LM2575-15TV_技术文档

Order this document by LM2575/D

EASY SWITCHER

1.0 A STEP–DOWN

VOLTAGE REGULATOR

SEMICONDUCTOR

TECHNICAL DATA

The LM2575 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 1.0 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 optimised for use with the LM2575 are offered by several different

inductor manufacturers.

Since the LM2575 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 many cases, the power

dissipated by the LM2575 regulator is so low, that no heatsink is required or

its size could be reduced dramatically.

The LM2575 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 80 µA typical standby current. The output switch includes

cycle–by–cycle current limiting, as well as thermal shutdown for full

protection under fault conditions.

T SUFFIX

PLASTIC PACKAGE

CASE 314D

1

5

Pin 1. V

in

2. Output

3. Ground

4. Feedback

5. ON/OFF

TV SUFFIX

1

PLASTIC PACKAGE

CASE 314B

5

Features

Heatsink surface

connected to Pin 3.

• 3.3 V, 5.0 V, 12 V, 15 V, and Adjustable Output Versions

• Adjustable Version Output Voltage Range of 1.23 V to 37 V ±4%

Maximum Over Line and Load Conditions

• Guaranteed 1.0 A Output Current

D2T SUFFIX

PLASTIC PACKAGE

• Wide Input Voltage Range: 4.75 V to 40 V

• Requires Only 4 External Components

• 52 kHz Fixed Frequency Internal Oscillator

• TTL Shutdown Capability, Low Power Standby Mode

• High Efficiency

1

CASE 936A

2

5

(D PAK)

Heatsink surface (shown as terminal 6 in case outline

drawing) is connected to Pin 3.

• Uses Readily Available Standard Inductors

• Thermal Shutdown and Current Limit Protection

DEVICE TYPE/NOMINAL OUTPUT VOLTAGE

Applications

LM2575–3.3

LM2575–5

LM2575–12

LM2575–15

LM2575–Adj

3.3 V

5.0 V

12 V

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

• Efficient Pre–Regulator for Linear Regulators

• On–Card Switching Regulators

15 V

1.23 V to 37 V

• Positive to Negative Converters (Buck–Boost)

• Negative Step–Up Converters

• Power Supply for Battery Chargers

ORDERING INFORMATION

Operating

Temperature Range

Device

Package

LM2575T–**

Straight Lead

Vertical Mount

Surface Mount

LM2575TV–** T = –40° to +125°C

J

LM2575D2T–**

** = Voltage Option, ie. 3.3, 5.0, 12, 15 V and

** =\Adjustable Output.

This document contains information on a new product. Specifications and information herein

Motorola, Inc. 1997

Rev 1

are subject to change without notice.

1

MOTOROLA ANALOG IC DEVICE DATA

LM2575

Figure 1. Block Diagram and Typical Application

Typical Application (Fixed Output Voltage Versions)

Feedback

4

L1

330

7.0 V – 40 V

Unregulated

DC Input

+V

in

LM2575

µ

H

Output

2

1

5.0 V Regulated

C

µ

in

F

Output 1.0 A Load

D1

1N5819

100

C

out

3

Gnd

5

ON/OFF

330 µF

Representative Block Diagram and Typical Application

+V

in

ON/OFF

Unregulated

DC Input

3.1 V Internal

Regulator

Output

Voltage Versions

R2

ON/OFF

(Ω)

1

5

C

3.3 V

5.0 V

12 V

15 V

1.7 k

3.1 k

8.84 k

11.3 k

in

4

Feedback

Current

Limit

For adjustable version

R1 = open, R2 = 0

R2

Fixed Gain

Error Amplifier

Ω

Comparator

Driver

Regulated

Output

R1

1.0 k

Latch

Freq

Shift

L1

V

out

Output

18 kHz

1.0 Amp

Switch

2

Gnd

1.235 V

C

D1

out

Band–Gap

Reference

Thermal

Shutdown

52 kHz

Oscillator

3

Reset

Load

This device contains 162 active transistors.

ABSOLUTE MAXIMUM RATINGS (Absolute Maximum Ratings indicate limits beyond

which damage to the device may occur.)

Rating

Symbol

Value

45

Unit

V

Maximum Supply Voltage

V

in

ON/OFF Pin Input Voltage

–

–0.3 V ≤ V ≤ +V

V

in

Output Voltage to Ground (Steady–State)

–

–1.0

V

Power Dissipation

Case 314B and 314D (TO–220, 5–Lead)

Thermal Resistance, Junction–to–Ambient

Thermal Resistance, Junction–to–Case

P

Internally Limited

W

D

R

65

°C/W

W

θ

JA

JC

R

5.0

Internally Limited

70

2

Case 936A (D PAK)

P

D

Thermal Resistance, Junction–to–Ambient

(Figure 34)

R

°C/W

θ

JA

Thermal Resistance, Junction–to–Case

R

5.0

–65 to +150

3.0

°C/W

°C

θ

JC

Storage Temperature Range

T

stg

Minimum ESD Rating (Human Body Model: C

–

kV

= 100 pF, R = 1.5 kΩ)

Lead Temperature (Soldering, 10 s)

Maximum Junction Temperature

–

260

150

°C

T

J

NOTE: ESD data available upon request.

2

MOTOROLA ANALOG IC DEVICE DATA

LM2575

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

Supply Voltage

Symbol

Value

–40 to +125

40

Unit

°C

T

J

V

in

V

SYSTEM PARAMETERS ([Note 1] Test Circuit Figure 14)

ELECTRICAL CHARACTERISTICS (Unless otherwise specified, V = 12 V for the 3.3 V, 5.0 V, and Adjustable version, V = 25 V for

in

the 12 V version, and V = 30 V for the 15 V version. I

= 200 mA. For typical values T = 25°C, for min/max values T is the operating

J J

in

Load

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

Characteristics

Symbol

Min

Typ

Max

Unit

LM2575–3.3 ([Note 1] Test Circuit Figure 14)

Output Voltage (V = 12 V, I

= 0.2 A, T = 25°C)

V

out

3.234

3.3

3.366

V

in

Load

J

Output Voltage (4.75 V ≤ V ≤ 40 V, 0.2 A ≤ I

≤ 1.0 A)

V

out

in

Load

T = 25°C

T = –40 to +125°C

J

3.168

3.135

3.3

–

3.432

3.465

J

Efficiency (V = 12 V, I

= 1.0 A)

Load

η

–

75

–

%

in

LM2575–5 ([Note 1] Test Circuit Figure 14)

Output Voltage (V = 12 V, I = 0.2 A, T = 25°C)

V

out

4.9

5.0

5.1

V

in

Load

J

Output Voltage (8.0 V ≤ V ≤ 40 V, 0.2 A ≤ I

≤ 1.0 A)

V

out

in

Load

T = 25°C

T = –40 to +125°C

J

4.8

4.75

5.0

–

5.2

5.25

J

Efficiency (V = 12 V, I

= 1.0 A)

Load

η

–

77

–

%

in

LM2575–12 ([Note 1] Test Circuit Figure 14)

Output Voltage (V = 25 V, I = 0.2 A, T = 25°C)

V

out

11.76

12

12.24

V

in

Load

J

Output Voltage (15 V ≤ V ≤ 40 V, 0.2 A ≤ I

≤ 1.0 A)

V

out

in

Load

T = 25°C

T = –40 to +125°C

J

11.52

11.4

12

–

12.48

12.6

J

Efficiency (V = 15V, I

= 1.0 A)

η

–

88

–

%

in

Load

LM2575–15 ([Note 1] Test Circuit Figure 14)

Output Voltage (V = 30 V, I = 0.2 A, T = 25°C)

V

out

14.7

15

15.3

V

in

Load

J

Output Voltage (18 V ≤ V ≤ 40 V, 0.2 A ≤ I

≤ 1.0 A)

V

out

in

Load

T = 25°C

T = –40 to +125°C

J

14.4

14.25

15

–

15.6

15.75

J

Efficiency (V = 18 V, I

= 1.0 A)

Load

η

–

88

–

%

in

LM2575 ADJUSTABLE VERSION ([Note 1] Test Circuit Figure 14)

Feedback Voltage (V = 12 V, I = 0.2 A, V = 5.0 V, T = 25°C)

V

1.217

1.23

1.243

V

in

Load

out

J

FB

Feedback Voltage (8.0 V ≤ V ≤ 40 V, 0.2 A ≤ I

≤ 1.0 A, V = 5.0 V)

V

FB

in

Load

out

T = 25°C

T = –40 to +125°C

J

1.193

1.18

1.23

–

1.267

1.28

J

Efficiency (V = 12 V, I

= 1.0 A, V = 5.0 V)

η

–

77

–

%

in

Load

out

NOTES: 1. External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the

LM2575 is used as shown in the Figure 14 test circuit, system performance will be as shown in system parameters section.

2. Tested junction temperature range for the LM2575:

T

low

= –40°C

T

high

= +125°C

3

MOTOROLA ANALOG IC DEVICE DATA

LM2575

DEVICE PARAMETERS

ELECTRICAL CHARACTERISTICS (Unless otherwise specified, V = 12 V for the 3.3 V, 5.0 V, and Adjustable version, V = 25 V for

in

the 12 V version, and V = 30 V for the 15 V version. I

= 200 mA. For typical values T = 25°C, for min/max values T is the operating

J J

in

Load

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

Characteristics

Symbol

Min

Typ

Max

Unit

ALL OUTPUT VOLTAGE VERSIONS

Feedback Bias Current (V = 5.0 V [Adjustable Version Only])

I

b

nA

out

T = 25°C

T = –40 to +125°C

J

–

25

–

100

200

J

Oscillator Frequency [Note 3]

f

kHz

V

osc

T = 25°C

–

47

42

52

–

58

63

J

T = 0 to +125°C

J

T = –40 to +125°C

J

Saturation Voltage (I = 1.0 A [Note 4])

V

sat

out

T = 25°C

T = –40 to +125°C

J

–

1.0

–

1.2

1.3

J

Max Duty Cycle (“on”) [Note 5]

DC

94

98

–

%

A

Current Limit (Peak Current [Notes 4 and 3])

I

CL

T = 25°C

T = –40 to +125°C

J

1.7

1.4

2.3

–

3.0

3.2

J

Output Leakage Current [Notes 6 and 7], T = 25°C

Output = 0 V

Output = –1.0 V

I

mA

µA

V

J

L

–

0.8

6.0

2.0

20

Quiescent Current [Note 6]

I

Q

T = 25°C

–

5.0

–

9.0

11

J

T = –40 to +125°C

J

Standby Quiescent Current (ON/OFF Pin = 5.0 V (“off”))

I

stby

T = 25°C

–

80

–

200

400

J

T = –40 to +125°C

J

ON/OFF Pin Logic Input Level (Test Circuit Figure 14)

V

out

= 0 V

V

IH

T = 25°C

2.2

2.4

1.4

–

J

T = –40 to +125°C

J

V

out

= Nominal Output Voltage

V

IL

T = 25°C

–

1.2

–

1.0

0.8

J

T = –40 to +125°C

J

ON/OFF Pin Input Current (Test Circuit Figure 14)

µA

ON/OFF Pin = 5.0 V (“off”), T = 25°C

I

–

15

0

30

5.0

J

IH

ON/OFF Pin = 0 V (“on”), T = 25°C

J

IL

NOTES: 3. 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 dissipation of the IC by lowering the

minimum duty cycle from 5% down to approximately 2%.

4. Output (Pin 2) sourcing current. No diode, inductor or capacitor connected to output pin.

5. Feedback (Pin 4) removed from output and connected to 0 V.

6. 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”.

7. V = 40 V.

in

4

MOTOROLA ANALOG IC DEVICE DATA

LM2575

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 14)

Figure 2. Normalized Output Voltage

Figure 3. Line Regulation

0.6

0.4

0.2

1.0

0.8

0.6

0.4

0.2

0

V

= 20 V

= 200 mA

in

I

T

= 200 mA

= 25°C

Load

I

Load

Normalized at

= 25

J

T

°C

J

3.3 V, 5.0 V and Adj

0

–0.2

12 V and 15 V

–0.4

–0.6

–0.2

–50

–25

0

25

50

75

C)

100

125

0

5.0

10

15

20

25

30

35

40

T , JUNCTION TEMPERATURE (

°

V , INPUT VOLTAGE (V)

J

in

Figure 4. Switch Saturation Voltage

Figure 5. Current Limit

1.2

1.1

1.0

3.0

2.5

2.0

1.5

0.9

0.8

0.7

–40°C

25°C

1.0

0.5

0

0.6

0.5

0.4

125°C

V

= 25 V

in

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

–50

–25

0

25

50

75

100

125

SWITCH CURRENT (A)

T , JUNCTION TEMPERATURE (

°C)

J

Figure 6. Dropout Voltage

Figure 7. Quiescent Current

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

20

V = 5.0 V

out

Measured at

Ground Pin

∆

V

= 5%

= 0.2 Ω

out

18

16

14

12

R

ind

I

= 1.0 A

Load

T

= 25°C

J

I

= 1.0 A

Load

10

8.0

6.0

4.0

= 200 mA

Load

I

= 200 mA

Load

–50

–25

0

25

50

75

100

125

0

5.0

10

15

20

25

30

35

40

T , JUNCTION TEMPERATURE (

°C)

V , INPUT VOLTAGE (V)

J

in

5

MOTOROLA ANALOG IC DEVICE DATA

LM2575

Figure 8. Standby Quiescent Current

Figure 9. Standby Quiescent Current

120

100

80

120

100

80

T

= 25°C

V

= 12 V

J

in

= 5.0 V

ON/OFF

60

40

20

0

20

0

5.0

10

15

20

25

30

35

40

–50

–25

0

25

50

75

C)

100

125

V , INPUT VOLTAGE (V)

T , JUNCTION TEMPERATURE (

°

in

J

Figure 10. Oscillator Frequency

Figure 11. Feedback Pin Current

2.0

0

40

20

Adjustable

Version Only

V

= 12 V

in

Normalized at 25°C

–2.0

–4.0

0

–20

–40

–6.0

–8.0

–10

–50

–25

0

25

50

75

C)

100

125

–50

–25

0

25

50

75

C)

100

125

T , JUNCTION TEMPERATURE (

°

T , JUNCTION TEMPERATURE (

°

J

Figure 12. Switching Waveforms

Figure 13. Load Transient Response

10 V

0

100

OUTPUT

VOLTAGE

(PIN 2)

0

OUTPUT

CURRENT

(PIN 2)

1.0 A

–100

0

1.0 A

0.5 A

1.0

0.5

INDUCTOR

CURRENT

0

OUTPUT

RIPPLE

VOLTAGE

20 mV

/DIV

5.0

µs/DIV

100 µs/DIV

6

MOTOROLA ANALOG IC DEVICE DATA

LM2575

Figure 14. Typical Test Circuit

5.0 Output Voltage Versions

Feedback

4

V

in

V

out

L1

LM2575–5

Regulated

Output

330

µH

+

1

Output

2

3

Gnd

5

ON/OFF

V

in

Unregulated

DC Input

8.0 V – 40 V

C

100

C

out

330

/16 V

in

µF/50 V

µ

F

D1

1N5819

Load

–

Adjustable Output Voltage Versions

Feedback

4

V

in

V

LM2575

Adjustable

out

L1

Regulated

Output

330

µH

+

1

Output

2

3

Gnd

5

ON/OFF

Unregulated

DC Input

8.0 V – 40 V

R2

R1

C

100

C

out

330

/16 V

in

µF/50 V

µ

F

D1

1N5819

Load

–

R2

R1

V

1

out

ref

V

out

R2

R1

– 1

V

ref

Where V = 1.23 V, R1

ref

between 1.0 k

Ω and 5.0 kΩ

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 14, 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.

On the other hand, the PCB area connected to the Pin 2

(emitter of the internal switch) of the LM2575 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

LM2575 regulator.

7

MOTOROLA ANALOG IC DEVICE DATA

LM2575

PIN FUNCTION DESCRIPTION

Symbol

Description (Refer to Figure 1)

1

V

in

This pin is the positive input supply for the LM2575 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 in Figure 1).

in

2

Output

This is the emitter of the internal switch. The saturation voltage V of this output switch is typically 1.0 V.

sat

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.

3

4

Gnd

Circuit ground pin. See the information about the printed circuit board layout.

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 LM2575 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

ON/OFF

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 1.4 V. Applying a

voltage above this value (up to +V ) shuts the regulator off. If the voltage applied to this pin is lower than

in

1.4 V or if this pin is connected to ground, the regulator will be in the “on” condition.

DESIGN PROCEDURE

Buck Converter Basics

V

– V

L

t

out

D

off

The LM2575 is a “Buck” or Step–Down Converter which is

the most elementary forward–mode converter. Its basic

schematic can be seen in Figure 15.

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:

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:

t

on

T

d

, where T is the period of switching.

For the buck converter with ideal components, the duty

cycle can also be described as:

V

out

d

V

– V

t

on

V

out

in

I

L(on)

L

Figure 16 shows the buck converter idealized waveforms

of the catch diode voltage and the inductor current.

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.

Figure 16. Buck Converter Idealized Waveforms

V

on(SW)

Figure 15. Basic Buck Converter

Power

Switch

Power

Switch

Off

Power

Switch

Off

Power

Switch

On

L

V

out

Power

Switch

On

C

out

D1

R

Load

V

in

Time

V (FWD)

D

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 catch dioded. Current now

flows through the catch diode thus maintaining the load

current loop. This removes the stored energy from the

inductor. The inductor current during this time is:

I

pk

I

(AV)

Load

I

min

Power

Switch

Power

Switch

Diode

Time

8

MOTOROLA ANALOG IC DEVICE DATA

LM2575

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)

Given Parameters:

= 5.0 V

V

out

V

out

V

= Maximum DC Input Voltage

V

= 20 V

in(max)

I

= Maximum Load Current

I

= 0.8 A

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.

According to the required input voltage, output voltage, current

polarity and current value, use the LM2575–5 controller IC

2. Input Capacitor Selection (C )

in

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

A 47 µF, 25 V aluminium electrolytic capacitor located near to

the input and ground pins provides sufficient bypassing.

input pin +V and ground pin Gnd. This capacitor should be

in

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. For this example the current rating of the diode is 1.0 A.

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

LM2575 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.

B. Use a 30 V 1N5818 Schottky diode, or any of the suggested

fast recovery diodes shown in the Table 4.

4. Inductor Selection (L1)

A. According to the required working conditions, select the

correct inductor value using the selection guide from

Figures 17 to 21.

A. Use the inductor selection guide shown in Figures 17 to 21.

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.

B. From the selection guide, the inductance area intersected

by the 20 V line and 0.8 A line is L330.

C. Select an appropriate inductor from the several different

manufacturers part numbers listed in Table 1 or Table 2.

When using Table 2 for selecting the right inductor 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:

C. Inductor value required is 330 µH. From the Table 1 or

Table 2, choose an inductor from any of the listed

manufacturers.

V

–V

t

on

out

2L

in

I

Load(max)

p(max)

where t is the “on” time of the power switch and

on

V

out

1

t

x

on

V

f

osc

in

For additional information about the inductor, see the

inductor section in the “External Components” section of

this data sheet.

9

MOTOROLA ANALOG IC DEVICE DATA

LM2575

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

)

out

)

out

A. Since the LM2575 is a forward–mode switching regulator

with voltage mode control, its open loop 2–pole–2–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.

A. C = 100 µF to 470 µF standard aluminium electrolytic.

out

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 8V is appropriate, and a 10 V or

16 V rating is recommended.

B. Capacitor voltage rating = 16 V.

Procedure (Adjustable Output Version: LM2575–Adj)

Procedure

Example

Given Parameters:

V

out

= Regulated Output Voltage

V

out

= 8.0 V

V

= Maximum DC Input Voltage

V

= 12 V

in(max)

I

= Maximum Load Current

I

= 1.0 A

Load(max)

1. Programming Output Voltage

1. Programming Output Voltage (selecting R1 and R2)

To select the right programming resistor R1 and R2 value (see

Figure 14) use the following formula:

Select R1 and R2:

R2

R1

V

1.23 1

Select R1 = 1.8 kΩ

out

R2

R1

V

1

where V = 1.23 V

ref

out

ref

V

out

8.0 V

1.8 k

R2

R1

1

Resistor R1 can be between 1.0 k and 5.0 kΩ. (For best

temperature coefficient and stability with time, use 1% metal

film resistors).

V

1.23 V

ref

R2 = 9.91 kΩ, choose a 9.88 k metal film resistor.

V

out

R2

R1

– 1

V

ref

2. Input Capacitor Selection (C )

in

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

A 100 µF aluminium electrolytic capacitor located near the

input and ground pin provides sufficient bypassing.

input pin +V and ground pin Gnd This capacitor should be

in

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

LM2575 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.

A. For this example, a 3.0 A current rating is adequate.

B. Use a 20 V 1N5820 or MBR320 Schottky diode or any

suggested fast recovery diode in the Table 4.

10

MOTOROLA ANALOG IC DEVICE DATA

LM2575

Procedure (Adjustable Output Version: LM2575–Adj) (continued)

Procedure

Example

A. Calculate E x T [V x µs] constant:

8.0

4. Inductor Selection (L1)

A. Use the following formula to calculate the inductor Volt x

microsecond [V x µs] constant:

V

6

10

F[Hz]

1000

52

out

(

)

E x T

12 – 8.0 x

x

51 [V x s]

E x T

V

– V

x

[V x s]

out

in

12

V

on

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 21. 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.

B. E x T = 51 [V x µs]

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 21.

C. I

= 1.0 A

Load(max)

Inductance Region = L220

D. From the inductor code, identify the inductor value. Then

select an appropriate inductor from the Table 1 or Table 2.

The inductor chosen must be rated for a switching

D. Proper inductor value = 220 µH

Choose the inductor from the Table 1 or Table 2.

frequency of 52 kHz and for a current rating of 1.15 x I

The inductor current rating can also be determined by

calculating the inductor peak current:

.

Ioad

V

– V

t

on

out

2L

in

I

Load(max)

p(max)

where t is the “on” time of the power switch and

on

V

out

1

t

x

on

V

f

osc

in

For additional information about the inductor, see the

inductor section in the “External Components” section of

this data sheet.

5. Output Capacitor Selection (C

)

out

5. Output Capacitor Selection (C

)

out

A. Since the LM2575 is a forward–mode switching regulator

with voltage mode control, its open loop 2–pole–2–zero

frequency characteristic has the dominant pole–pair

determined by the output capacitor and inductor values.

A.

12

8.220

C

7.785

53 µF

out

To achieve an acceptable ripple voltage, select

C

= 100 µF electrolytic capacitor.

out

For stable operation, the capacitor must satisfy the

following requirement:

V

in(max)

C

7.785

[µF]

out

V

x L [µH]

out

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 8V is appropriate, and a 10 V

or 16 V rating is recommended.

11

MOTOROLA ANALOG IC DEVICE DATA

LM2575

INDUCTOR VALUE SELECTION GUIDE

Figure 17. LM2575–3.3

Figure 18. LM2575–5.0

60

20

60

40

25

20

H1000

H1500

L680

15

10

H1000

L680

L470

15

12

8.0

7.0

L470

L330

L220

10

L150

6.0

9.0

L220

8.0

L100

L150

5.0

7.0

0.2

0.3

0.4

0.5

0.6

0.8

1.0

0.2

0.3

0.4

0.5

0.6

0.7 0.8 0.9 1.0

I , MAXIMUM LOAD CURRENT (A)

L

I , MAXIMUM LOAD CURRENT (A)

L

Figure 19. LM2575–12

Figure 20. LM2575–15

60

H2200

40

30

40

35

30

H1500

H1000

H1500

H1000

25

H680

25

22

H680

H470

L220

20

18

17

20

19

L680

L470

16

L470

0.5

L330

15

18

L220

14

17

0.2

0.3

0.4

0.6

0.7 0.8 0.9 1.0

0.2

0.3

0.4

0.5

0.6

0.7 0.8 0.9 1.0

I , MAXIMUM LOAD CURRENT (A)

L

I , MAXIMUM LOAD CURRENT (A)

L

Figure 21. LM2575–Adj

200

150

125

H2200

H1500

H1000

H680

100

H470

80

70

60

L680

50

L470

L330

L220

40

30

20

L150

L100

0.2

0.3

0.4

0.5

0.6

0.7 0.8 0.9 1.0

I , MAXIMUM LOAD CURRENT (A)

L

NOTE: This Inductor Value Selection Guide is applicable for continuous mode only.

12

MOTOROLA ANALOG IC DEVICE DATA

LM2575

Table 1. Inductor Selection Guide

Inductor

Code

Inductor

Value

Pulse Eng

PE–92108

PE–53113

PE–52626

PE–52627

PE–53114

PE–52629

PE–53115

PE–53116

PE–53117

PE–53118

PE–53119

PE–53120

PE–53121

PE–53122

Renco

RL2444

RL1954

RL1953

RL1952

RL1951

RL1950

RL2445

RL2446

RL2447

RL1961

RL1960

RL1959

RL1958

RL2448

AIE

Tech 39

77 308 BV

77 358 BV

77 408 BV

77 458 BV

–

L100

L150

100 µH

150 µH

220 µH

330 µH

470 µH

680 µH

150 µH

220 µH

330 µH

470 µH

680 µH

1000 µH

1500 µH

2200 µH

415–0930

415–0953

415–0922

415–0926

415–0927

415–0928

415–0936

430–0636

430–0635

430–0634

415–0935

415–0934

415–0933

415–0945

L220

L330

L470

L680

77 508 BV

77 368 BV

77 410 BV

77 460 BV

–

H150

H220

H330

H470

H680

H1000

H1500

H2200

77 510 BV

77 558 BV

–

77 610 BV

Table 2. Inductor Selection Guide

Renco

Inductance Current

Schott

Pulse Engineering

Coilcraft

(µH)

(A)

THT

SMT

THT

SMT

RL1500–68

–

THT

SMT

0.32

0.58

0.99

1.78

0.48

0.82

1.47

0.39

0.66

1.20

0.32

0.55

1.00

0.42

0.80

67143940

67143990

67144070

67144140

67143980

67144060

67144130

–

67144310

67144360

67144450

67144520

67144350

67144440

67144510

67144340

67144430

67144500

67144330

67144420

67144490

67144410

67144480

RL–1284–68–43

RL–5470–6

RL–5471–5

RL–5470–5

RL–5471–4

RL–5470–4

RL–5471–3

RL–5470–3

RL–5471–2

RL–5471–1

PE–53804

PE–53812

PE–53821

PE–53830

PE–53811

PE–53820

PE–53829

PE–53810

PE–53819

PE–53828

PE–53809

PE–53818

PE–53827

PE–53817

PE–53826

PE–53804–S

DO1608–68

PE–53812–S DO3308–683

PE–53821–S DO3316–683

PE–53830–S DO5022P–683

68

RL1500–100

–

PE–53811–S

DO3308–104

100

150

PE–53820–S DO3316–104

PE–53829–S DO5022P–104

PE–53810–S DO3308–154

PE–53819–S DO3316–154

PE–53828–S DO5022P–154

PE–53809–S DO3308–224

PE–53818–S DO3316–224

PE–53827–S DO5022P–224

PE–53817–S DO3316–334

PE–53826–S DO5022P–334

RL1500–150

–

67144050

67144120

67143960

67144040

67144110

67144030

67144100

RL1500–220

–

220

330

RL1500–330

–

NOTE: Table 1 and Table 2 of this Indicator Selection Guide shows some examples of different manufacturer products suitable for design with the LM2575.

13

MOTOROLA ANALOG IC DEVICE DATA

LM2575

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

Phone

Fax

+ 1–619–674–8100

+ 1–619–674–8262

Pulse Engineering Inc.

Pulse Engineering Inc. Europe

Renco Electronics Inc.

AIE Magnetics

Phone

Fax

+ 353 93 24 107

+ 353 93 24 459

Phone

Fax

+ 1–516–645–5828

+ 1–516–586–5562

Phone

Fax

+ 1–813–347–2181

Phone

Fax

+ 1–708–322–2645

+ 1–708–639–1469

Coilcraft Inc.

Phone

Fax

+ 44 1236 730 595

+ 44 1236 730 627

Coilcraft Inc., Europe

Tech 39

Phone

Fax

+ 33 8425 2626

+ 33 8425 2610

Phone

Fax

+ 1–612–475–1173

+ 1–612–475–1786

Schott Corp.

Table 4. Diode Selection Guide gives an overview about both surface–mount and through–hole diodes for an

effective design. Device listed in bold are available from Motorola.

Schottky

Ultra–Fast Recovery

1.0 A

3.0 A

1.0 A

3.0 A

V

R

SMT

THT

SMT

THT

SMT

THT

SMT

THT

20 V

SK12

1N5817

SR102

SK32

MBRD320

1N5820

MBR320

SR302

30 V

MBRS130LT3

1N5818

SR103

11DQ03

SK33

MBRD330

1N5821

MBR330

SR303

MURS320T3

SK13

MURS120T3

MUR120

11DF1

31DQ03

HER102

40 V

50 V

MBRS140T3

SK14

10BQ040

10MQ040

1N5819

SR104

11DQ04

MBRS340T3

MBRD340

30WQ04

SK34

1N5822

MBR340

SR304

10BF10

MURD320

MUR320

30WF10

MUR420

31DQ04

MBRS150

10BQ050

MBR150

SR105

MBRD350

SK35

MBR350

SR305

31DF1

HER302

11DQ05

30WQ05

11DQ05

14

MOTOROLA ANALOG IC DEVICE DATA

LM2575

EXTERNAL COMPONENTS

Input Capacitor (C_in)

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.

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.

Catch Diode

RMS Current Rating of C

Locate the Catch Diode Close to the LM2575

The LM2575 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 LM2575 using short leads and short printed circuit traces

to avoid EMI problems.

in

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 consequence of operating an electrolytic

capacitor above 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:

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 a quality, low noise design requirements.

Table 4 provides a list of suitable diodes for the LM2575

regulator. Standard 50/60 Hz rectifier diodes such as the

1N4001 series or 1N5400 series are NOT suitable.

I

_rms> 1.2 x d x I_Load

where d is the duty cycle, for a buck regulator

V

t

on

T

out

d

V

in

for a buck boost regulator.

|V

|

t

on

T

out

and d

|V

|

V

out

in

Output Capacitor (C_out

)

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.

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 has a great influence on the reliability of the overall

power supply.

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 voltage

ratings may be needed to provide low ESR values that are

required for low output ripple voltage.

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.

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

0.05 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.

Continuous and Discontinuous Mode of Operation

The LM2575 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

15

MOTOROLA ANALOG IC DEVICE DATA

LM2575

conditions, the circuit will be forced to the discontinuous

toroid and bobbin core, as well as different core materials

such as ferrites and powdered iron from different

manufacturers.

mode when inductor current falls to zero for certain period of

time (see Figure 22 and Figure 23). 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 LM2575 regulator was added to this

data sheet (Figures 17 through 21). 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 200 mA) 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.

For high quality design regulators the toroid core seems to

be the best choice. Since the magnetic flux is completely

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 completely 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 LM2575 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

LM2575. Different inductor types have different saturation

characteristics, and this should be kept in mind when

selecting an inductor.

Figure 22. Continuous Mode Switching

Current Waveforms

1.0

0

Figure 23. Discontinuous Mode Switching

Current Waveforms

1.0

0.1

0

HORTIZONTAL TIME BASE: 5.0 µs/DIV

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. The inductor selection guide

covers different styles of inductors, such as pot core, E–core,

0.1

0

HORTIZONTAL TIME BASE: 5.0 µs/DIV

16

MOTOROLA ANALOG IC DEVICE DATA

LM2575

GENERAL RECOMMENDATIONS

a heatsink for ambient temperatures up to approximately

Output Voltage Ripple and Transients

Source of the Output Ripple

50°C (depending on the output voltage and load current).

Higher ambient temperatures require some heatsinking,

either to the printed circuit (PC) board or an external heatsink.

Since the LM2575 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.

The Surface Mount Package D²PAK and its Heatsinking

The other type of package, the surface mount D²PAK, is

designed to be soldered to the copper on the PC board. The

copper and the board are the heatsink for this package and

the other heat producing components, such as the catch

diode and inductor. The PC board copper area that the

package is soldered to should be at least 0.4 in²(or

100 mm²) and ideally should have 2 or more square inches

(1300 mm²) of 0.0028 inch copper. Additional increasing of

copper area beyond approximately 3.0 in²(2000 mm²) will

not improve heat dissipation significantly. If further thermal

improvements are needed, double sided or multilayer PC

boards with large copper areas should be considered.

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 24). 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 minimise 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.

Thermal Analysis and Design

The following procedure must be performed to determine

whether or not a heatsink will be required. First determine:

1. P_D(max)

maximum regulator power dissipation in the

application.

2. T_A(max

)

maximum ambient temperature in the

application.

Figure 24. Output Ripple Voltage Waveforms

Voltage spikes caused by switching action of the output

switch and the parasitic inductance of the output capacitor

3. T_J(max)

maximum allowed junction temperature

(125°C for the LM2575). 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.

UNFILITERED

OUTPUT

VOLTAGE

VERTICAL

RESOLUTION:

20 mV/DIV

4. R_θ

package thermal resistance junction–case.

package thermal resistance junction–ambient.

JC

5. R_θ

JA

FILITERED

OUTPUT

VOLTAGE

(Refer to Absolute Maximum Ratings in this data sheet or

R_θand R_θ_JAvalues).

JC

The following formula is to calculate the total power

dissipated by the LM2575:

HORTIZONTAL TIME BASE: 10 µs/DIV

P_D= (V_inx I_Q) + d x I_Loadx V_sat

Minimizing the Output Ripple

where d is the duty cycle and for buck converter

In order to minimise 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 33) 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 24

shows the difference between filtered and unfiltered output

waveforms of the regulator shown in Figure 33.

V

t

on

T

O

in

d

,

V

I_Q

(quiescent current) and V_satcan be found in the

LM2575 data sheet,

V_inis minimum input voltage applied,

V_Ois the regulator output voltage,

I_Loadis the load current.

The dynamic switching losses during turn–on and turn–off

can be neglected if proper type catch diode is used.

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.

Packages Not on a Heatsink (Free–Standing)

For a free–standing application when no heatsink is used,

the junction temperature can be determined by the following

expression:

Heatsinking and Thermal Considerations

The Through–Hole Package TO–220

The LM2575 is available in two packages, a 5–pin

TO–220(T, TV) and a 5–pin surface mount D²PAK(D2T).

There are many applications that require no heatsink to keep

the LM2575 junction temperature within the allowed

operating range. The TO–220 package can be used without

T_J= (R_θ_JA) (P_D) + T_A

where (R_θ_JA)(P_D) represents the junction temperature rise

caused by the dissipated power and T_Ais the maximum

ambient temperature.

17

MOTOROLA ANALOG IC DEVICE DATA

LM2575

Packages on a Heatsink

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.

If the actual operating junction temperature is greater than

the selected safe operating junction temperature determined

in step 3, than a heatsink is required. The junction

temperature will be calculated as follows:

This circuit configuration is able to deliver approximately

0.35 A to the output when the input voltage is 12 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 1.5 A.

Such an amount of input startup current is needed for at

least 2.0 ms or more. The actual time depends on the output

voltage and size of the output capacitor.

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.

Using a delayed startup arrangement, the input capacitor

can charge up to a higher voltage before the switch–mode

regulator begins to operate.

T_J= P_D(R_θ+ R_θ+ R_θ_SA) + T_A

JA

CS

where R_θ_JCis the thermal resistance junction–case,

R_θ_CSis the thermal resistance case–heatsink,

R_θ_SAis the thermal resistance heatsink–ambient.

If the actual operating temperature is greater than the

selected safe operating junction temperature, then a larger

heatsink is required.

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.

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 colour of

the traces.

The size, quantity and spacing of other components on

the board can also influence its effectiveness to dissipate

the heat.

The high input current needed for startup is now partially

supplied by the input capacitor C_in.

Design Recommendations:

The inverting regulator operates in a different manner than

the buck converter and so a different design procedure has to

Figure 25. Inverting Buck–Boost Regulator Using the

LM2575–12 Develops –12 V @ 0.35 A

be used to select the inductor L1 or the output capacitor C_out

.

Unregulated

Feedback

The output capacitor values must be larger than 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.

DC Input

L1

100

+V

4

12 V to 25 V

in

LM2575–12

µH

Output

1

C

µ

in

F

100

2

C

1800

/16 V

out

D1

1N5819

/50 V

3

Gnd

5

ON/OFF

µF

Regulated

Output

–12 V @ 0.35 A

The following formula is used to obtain the peak inductor

current:

I

(V

|V |)

V

x t

on

Load in

O

ADDITIONAL APPLICATIONS

Inverting Regulator

An inverting buck–boost regulator using the LM2575–12 is

shown in Figure 25. 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

2L

I

peak

on

V

1

in

|V |

O

1

where t

x

, and f

52 kHz.

osc

V

|V |

f

osc

in

O

Under normal continuous inductor current operating

conditions, the worst case occurs when V_inis minimal.

Note that the voltage appearing across the regulator is the

absolute sum of the input and output voltage, and must not

exceed 40 V.

In this example the LM2575–12 is used to generate a

–12 V output. The maximum input voltage in this case

18

MOTOROLA ANALOG IC DEVICE DATA

LM2575

Figure 26. Inverting Buck–Boost

Regulator with Delayed Startup

Figure 28. Inverting Buck–Boost Regulator Shut Down

Circuit Using a PNP Transistor

Unregulated

DC Input

12 V to 25 V

Shutdown

Input

+V

0

Feedback

Off

+V

L1

100

in

4

On

LM2575–12

µH

Output

1

R2

C

µ

in

F

5.6 k

C1

100

2

0.1

µF

+V

in

/50 V

5

ON/OFF 3 Gnd

C

1800

/16 V

out

D1

1N5819

1

R1

47 k

µF

LM2575–XX

C

in

R2

47 k

100 µF

Q1

2N3906

5

ON/OFF

3

Gnd

Regulated

Output

–12 V @ 0.35 A

R1

12 k

–V

out

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 26.

Figure 32 in the “Undervoltage Lockout” section describes an

undervoltage lockout feature for the same converter

topology.

NOTE: This picture does not show the complete circuit.

Negative Boost Regulator

This example is a variation of the buck–boost topology and

is called a 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.

The circuit in Figure 29 shows the negative boost

configuration. The input voltage in this application ranges

from –5.0 V 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.

Figure 27. Inverting Buck–Boost Regulator Shut Down

Circuit Using an Optocoupler

+V

in

LM2575–XX

1

C

100

R1

F 47 k

in

µ

Figure 29. Negative Boost Regulator

5

ON/OFF

3

Gnd

Shutdown

Input

5.0 V

0

Off

R3

470

On

C

1000

/16 V

R2

47 k

out

4

µF

+V

in

–V

Feedback

Output

2

out

LM2575–12

D1

1

MOC8101

C

µ

/50 V

in

F

1N5817

100

3

5

Gnd

ON/OFF

Regulated

Output

NOTE: This picture does not show the complete circuit.

V

= –12 V

out

With the inverting configuration, the use of the ON/OFF pin

requires some level shifting techniques. This is caused by the

fact, that the ground pin of the converter IC is no longer at

ground. Now, the ON/OFF pin threshold voltage (1.4 V

approximately) has to be related to the negative output

voltage level. There are many different possible shut down

methods, two of them are shown in Figures 27 and 28.

Load Current from

200 mA for V = –5.2 V

L1

in

to 500 mA for V = –7.0 V

in

150 µH

Unregulated

DC Input

–V = –5.0 V to –12 V

in

19

MOTOROLA ANALOG IC DEVICE DATA

LM2575

Design Recommendations:

shown in Figure 32. 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:

The same design rules as for the previous inverting

buck–boost converter can be applied. The output capacitor

C_outmust be chosen larger than 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.

R2

R1

( )

Q1

V

1

V

th

Z1

BE

Figure 31. Undervoltage Lockout Circuit for

Buck Converter

Another important point is that these negative boost

converters cannot provide 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.

+V

in

LM2575–5.0

1

Delayed Startup

C

100

in

R2

10 k

R3

47 k

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 the input voltage

is applied and the time when the output voltage comes up,

the circuit in Figure 30 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 1.4 V, the regulator starts

up. Resistor R1 is included to limit the maximum voltage

applied to the ON/OFF pin, reduces the power supply noise

sensitivity, and also limits the capacitor C1 discharge current,

but its use is not mandatory.

µF

5

ON/OFF

3

Gnd

Z1

1N5242B

Q1

2N3904

R1

10 k

V

≈ 13 V

th

NOTE: This picture does not show the complete circuit.

Figure 32. Undervoltage Lockout Circuit for

Buck–Boost Converter

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

LM2575–5.0

1

Figure 30. Delayed Startup Circuitry

C

100

in

R2

15 k

R3

68 k

5

ON/OFF

3

Gnd

µF

+V

in

LM2575–XX

Z1

1N5242B

1

V

≈ 13 V

th

C1

0.1

Q1

2N3904

5

ON/OFF

3

Gnd

µF

C

µ

R1

15 k

in

F

100

V

= –5.0 V

R1

47 k

out

R2

47 k

NOTE: This picture does not show the complete circuit.

Adjustable Output, Low–Ripple Power Supply

A 1.0 A output current capability power supply that

features an adjustable output voltage is shown in Figure 33.

This regulator delivers 1.0 A into 1.2 V to 35 V output. The

input voltage ranges from roughly 8.0 V 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.

NOTE: This picture does not show the complete circuit.

Undervoltage Lockout

Some applications require the regulator to remain off until

the input voltage reaches a certain threshold level. Figure 31

shows an undervoltage lockout circuit applied to a buck

regulator. A version of this circuit for buck–boost converter is

20

MOTOROLA ANALOG IC DEVICE DATA

LM2575

Figure 33. Adjustable Power Supply with Low Ripple Voltage

Feedback

4

Unregulated

DC Input

+

+V

in

L1

150

L2

LM2575–Adj

Regulated

Output Voltage

µ

H

20

µ

H

1

Output

2

1.2 V to 35 V @1.0 A

R2

50 k

3

Gnd

5

ON/OFF

C1

100 µF

C

µ

/50 V

in

F

100

C

out

2200 µF

D1

1N5819

R1

1.1 k

Optional Output

Ripple Filter

Figure 34. D²PAK Thermal Resistance and Maximum

Power Dissipation versus P.C.B. Copper Length

80

3.5

P

for T = 50°C

A

D(max)

70

60

50

40

30

3.0

Free Air

Mounted

Vertically

2.0 oz. Copper

L

2.5

2.0

1.5

1.0

Minimum

Size Pad

L

R

θ

JA

0

5.0

10

15

20

25

30

L, LENGTH OF COPPER (mm)

21

MOTOROLA ANALOG IC DEVICE DATA

LM2575

THE LM2575–5.0 STEP–DOWN VOLTAGE REGULATOR WITH 5.0 V @ 1.0 A OUTPUT POWER CAPABILITY.

TYPICAL APPLICATION WITH THROUGH–HOLE PC BOARD LAYOUT

Figure 35. Schematic Diagram of the LM2575–5.0 Step–Down Converter

Feedback

4

+V

in

Unregulated

DC Input

+V = +7.0 V to +40 V

L1

330

LM2575–5.0

µ

H

1

Output

2

Regulated Output

+V = 5.0 V @ 1.0 A

in

out1

3

Gnd

5

ON/OFF

C1

100 µF

J1

C

330

/16 V

out

D1

1N5819

/50 V

µ

F

Gnd

in

Gnd

out

C1

C2

D1

L1

–

100 µF, 50 V, Aluminium Electrolytic

330 µF, 16 V, Aluminium Electrolytic

1.0 A, 40 V, Schottky Rectifier, 1N5819

330 µH, Tech 39: 77 458 BV, Toroid Core, Through–Hole, Pin 3 = Start, Pin 7 = Finish

Figure 36. Printed Circuit Board

Component Side

Figure 37. Printed Circuit Board

Copper Side

Gnd

in

out

U1 LM2575

C1

J1

C2

D1

L1

DC–DC Converter

+V

out1

in

NOTE: Not to scale.

22

MOTOROLA ANALOG IC DEVICE DATA

LM2575

THE LM2575–ADJ STEP–DOWN VOLTAGE REGULATOR WITH 8.0 V @ 1.0 A OUTPUT POWER

CAPABILITY. TYPICAL APPLICATION WITH THROUGH–HOLE PC BOARD LAYOUT

Figure 38. Schematic Diagram of the 8.0 V @ 1.0 V Step–Down Converter Using the LM2575–Adj

(An additional LC filter is included to achieve low output ripple voltage)

Regulated

Output Unfiltered

V

= 8.0 V @1.0 A

out1

4

Feedback

Unregulated

DC Input

+V

in

L1

330

L2

LM2575–Adj

Regulated

Output Filtered

µ

H

1

25

µ

H

+V = +10 V to + 40 V

in

Output

2

V

= 8.0 V @1.0 A

out2

3

Gnd

5

ON/OFF

R2

10 k

C3

C1

100 µF

/50 V

100 µF

/16 V

C2

330 µF

/16 V

D1

1N5819

R1

1.8 k

R2

R1

V

1

out

= 1.23 V

ref

V

ref

C1

C2

C3

D1

L1

L2

R1

R2

–

100 µF, 50 V, Aluminium Electrolytic

330 µF, 16 V, Aluminium Electrolytic

100 µF, 16 V, Aluminium Electrolytic

1.0 A, 40 V, Schottky Rectifier, 1N5819

330 µH, Tech 39: 77 458 BV, Toroid Core, Through–Hole, Pin 3 = Start, Pin 7 = Finish

25 µH, TDK: SFT52501, Toroid Core, Through–Hole

R1 is between 1.0 k and 5.0 k

1.8 k

10 k

Figure 39. PC Board Component Side

Figure 40. PC Board Copper Side

Gnd

C3

Gnd

U1 LM2575

out

in

C1

C2

D1

J1

L1

L2

+V

out2

+V

in

+V

out1

R2 R1

MOTOROLA

NOTE: Not to scale.

References

• National Semiconductor LM2575 Data Sheet and Application Note

• National Semiconductor LM2595 Data Sheet and Application Note

• Marty Brown “Pratical Switching Power Supply Design”, Academic Press, Inc., San Diego 1990

• Ray Ridley “High Frequency Magnetics Design”, Ridley Engineering, Inc. 1995

23

MOTOROLA ANALOG IC DEVICE DATA

LM2575

OUTLINE DIMENSIONS

T SUFFIX

PLASTIC PACKAGE

CASE 314D–03

ISSUE D

SEATING

–T–

NOTES:

1. DIMENSIONING AND TOLERANCING PER ANSI

PLANE

C

Y14.5M, 1982.

–Q–

2. CONTROLLING DIMENSION: INCH.

3. DIMENSION D DOES NOT INCLUDE

INTERCONNECT BAR (DAMBAR) PROTRUSION.

DIMENSION D INCLUDING PROTRUSION SHALL

NOT EXCEED 10.92 (0.043) MAXIMUM.

B

E

U

INCHES

MIN MAX

0.613 14.529 15.570

MILLIMETERS

MIN MAX

A

S

DIM

A

B

C

D

E

G

H

J

K

L

Q

U

S

L

0.572

0.390

0.170

0.025

0.048

0.415

0.180

0.038

0.055

9.906 10.541

1

2

3

4 5

4.318

0.635

1.219

4.572

0.965

1.397

K

0.067 BSC

1.702 BSC

0.087

0.015

1.020

0.320

0.140

0.105

0.543

0.112

0.025

2.210

0.381

2.845

0.635

1.065 25.908 27.051

0.365

0.153

0.117

8.128

3.556

2.667

9.271

3.886

2.972

J

G

0.582 13.792 14.783

H

D 5 PL

M

0.356 (0.014)

T

Q

TV SUFFIX

PLASTIC PACKAGE

CASE 314B–05

ISSUE J

NOTES:

C

1. DIMENSIONING AND TOLERANCING PER ANSI

Y14.5M, 1982.

B

–P–

OPTIONAL

CHAMFER

Q

F

2. CONTROLLING DIMENSION: INCH.

3. DIMENSION D DOES NOT INCLUDE

INTERCONNECT BAR (DAMBAR) PROTRUSION.

DIMENSION D INCLUDING PROTRUSION SHALL

NOT EXCEED 0.043 (1.092) MAXIMUM.

E

A

INCHES

MIN MAX

0.613 14.529 15.570

MILLIMETERS

MIN MAX

U

DIM

A

B

C

D

E

L

S

V

0.572

0.390

0.170

0.025

0.048

0.850

W

0.415

0.180

0.038

0.055

9.906 10.541

K

4.318

0.635

1.219

4.572

0.965

1.397

F

0.935 21.590 23.749

G

H

J

K

L

N

Q

S

0.067 BSC

0.166 BSC

1.702 BSC

4.216 BSC

0.015

0.900

0.320

0.025

0.381

0.635

1.100 22.860 27.940

0.365

5X J

8.128

8.128 BSC

3.556

9.271

3.886

G

M

0.24 (0.610)

T

H

0.320 BSC

5X D

0.140

–––

0.153

0.620

––– 15.748

N

M

0.10 (0.254)

T P

U

V

W

0.468

–––

0.090

0.505 11.888 12.827

0.735

0.110

––– 18.669

2.286 2.794

SEATING

PLANE

–T–

24

MOTOROLA ANALOG IC DEVICE DATA

LM2575

OUTLINE DIMENSIONS

D2T SUFFIX

PLASTIC PACKAGE

CASE 936A–02

2

(D PAK)

ISSUE A

NOTES:

1

DIMENSIONING AND TOLERANCING PER ANSI

Y14.5M, 1982.

–T–

TERMINAL 6

2

3

CONTROLLING DIMENSION: INCH.

TAB CONTOUR OPTIONAL WITHIN DIMENSIONS

A AND K.

DIMENSIONS U AND V ESTABLISH A MINIMUM

MOUNTING SURFACE FOR TERMINAL 6.

DIMENSIONS A AND B DO NOT INCLUDE MOLD

FLASH OR GATE PROTRUSIONS. MOLD FLASH

AND GATE PROTRUSIONS NOT TO EXCEED

0.025 (0.635) MAXIMUM.

OPTIONAL

CHAMFER

A

E

U

4

5

S

K

V

B

H

1

2

3

4 5

INCHES

MILLIMETERS

MIN MAX

9.804 10.236

M

L

DIM

A

B

C

D

E

MIN

MAX

0.403

0.368

0.180

0.036

0.055

0.386

0.356

0.170

0.026

0.045

9.042

4.318

0.660

1.143

9.347

4.572

0.914

1.397

D

P

N

M

0.010 (0.254)

T

G

R

G

H

K

L

M

N

P

R

S

U

V

0.067 BSC

0.539

0.050 REF

1.702 BSC

0.579 13.691 14.707

1.270 REF

0.000

0.088

0.018

0.058

5

0.010

0.102

0.026

0.078

0.000

0.254

2.591

0.660

1.981

2.235

0.457

1.473

5

C

REF

0.116 REF

0.200 MIN

0.250 MIN

2.946 REF

5.080 MIN

6.350 MIN

25

MOTOROLA ANALOG IC DEVICE DATA

LM2575

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the suitability of its products for any particular purpose, nor does Motorola assume any liability arising out of the application or use of any product or circuit, and

specificallydisclaims any and all liability, including without limitation consequential or incidental damages. “Typical” parameters which may be provided in Motorola

datasheetsand/orspecificationscananddovaryindifferentapplicationsandactualperformancemayvaryovertime. Alloperatingparameters,including“Typicals”

must be validated for each customer application by customer’s technical experts. Motorola does not convey any license under its patent rights nor the rights of

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applicationsintended to support or sustain life, or for any other application in which the failure of the Motorola product could create a situation where personal injury

ordeathmayoccur. ShouldBuyerpurchaseoruseMotorolaproductsforanysuchunintendedorunauthorizedapplication,BuyershallindemnifyandholdMotorola

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Mfax is a trademark of Motorola, Inc.

How to reach us:

USA/EUROPE/Locations Not Listed: Motorola Literature Distribution;

P.O. Box 5405, Denver, Colorado 80217. 303–675–2140 or 1–800–441–2447

JAPAN: Nippon Motorola Ltd.: SPD, Strategic Planning Office, 4–32–1,

Nishi–Gotanda, Shinagawa–ku, Tokyo 141, Japan. 81–3–5487–8488

Mfax : RMFAX0@email.sps.mot.com – TOUCHTONE 602–244–6609

ASIA/PACIFIC: Motorola Semiconductors H.K. Ltd.; 8B Tai Ping Industrial Park,

– US & Canada ONLY 1–800–774–1848 51 Ting Kok Road, Tai Po, N.T., Hong Kong. 852–26629298

INTERNET: http://motorola.com/sps

LM2575/D

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26

MOTOROLA ANALOG IC DEVICE DATA


型号：	LM2575-15TV
厂家：	ETC
描述：	Easy Switcher 1.0 A Step-Down Voltage Regulator(389.50 k) 轻松切换器1.0降压稳压器（ 389.50 K）\n 稳压器
文件：	总26页 (文件大小：392K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM2575-15TV [ETC]

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