ON Semiconductor LM2574, NCV2574 Technical data

LM2574, NCV2574

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

16

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 mA (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

• NCV Prefix for Automotive and Other Applications Requiring Site

and Control Changes

* No internal connection, but should be soldered to

* PC board for best heat transfer.

• Pb−Free Packages are Available*

Applications

• Simple and High−Efficiency Step−Down (Buck) Regulators

• Efficient Pre−regulator for Linear Regulators

See detailed ordering and shipping information in the package
dimensions section on page 24 of this data sheet.

• On−Card Switching Regulators

• Positive to Negative Converters (Buck−Boost)

• Negative Step−Up Converters

DEVICE MARKING INFORMATION

See general marking information in the device marking
section on page 24 of this data sheet.

• Power Supply for Battery Chargers

*For additional information on our Pb−Free strategy and soldering details, please download the ON Semiconductor Soldering and Mounting

Techniques Reference Manual, SOLDERRM/D.

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SO−16 WB

DW SUFFIX

1

8

1

CASE 751G

PDIP−8

N SUFFIX

CASE 626

PIN CONNECTIONS

*

16

15

14

13

12

11

10

9

8

7

6

5

*
Output
*
V
*
*
*

*
Output

*
V

FB

Sig Gnd

/OFF

ON
Pwr Gnd

FB

Sig Gnd

ON

/OFF

Pwr Gnd

1

*

2

*

3

4

5

6

7

*

8

*

(Top View)

1

2

3

4

(Top View)

ORDERING INFORMATION

in

in

© Semiconductor Components Industries, LLC, 2006

August, 2006 − Rev. 8

1 Publication Order Number:

LM2574/D

Typical Application (Fixed Output Voltage Versions)

Unregulated

DC Input

LM2574, NCV2574

Feedback

(3)

(14)

/OFF3

1

Output

7

7.0 − 40 V

Unregulated

DC Input

C

22 mF

+V

in

LM2574

5

in

(12)

Sig

2

Gnd

Pwr

4ON

Gnd

(4) (6) (5)

Representative Block Diagram and Typical Application

+V

in

5

C

in

(12)

1

(3)

Feedback

Fixed Gain

R2

Error Amplifier

R1

1.0 k

Freq
Shift

18 kHz

Sig Gnd

2

1.235 V

Band−Gap

Reference

(4)

3.1 V Internal

Comparator

52 kHz

Oscillator

Regulator

Current

Latch

Reset

ON

Limit

/OFF

330 mH

D1
1N5819

Driver

Thermal

Shutdown

L1

C
220 mF

1.0 Amp
Switch

out

5.0 V Regulated
Output 0.5 A Load

ON

/OFF

3

Voltage Versions

(5)

For adjustable version
R1 = open, R2 = 0 W

Output

7

(14)

Pwr Gnd

4

(6)

Output

D1

3.3 V

5.0 V
12 V
15 V

L1

R2

(W)

1.7 k

3.1 k

8.84 k

11.3 k

V

out

C

out

Load

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).

Rating

Maximum Supply Voltage V
ON/OFF Pin Input Voltage − −0.3 V ≤ V ≤ +V
Output Voltage to Ground (Steady State) − −1.0 V
DW Suffix, Plastic Package Case 751G

Max Power Dissipation P

Thermal Resistance, Junction−to−Air

N Suffix, Plastic Package Case 626
Max Power Dissipation P

Thermal Resistance, Junction−to−Ambient

Thermal Resistance, Junction−to−Case
Storage Temperature Range T
Minimum ESD Rating − 2.0 kV

(Human Body Model: C = 100 pF, R = 1.5 kW)
Lead Temperature (Soldering, 10 seconds) − 260 °C

Maximum Junction Temperature T

Maximum ratings are those values beyond which device damage can occur. Maximum ratings applied to the device are individual stress limit
values (not normal operating conditions) and are not valid simultaneously . If these limits are exceeded, device functional operation is not implied,
damage may occur and reliability may be affected.

NOTE: ESD data available upon request.

Symbol Value Unit

in

D

R

q

JA

D

R

q

JA

R

q

JC

stg

J

45 V

in

Internally Limited W

145 °C/W

Internally Limited W

100 °C/W

5.0 °C/W

−65°C to +150°C °C

150 °C

V

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2

LM2574, NCV2574

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

Operating Junction Temperature Range T
Supply Voltage V

SYSTEM PARAMETERS ([Note 1] Test Circuit Figure 16)

ELECTRICAL CHARACTERISTICS (Unless otherwise specified, V

version, V
T

is the operating junction temperature range that applies [Note 2], unless otherwise noted).

J

LM2574−3.3 ([Note 1] Test Circuit Figure 16)

Output Voltage (V
Output Voltage (4.75 V ≤ Vin ≤ 40 V, 0.1 A ≤ I

Efficiency (Vin = 12 V, I

LM2574−5 ([Note 1] Test Circuit Figure 16)

Output Voltage (Vin = 12 V, I
Output Voltage (7.0 V ≤ Vin ≤ 40 V, 0.1 A ≤ I

Efficiency (Vin = 12 V, I

LM2574−12 ([Note 1] Test Circuit Figure 16)

Output Voltage (Vin = 25 V, I
Output Voltage (15 V ≤ Vin ≤ 40 V, 0.1 A ≤ I

Efficiency (Vin = 15 V, I

LM2574−15 ([Note 1] Test Circuit Figure 16)

Output Voltage (V
Output Voltage (18 V < Vin < 40 V, 0.1 A < I

Efficiency (Vin = 18 V, I

LM2574 ADJUSTABLE VERSION ([Note 1] Test Circuit Figure 16)

Feedback Voltage Vin = 12 V, I
Feedback Voltage 7.0 V ≤ Vin ≤ 40 V, 0.1 A ≤ I

V

Efficiency (Vin = 12 V, I

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, NCV2574: T

= 25 V for the 12 V version, Vin = 30 V for the 15 V version. I

in

Characteristic

= 12 V, I

in

= 100 mA, TJ = 25°C) V

Load

≤ 0.5 A) V

Load

TJ = 25°C 3.168 3.3 3.432
TJ = −40 to +125°C 3.135 − 3.465

= 0.5 A) η − 72 − %

Load

= 100 mA, TJ = 25°C) V

Load

≤ 0.5 A) V

T

= 25°C

J

T

= −40 to +125°C

J

T

= 25°C

J

T

= −40 to +125°C

J

Load

Load

= 30 V, I

in

= 0.5 A) η − 77 − %

= 100 mA, TJ = 25°C) V

Load

= 0.5 A) η − 88 − %

= 100 mA, TJ = 25°C) V

Load

Load

≤ 0.5 A) V

Load

< 0.5 A) V

Load

TJ = 25°C 14.4 15 15.6
TJ = −40 to +125°C 14.25 15.75

= 0.5 A) η − 88 − %

Load

= 100 mA, V

Load

= 5.0 V, TJ = 25°C V

out

Load

≤ 0.5 A, V

out

= 5.0

TJ = 25°C 1.193 1.23 1.267
TJ = −40 to +125°C 1.18 1.28

= 0.5 A, V

Load

= 5.0 V) η − 77 − %

out

= 12 V for the 3.3 V, 5.0 V, and Adjustable

in

Load

= −40°C T

low

Symbol Value Unit

J

in

−40 to +125 °C
40 V

= 100 mA. For typical values TJ = 25°C, for min/max values

Symbol Min Typ Max Unit

out
out

out
out

3.234 3.3 3.366 V
V

4.9 5.0 5.1 V
V

4.8 5.0 5.2

4.75 5.25

out
out

11.76 10 12.24 V
V

11.52 12 12.48

11.4 − 12.6

14.7 15 15.3 V
V

1.217 1.23 1.243 V
V

V

out
out

FB

FBT

= +125°C.

high

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3

LM2574, NCV2574

SYSTEM PARAMETERS ([Note 3] Test Circuit Figure 16)

ELECTRICAL CHARACTERISTICS (continued) (Unless otherwise specified, V

Adjustable version, V
min/max values T

ALL OUTPUT VOLTAGE VERSIONS

Feedback Bias Current V

TJ = 25°C − 25 100
TJ = −40 to +125°C − − 200

Oscillator Frequency (Note 5) f

TJ = 25°C − 52 −
TJ = 0 to +125°C 47 52 58
TJ = −40 to +125°C 42 − 63

Saturation Voltage (I

TJ = 25°C − 1.0 1.2

TJ = −40 to +125°C − − 1.4
Max Duty Cycle (“on”) (Note 7) DC 93 98 − %
Current Limit Peak Current (Notes 5 and 6) I

TJ = 25°C 0.7 1.0 1.6

TJ = −40 to +125°C 0.65 − 1.8
Output Leakage Current (Notes 8 and 9), TJ = 25°C I

Output = 0 V − 0.6 2.0

Output = − 1.0 V − 10 30
Quiescent Current (Note 8) I

TJ = 25°C − 5.0 9.0

TJ = −40 to +125°C − − 11
Standby Quiescent Current (ON/OFF Pin = 5.0 V (“off”)) I

TJ = 25°C − 60 200

TJ = −40 to +125°C − − 400
ON/OFF Pin Logic Input Level V

V

= 0 V V

out

TJ = 25°C 2.2 1.4 −

TJ = −40 to +125°C 2.4 − −

Nominal Output Voltage V

TJ = 25°C − 1.2 1.0

TJ = −40 to +125°C − − 0.8
ON/OFF Pin Input Current

ON/OFF Pin = 5.0 V (“off”), T

ON/OFF Pin = 0 V (“on”), TJ = 25°C I

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, NCV2574: T

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.

= 40 V.

9. V

in

= 25 V for the 12 V version, Vin = 30 V for the 15 V version. I

in

is the operating junction temperature range that applies [Note 4], unless otherwise noted).

J

Characteristic

= 5.0 V (Adjustable Version Only) I

out

= 0.5 A, [Note 6]) V

out

= 25°C I

J

low

Symbol Min Typ Max Unit

O

sat

CL

Q

stby

IH
IL

= −40°C T

= 12 V for the 3.3 V, 5.0 V, and

in

= 100 mA. For typical values TJ = 25°C, for

Load

b

L

IH

IL

− 15 30

− 0 5.0

= +125°C.

high

nA

kHz

V

A

mA

mA

mA

mA

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4

LM2574, NCV2574

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 16)

1.0

Vin = 20 V

0.8
I

Load

0.6
Normalized at TJ = 25°C

0.4

0.2

0

−0.2

−0.4

−0.6

, OUTPUT VOLTAGE CHANGE (%)

out

−0.8

V

−1.0

2.0

L = 300 mH

1.5

1.0

0.5

INPUT − OUTPUT DIFFERENTIAL (V)

0

= 100 mA

TJ, JUNCTION TEMPERATURE (°C)

Figure 2. Normalized Output Voltage

I

= 500 mA

Load

I

= 100 mA

Load

TJ, JUNCTION TEMPERATURE (°C)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

−0.2

, OUTPUT VOLTAGE CHANGE (%)

out

−0.4

V

1251007560250−25−50 403530252015105.00

−0.6

1.4

1.3

1.2

1.1

1.0

0.9

, OUTPUT CURRENT (A)

O

I

0.8

1251007560250−25−50 1251007560250−25−50

0.7

I

Load

T

= 25°C

J

= 100 mA

3.3 V, 5.0 V and ADJ

, INPUT VOLTAGE (V)

V

in

Figure 3. Line Regulation

TJ, JUNCTION TEMPERATURE (°C)

12 V and 15 V

Vin = 25 V

, QUIESCENT CURRENT (mA)

Q

I

8.0

6.0

4.0

Figure 4. Dropout Voltage Figure 5. Current Limit

, STANDBY QUIESCENT CURRENT ( A)μ

I

stby

200

180

160

140

120

100

V

= 5.0 V

ON/OFF

Vin = 40 V

80

60

40

Vin = 12 V

20

0

TJ, JUNCTION TEMPERATURE (°C)

20

V

= 5.0 V

18

16

14

I

12

Load

= 500 A

out

Measured at
Ground Pin
T

= 25°C

J

10

I

= 100 mA

Load

403530252015105.00 1251007560250−25−50

V

, INPUT VOLTAGE (V)

in

Figure 6. Quiescent Current Figure 7. Standby Quiescent Current

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5

LM2574, NCV2574

out

s

out

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 16) (continued)

8.0

6.0

Vin = 12 V
Normalized at 25°C

4.0

2.0

0

−2.0

−4.0

−6.0

NORMALIZED FREQUENCY (%)

−8.0

10

TJ, JUNCTION TEMPERATURE (°C)

1251007550250−25−50 0 0.1 0.2 0.3 0.4 0.5

Figure 8. Oscillator Frequency

5.0

4.5

4.0

3.5

3.0

2.5

2.0

, INPUT VOLTAGE (V)V

1.5

in

1.0

0.5

0

Vin = 1.23 V
I

= 100 mA

Load

TJ, JUNCTION TEMPERATURE (°C)

Adjustable Version Only

1251007550250−25−50 1251007550250−25−50

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

, SATURATION VOLTAGE (V)

0.5

sat

V

0.4

0.3

100

80

60

40

20

0

−20

−40

−60

, FEEDBACK PIN CURRENT (nA)

FB

I

−80

−100

−40°C

25°C

125°C

Figure 9. Switch Saturation Voltage

, JUNCTION TEMPERATURE (°C)

T

J

Figure 10. Minimum Operating Voltage Figure 11. Feedback Pin Current

SWITCH CURRENT (A)

Adjustable Version Only

20 V

A

10 V

0

0.6 A

0.4 A

B

0.2 A

0

20 mV

C

AC

5 ms/DIV

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

V

= 5.0 V, 500 mA Load Current, L = 330 mH

A

B

20 mV

C

Figure 13. Discontinuous Mode Switching Waveform

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6

20 V

10 V

0

0.4 A

0.2 A

0

AC

5 ms/DIV

A: Output Pin Voltage, 10 V/DIV.
B: Inductor Current, 0.2 A/DIV.
C: Output Ripple Voltage, 20 mV/DIV, AC−Coupled

V

= 5.0 V, 100 mA Load Current, L = 100 mH

LM2574, NCV2574

out

out

F

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 16) (continued)

50 mV

A

AC

50 mV

A

AC

500 mA

B

0

A: Output Voltage, 50 mV/DIV, AC Coupled
B: 100 mA to 500 mA Load Pulse

Figure 14. 500 mA Load Transient Response for

Continuous Mode Operation, L = 330 mH, C

= 300 mF

200 mA

B

100 mA

0

200 ms/DIV200 ms/DIV

A: Output Voltage, 50 mV/DIV, AC Coupled
B: 50 mA to 250 mA Load Pulse

Figure 15. 250 mA Load Transient Response for

Discontinuous Mode Operation, L = 68 mH, C

= 470 m

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7

Fixed Output Voltage Versions

7.0 − 40 V

Unregulated

DC Input

LM2574, NCV2574

(3)

V

in

1

(12)

C

in

22 mF

Cin−22 mF, 60 V, Aluminium Electrolytic

− 220 mF, 25 V, Aluminium Electrolytic

C

out

D1 − Schottky, 1N5819
L1 − 330 mH, (For 5.0 V
R1 − 2.0 k, 0.1%
R2 − 6.12 k, 0.1%

LM2574

Fixed Output

2 Sig

Gnd

Gnd

(6) (4)

(14)

34ON

, 3.3 V

in

out

Feedback

1

Output

7

/OFFPwr

(5)

, use 100 mH)

L1

330 mH

D1
1N5819

C

out

220 mF

V

out

Load

7.0 V − 40 V
Unregulated

DC Input

V

in

1

C

in

22 mF

Adjustable Output Voltage Versions

Feedback

(3)

(14)

34ON/OFFPwr

+ V

ref

= 1.23 V, R1

ref

(5)

V

V

1

Output

7

ǒ

1.0 )

out

ref

–1.0Ǔ

R2
R1

Ǔ

LM2574

Adjustable

(12)

2 Sig

Gnd

NOTE: Pin numbers in ( ) are for the SO−16W package.

Gnd

(6) (4)

V

out

R2 + R1ǒ

Where V
between 1.0 kW and 5.0 kW

Figure 16. Test Circuit and Layout Guidelines

L1

330 mH

D1
1N5819

C

out

220 mF

R2

6.12 k

R1

2.0 k

V

5.0 V

Load

out

PCB LAYOUT GUIDELINES

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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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.

8

LM2574, NCV2574

PIN FUNCTION DESCRIPTION

Pin

SO−16W PDIP−8

12 5 V

14 7 Output This is the emitter of the internal switch. The saturation voltage V

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

5 3 ON/OFF It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the

Buck Converter Basics

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)

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

Symbol Description (Refer to Figure 1)

in

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 (C

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.

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.

total input supply current to approximately 80 mA. The input threshold voltage is typically 1.5 V.
Applying a voltage above this value (up to +V
pin is lower than 1.5 V or if this pin is left open, the regulator will be in the “on” condition.

in Figure 1).

in

) shuts the regulator off. If the voltage applied to this

in

DESIGN PROCEDURE

current loop. This removes the stored energy from the
inductor. The inductor current during this time is:

I

L(off)

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:

t

on

d +

, where T is the period of switching.

T

For the buck converter with ideal components, the duty

+

ǒ

Vin–V

L

out

Ǔ

t

on

cycle can also be described as:

Figure 18 shows the buck converter idealized waveforms
of the catch diode voltage and the inductor current.

V

L

on(SW)

+

of this output switch is

sat

ǒ

V

–V

out

D

L

V

out

d +

V

in

Ǔ

t

off

in

DV

C

out

R

Load

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

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9

Power

Switch

Off

VD(FWD)

Diode VoltageInductor Current

I

min

Diode Diode

Switch

Power

On

Power

Switch

Power
Switch

Off

Power
Switch

On

I

pk

Power
Switch

Figure 18. Buck Converter Idealized Waveforms

I

Load

Time

(AV)

Time

LM2574, NCV2574

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:

= Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V)

V

out

= Maximum Input Voltage

V

in(max)

I

Load(max)

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.

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 +V
located close to the IC using short leads. This capacitor should
have a low ESR (Equivalent Series Resistance) value.

3. Catch Diode Selection (D1)

A.Since the diode maximum peak current exceeds the

B.The reverse voltage rating of the diode should be at least

4. Inductor Selection (L1)

A.According to the required working conditions, select the

B.From the appropriate inductor selection guide, identify the

C.Select an appropriate inductor from the several different

= Maximum Load Current

and ground pin Gnd. This capacitor should be

in

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.

1.25 times the maximum input voltage.

correct inductor value using the selection guide from
Figures 19 to 23.

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.

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

)

)

ǒ

Vin* V

2L

out

Ǔ

t

on

Given Parameters:

= 5.0 V

V

out

= 15 V

V

in(max)

I

Load(max)

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)

A 22 mF, 25 V aluminium electrolytic capacitor located near
to the input and ground pins provides sufficient bypassing.

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

4. Inductor Selection (L1)

A.Use the inductor selection guide shown in Figure 20.

B.From the selection guide, the inductance area

C.Inductor value required is 330 mH. From Table 2, choose

= 0.4 A

suggested fast recovery diodes shown in Table 1.

intersected by the 15 V line and 0.4 A line is 330.

an inductor from any of the listed manufacturers.

where t

For additional information about the inductor, see the inductor
section in the “EXTERNAL COMPONENTS” section of this
data sheet.

is the “on” time of the power switch and

on

V

out

t

+

on

1.0

x

V

f

osc

in

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10

LM2574, NCV2574

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 (C

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 mF and 470 mF 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.

out

)

Procedure (Adjustable Output Version: LM2574−ADJ)

Procedure Example

Given Parameters:

= Regulated Output Voltage

V

out

= Maximum DC Input Voltage

V

in(max)

I

Load(max)

1. Programming Output Voltage

To select the right programming resistor R1 and R2 value (see
Figure 2) use the following formula:

where V

Resistor R1 can be between 1.0 kW and 5.0 kW. (For best
temperature coefficient and stability with time, use 1% metal
film resistors).

= Maximum Load Current

R2

ǒ

+ V

V

out

1.0 )

ref

R2 + R1

R1

V

out

ǒ

V

ref

Ǔ

* 1.0

Ǔ

= 1.23 V

ref

5. Output Capacitor Selection (C

A.C

= 100 mF to 470 mF standard aluminium electrolytic.

out

B.Capacitor voltage rating = 20 V.

Given Parameters:

= 24 V

V

out

= 40 V

V

in(max)

I

Load(max)

1. Programming Output Voltage (selecting R1 and R2)

Select R1 and R2 :

R2 + R1

R2 = 18.51 kW, choose a 18.7 kW metal film resistor.

= 0.4 A

R2

ǒ

1.0 )

= 1.23 Select R1 = 1.0 kW

V

out

V

out

ǒ

V

ref

Ǔ

R1

* 1.0Ǔ+ 1.0 k

out

)

10 V

ǒ

1.23 V

* 1.0

Ǔ

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 +V
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.

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.

and ground pin Gnd. This capacitor should be

in

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2. Input Capacitor Selection (Cin)

A 22 mF aluminium electrolytic capacitor located near the
input and ground pin provides sufficient bypassing.

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.

11

LM2574, NCV2574

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 ms] constant:

V

ExT+ (Vin* V

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 I
rating can also be determined by calculating the inductor
peak current:

I

+ I

)

p(max

where t

For additional information about the inductor, see the inductor
section in the “External Components” section of this data
sheet.

is the “on” time of the power switch and

on

)

out

Load(max

t

+

on

out

V

in

V

out

V

in

x

)

)

6

10

ƪ

Vxms

]

F[Hz

. The inductor current

Load

ǒ

Vin* V

2L

1.0

x

f

osc

out

ƫ

Ǔ

t

on

4. Inductor Selection (L1)
A.

Calculate E x TƪVxmsƫconstant :

ExT+ (40 * 24) x

B.

ExT+ 185ƪVxms

C.I

Load(max)

Inductance Region = 1000

D.Proper inductor value = 1000 mH

Choose the inductor from Table 2.

= 0.4 A

24
40

ƫ

x

1000

+ 105ƪVxms

52

ƫ

5. Output Capacitor Selection (C

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:

C

w 13,300

out

B.Capacitor values between 10 mF and 2000 mF 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.

)

out

V

in

(

)

max

ƪmFƫ

xLƪmH

ƫ

V

out

5. Output Capacitor Selection (C
A.

C

w 13,300 x

out

To achieve an acceptable ripple voltage, select
C

= 100 mF electrolytic capacitor.

out

24 x 1000

40

)

out

+ 22.2 mF

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12

LM2574, NCV2574

LM2574 Series Buck Regulator Design Procedures (continued)

Indicator Value Selection Guide (For Continuous Mode Operation)

60
20

15
12

10

9.0

8.0

7.0

6.0

, MAXIMUM INPUT VOLTAGE (V)

in

5.0

60

40
30

25

20

18
17

16

, MAXIMUM INPUT VOLTAGE (V) V

15

in

V

14

680

470

330

220

150

100

0.50.40.30.20.150.1 0.50.40.30.20.150.1

IL, MAXIMUM LOAD CURRENT (A)

60

1000

30

20
15

12

10

9.0

8.0

, MAXIMUM INPUT VOLTAGE (V)V

in

V

7.0

Figure 19. LM2574−3.3

2200

1500

1000

680

I

, MAXIMUM LOAD CURRENT (A)

L

470

330

220

0.50.40.30.20.150.1 0.50.40.30.20.150.1

60

40

30

25

22

20

19

, MAXIMUM INPUT VOLTAGE (V)

18

in

17

2200

1500

Figure 21. LM2574−12 Figure 22. LM2574−15

680

470

330

I

, MAXIMUM LOAD CURRENT (A)

L

Figure 20. LM2574−5

1000

680

470

IL, MAXIMUM LOAD CURRENT (A)

220

150

330

220

250
200

2200

150

100

80

60
50
40

30

ET, VOLTAGE TIME (V s)μ

20
15

10

1500

1000

680

470

I

, MAXIMUM LOAD CURRENT (A)

L

330

220

150

100

68

0.50.40.30.20.150.1

Figure 23. LM2574−ADJ

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13

LM2574, NCV2574

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

V

R

Schottky Fast Recovery

1.0 Amp Diodes

Inductor

Value

68 mH
100 mH
150 mH
220 mH
330 mH

20 V

30 V

40 V

50 V MBR150
60 V MBR160

1N5817

MBR120P

1N5818

MBR130P

1N5819

MBR140P

MUR110

(rated to 100 V)

Table 2. Inductor Selection Guide

Pulse Engineering Tech 39 Renco NPI

* 55 258 SN RL−1284−68 NP5915

* 55 308 SN RL−1284−100 NP5916
52625 55 356 SN RL−1284−150 NP5917
52626 55 406 SN RL−1284−220 NP5918/5919
52627 55 454 SN RL−1284−330 NP5920/5921

470 mH

680 mH
1000 mH
1500 mH
2200 mH

* : Contact Manufacturer

52628 * RL−1284−470 NP5922
52629 55 504 SN RL−1284−680 NP5923
52631 55 554 SN RL−1284−1000 *

* * RL−1284−1500 *
* * RL−1284−2200 *

Table 3. Example of Several Inductor Manufacturers Phone/Fax Numbers

Pulse Engineering Inc.

Pulse Engineering Inc. Europe

Renco Electronics Inc.

Tech 39

NPI/APC

Phone
Fax

Phone
Fax

Phone
Fax

Phone
Fax

Phone
Fax

+ 1−619−674−8100
+ 1−619−674−8262

+ 353−9324−107
+ 353−9324−459

+ 1−516−645−5828
+ 1−516−586−5562

+ 33−1−4115−1681
+ 33−1−4709−5051

+ 44−634−290−588

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14

LM2574, NCV2574

EXTERNAL COMPONENTS

Input Capacitor (Cin)

The Input Capacitor Should Have a Low ESR

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.

RMS Current Rating of C

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

where d is the duty cycle, for a continuous mode buck
regulator

and

Output Capacitor (C

d +

t

on

+

|V

T

out

For low output ripple voltage and good stability , low ES R
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 a nd
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

in

u 1.2 x d x I

t

d +

|V

|

out

| ) V

)

out

Load

V

on

out

+

V

T

in

for a buck−boost regulator.

in

voltage ratings may be needed to provide low ESR values,
that are required for low output ripple voltage.

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 W), 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.

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.

Use a Schottky or a Soft Switching
Ultra−Fast Recovery Diode

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.

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LM2574, NCV2574

Inductor

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.

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

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.

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.

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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LM2574, NCV2574

0.5 A

Inductor

Current

Waveform

0 A

0.5 A

Power

Switch

Current

Waveform

0 A

HORIZONTAL TIME BASE: 5.0 ms/DIV

Figure 24. Continuous Mode Switching

Current Waveforms

GENERAL RECOMMENDATIONS

Output Voltage Ripple and Transients

Source of the Output Ripple

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.

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 ms/DIV

Figure 26. Output Ripple Voltage Waveforms

VERTRICAL RESOLUTION 20 mV/DIV

0.1 A

Inductor

Current

Waveform

Current

Waveform

VERTRICAL RESOLUTION 200 mADV

Minimizing the Output Ripple

Power

Switch

0 A

0.1 A

0 A

HORIZONTAL TIME BASE: 5.0 ms/DIV

Figure 25. Discontinuous Mode Switching

Current Waveforms

In order to minimize t he o utput r ipple v oltage i t is p ossible
to enlarge the inductance value of the inductor L1 and/or to
use a larger value o utput c apacitor. There is also another way
to smooth the output by means of an additional LC filter
(20 mH, 100 mF), 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.

Heatsinking and Thermal Considerations

The LM2574 is available in both 8−pin DIP and SO−16L
packages. When used i n t he t ypical a pplication t he c opper l ead
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.

VERTICAL RESOLUTION 100 mADV

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17

LM2574, NCV2574

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.

Thermal Analysis and Design

The following procedure must be performed to determine
the operating junction temperature. First determine:

1. P

D(max)

− maximum regulator power dissipation in
the application.

2. T

A(max)

− maximum ambient temperature in the
application.

3. T

J(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. R

5. R

q

q

− package thermal resistance junction−case.

JC

− package thermal resistance junction−ambient.

JA

(Refer to Absolute Maximum Ratings on page 2 of this data
sheet or R

JC

q

and R

JA

q

values).

The following formula is to calculate the approximate

total power dissipated by the LM2574:

PD = (Vin x IQ) + d x I

Load

x V

sat

where d is the duty cycle and for buck converter

V

t

on

d +

IQ (quiescent current) and V

O

+

,

V

T

in

can be found in the

sat

LM2574 data sheet,

V

is minimum input voltage applied,

in

V

is the regulator output voltage,

O

I

is the load current.

Load

8.0 to 25 V

Unregulated

DC Input

+V

in

LM2574−12

5

C

in

22 mF

Figure 27. Inverting Buck−Boost Develops −12 V

(12)

Gnd

(6)

2 Sig

Gnd

(4)

Feedback

(3)

1

Output

(14)

7

34ON

/OFFPwr

(5)

L1

68 mH

D1
MBR150

−12 V @ 100 mA
Regulated

Output

C

out

680 mF

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:

TJ = (R

where (R

)(PD) represents the junction temperature rise

JA

q

caused by the dissipated power and T

)(PD) + T

q

JA

A

is the maximum

A

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.

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 t o g enerate 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.

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18

LM2574, NCV2574

n

n

The high input current needed for startup is now partially

supplied by the input capacitor C

Design Recommendations:

.

in

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 C

out

.

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 mF).

The recommended range of inductor values for the
inverting converter design is between 68 mH and 220 mH. To
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.

12 to 25 V

Unregulated

DC Input

C

in

22 mF

0.1 mF

/50 V

C1

+V

in

5

R1

47 k

LM2574−12

(12)

(5)

R2
47 k

43ON/OFF Pwr

Gnd

Feedback

(3)

1

Output

(14)

7

2 Sig

Gnd

(6) (4)

L1

68 mH

D1
MBR150

C

out

680 mF
/16 V

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.

+V

in

C

in

Shutdown

5.0 V

0

Input

Off

On

NOTE: This picture does not show the complete circuit.

R3

470

22 mF

Figure 29. Inverting Buck−Boost Regulator Shutdow

Circuit Using an Optocoupler

R1

47 k

MOC8101

+V

in

LM2574−XX

(12)

5

3 Gnds

ON/OFF

(5)

R2
47 k

and

2

Pins

4

(4)

and

(6)

−V

out

−12 V @ 100 mA
Regulated

Output

Figure 28. Inverting Buck−Boost Regulator with

Delayed Startup

The following formula is used to obtain the peak inductor

current:

where

I

peak

t

on

[

+

Vin) |VO|

I

Load

|VO|

ǒ

Vin) |VO|

V

in

x

1.0

f

osc

Ǔ

)

, and f

Vinxt

2L

= 52 kHz.

osc

on

1

Under normal continuous inductor current operating

conditions, the worst case occurs when V

is minimal.

in

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

5.6 k

R2

Shutdown
Input

+V

Q1
2N3906

in

(12)

5

LM2574−XX

3 Gnds

ON/OFF

(5)

R1
12 k

and

2

4

Pins

−V

(4)

and

(6)

out

+V

+V

in

NOTE: This picture does not show the complete circuit.

Off

0

On

C

in

22 mF

Figure 30. Inverting Buck−Boost Regulator Shutdow

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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19

LM2574, NCV2574

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.

1

C

in

22 mF

V

in

−5.0 to −12 V

+V

in

5

LM2574−12

(12)

Gnd

330 mH

(3)

Feedback

Output

(14)

7

34

2

L1

Sig
Gnd

(4)(6)

ON

/OFFPwr

(5)

D1

1N5817

Load Current
60 mA for V
120 mA for V

C

out

1000 mF

V

out

= −5.2 V

in

= −7.0 V

in

= −12 V

Figure 31. Negative Boost Regulator

Design Recommendations:

The same design rules as for the previous inverting
buck−boost converter can be applied. The output capacitor
C

must be chosen larger than what would be required for

out

a standard buck converter. Low input voltages or high output
currents require a large value output capacitor (in the range
of thousands of mF). 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.

Delayed Startup

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.

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.

+V

in

C

in

22 mF

NOTE: This picture does not show the complete circuit.

Figure 32. Delayed Startup Circuitry

Undervoltage Lockout

+V

C1

0.1 mF

R1

47 k

in

LM2574−XX

5

(12)

3 Gnds

ON/OFF

(5)

R2
47 k

and

Pins

(4)

and

(6)

2

4

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:

R2

Vth[ V

+V

in

R3

R1

10 k

1N5242B

10 k

NOTE: This picture does not show the complete circuit.

47 k

Z1

Q1
2N3904

R2

Z1

)

ǒ

1.0 )

+V

C

in

22 mF

Figure 33. Undervoltage Lockout Circuit for

Buck Converter

Ǔ

V

BE

R1

in

LM2574−XX

5

(12)

3 Gnds

ON/OFF

(5)

(Q1)

2

and

4

Pins

(4)

and

(6)

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20

LM2574, NCV2574

+V

in

R2

15 k

1N5242

15 k

NOTE: This picture does not show the complete circuit (see Figure 27).

R3

68 k

Z1

Q1
2N3904

R1

Figure 34. Undervoltage Lockout Circuit for

Buck−Boost Converter

+V

in

LM2574−XX

(12)

5

C

in

3 Gnds

22 mF

ON/OFF

(5)

and

Pins

(4)

and

(6)

−V

out

2

4

Adjustable Output, Low−Ripple Power Supply

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.

40 V Max
Unregulated
DC Input

C

22 mF

Feedback

(3)

+V

in

LM2574−ADJ

5

(12)

in

2 Sig

Gnd

(6) (4)

Gnd

1

Output

(14)

7

/OFFPwr

34ON

(5)

L1

150 mH

D1
1N5819

C

out

1000 mF

R2
50 k

R1

1.1 k

Figure 35. 1.2 to 35 V Adjustable 500 mA Power Supply with Low Output Ripple

L2

20 mH

C1

100 mF

Optional Output

Ripple Filter

Output

Voltage

1.2 to 35 V @ 0.5 A

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21

LM2574, NCV2574

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)

Unregulated

+V

= 7.0 to 40 V

in

+V

DC Input

in

5

(12)

LM2574−5

2 Sig

Gnd

Gnd

(6)

(4)

C1

22 mF

Gnd Gnd

C1 − 22 mF, 63 V, Aluminium Electrolytic
C2 − 220 mF, 16 V, Aluminium Electrolytic
D1 − 1.0 A, 40 V, Schottky Rectifier, 1N5819
L1 − 330 mH, RL−1284−330, Renco Electronics

Figure 36. Schematic Diagram of the LM2574−5 Step−Down Converter

1

Output

(14)

7

/OFFPwr

34ON

330 mH

(5)

D1
1N5819

L1

Regulated Output
+V

= 5.0 V @ 0.5 A

out

C2
220 mF

LM2574−5.0

+

+V

C1 C2

in

U1

D1

L1

Gnd

NOTE: Not to scale.

Figure 37. PC Board Layout Component Side

Gnd

+

V

out

NOTE: Not to scale.

Figure 38. PC Board Layout Copper Side

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22

LM2574, NCV2574

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

Feedback

Unregulated
DC Input

+Vin = 7.0 to 40 V

C1

22 mF

Gnd Gnd

+V

in

5

(12)

LM2574−ADJ

2 Sig

Gnd

(6)

(3)

1

Output

(14)

7

34ON

Gnd

(4)

/OFFPwr

(5)

330 mH

D1
1N5819

L1

R2

6.12 kW

C2
220 mF

R1

2.0 kW

L2

22 mH

100 mF

C3

Regulated
Output Filtered

V

= 5.0 V @ 0.5 A

out

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)

LM2574

U1

D1

NOTE: Not to scale.

C2

R1 R2

L1

+V

Gnd

+

C1

in

Figure 40. PC Board Layout Component Side

C1 − 22 mF, 63 V, Aluminium Electrolytic
C2 − 220 mF, 16 V, Aluminium Electrolytic
C3 − 100 mF, 16 V Aluminium Electrolytic
D1 − 1.0 A, 40 V, Schottky Rectifier, 1N5819
L1 − 330 mH, RL−1284−330, Renco Electronics
L2 − 25 mH, SFT52501, TDK
R1 − 2.0 kW, 0.1%, 0.25 W
R2 − 6.12 kW, 0.1%, 0.25 W

C3+Gnd

+

L2

V

out

Figure 41. PC Board Layout Copper Side

Output

Ripple Filter

NOTE: Not to scale.

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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23

LM2574, NCV2574

ORDERING INFORMATION

Nominal Output

Device

LM2574DW−ADJ
LM2574DW−ADJR2 SO−16 WB
LM2574DW−ADJR2G SO−16 WB

LM2574N−ADJ PDIP−8
LM2574N−ADJG PDIP−8

NCV2574DW−ADJR2 SO−16 WB
NCV2574DW−ADJR2G SO−16 WB

LM2574N−3.3
LM2574N−3.3G PDIP−8

LM2574N−5
LM2574N−5G PDIP−8

LM2574N−12
LM2574N−12G PDIP−8

LM2574N−15
LM2574N−15G

†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging

Specifications Brochure, BRD8011/D.

*NCV devices: T

change control.

= −40°C, T

low

Voltage

1.23 V to 37 V TJ = −40° to +125°C

3.3 V TJ = −40° to +125°C

5.0 V TJ = −40° to +125°C

12 V TJ = −40° to +125°C

15 V TJ = −40° to +125°C

= +125°C. Guaranteed by Design. NCV prefix is for automotive and other applications requiring site and

high

Operating Junction

Temperature Range

Package Shipping

SO−16 WB 47 Units/Rail

1000 Units/Tape & Reel

(Pb−Free)

50 Units/Rail

(Pb−Free)

1000 Units/Tape & Reel

(Pb−Free)

PDIP−8

(Pb−Free)

PDIP−8

(Pb−Free)

PDIP−8

(Pb−Free)

PDIP−8
PDIP−8

(Pb−Free)

50 Units/Rail

16

LM2574DW−A

AWLYYWWG

1

DJ

SO−16 WB
DW SUFFIX
CASE 751G

16

1

MARKING DIAGRAMS

*NCV part

CV2574DW−A

DJ

AWLYYWWG

xxx = 3.3, 5.0, 12, 15, or ADJ
A = Assembly Location
WL = Wafer Lot
Y = Year
WW = Work Week
G = Pb−Free Package

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24

8

2574−xxx

1

AWL

YYWWG

PDIP−8

N SUFFIX

CASE 626

8

2574N−xxx

AWL

YYWWG

1

LM2574, NCV2574

PACKAGE DIMENSIONS

SO−16 WB

DW SUFFIX

CASE 751G−03

ISSUE C

16 9

M

B

H8X

M

0.25

0.25 B

14X

D

A

q

E

_

h X 45

81

B16X

M

S

A

T

B

S

A

SEATING

T

PLANE

C

e

A1

L

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.

MILLIMETERS

DIM MIN MAX

A 2.35 2.65

A1 0.10 0.25

B 0.35 0.49
C 0.23 0.32
D 10.15 10.45
E 7.40 7.60
e 1.27 BSC
H 10.05 10.55
h 0.25 0.75
L 0.50 0.90
q 0 7

__

PDIP−8

N SUFFIX

CASE 626−05

ISSUE L

NOTE 2

−T−

SEATING
PLANE

H

58

−B−

14

F

−A−

C

N

D

G

0.13 (0.005) B

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.

DIM MIN MAX MIN MAX

A 9.40 10.16 0.370 0.400
B 6.10 6.60 0.240 0.260
C 3.94 4.45 0.155 0.175

L

J

K

M

M

A

T

M

M

D 0.38 0.51 0.015 0.020
F 1.02 1.78 0.040 0.070
G 2.54 BSC 0.100 BSC
H 0.76 1.27 0.030 0.050

J 0.20 0.30 0.008 0.012
K 2.92 3.43 0.115 0.135
L 7.62 BSC 0.300 BSC
M −−− 10 −−− 10
N 0.76 1.01 0.030 0.040

INCHESMILLIMETERS

__

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25

LM2574, NCV2574

ON Semiconductor and are registered 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 purpose, nor does SCILLC assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All
operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights
nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications
intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should
Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates,
and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death
associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal
Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.

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For additional information, please contact your
local Sales Representative.

LM2574/D

26