CASE 751G
16
1
PDIP–8
N SUFFIX
CASE 626
8
1
PIN CONNECTIONS
*
*
FB
Sig Gnd
ON/OFF
Pwr Gnd
*
*
1
16
2
15
3
14
4
13
5
12
6
11
7
10
8
9
*
*
Output
*
Vin
*
*
*
(Top View)
FB
Sig Gnd
ON/OFF
Pwr Gnd
1
8
2
7
3
6
4
5
*
Output
*
Vin
(Top View)
* No internal connection, but should be soldered to
* PC board for best heat transfer.
Applications
•
•
•
•
•
•
Simple and High–Efficiency Step–Down (Buck) Regulators
Efficient Pre–regulator for Linear Regulators
On–Card Switching Regulators
Positive to Negative Converters (Buck–Boost)
Negative Step–Up Converters
Power Supply for Battery Chargers
 Semiconductor Components Industries, LLC, 2002
February, 2002 – Rev. 5
1
ORDERING INFORMATION
See detailed ordering and shipping information in the package
dimensions section on page 24 of this data sheet.
DEVICE MARKING INFORMATION
See general marking information in the device marking
section on page 24 of this data sheet.
Publication Order Number:
LM2574/D
LM2574
Typical Application (Fixed Output Voltage Versions)
7.0 - 40 V
Unregulated
DC Input
(3)
+Vin
LM2574
5
(12)
Cin
22 µF
(14)
2
Sig
Gnd
(4)
4
L1
330 µH
1
Output
D1
1N5819
7
ON/OFF
Pwr 3
Gnd
(6)
Feedback
Cout
220 µF
5.0 V Regulated
Output 0.5 A Load
(5)
Representative Block Diagram and Typical Application
+Vin
Unregulated
DC Input
Cin
5
(12)
1
(3)
3.1 V Internal
Regulator
Feedback
(4)
R2
(Ω)
3.3 V
5.0 V
12 V
15 V
1.7 k
3.1 k
8.84 k
11.3 k
For adjustable version
R1 = open, R2 = 0 Ω
Latch
Freq
Shift
18 kHz
1.235 V
Band-Gap
Reference
2
Output
Voltage Versions
Driver
R1
1.0 k
Sig Gnd
3
(5)
Current
Limit
Fixed Gain
Error Amplifier Comparator
R2
ON/OFF
ON/OFF
52 kHz
Oscillator
Reset
L1
Output
Thermal
Shutdown
1.0 Amp
Switch
7 (14)
Pwr Gnd
D1
Vout
Cout
4
Load
(6)
NOTE: Pin numbers in ( ) are for the SO–16W package.
Figure 1. Block Diagram and Typical Application
ABSOLUTE MAXIMUM RATINGS (Absolute Maximum Ratings indicate limits beyond which damage to the device may occur).
Symbol
Value
Unit
Maximum Supply Voltage
Vin
45
V
ON/OFF Pin Input Voltage
–
–0.3 V ≤ V ≤ +Vin
V
Output Voltage to Ground (Steady State)
–
–1.0
V
DW Suffix, Plastic Package Case 751G
Max Power Dissipation
Thermal Resistance, Junction–to–Air
PD
RθJA
Internally Limited
145
W
°C/W
N Suffix, Plastic Package Case 626
Max Power Dissipation
Thermal Resistance, Junction–to–Ambient
Thermal Resistance, Junction–to–Case
PD
RθJA
RθJC
Internally Limited
100
5.0
W
°C/W
°C/W
Storage Temperature Range
Tstg
–65°C to +150°C
°C
–
2.0
kV
Lead Temperature (Soldering, 10 seconds)
–
260
°C
Maximum Junction Temperature
TJ
150
°C
Rating
Minimum ESD Rating
(Human Body Model: C = 100 pF, R = 1.5 kΩ)
NOTE: ESD data available upon request.
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2
LM2574
OPERATING RATINGS (Operating Ratings indicate conditions for which the device is intended to be functional, but do not guarantee
specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics).
Rating
Symbol
Value
Unit
Operating Junction Temperature Range
TJ
–40 to +125
°C
Supply Voltage
Vin
40
V
SYSTEM PARAMETERS ([Note 1] Test Circuit Figure 16)
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and Adjustable
version, Vin = 25 V for the 12 V version, Vin = 30 V for the 15 V version. ILoad = 100 mA. For typical values TJ = 25°C, for min/max values
TJ is the operating junction temperature range that applies [Note 2], unless otherwise noted).
Characteristic
Symbol
Min
Typ
Max
Unit
Output Voltage (Vin = 12 V, ILoad = 100 mA, TJ = 25°C)
Vout
3.234
3.3
3.366
V
Output Voltage (4.75 V ≤ Vin ≤ 40 V, 0.1 A ≤ ILoad ≤ 0.5 A)
TJ = 25°C
TJ = –40 to +125°C
Vout
3.168
3.135
3.3
–
3.432
3.465
η
–
72
–
%
Output Voltage (Vin = 12 V, ILoad = 100 mA, TJ = 25°C)
Vout
4.9
5.0
5.1
V
Output Voltage (7.0 V ≤ Vin ≤ 40 V, 0.1 A ≤ ILoad ≤ 0.5 A)
TJ = 25°C
TJ = –40 to +125°C
Vout
4.8
4.75
5.0
5.2
5.25
η
–
77
–
%
Output Voltage (Vin = 25 V, ILoad = 100 mA, TJ = 25°C)
Vout
11.76
10
12.24
V
Output Voltage (15 V ≤ Vin ≤ 40 V, 0.1 A ≤ ILoad ≤ 0.5 A)
TJ = 25°C
TJ = –40 to +125°C
Vout
11.52
11.4
12
–
12.48
12.6
η
–
88
–
%
Output Voltage (Vin = 30 V, ILoad = 100 mA, TJ = 25°C)
Vout
14.7
15
15.3
V
Output Voltage (18 V < Vin < 40 V, 0.1 A < ILoad < 0.5 A)
TJ = 25°C
TJ = –40 to +125°C
Vout
14.4
14.25
15
15.6
15.75
η
–
88
–
%
Feedback Voltage Vin = 12 V, ILoad = 100 mA, Vout = 5.0 V, TJ = 25°C
VFB
1.217
1.23
1.243
V
Feedback Voltage 7.0 V ≤ Vin ≤ 40 V, 0.1 A ≤ ILoad ≤ 0.5 A, Vout = 5.0
V
TJ = 25°C
TJ = –40 to +125°C
VFBT
LM2574–3.3 ([Note 1] Test Circuit Figure 16)
Efficiency (Vin = 12 V, ILoad = 0.5 A)
V
LM2574–5 ([Note 1] Test Circuit Figure 16)
Efficiency (Vin = 12 V, ILoad = 0.5 A)
V
LM2574–12 ([Note 1] Test Circuit Figure 16)
Efficiency (Vin = 15 V, ILoad = 0.5 A)
V
LM2574–15 ([Note 1] Test Circuit Figure 16)
Efficiency (Vin = 18 V, ILoad = 0.5 A)
V
LM2574 ADJUSTABLE VERSION ([Note 1] Test Circuit Figure 16)
η
Efficiency (Vin = 12 V, ILoad = 0.5 A, Vout = 5.0 V)
V
1.193
1.18
1.23
1.267
1.28
–
77
–
%
1. External components such as the catch diode, inductor, input and output capacitors can affect the switching regulator system performance.
When the LM2574 is used as shown in the Figure 16 test circuit, the system performance will be as shown in the system parameters section
of the Electrical Characteristics.
2. Tested junction temperature range for the LM2574: Tlow = –40°C Thigh = +125°C.
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3
LM2574
SYSTEM PARAMETERS ([Note 3] Test Circuit Figure 16)
ELECTRICAL CHARACTERISTICS (continued) (Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and
Adjustable version, Vin = 25 V for the 12 V version, Vin = 30 V for the 15 V version. ILoad = 100 mA. For typical values TJ = 25°C, for
min/max values TJ is the operating junction temperature range that applies [Note 4], unless otherwise noted).
Characteristic
Symbol
Min
Typ
Max
–
–
25
–
100
200
–
47
42
52
52
–
–
58
63
–
–
1.0
–
1.2
1.4
93
98
–
0.7
0.65
1.0
–
1.6
1.8
–
–
0.6
10
2.0
30
–
–
5.0
–
9.0
11
–
–
60
–
200
400
Unit
ALL OUTPUT VOLTAGE VERSIONS
Feedback Bias Current Vout = 5.0 V (Adjustable Version Only)
TJ = 25°C
TJ = –40 to +125°C
Ib
Oscillator Frequency (Note 5)
TJ = 25°C
TJ = 0 to +125°C
TJ = –40 to +125°C
fO
Saturation Voltage (Iout = 0.5 A, [Note 6])
TJ = 25°C
TJ = –40 to +125°C
Vsat
Max Duty Cycle (“on”) (Note 7)
DC
Current Limit Peak Current (Notes 5 and 6)
TJ = 25°C
TJ = –40 to +125°C
ICL
Output Leakage Current (Notes 8 and 9), TJ = 25°C
Output = 0 V
Output = – 1.0 V
IL
Quiescent Current (Note 8)
TJ = 25°C
TJ = –40 to +125°C
IQ
Standby Quiescent Current (ON/OFF Pin = 5.0 V (“off”))
TJ = 25°C
TJ = –40 to +125°C
nA
kHz
V
A
mA
mA
µA
Istby
ON/OFF Pin Logic Input Level
Vout = 0 V
TJ = 25°C
TJ = –40 to +125°C
Nominal Output Voltage
TJ = 25°C
TJ = –40 to +125°C
%
V
VIH
2.2
2.4
1.4
–
–
–
–
–
1.2
–
1.0
0.8
–
–
15
0
30
5.0
VIL
µA
ON/OFF Pin Input Current
ON/OFF Pin = 5.0 V (“off”), TJ = 25°C
ON/OFF Pin = 0 V (“on”), TJ = 25°C
IIH
IIL
3. External components such as the catch diode, inductor, input and output capacitors can affect the switching regulator system performance.
When the LM2574 is used as shown in the Figure 16 test circuit, the system performance will be as shown in the system parameters section
of the Electrical Characteristics.
4. Tested junction temperature range for the LM2574: Tlow = –40°C Thigh = +125°C.
5. 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%.
6. Output (Pin 2) sourcing current. No diode, inductor or capacitor connected to the output pin.
7. Feedback (Pin 4) removed from output and connected to 0 V.
8. Feedback (Pin 4) removed from output and connected to 12 V for the Adjustable, 3.3 V, and 5.0 V versions, and 25 V for the 12 V and 15 V
versions, to force the output transistor OFF.
9. Vin = 40 V.
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LM2574
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 16)
1.4
Vout , OUTPUT VOLTAGE CHANGE (%)
Vout , OUTPUT VOLTAGE CHANGE (%)
1.0
Vin = 20 V
ILoad = 100 mA
Normalized at TJ = 25°C
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1.0
-50
-25
0
25
60
75
100
1.2
0.8
0.4
0.2
-0.2
-0.4
0
5.0
10
15
20
25
30
TJ, JUNCTION TEMPERATURE (°C)
Vin, INPUT VOLTAGE (V)
Figure 2. Normalized Output Voltage
Figure 3. Line Regulation
35
40
1.4
L = 300 µH
1.5
I O, OUTPUT CURRENT (A)
INPUT - OUTPUT DIFFERENTIAL (V)
12 V and 15 V
0
2.0
ILoad = 500 mA
1.0
ILoad = 100 mA
0.5
Vin = 25 V
1.3
1.2
1.1
1.0
0.9
0.8
0
-50
-25
0
25
60
75
100
0.7
-50
125
Istby , STANDBY QUIESCENT CURRENT (µA)
14
ILoad = 500 A
12
10
ILoad = 100 mA
6.0
5.0
60
75
Figure 5. Current Limit
16
0
25
Figure 4. Dropout Voltage
18
4.0
0
TJ, JUNCTION TEMPERATURE (°C)
Vout = 5.0 V
Measured at
Ground Pin
TJ = 25°C
8.0
-25
TJ, JUNCTION TEMPERATURE (°C)
20
IQ , QUIESCENT CURRENT (mA)
3.3 V, 5.0 V and ADJ
0.6
-0.6
125
ILoad = 100 mA
TJ = 25°C
1.0
10
15
20
25
30
35
40
100
125
100
125
200
180
VON/OFF = 5.0 V
160
140
120
Vin = 40 V
100
80
60
Vin = 12 V
40
20
0
-50
-25
0
25
60
75
Vin, INPUT VOLTAGE (V)
TJ, JUNCTION TEMPERATURE (°C)
Figure 6. Quiescent Current
Figure 7. Standby Quiescent Current
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LM2574
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 16) (continued)
1.3
6.0
Vin = 12 V
Normalized at 25°C
4.0
Vsat , SATURATION VOLTAGE (V)
NORMALIZED FREQUENCY (%)
8.0
2.0
0
-2.0
-4.0
-6.0
-8.0
10
-50
-25
0
25
50
75
100
125
1.2
1.1
1.0
0.9
-40°C
0.8
25°C
0.7
125°C
0.6
0.5
0.4
0.3
0
0.1
0.2
TJ, JUNCTION TEMPERATURE (°C)
100
Adjustable Version Only
IFB , FEEDBACK PIN CURRENT (nA)
V in , INPUT VOLTAGE (V)
4.5
4.0
3.5
3.0
2.5
2.0
1.5
Vin = 1.23 V
ILoad = 100 mA
1.0
0.5
0
-50
-25
0
25
50
75
100
80
Adjustable Version Only
60
40
20
0
-20
-40
-60
-80
-100
-50
125
-25
0
25
50
75
100
TJ, JUNCTION TEMPERATURE (°C)
TJ, JUNCTION TEMPERATURE (°C)
Figure 10. Minimum Operating Voltage
Figure 11. Feedback Pin Current
125
20 V
20 V
A
10 V
10 V
0
0
0.6 A
0.6 A
B
0.4 A
0.4 A
0.2 A
0.2 A
C
0.5
Figure 9. Switch Saturation Voltage
5.0
B
0.4
SWITCH CURRENT (A)
Figure 8. Oscillator Frequency
A
0.3
0
0
20 mV
AC
20 mV
AC
C
5 µs/DIV
5 µs/DIV
A: Output Pin Voltage, 10 V/DIV.
B: Inductor Current, 0.2 A/DIV.
C: Output Ripple Voltage, 20 mV/DIV, AC–Coupled
A: Output Pin Voltage, 10 V/DIV.
B: Inductor Current, 0.2 A/DIV.
C: Output Ripple Voltage, 20 mV/DIV, AC–Coupled
Figure 12. Continuous Mode Switching Waveforms
Vout = 5.0 V, 500 mA Load Current, L = 330 µH
Figure 13. Discontinuous Mode Switching Waveforms
Vout = 5.0 V, 100 mA Load Current, L = 100 µH
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LM2574
TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 16) (continued)
A
50 mV
AC
A
B
500 mA
50 mV
AC
200 mA
B
0
100 mA
0
200 µs/DIV
200 µs/DIV
A: Output Voltage, 50 mV/DIV, AC Coupled
B: 100 mA to 500 mA Load Pulse
A: Output Voltage, 50 mV/DIV, AC Coupled
B: 50 mA to 250 mA Load Pulse
Figure 14. 500 mA Load Transient Response for
Continuous Mode Operation, L = 330 µH, Cout = 300 µF
Figure 15. 250 mA Load Transient Response for
Discontinuous Mode Operation, L = 68 µH, Cout = 470 µF
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LM2574
Fixed Output Voltage Versions
Feedback
(3)
Vin
1 (12)
7.0 - 40 V
Unregulated
DC Input
Pwr 2
Gnd
(6)
Cin
22 µF
–
–
–
–
–
–
Output
(14)
4
Cin
Cout
D1
L1
R1
R2
1
LM2574
Fixed Output
L1
330 µH
Vout
7
ON/OFF
(5)
Sig 3
Gnd
(4)
Cout
220 µF
D1
1N5819
Load
22 µF, 60 V, Aluminium Electrolytic
220 µF, 25 V, Aluminium Electrolytic
Schottky, 1N5819
330 µH, (For 5.0 Vin, 3.3 Vout, use 100 µH)
2.0 k, 0.1%
6.12 k, 0.1%
Adjustable Output Voltage Versions
Feedback
(3)
Vin
1 (12)
7.0 V - 40 V
Unregulated
DC Input
4
Cin
22 µF
1
LM2574
Adjustable
Output
(14)
Pwr 2
Gnd
(6)
Sig 3
Gnd
(4)
L1
330 µH
7
ON/OFF
(5)
V out V
R2
6.12 k
D1
1N5819
ref
R2 R1
Cout
220 µF
Load
R1
2.0 k
1.0 R2
R1
V out
V
Vout
5.0 V
ref
1.0
Where Vref = 1.23 V, R1
between 1.0 kΩ and 5.0 kΩ
NOTE: Pin numbers in ( ) are for the SO–16W package.
Figure 16. Test Circuit and Layout Guidelines
PCB LAYOUT GUIDELINES
On the other hand, the PCB area connected to the Pin 7
(emitter of the internal switch) of the LM2574 should be
kept to a minimum in order to minimize coupling to sensitive
circuitry.
Another sensitive part of the circuit is the feedback. It is
important to keep the sensitive feedback wiring short. To
assure this, physically locate the programming resistors near
to the regulator, when using the adjustable version of the
LM2574 regulator.
As in any switching regulator, the layout of the printed
circuit board is very important. Rapidly switching currents
associated with wiring inductance, stray capacitance and
parasitic inductance of the printed circuit board traces can
generate voltage transients which can generate
electromagnetic interferences (EMI) and affect the desired
operation. As indicated in the Figure 16, to minimize
inductance and ground loops, the length of the leads
indicated by heavy lines should be kept as short as possible.
For best results, single–point grounding (as indicated) or
ground plane construction should be used.
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LM2574
PIN FUNCTION DESCRIPTION
Pin
SO–16W
PDIP–8
Symbol
12
5
Vin
14
7
Output
4
2
Sig Gnd
Circuit signal ground pin. See the information about the printed circuit board layout.
6
4
Pwr Gnd
Circuit power ground pin. See the information about the printed circuit board layout.
3
1
Feedback
This pin senses regulated output voltage to complete the feedback loop. The signal is divided by
the internal resistor divider network R2, R1 and applied to the non–inverting input of the internal
error amplifier. In the Adjustable version of the LM2574 switching regulator, this pin is the direct
input of the error amplifier and the resistor network R2, R1 is connected externally to allow
programming of the output voltage.
5
3
ON/OFF
Description (Refer to Figure 1)
This pin is the positive input supply for the LM2574 step–down switching regulator. In order to
minimize voltage transients and to supply the switching currents needed by the regulator, a
suitable input bypass capacitor must be present (Cin in Figure 1).
This is the emitter of the internal switch. The saturation voltage Vsat of this output switch is
typically 1.0 V. It should be kept in mind that the PCB area connected to this pin should be kept
to a minimum in order to minimize coupling to sensitive circuitry.
It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the
total input supply current to approximately 80 µA. The input threshold voltage is typically 1.5 V.
Applying a voltage above this value (up to +Vin) shuts the regulator off. If the voltage applied to this
pin is lower than 1.5 V or if this pin is left open, the regulator will be in the “on” condition.
DESIGN PROCEDURE
Buck Converter Basics
current loop. This removes the stored energy from the
inductor. The inductor current during this time is:
The LM2574 is a “Buck” or Step–Down Converter which
is the most elementary forward–mode converter. Its basic
schematic can be seen in Figure 17.
The operation of this regulator topology has two distinct
time periods. The first one occurs when the series switch is
on, the input voltage is connected to the input of the inductor.
The output of the inductor is the output voltage, and the
rectifier (or catch diode) is reverse biased. During this
period, since there is a constant voltage source connected
across the inductor, the inductor current begins to linearly
ramp upwards, as described by the following equation:
I
L(on)
V
I
t
off
L
For the buck converter with ideal components, the duty
cycle can also be described as:
– V out t on
in
L
V
d out
V
in
Figure 18 shows the buck converter idealized waveforms
of the catch diode voltage and the inductor current.
Von(SW)
L
D
D
t
d on , where T is the period of switching.
T
Cout
Diode Voltage
Vin
V out – V
This period ends when the power switch is once again
turned on. Regulation of the converter is accomplished by
varying the duty cycle of the power switch. It is possible to
describe the duty cycle as follows:
During this “on” period, energy is stored within the core
material in the form of magnetic flux. If the inductor is
properly designed, there is sufficient energy stored to carry
the requirements of the load during the “off” period.
Power
Switch
L(off)
RLoad
Power
Switch
Off
VD(FWD)
Power
Switch
On
Power
Switch
Off
Power
Switch
On
Time
Inductor Current
Figure 17. Basic Buck Converter
The next period is the “off” period of the power switch.
When the power switch turns off, the voltage across the
inductor reverses its polarity and is clamped at one diode
voltage drop below ground by the catch diode. Current now
flows through the catch diode thus maintaining the load
Ipk
Imin
Diode
Power
Switch
Diode
ILoad(AV)
Power
Switch
Time
Figure 18. Buck Converter Idealized Waveforms
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LM2574
Procedure (Fixed Output Voltage Version) In order to simplify the switching regulator design, a step–by–step design
procedure and example is provided.
Procedure
Example
Given Parameters:
Vout = Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V)
Vin(max) = Maximum Input Voltage
ILoad(max) = Maximum Load Current
Given Parameters:
Vout = 5.0 V
Vin(max) = 15 V
ILoad(max) = 0.4 A
1. Controller IC Selection
According to the required input voltage, output voltage and
current, select the appropriate type of the controller IC output
voltage version.
1. Controller IC Selection
According to the required input voltage, output voltage,
current polarity and current value, use the LM2574–5
controller IC.
2. Input Capacitor Selection (Cin)
To prevent large voltage transients from appearing at the input
and for stable operation of the converter, an aluminium or
tantalum electrolytic bypass capacitor is needed between the
input pin +Vin and ground pin Gnd. This capacitor should be
located close to the IC using short leads. This capacitor should
have a low ESR (Equivalent Series Resistance) value.
2. Input Capacitor Selection (Cin)
A 22 µF, 25 V aluminium electrolytic capacitor located near
to the input and ground pins provides sufficient bypassing.
3. Catch Diode Selection (D1)
A. Since the diode maximum peak current exceeds the
regulator maximum load current, the catch diode current
rating must be at least 1.2 times greater than the maximum
load current. For a robust design the diode should have a
current rating equal to the maximum current limit of the
LM2574 to be able to withstand a continuous output short.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
3. Catch Diode Selection (D1)
A. For this example the current rating of the diode is 1.0 A.
B. Use a 20 V 1N5817 Schottky diode, or any of the
suggested fast recovery diodes shown in Table 1.
4. Inductor Selection (L1)
A. According to the required working conditions, select the
correct inductor value using the selection guide from
Figures 19 to 23.
B. From the appropriate inductor selection guide, identify the
inductance region intersected by the Maximum Input Voltage
line and the Maximum Load Current line. Each region is
identified by an inductance value and an inductor code.
C. Select an appropriate inductor from the several different
manufacturers part numbers listed in Table 2. The designer
must realize that the inductor current rating must be higher
than the maximum peak current flowing through the inductor.
This maximum peak current can be calculated as follows:
I p(max) I Load(max) 4. Inductor Selection (L1)
A. Use the inductor selection guide shown in Figure 20.
B. From the selection guide, the inductance area
intersected by the 15 V line and 0.4 A line is 330.
C. Inductor value required is 330 µH. From Table 2, choose
an inductor from any of the listed manufacturers.
V in V out t on
2L
where ton is the “on” time of the power switch and
V
t on out x 1.0
V in
f osc
For additional information about the inductor, see the inductor
section in the “EXTERNAL COMPONENTS” section of this
data sheet.
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LM2574
Procedure (Fixed Output Voltage Version) (continued) In order to simplify the switching regulator design, a step–by–step
design procedure and example is provided.
Procedure
Example
5. Output Capacitor Selection (Cout)
A. Since the LM2574 is a forward–mode switching regulator
with voltage mode control, its open loop 2–pole–1–zero
frequency characteristic has the dominant pole–pair
determined by the output capacitor and inductor values. For
stable operation and an acceptable ripple voltage,
(approximately 1% of the output voltage) a value between
100 µF and 470 µF is recommended.
B. Due to the fact that the higher voltage electrolytic capacitors
generally have lower ESR (Equivalent Series Resistance)
numbers, the output capacitor’s voltage rating should be at
least 1.5 times greater than the output voltage. For a 5.0 V
regulator, a rating at least 8.0 V is appropriate, and a 10 V
or 16 V rating is recommended.
5. Output Capacitor Selection (Cout)
A. Cout = 100 µF to 470 µF standard aluminium electrolytic.
B. Capacitor voltage rating = 20 V.
Procedure (Adjustable Output Version: LM2574–ADJ)
Procedure
Example
Given Parameters:
Vout = Regulated Output Voltage
Vin(max) = Maximum DC Input Voltage
ILoad(max) = Maximum Load Current
Given Parameters:
Vout = 24 V
Vin(max) = 40 V
ILoad(max) = 0.4 A
1. Programming Output Voltage
To select the right programming resistor R1 and R2 value (see
Figure 2) use the following formula:
1. Programming Output Voltage (selecting R1 and R2)
Select R1 and R2 :
R2
1.0 V out V
ref
R1
Vout = 1.23 1.0 where Vref = 1.23 V
Resistor R1 can be between 1.0 kΩ and 5.0 kΩ. (For best
temperature coefficient and stability with time, use 1% metal
film resistors).
R2 R1
V out
V
R2 R1
V out
V
ref
R2
R1
1.0
Select R1 = 1.0 kΩ
1.0 k
10 V
1.0
1.23 V
R2 = 18.51 kΩ, choose a 18.7 kΩ metal film resistor.
1.0
ref
2. Input Capacitor Selection (Cin)
To prevent large voltage transients from appearing at the input
and for stable operation of the converter, an aluminium or
tantalum electrolytic bypass capacitor is needed between the
input pin +Vin and ground pin Gnd. This capacitor should be
located close to the IC using short leads. This capacitor should
have a low ESR (Equivalent Series Resistance) value.
For additional information see input capacitor section in the
“EXTERNAL COMPONENTS” section of this data sheet.
2. Input Capacitor Selection (Cin)
A 22 µF aluminium electrolytic capacitor located near the
input and ground pin provides sufficient bypassing.
3. Catch Diode Selection (D1)
A. Since the diode maximum peak current exceeds the
regulator maximum load current the catch diode current
rating must be at least 1.2 times greater than the maximum
load current. For a robust design, the diode should have a
current rating equal to the maximum current limit of the
LM2574 to be able to withstand a continuous output short.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
3. Catch Diode Selection (D1)
A. For this example, a 1.0 A current rating is adequate.
B. Use a 50 V MBR150 Schottky diode or any suggested
fast recovery diodes in Table 1.
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LM2574
Procedure (Adjustable Output Version: LM2574–ADJ)
Procedure
Example
4. Inductor Selection (L1)
A. Use the following formula to calculate the inductor Volt x
microsecond [V x µs] constant:
4. Inductor Selection (L1)
A. Calculate E x T V x s constant :
E x T (40 24) x 24 x 1000 105V x s
52
40
V out
6
E x T (V V out)
x 10 V x s
in
[Hz]
V
F
in
B. Match the calculated E x T value with the corresponding
number on the vertical axis of the Inductor Value Selection
Guide shown in Figure 23. This E x T constant is a measure
of the energy handling capability of an inductor and is
dependent upon the type of core, the core area, the number
of turns, and the duty cycle.
C. Next step is to identify the inductance region intersected by
the E x T value and the maximum load current value on the
horizontal axis shown in Figure 27.
D. From the inductor code, identify the inductor value. Then
select an appropriate inductor from Table 2. The inductor
chosen must be rated for a switching frequency of 52 kHz
and for a current rating of 1.15 x ILoad. The inductor current
rating can also be determined by calculating the inductor
peak current:
I p(max) I Load(max) B. E x T 185V x s
C. ILoad(max) = 0.4 A
Inductance Region = 1000
D. Proper inductor value = 1000 µH
Choose the inductor from Table 2.
V in V out t on
2L
where ton is the “on” time of the power switch and
V
t on out x 1.0
V in
f osc
For additional information about the inductor, see the inductor
section in the “External Components” section of this data
sheet.
5. Output Capacitor Selection (Cout)
A. Since the LM2574 is a forward–mode switching regulator with
voltage mode control, its open loop 2–pole–1–zero frequency
characteristic has the dominant pole–pair determined by the
output capacitor and inductor values.
For stable operation, the capacitor must satisfy the following
requirement:
V
in (max)
F
C out 13, 300
V out x LH
5. Output Capacitor Selection (Cout)
A.
40
C out 13, 300 x
22.2 F
24 x 1000
To achieve an acceptable ripple voltage, select
Cout = 100 µF electrolytic capacitor.
B. Capacitor values between 10 µF and 2000 µF will satisfy the
loop requirements for stable operation. To achieve an
acceptable output ripple voltage and transient response, the
output capacitor may need to be several times larger than the
above formula yields.
C. Due to the fact that the higher voltage electrolytic capacitors
generally have lower ESR (Equivalent Series Resistance)
numbers, the output capacitor’s voltage rating should be at
least 1.5 times greater than the output voltage. For a 5.0 V
regulator, a rating of at least 8.0 V is appropriate, and a 10 V
or 16V rating is recommended.
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LM2574
LM2574 Series Buck Regulator Design Procedures (continued)
470
330
220
150
6.0
100
5.0
0.1
40
30
25
0.15
0.2
0.3
0.4
20
15
680
470
12
10
330
9.0
220
8.0
150
0.5
0.15
0.2
0.3
IL, MAXIMUM LOAD CURRENT (A)
IL, MAXIMUM LOAD CURRENT (A)
Figure 19. LM2574–3.3
Figure 20. LM2574–5
0.4
0.5
60
2200
1500
1000
20
680
18
17
470
16
330
15
14
0.1
1000
30
7.0
0.1
Vin , MAXIMUM INPUT VOLTAGE (V)
60
Vin , MAXIMUM INPUT VOLTAGE (V)
60
680
Vin , MAXIMUM INPUT VOLTAGE (V)
60
20
15
12
10
9.0
8.0
7.0
220
0.15
0.2
0.3
0.4
2200
40
30
1500
25
1000
680
22
20
470
19
330
18
220
17
0.1
0.5
0.15
0.2
0.3
IL, MAXIMUM LOAD CURRENT (A)
IL, MAXIMUM LOAD CURRENT (A)
Figure 21. LM2574–12
Figure 22. LM2574–15
ET, VOLTAGE TIME (Vµ s)
Vin , MAXIMUM INPUT VOLTAGE (V)
Indicator Value Selection Guide (For Continuous Mode Operation)
250
200
150
100
80
2200
1500
1000
680
60
50
40
30
470
330
220
150
20
15
10
0.1
100
68
0.15
0.2
0.3
IL, MAXIMUM LOAD CURRENT (A)
Figure 23. LM2574–ADJ
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0.4
0.5
0.4
0.5
LM2574
Table 1. Diode Selection Guide gives an overview about through–hole diodes for
an effective design. Device listed in bold are available from ON Semiconductor
1.0 Amp Diodes
VR
Schottky
20 V
1N5817
MBR120P
30 V
1N5818
MBR130P
40 V
1N5819
MBR140P
50 V
MBR150
60 V
MBR160
Fast Recovery
MUR110
(rated to 100 V)
Table 2. Inductor Selection Guide
Inductor
Value
Pulse Engineering
Tech 39
Renco
NPI
68 µH
*
55 258 SN
RL–1284–68
NP5915
100 µH
*
55 308 SN
RL–1284–100
NP5916
150 µH
52625
55 356 SN
RL–1284–150
NP5917
220 µH
52626
55 406 SN
RL–1284–220
NP5918/5919
330 µH
52627
55 454 SN
RL–1284–330
NP5920/5921
470 µH
52628
*
RL–1284–470
NP5922
680 µH
52629
55 504 SN
RL–1284–680
NP5923
1000 µH
52631
55 554 SN
RL–1284–1000
*
1500 µH
*
*
RL–1284–1500
*
2200 µH
*
*
RL–1284–2200
*
* : Contact Manufacturer
Table 3. Example of Several Inductor Manufacturers Phone/Fax Numbers
Pulse Engineering Inc.
Phone
Fax
+ 1–619–674–8100
+ 1–619–674–8262
Pulse Engineering Inc. Europe
Phone
Fax
+ 353–9324–107
+ 353–9324–459
Renco Electronics Inc.
Phone
Fax
+ 1–516–645–5828
+ 1–516–586–5562
Tech 39
Phone
Fax
+ 33–1–4115–1681
+ 33–1–4709–5051
NPI/APC
Phone
Fax
+ 44–634–290–588
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LM2574
EXTERNAL COMPONENTS
Input Capacitor (Cin)
The Input Capacitor Should Have a Low ESR
voltage ratings may be needed to provide low ESR values,
that are required for low output ripple voltage.
For stable operation of the switch mode converter a low
ESR (Equivalent Series Resistance) aluminium or solid
tantalum bypass capacitor is needed between the input pin
and the ground pin, to prevent large voltage transients from
appearing at the input. It must be located near the regulator
and use short leads. With most electrolytic capacitors, the
capacitance value decreases and the ESR increases with
lower temperatures. For reliable operation in temperatures
below –25°C larger values of the input capacitor may be
needed. Also paralleling a ceramic or solid tantalum
capacitor will increase the regulator stability at cold
temperatures.
The Output Capacitor Requires an ESR Value that has
an Upper and Lower Limit
As mentioned above, a low ESR value is needed for low
output ripple voltage, typically 1% to 2% of the output
voltage. But if the selected capacitor’s ESR is extremely low
(below 0.03 Ω), there is a possibility of an unstable feedback
loop, resulting in oscillation at the output. This situation can
occur when a tantalum capacitor, that can have a very low
ESR, is used as the only output capacitor.
At Low Temperatures, Put in Parallel Aluminium
Electrolytic Capacitors with Tantalum Capacitors
Electrolytic capacitors are not recommended for
temperatures below –25°C. The ESR rises dramatically at
cold temperatures and typically rises 3 times at –25°C and
as much as 10 times at –40°C. Solid tantalum capacitors
have much better ESR spec at cold temperatures and are
recommended for temperatures below –25°C. They can be
also used in parallel with aluminium electrolytics. The value
of the tantalum capacitor should be about 10% or 20% of the
total capacitance. The output capacitor should have at least
50% higher RMS ripple current rating at 52 kHz than the
peak–to–peak inductor ripple current.
RMS Current Rating of Cin
The important parameter of the input capacitor is the RMS
current rating. Capacitors that are physically large and have
large surface area will typically have higher RMS current
ratings. For a given capacitor value, a higher voltage
electrolytic capacitor will be physically larger than a lower
voltage capacitor, and thus be able to dissipate more heat to
the surrounding air, and therefore will have a higher RMS
current rating. The consequences of operating an
electrolytic capacitor beyond the RMS current rating is a
shortened operating life. In order to assure maximum
capacitor operating lifetime, the capacitor’s RMS ripple
current rating should be:
I rms 1.2 x d x I
Catch Diode
Locate the Catch Diode Close to the LM2574
The LM2574 is a step–down buck converter, it requires a
fast diode to provide a return path for the inductor current
when the switch turns off. This diode must be located close
to the LM2574 using short leads and short printed circuit
traces to avoid EMI problems.
Load
where d is the duty cycle, for a continuous mode buck
regulator
V
t
d on out
V
T
in
and
|V out|
t
d on |V out| V
T
Use a Schottky or a Soft Switching
Ultra–Fast Recovery Diode
for a buck–boost regulator.
Since the rectifier diodes are very significant source of
losses within switching power supplies, choosing the
rectifier that best fits into the converter design is an
important process. Schottky diodes provide the best
performance because of their fast switching speed and low
forward voltage drop.
They provide the best efficiency especially in low output
voltage applications (5.0 V and lower). Another choice
could be Fast–Recovery, or Ultra–Fast Recovery diodes. It
has to be noted, that some types of these diodes with an
abrupt turnoff characteristic may cause instability or EMI
troubles.
A fast–recovery diode with soft recovery characteristics
can better fulfill some quality, low noise design
requirements. Table 1 provides a list of suitable diodes for
the LM2574 regulator. Standard 50/60 Hz rectifier diodes,
such as the 1N4001 series or 1N5400 series are NOT
suitable.
in
Output Capacitor (Cout)
For low output ripple voltage and good stability, low ESR
output capacitors are recommended. An output capacitor
has two main functions: it filters the output and provides
regulator loop stability. The ESR of the output capacitor and
the peak–to–peak value of the inductor ripple current are the
main factors contributing to the output ripple voltage value.
Standard aluminium electrolytics could be adequate for
some applications but for quality design, low ESR types are
recommended.
An aluminium electrolytic capacitor’s ESR value is
related to many factors, such as the capacitance value, the
voltage rating, the physical size and the type of construction.
In most cases, the higher voltage electrolytic capacitors have
lower ESR value. Often capacitors with much higher
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LM2574
Inductor
current increases. This discontinuous mode of operation is
perfectly acceptable for this type of switching converter.
Any buck regulator will be forced to enter discontinuous
mode if the load current is light enough.
The magnetic components are the cornerstone of all
switching power supply designs. The style of the core and
the winding technique used in the magnetic component’s
design have a great influence on the reliability of the overall
power supply.
Using an improper or poorly designed inductor can cause
high voltage spikes generated by the rate of transitions in
current within the switching power supply, and the
possibility of core saturation can arise during an abnormal
operational mode. Voltage spikes can cause the
semiconductors to enter avalanche breakdown and the part
can instantly fail if enough energy is applied. It can also
cause significant RFI (Radio Frequency Interference) and
EMI (Electro–Magnetic Interference) problems.
Selecting the Right Inductor Style
Some important considerations when selecting a core type
are core material, cost, the output power of the power supply,
the physical volume the inductor must fit within, and the
amount of EMI (Electro–Magnetic Interference) shielding
that the core must provide. There are many different styles
of inductors available, such as pot core, E–core, toroid and
bobbin core, as well as different core materials such as
ferrites and powdered iron from different manufacturers.
For high quality design regulators the toroid core seems to
be the best choice. Since the magnetic flux is contained
within the core, it generates less EMI, reducing noise
problems in sensitive circuits. The least expensive is the
bobbin core type, which consists of wire wound on a ferrite
rod core. This type of inductor generates more EMI due to
the fact that its core is open, and the magnetic flux is not
contained within the core.
When multiple switching regulators are located on the
same printed circuit board, open core magnetics can cause
interference between two or more of the regulator circuits,
especially at high currents due to mutual coupling. A toroid,
pot core or E–core (closed magnetic structure) should be
used in such applications.
Continuous and Discontinuous Mode of Operation
The LM2574 step–down converter can operate in both the
continuous and the discontinuous modes of operation. The
regulator works in the continuous mode when loads are
relatively heavy, the current flows through the inductor
continuously and never falls to zero. Under light load
conditions, the circuit will be forced to the discontinuous
mode when inductor current falls to zero for certain period
of time (see Figure 24 and Figure 25). Each mode has
distinctively different operating characteristics, which can
affect the regulator performance and requirements. In many
cases the preferred mode of operation is the continuous
mode. It offers greater output power, lower peak currents in
the switch, inductor and diode, and can have a lower output
ripple voltage. On the other hand it does require larger
inductor values to keep the inductor current flowing
continuously, especially at low output load currents and/or
high input voltages.
To simplify the inductor selection process, an inductor
selection guide for the LM2574 regulator was added to this
data sheet (Figures 19 through 23). This guide assumes that
the regulator is operating in the continuous mode, and
selects an inductor that will allow a peak–to–peak inductor
ripple current to be a certain percentage of the maximum
design load current. This percentage is allowed to change as
different design load currents are selected. For light loads
(less than approximately 0.2 A) it may be desirable to
operate the regulator in the discontinuous mode, because the
inductor value and size can be kept relatively low.
Consequently, the percentage of inductor peak–to–peak
Do Not Operate an Inductor Beyond its Maximum
Rated Current
Exceeding an inductor’s maximum current rating may
cause the inductor to overheat because of the copper wire
losses, or the core may saturate. Core saturation occurs when
the flux density is too high and consequently the cross
sectional area of the core can no longer support additional
lines of magnetic flux.
This causes the permeability of the core to drop, the
inductance value decreases rapidly and the inductor begins
to look mainly resistive. It has only the dc resistance of the
winding. This can cause the switch current to rise very
rapidly and force the LM2574 internal switch into
cycle–by–cycle current limit, thus reducing the dc output
load current. This can also result in overheating of the
inductor and/or the LM2574. Different inductor types have
different saturation characteristics, and this should be kept
in mind when selecting an inductor.
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0.5 A
Inductor
Current
Waveform
0A
0.5 A
Power
Switch
Current
Waveform
0A
VERTICAL RESOLUTION 100 mADV
VERTRICAL RESOLUTION 200 mADV
LM2574
Inductor 0.1 A
Current
Waveform 0 A
Power
Switch 0.1 A
Current
0A
Waveform
HORIZONTAL TIME BASE: 5.0 µs/DIV
HORIZONTAL TIME BASE: 5.0 µs/DIV
Figure 24. Continuous Mode Switching
Current Waveforms
Figure 25. Continuous Mode Switching
Current Waveforms
GENERAL RECOMMENDATIONS
Minimizing the Output Ripple
Output Voltage Ripple and Transients
Source of the Output Ripple
In order to minimize the output ripple voltage it is possible
to enlarge the inductance value of the inductor L1 and/or to
use a larger value output capacitor. There is also another way
to smooth the output by means of an additional LC filter
(20 µH, 100 µF), that can be added to the output (see
Figure 35) to further reduce the amount of output ripple and
transients. With such a filter it is possible to reduce the
output ripple voltage transients 10 times or more. Figure 26
shows the difference between filtered and unfiltered output
waveforms of the regulator shown in Figure 34.
The upper waveform is from the normal unfiltered output
of the converter, while the lower waveform shows the output
ripple voltage filtered by an additional LC filter.
Since the LM2574 is a switch mode power supply
regulator, its output voltage, if left unfiltered, will contain a
sawtooth ripple voltage at the switching frequency. The
output ripple voltage value ranges from 0.5% to 3% of the
output voltage. It is caused mainly by the inductor sawtooth
ripple current multiplied by the ESR of the output capacitor.
Short Voltage Spikes and How to Reduce Them
The regulator output voltage may also contain short
voltage spikes at the peaks of the sawtooth waveform (see
Figure 26). These voltage spikes are present because of the
fast switching action of the output switch, and the parasitic
inductance of the output filter capacitor. There are some
other important factors such as wiring inductance, stray
capacitance, as well as the scope probe used to evaluate these
transients, all these contribute to the amplitude of these
spikes. To minimize these voltage spikes, low inductance
capacitors should be used, and their lead lengths must be
kept short. The importance of quality printed circuit board
layout design should also be highlighted.
Heatsinking and Thermal Considerations
The LM2574 is available in both 8–pin DIP and SO–16L
packages. When used in the typical application the copper lead
frame conducts the majority of the heat from the die, through
the leads, to the printed circuit copper. The copper and the
board are the heatsink for this package and the other heat
producing components, such as the catch diode and inductor.
For the 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 to the surrounding air. One
exception to this is the output (switch) pin, which should not
have large areas of copper in order to minimize coupling to
sensitive circuitry.
Additional improvement in heat dissipation can be
achieved even by using of double sided or multilayer boards
which can provide even better heat path to the ambient.
Using a socket for the 8–pin DIP package is not
recommended because socket represents an additional
thermal resistance, and as a result the junction temperature
will be higher.
VERTRICAL RESOLUTION 20 mV/DIV
Voltage spikes caused by switching action of the output
switch and the parasitic inductance of the output capacitor
Unfiltered
Output
Voltage
Filtered
Output
Voltage
HORIZONTAL TIME BASE: 5.0 µs/DIV
Figure 26. Output Ripple Voltage Waveforms
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LM2574
TJ = (RθJA)(PD) + TA
Since the current rating of the LM2574 is only 0.5 A, the
total package power dissipation for this switcher is quite
low, ranging from approximately 0.1 W up to 0.75 W under
varying conditions. In a carefully engineered printed circuit
board, the through–hole DIP package can easily dissipate up
to 0.75 W, even at ambient temperatures of 60°C, and still
keep the maximum junction temperature below 125°C.
where (RθJA)(PD) represents the junction temperature rise
caused by the dissipated power and TA is the maximum
ambient temperature.
Some Aspects That can Influence Thermal Design
It should be noted that the package thermal resistance and
the junction temperature rise numbers are all approximate,
and there are many factors that will affect these numbers,
such as PC board size, shape, thickness, physical position,
location, board temperature, as well as whether the
surrounding air is moving or still. At higher power levels the
thermal resistance decreases due to the increased air current
activity.
Other factors are trace width, total printed circuit copper
area, copper thickness, single– or double–sided, multilayer
board, the amount of solder on the board or even color of the
traces.
The size, quantity and spacing of other components on the
board can also influence its effectiveness to dissipate the
heat. Some of them, like the catch diode or the inductor will
generate some additional heat.
Thermal Analysis and Design
The following procedure must be performed to determine
the operating junction temperature. First determine:
1. PD(max) – maximum regulator power dissipation in
the application.
2. TA(max) – maximum ambient temperature in the
application.
3. TJ(max) – maximum allowed junction temperature
(125°C for the LM2574). For a conservative
design, the maximum junction temperature
should not exceed 110°C to assure safe
operation. For every additional +10°C
temperature rise that the junction must
withstand, the estimated operating lifetime
of the component is halved.
4. RθJC
– package thermal resistance junction–case.
5. RθJA
– package thermal resistance junction–ambient.
(Refer to Absolute Maximum Ratings on page 2 of this data
sheet or RθJC and RθJA values).
ADDITIONAL APPLICATIONS
Inverting Regulator
An inverting buck–boost regulator using the LM2574–12
is shown in Figure 27. This circuit converts a positive input
voltage to a negative output voltage with a common ground
by bootstrapping the regulators ground to the negative
output voltage. By grounding the feedback pin, the regulator
senses the inverted output voltage and regulates it.
In this example the LM2574–12 is used to generate a –12 V
output. The maximum input voltage in this case cannot
exceed 28 V because the maximum voltage appearing across
the regulator is the absolute sum of the input and output
voltages and this must be limited to a maximum of 40 V.
This circuit configuration is able to deliver approximately
0.1 A to the output when the input voltage is 8.0 V or higher.
At lighter loads the minimum input voltage required drops
to approximately 4.7 V, because the buck–boost regulator
topology can produce an output voltage that, in its absolute
value, is either greater or less than the input voltage.
Since the switch currents in this buck–boost configuration
are higher than in the standard buck converter topology, the
available output current is lower.
This type of buck–boost inverting regulator can also
require a larger amount of startup input current, even for
light loads. This may overload an input power source with
a current limit less than 0.6 A.
Because of the relatively high startup currents required by
this inverting regulator topology, the use of a delayed startup
or an undervoltage lockout circuit is recommended.
While using a delayed startup arrangement, the input
capacitor can charge up to a higher voltage before the
switch–mode regulator begins to operate.
The following formula is to calculate the approximate
total power dissipated by the LM2574:
PD = (Vin x IQ) + d x ILoad x Vsat
where d is the duty cycle and for buck converter
V
t
d on O ,
V
T
in
IQ (quiescent current) and Vsat can be found in the
LM2574 data sheet,
Vin is minimum input voltage applied,
VO is the regulator output voltage,
ILoad is the load current.
8.0 to 25 V
Unregulated
DC Input +Vin
Cin
22 µF
(3)
5
(12)
4
LM2574–12
Pwr 2
Gnd
(6)
(14)
Sig 3
Gnd
(4)
Feedback
1
Output
7
ON/OFF
(5)
L1
68 µH
D1
MBR150
Cout
680 µF
-12 V @ 100 mA
Regulated
Output
Figure 27. Inverting Buck–Boost Develops –12 V
The dynamic switching losses during turn–on and
turn–off can be neglected if a proper type catch diode is used.
The junction temperature can be determined by the
following expression:
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LM2574
The high input current needed for startup is now partially
supplied by the input capacitor Cin.
by the fact, that the ground pin of the converter IC is no
longer at ground. Now, the ON/OFF pin threshold voltage
(1.3 V approximately) has to be related to the negative
output voltage level. There are many different possible
shutdown methods, two of them are shown in Figures 29
and 30.
Design Recommendations:
The inverting regulator operates in a different manner
than the buck converter and so a different design procedure
has to be used to select the inductor L1 or the output
capacitor Cout.
The output capacitor values must be larger than what is
normally required for buck converter designs. Low input
voltages or high output currents require a large value output
capacitor (in the range of thousands of µF).
The recommended range of inductor values for the
inverting converter design is between 68 µH and 220 µH. To
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.
12 to 25 V
Unregulated
DC Input
(3)
+Vin
Cin
C1
22 µF
/50 V 0.1 µF
5 (12)
3
R1
47 k
LM2574–12
(14)
ON/OFF 4
(5)
R2
47 k
Pwr 2
Gnd
(6)
5.0 V
0
Off
where
peak
t on Load
3
ON/OFF 2
(5) and
4
R2
47 k
1
Output
7
Sig
Gnd
(4)
Gnds
Pins
(4)
and
(6)
-Vout
Feedback
L1
68 µH
MOC8101
NOTE: This picture does not show the complete circuit.
D1
MBR150
Cout
680 µF
/16 V
Figure 29. Inverting Buck–Boost Regulator Shutdown
Circuit Using an Optocoupler
+V
0
+Vin
The following formula is used to obtain the peak inductor
current:
I
Cin
R1
22 µF 47 k
R3
470
On
Figure 28. Inverting Buck–Boost Regulator with
Delayed Startup
LM2574–XX
5 (12)
Shutdown
Input
-12 V @ 100 mA
Regulated
Output
I
+Vin
+Vin
Off
On
Shutdown
Input
R2
5.6 k
+Vin
5
(12)
LM2574–XX
Cin
22 µF
|V |
V x t on
in
O
in
2L 1
V
in
V
Q1
2N3906
, and fosc = 52 kHz.
|V |
O
x 1.0
V |V | f osc
in
O
3
ON/OFF 2
(5) and
4
R1
12 k
Gnds (4)
Pins and
(6)
-Vout
NOTE: This picture does not show the complete circuit.
Under normal continuous inductor current operating
conditions, the worst case occurs when Vin is minimal.
It has been already mentioned above, that in some
situations, the delayed startup or the undervoltage lockout
features could be very useful. A delayed startup circuit
applied to a buck–boost converter is shown in Figure 28.
Figure 34 in the “Undervoltage Lockout” section describes
an undervoltage lockout feature for the same converter
topology.
With the inverting configuration, the use of the ON/OFF
pin requires some level shifting techniques. This is caused
Figure 30. Inverting Buck–Boost Regulator Shutdown
Circuit Using a PNP Transistor
Negative Boost Regulator
This example is a variation of the buck–boost topology
and it is called negative boost regulator. This regulator
experiences relatively high switch current, especially at low
input voltages. The internal switch current limiting results in
lower output load current capability.
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LM2574
When a high 50 Hz or 60 Hz (100 Hz or 120 Hz
respectively) ripple voltage exists, a long delay time can
cause some problems by coupling the ripple into the
ON/OFF pin, the regulator could be switched periodically
on and off with the line (or double) frequency.
The circuit in Figure 31 shows the negative boost
configuration. The input voltage in this application ranges
from –5.0 to –12 V and provides a regulated –12 V output.
If the input voltage is greater than –12 V, the output will rise
above –12 V accordingly, but will not damage the regulator.
(3)
+Vin
5 (12)
Cin
22 µF
4
Pwr 2
Gnd
(6)
Sig 3
Gnd
(4)
L1
Vin
-5.0 to -12 V
330 µH
Output
7
ON/OFF
(5)
+Vin
Cout
1000 µF
Feedback
LM2574–12
(14)
+Vin
1
5
C1
0.1 µF
D1
Cin
22 µF
1N5817
LM2574–XX
(12)
3
R1
47 k
Vout = -12 V
ON/OFF 2
(5) and
4
Gnds (4)
Pins and
(6)
R2
47 k
Load Current
60 mA for Vin = -5.2 V
120 mA for Vin = -7.0 V
NOTE: This picture does not show the complete circuit.
Figure 31. Negative Boost Regulator
Figure 32. Delayed Startup Circuitry
Design Recommendations:
Undervoltage Lockout
The same design rules as for the previous inverting
buck–boost converter can be applied. The output capacitor
Cout must be chosen larger than what would be required for
a standard buck converter. Low input voltages or high output
currents require a large value output capacitor (in the range
of thousands of µF). The recommended range of inductor
values for the negative boost regulator is the same as for
inverting converter design.
Another important point is that these negative boost
converters cannot provide any current limiting load
protection in the event of a short in the output so some other
means, such as a fuse, may be necessary to provide the load
protection.
Some applications require the regulator to remain off until
the input voltage reaches a certain threshold level. Figure 33
shows an undervoltage lockout circuit applied to a buck
regulator. A version of this circuit for buck–boost converter
is shown in Figure 34. Resistor R3 pulls the ON/OFF pin
high and keeps the regulator off until the input voltage
reaches a predetermined threshold level, which is
determined by the following expression:
V
th
V
Z1
1.0 R2 V (Q1)
R1 BE
+Vin
+Vin
Delayed Startup
5
There are some applications, like the inverting regulator
already mentioned above, which require a higher amount of
startup current. In such cases, if the input power source is
limited, this delayed startup feature becomes very useful.
To provide a time delay between the time when the input
voltage is applied and the time when the output voltage
comes up, the circuit in Figure 32 can be used. As the input
voltage is applied, the capacitor C1 charges up, and the
voltage across the resistor R2 falls down. When the voltage
on the ON/OFF pin falls below the threshold value 1.3 V, the
regulator starts up. Resistor R1 is included to limit the
maximum voltage applied to the ON/OFF pin. It reduces the
power supply noise sensitivity, and also limits the capacitor
C1 discharge current, but its use is not mandatory.
R1
10 k
Cin
22 µF
R3
47 k
LM2574–XX
(12)
3
ON/OFF 2
(5) and
4
Gnds (4)
Pins and
(6)
Z1
1N5242B
Q1
2N3904
R2
10 k
NOTE: This picture does not show the complete circuit.
Figure 33. Undervoltage Lockout Circuit for
Buck Converter
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LM2574
Adjustable Output, Low–Ripple Power Supply
R2
15 k
A 0.5 A output current capability power supply that
features an adjustable output voltage is shown in Figure 35.
This regulator delivers 0.5 A into 1.2 to 35 V output. The
input voltage ranges from roughly 3.0 to 40 V. In order to
achieve a 10 or more times reduction of output ripple, an
additional L–C filter is included in this circuit.
+Vin
+Vin
LM2574–XX
5 (12)
Cin
22 µF
R3
68 k
3
ON/OFF 2
(5)
and
4
Gnds (4)
Pins and
(6)
Z1
1N5242
Q1
2N3904
R1
15 k
-Vout
NOTE: This picture does not show the complete circuit (see Figure 27).
Figure 34. Undervoltage Lockout Circuit for
Buck–Boost Converter
40 V Max
Unregulated
DC Input
Feedback
(3)
+Vin
5
1
LM2574–ADJ
(12)
(14)
Cin
22 µF
4
Pwr 2
Gnd
(6)
Sig 3
Gnd
(4)
Output
L1
150 µH
L2
20 µH
7
ON/OFF
(5)
1.2 to 35 V @ 0.5 A
R2
50 k
Cout
1000 µF
D1
1N5819
R1
1.1 k
Output
Voltage
C1
100 µF
Optional Output
Ripple Filter
Figure 35. 1.2 to 35 V Adjustable 500 mA Power Supply with Low Output Ripple
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LM2574
The LM2574–5 Step–Down Voltage Regulator with 5.0 V @ 0.5 A Output Power Capability.
Typical Application With Through–Hole PC Board Layout
Feedback
(3)
+Vin
Unregulated
DC Input
+Vin = 7.0 to 40 V
5
(12)
1
LM2574–5
(14)
4
C1
22 µF
Pwr 2
Gnd
(6)
Sig 3
Gnd
(4)
Output
L1
330 µH
Regulated Output
+Vout = 5.0 V @ 0.5 A
7
ON/OFF
(5)
D1
1N5819
Gnd
C2
220 µF
Gnd
C1
C2
D1
L1
–
–
–
–
22 µF, 63 V, Aluminium Electrolytic
220 µF, 16 V, Aluminium Electrolytic
1.0 A, 40 V, Schottky Rectifier, 1N5819
330 µH, RL–1284–330, Renco Electronics
Figure 36. Schematic Diagram of the LM2574–5 Step–Down Converter
LM2574-5.0
+Vin
+
C1
C2
U1
Gnd
+
D1
Vout
L1
Gnd
NOTE: Not to scale.
NOTE: Not to scale.
Figure 38. PC Board Layout Copper Side
Figure 37. PC Board Layout Component Side
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LM2574
The LM2574–ADJ Step–Down Voltage Regulator with 5.0 V @ 0.5 A Output Power Capability Typical
Application With Through–Hole PC Board Layout
(3)
Unregulated
DC Input
+Vin
5 (12)
+Vin = 7.0 to 40 V
C1
22 µF
L1
330 µH
LM2574–ADJ
(14)
4
Feedback
1
Pwr
Gnd
(6)
2
Output
L2
22 µH
7
ON/OFF
(5)
Sig 3
Gnd
(4)
Regulated
Output Filtered
Vout = 5.0 V @ 0.5 A
R2
6.12 kΩ
D1
1N5819
C2
220 µF
C3
100 µF
R1
2.0 kΩ
Gnd
Gnd
C1
C2
C3
D1
L1
L2
R1
R2
–
–
–
–
–
–
–
–
22 µF, 63 V, Aluminium Electrolytic
220 µF, 16 V, Aluminium Electrolytic
100 µF, 16 V Aluminium Electrolytic
1.0 A, 40 V, Schottky Rectifier, 1N5819
330 µH, RL–1284–330, Renco Electronics
25 µH, SFT52501, TDK
2.0 kΩ, 0.1%, 0.25 W
6.12 kΩ, 0.1%, 0.25 W
Output
Ripple Filter
Figure 39. Schematic Diagram of the 5.0 V @ 0.5 A Step–Down Converter Using the LM2574–ADJ
(An additional LC filter is included to achieve low output ripple voltage)
+Vin
+
LM2574
C1
C2
U1
D1
Gnd
C3
+
+
Gnd
R1 R2
L2
Vout
L1
NOTE: Not to scale.
NOTE: Not to scale.
Figure 40. PC Board Layout Component Side
Figure 41. PC Board Layout Copper Side
References
• Marty Brown “Practical Switching Power Supply Design”, Academic Press, Inc., San Diego 1990
• Ray Ridley “High Frequency Magnetics Design”, Ridley Engineering, Inc. 1995
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LM2574
ORDERING INFORMATION
Device
Nominal Output
Voltage
Operating Junction
Temperature Range
LM2574DW–ADJ
LM2574DW–ADJR2
1.23 V to 37 V
LM2574N–ADJ
Package
Shipping
SO–16W
47 Units/Rail
SO–16W
1000 Units/Tape & Reel
PDIP–8
50 Units/Rail
PDIP–8
50 Units/Rail
LM2574N–3.3
3.3 V
LM2574N–5
5.0 V
PDIP–8
50 Units/Rail
LM2574N–12
12 V
PDIP–8
50 Units/Rail
LM2574N–15
15 V
PDIP–8
50 Units/Rail
TJ = –40° to +125°C
MARKING DIAGRAMS
SO–16W
DW SUFFIX
CASE 751G
16
2574DW–ADJ
AWL
YYWW
1
PDIP–8
N SUFFIX
CASE 626
8
8
2574–ADJ
AWL
YYWW
1
8
2574–3.3
AWL
YYWW
8
LM2574N–5
AWL
YYWW
1
1
A
WL
YY
WW
2574–12
AWL
YYWW
1
= Assembly Location
= Wafer Lot
= Year
= Work Week
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8
2574–15
AWL
YYWW
1
LM2574
PACKAGE DIMENSIONS
SO–16W
DW SUFFIX
CASE 751G–03
ISSUE B
A
D
9
1
8
NOTES:
1. DIMENSIONS ARE IN MILLIMETERS.
2. INTERPRET DIMENSIONS AND TOLERANCES
PER ASME Y14.5M, 1994.
3. DIMENSIONS D AND E DO NOT INLCUDE MOLD
PROTRUSION.
4. MAXIMUM MOLD PROTRUSION 0.15 PER SIDE.
5. DIMENSION B DOES NOT INCLUDE DAMBAR
PROTRUSION. ALLOWABLE DAMBAR
PROTRUSION SHALL BE 0.13 TOTAL IN EXCESS
OF THE B DIMENSION AT MAXIMUM MATERIAL
CONDITION.
h X 45 H
E
0.25
8X
M
B
M
16
16X
M
T A
S
B
DIM
A
A1
B
C
D
E
e
H
h
L
S
e
SEATING
PLANE
A1
14X
L
A
0.25
B
B
C
T
MILLIMETERS
MIN
MAX
2.35
2.65
0.10
0.25
0.35
0.49
0.23
0.32
10.15
10.45
7.40
7.60
1.27 BSC
10.05
10.55
0.25
0.75
0.50
0.90
0
7
PDIP–8
N SUFFIX
CASE 626–05
ISSUE L
8
NOTES:
1. DIMENSION L TO CENTER OF LEAD WHEN
FORMED PARALLEL.
2. PACKAGE CONTOUR OPTIONAL (ROUND OR
SQUARE CORNERS).
3. DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.
5
–B–
1
4
F
–A–
NOTE 2
L
C
J
–T–
N
SEATING
PLANE
D
H
M
K
G
0.13 (0.005)
M
T A
M
B
M
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DIM
A
B
C
D
F
G
H
J
K
L
M
N
MILLIMETERS
MIN
MAX
9.40
10.16
6.10
6.60
3.94
4.45
0.38
0.51
1.02
1.78
2.54 BSC
0.76
1.27
0.20
0.30
2.92
3.43
7.62 BSC
--10
0.76
1.01
INCHES
MIN
MAX
0.370
0.400
0.240
0.260
0.155
0.175
0.015
0.020
0.040
0.070
0.100 BSC
0.030
0.050
0.008
0.012
0.115
0.135
0.300 BSC
--10
0.030
0.040
LM2574
Notes
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LM2574
Notes
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LM2574
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are trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes
without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular
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including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or
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LM2574/D