Datasheet LM2574N Datasheet (Motorola)

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The LM2574 series of regulators are monolithic integrated circuits ideally suited for easy and convenient design of a step–down switching regulator (buck converter). All circuits of this series are capable of driving a 0.5 A load with excellent line and load regulation. These devices are available in fixed output voltages of 3.3 V, 5.0 V, 12 V, 15 V, and an adjustable output version.

These regulators were designed to minimize the number of external components to simplify the power supply design. Standard series of inductors optimized for use with the LM2574 are offered by several different inductor manufacturers.

Since the LM2574 converter is a switch–mode power supply , its efficiency is significantly higher in comparison with popular three–terminal linear regulators, especially with higher input voltages. In most cases, the power dissipated by the LM2574 regulator is so low, that the copper traces on the printed circuit board are normally the only heatsink needed and no additional heatsinking is required.

The LM2574 features include a guaranteed ±4% tolerance on output voltage within specified input voltages and output load conditions, and ±10% on the oscillator frequency (±2% over 0°C to +125°C). External shutdown is included, featuring 60 µA (typical) standby current. The output switch includes cycle–by–cycle current limiting, as well as thermal shutdown for full protection under fault conditions.

Features

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

• Adjustable Version Output Voltage Range, 1.23 to 37 V ±4% max over

Line and Load Conditions

• Guaranteed 0.5 A Output Current

• Wide Input Voltage Range: 4.75 to 40 V

• Requires Only 4 External Components

• 52 kHz Fixed Frequency Internal Oscillator

• TTL Shutdown Capability , Low Power Standby Mode

• High Efficiency

• Uses Readily Available Standard Inductors

• Thermal Shutdown and Current Limit Protection

EASY SWITCHER

0.5 A STEP–DOWN

VOLTAGE REGULATOR

SEMICONDUCTOR

TECHNICAL DATA

N SUFFIX

PLASTIC PACKAGE

CASE 626

PIN CONNECTIONS

Sig Gnd

/OFF

3 4

(Top View)

ON Pwr Gnd

* No internal connection, but should be soldered to

* PC board for best heat transfer.

8 7 6 5

* Output

* V

Applications

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

• Efficient Pre–regulator for Linear Regulators

• On–Card Switching Regulators

• Positive to Negative Converters (Buck–Boost)

• Negative Step–Up Converters

• Power Supply for Battery Chargers

This document contains information on a new product. Specifications and information herein are subject to change without notice.

DEVICE TYPE/NOMINAL OUTPUT VOLTAGE

LM2574–3.3 LM2574–5 LM2574–12 LM2574–15 LM2574–ADJ

ORDERING INFORMATION

Operating

Device

LM2574N–XX TA = –40° to +125°C DIP–8

XX = Voltage Option, i.e, 3.3, 5, 12, 15 V; and ADJ for Adjustable Output.

 Motorola, Inc. 1997 Rev 0

Temperature Range

3.3 V

5.0 V 12 V 15 V

1.23 V to 37 V

Package

Page 2

7.0 – 40 V

Unregulated

DC Input

LM2574

Figure 1. Block Diagram and T ypical Application

T ypical Application (Fixed Output Voltage Versions)

Feedback 1

Output 7

Sig Gnd

LM2574

4ON/OFF3

Pwr Gnd

Representative Block Diagram and T ypical Application

330

D1 1N5819

out

220

5.0 V Regulated Output 0.5 A Load

Unregulated

DC Input

Feedback

Sig Gnd

Fixed Gain Error Amplifier

1.0 k 18 kHz

1.235 V Band–Gap Reference

Freq Shift

3.1 V Internal Regulator

Comparator

52 kHz

Oscillator

Current

Latch

Reset

Limit

/OFF

Driver

1.0 Amp Switch

Thermal

Shutdown

ABSOLUTE MAXIMUM RATINGS (Absolute Maximum Ratings indicate limits beyond which

damage to the device may occur).

Rating Symbol Value Unit

Maximum Supply Voltage V ON/OFF Pin Input Voltage – –0.3 V ≤ V ≤ +V Output Voltage to Ground (Steady State) – –1.0 V Power Dissipation P

Thermal Resistance, Junction–to–Ambient R

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

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

Maximum Junction Temperature T

NOTE: ESD data available upon request.

θJA θJC

stg

45 V

Internally Limited W

100 °C/W

5.0 °C/W

–65°C to +150°C °C

150 °C

/OFF

ON 3

Output 7

Pwr Gnd 4

Output

Voltage Versions

3.3 V

5.0 V 12 V 15 V

For adjustable version R1 = open, R2 = 0

(

1.7 k

3.1 k

8.84 k

11.3 k

Ω

out

Ω

)

out

Load

MOTOROLA ANALOG IC DEVICE DATA

Page 3

LM2574

OPERATING RATINGS (Operating Ratings indicate conditions for which the device is

intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics).

Rating

Operating Junction Temperature Range T Supply Voltage V

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

ELECTRICAL CHARACTERISTICS

Vin = 25 V for the 12 V version, Vin = 30 V for the 15 V version. I operating junction temperature range that applies [Note 2], unless otherwise noted).

Characteristic

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

Output Voltage (Vin = 12 V, I Output Voltage (4.75 V ≤ Vin ≤ 40 V, 0.1 A ≤ I

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

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

= 25°C

= –40 to +125°C

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

= 25°C

= –40 to +125°C

Efficiency (Vin = 15 V, I

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

Output Voltage (Vin = 30 V, I Output Voltage (18 V < Vin < 40 V, 0.1 A < I

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

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

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

Efficiency (Vin = 12 V, I

NOTES: 1. External components such as the catch diode, inductor, input and output capacitors can af fect 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: T

Load

= 0.5 A) η – 72 – %

Load

= 0.5 A) η – 77 – %

Load

= 0.5 A) η – 88 – %

Load

= 0.5 A) η – 88 – %

Load

= 0.5 A, V

Load

(Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and Adjustable version,

= 100 mA, TJ = 25°C) V

Load

= 100 mA, TJ = 25°C) V

Load

= 100 mA, TJ = 25°C) V

Load

= 100 mA, TJ = 25°C) V

Load

= 100 mA, V

Load

= 5.0 V) η – 77 – %

out

Symbol Value Unit

≤ 0.5 A) V

< 0.5 A) V

= 5.0 V, TJ = 25°C V

out

≤ 0.5 A, V

= –40°C T

low

–40 to +125 °C

40 V

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

Load

Symbol Min Typ Max Unit

= 5.0 V V

out

= +125°C

high

out out

FBT

3.234 3.3 3.366 V

4.9 5.0 5.1 V

4.8 5.0 5.2

4.75 5.25

11.76 10 12.24 V

11.52 12 12.48

11.4 – 12.6

14.7 15 15.3 V

1.217 1.23 1.243 V

MOTOROLA ANALOG IC DEVICE DATA

Page 4

LM2574

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

ELECTRICAL CHARACTERISTICS

Vin = 25 V for the 12 V version, Vin = 30 V for the 15 V version. I operating junction temperature range that applies [Note 2], unless otherwise noted).

Characteristic UnitMaxTypMinSymbol

ALL OUTPUT VOLTAGE VERSIONS

Feedback Bias Current V

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

Oscillator Frequency (Note 3) 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 5] DC 93 98 – % Current Limit Peak Current (Notes 3 and 4) I

TJ = 25°C 0.7 1.0 1.6

TJ = –40 to +125°C 0.65 – 1.8 Output Leakage Current (Notes 6 and 7), TJ = 25°C I

Output = 0 V – 0.6 2.0

Output = – 1.0 V – 10 30 Quiescent Current (Note 6) 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

= 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 µA

ON/OFF Pin = 5.0 V (“off”), TJ = 25°C I

ON/OFF Pin = 0 V (“on”), TJ = 25°C I

NOTES: 1. External components such as the catch diode, inductor, input and output capacitors can af fect 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: T

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 power 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 the 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.Vin = 40 V.

= 5.0 V (Adjustable Version Only) I

out

= 0.5 A, [Note 4]) V

out

(continued) (Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and Adjustable version,

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

Load

sat

stby

– 15 30 – 0 5.0

= –40°C T

low

high

= +125°C

kHz

µA

MOTOROLA ANALOG IC DEVICE DATA

Page 5

LM2574

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 16)

1.0

Figure 2. Normalized Output Voltage

Vin = 20 V

0.8 I

= 100 mA

Load

0.6

Normalized at TJ = 25

0.4

0.2

0 –0.2 –0.4 –0.6

, OUTPUT VOLTAGE CHANGE (%)

out

–0.8

–1.0

2.0 L = 300 µH

1.5

1.4

1.2

= 100 mA

Load

TJ = 25

TJ, JUNCTION TEMPERATURE (°C)

1251007560250–25–50 403530252015105.00

1.0

0.8

0.6

0.4

0.2 0

–0.2

, OUTPUT VOLTAGE CHANGE (%)

out

–0.4

–0.6

3.3 V, 5.0 V and ADJ

12 V and 15 V

Vin, INPUT VOLTAGE (V)

Figure 4. Dropout Voltage Figure 5. Current Limit

1.4

1.3

Figure 3. Line Regulation

Load

= 500 mA

1.2

Vin = 25 V

INPUT – OUTPUT DIFFERENTIAL (V)

, QUIESCENT CURRENT (mA)

1.0

0.5

20 18 16 14 12 10

8.0

6.0

4.0

1.1

, OUTPUT CURRENT (A)

1.0

0.9

0.8

0.7

TJ, JUNCTION TEMPERATURE (°C)

= 100 mA

Load

TJ, JUNCTION TEMPERATURE (°C)

1251007560250–25–50 1251007560250–25–50

Figure 6. Quiescent Current Figure 7. Standby Quiescent Current

200

, STANDBY QUIESCENT CURRENT ( A)

stby

180 160 140 120 100

80 60 40 20

= 5.0 V

/OFF

Vin = 40 V

Vin = 12 V

TJ, JUNCTION TEMPERATURE (°C)

Load

= 100 mA

Load

Vin, INPUT VOLTAGE (V)

= 500 A

= 5.0 V

out

Measured at Ground Pin

TJ = 25

403530252015105.00 1251007560250–25–50

MOTOROLA ANALOG IC DEVICE DATA

Page 6

LM2574

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

NORMALIZED FREQUENCY (%)

, INPUT VOLTAGE (V)V

8.0

6.0

4.0

2.0

–2.0 –4.0 –6.0 –8.0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

Vin = 12 V Normalized at 25

Figure 8. Oscillator Frequency

TJ, JUNCTION TEMPERATURE (°C)

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

, SATURATION VOLTAGE (V)I

0.5

sat

0.4

1251007550250–25–50 0 0.1 0.2 0.3 0.4 0.5

0.3

Figure 9. Switch Saturation Voltage

–40°C

25°C

125°C

SWITCH CURRENT (A)

Figure 10. Minimum Operating V oltage Figure 11. Feedback Pin Current

100

–20 –40 –60

, FEEDBACK PIN CURRENT (nA)

–80

–100

80 60 40 20

TJ, JUNCTION TEMPERATURE (

Vin = 1.23 V I

= 100 mA

Load

TJ, JUNCTION TEMPERATURE (

Adjustable Version Only

1251007550250–25–50 1251007550250–25–50

Adjustable Version Only

Figure 12. Continuous Mode Switching Waveforms

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

out

20 V

10 V

0.6 A

0.4 A

0.2 A 0

20 mV

s/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 13. Discontinuous Mode Switching Waveforms

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

out

20 V

10 V

0.6 A

0.4 A

0.2 A 0

20 mV

s/DIV

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

MOTOROLA ANALOG IC DEVICE DATA

Page 7

LM2574

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

Figure 14. 500 mA Load Transient Response for

Continuous Mode Operation, L = 330 µH, C

50 mV

500 mA

A: Output Voltage, 50 mV/DIV, AC Coupled

B: 100 mA to 500 mA Load Pulse

= 300 µF

out

Figure 15. 250 mA Load Transient Response for

Discontinuous Mode Operation, L = 68 µH, C

50 mV

200 mA

100 mA

200

s/DIV200 µs/DIV

A: Output Voltage, 50 mV/DIV, AC Coupled

B: 50 mA to 250 mA Load Pulse

= 470 µF

out

MOTOROLA ANALOG IC DEVICE DATA

Page 8

7.0 – 40 V

Unregulated

DC Input

7.0 V – 40 V Unregulated

DC Input

Figure 16. T est Circuit and Layout Guidelines

Fixed Output Voltage Versions

in 1

Cin–22 µF, 60 V, Aluminium Electrolytic C

– 220 µF, 25 V, Aluminium Electrolytic

out

D1 – Schottky, 1N5819 L1 – 330 µH, (For 5.0 Vin, 3.3 V R1 – 2.0 k, 0.1% R2 – 6.12 k, 0.1%

LM2574

Fixed Output

2 Sig

Gnd

Adjustable Output Voltage Versions

C 22

LM2574

Adjustable

2 Sig

Gnd

34ON/OFFPwr

Gnd

LM2574

Feedback

Output 7

/OFFPwr

34ON

Gnd

, use 100 µH)

out

Feedback

Output

330

D1 1N5819

330

D1 1N5819

out

220

out

220

Load

out

5.0 V

6.12 k Load

2.0 k

out

ref

R2+R1

Where V between 1.0 k

= 1.23 V, R1

ref

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.

1.0

out

ref

Ω

and 5.0 k

)

–1.0

Ω

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.

MOTOROLA ANALOG IC DEVICE DATA

Page 9

LM2574

PIN FUNCTION DESCRIPTION

Pin Symbol Description (Refer to Figure 1)

5 V

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

2 Sig Gnd Circuit signal ground pin. See the information about the printed circuit board layout. 4 Pwr Gnd Circuit power ground pin. See the information about the printed circuit board layout. 1 Feedback This pin senses regulated output voltage to complete the feedback loop. The signal is divided by the

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

This pin is the positive input supply for the LM2574 step–down switching regulator. In order to minimize voltage transients and to supply the switching currents needed by the regulator, a suitable input bypass capacitor must be present (Cin in

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.

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.

input supply current to approximately 80 µA. The input threshold voltage is typically 1.5 V. Applying a voltage above this value (up to +Vin) shuts the regulator off. If the voltage applied to this pin is lower than

1.5 V or if this pin is left open, the regulator will be in the “on” condition.

Figure 1).

of this output switch is typically 1.0 V.

sat

DESIGN PROCEDURE

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:

L(on)

Vin–V

out

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 17. Basic Buck Converter

Power Switch

out

Load

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

L(off)

out

–V

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:

, where T is the period of switching.

For the buck converter with ideal components, the duty cycle can also be described as:

out

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

Figure 18. Buck Converter Idealized Waveforms

on(SW)

Power Switch

Off

VD(FWD)

Diode VoltageInductor Current

Power Switch

Off

Power Switch

Time

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

MOTOROLA ANALOG IC DEVICE DATA

min

Diode Diode

Power Switch

Load

(AV)

Time

Page 10

LM2574

Procedure

(Fixed Output Voltage Version) In order to simplify the switching regulator design, a step–by–step

design procedure and example is provided.

Procedure Example

Given Parameters:

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

out

V I

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 +Vin and ground pin Gnd. This capacitor should be located close to the IC using short leads. This capacitor should have a low ESR (Equivalent Series Resistance) value.

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

= Maximum Input Voltage

in(max)

Load(max)

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.

= Maximum Load Current

Given Parameters:

= 5.0 V

out

V 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 µF, 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

= 15 V

in(max)

suggested fast recovery diodes shown in Table 1.

= 0.4 A

4. Inductor Selection (L1)

A.According to the required working conditions, select the

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

B.From the appropriate inductor selection guide, identify the

inductance region intersected by the Maximum Input Voltage line and the Maximum Load Current line. Each region is identified by an inductance value and an inductor code.

C.Select an appropriate inductor from the several different

manufacturers part numbers listed in 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:

p(max

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

Load(max

)

ton+

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

)

out

Table 2. The designer

Vin*

out

1.0

osc

4. Inductor Selection (L1)

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

B.From the selection guide, the inductance area intersected

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

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

inductor from any of the listed manufacturers.

MOTOROLA ANALOG IC DEVICE DATA

Page 11

LM2574

Procedure (Fixed Output Voltage Version) (continued) In order to simplify the switching regulator design, a step–by–step

design procedure and example is provided.

Procedure Example

5. Output Capacitor Selection (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 µF and 470 µF is recommended.

B.Due to the fact that the higher voltage electrolytic capacitors

generally have lower ESR (Equivalent Series Resistance) numbers, the output capacitor’s voltage rating should be at least 1.5 times greater than the output voltage. For a 5.0 V regulator, a rating at least 8.0 V is appropriate, and a 10 V or 16 V rating is recommended.

out

)

Procedure (Adjustable Output Version: LM2574–ADJ)

Procedure Example

Given Parameters:

= Regulated Output Voltage

out

V I

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 kΩ and 5.0 kΩ. (For best temperature coefficient and stability with time, use 1% metal film resistors).

= Maximum DC Input Voltage

in(max)

Load(max)

= Maximum Load Current

out

ref

R2+R1

1.0

)

out

1.0

ref

= 1.23 V

ref

5. Output Capacitor Selection (C

A.C

= 100 µF to 470 µF standard aluminium electrolytic.

out

B.Capacitor voltage rating = 20 V.

Given Parameters:

= 24 V

out

V I

Load(max)

1. Programming Output Voltage (selecting R1 and R2)

Select R1 and R2 :

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

= 40 V

in(max)

R2+R1

= 0.4 A

= 1.23 Select R1 = 1.0 kΩ

out

1.0

out

ref

)

1.0Ǔ+

out

1.0 k

)

10 V

1.23 V

1.0

2. Input Capacitor Selection (Cin)

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.

MOTOROLA ANALOG IC DEVICE DATA

2. Input Capacitor Selection (Cin)

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

3. Catch Diode Selection (D1)

A. 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.

Page 12

LM2574

Procedure

4. Inductor Selection (L1)

(Adjustable Output Version: LM2574–ADJ)

Procedure Example

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

microsecond [V x µs] constant:

ExT+(Vin*

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:

)

p(max

)

out

Load(max

out

)

Vxms

]

F[Hz

. The inductor current

Load

Vin*

out

4. Inductor Selection (L1) A.

Calculate E x TƪVxmsƫconstant :

ExT+(40*24) x

ExT+185ƪVxms

C.I

Load(max)

Inductance Region = 1000

D.Proper inductor value = 1000 µH

Choose the inductor from Table 2.

= 0.4 A

24 40

1000

105ƪVxms

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

out

ton+

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

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:

13,300

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 8.0 V is appropriate, and a 10 V or 16V rating is recommended.

1.0

osc

)

out

(

)

max

xLƪm

out

5. Output Capacitor Selection (C A.

13,300 x

out

To achieve an acceptable ripple voltage, select C

= 100 µF electrolytic capacitor.

out

24 x 1000

)

22.2mF

MOTOROLA ANALOG IC DEVICE DATA

Page 13

LM2574

LM2574 Series Buck Regulator Design Procedures (continued)

Indicator Value Selection Guide (For Continuous Mode Operation)

60 20

15 12

9.0

8.0

7.0

6.0

, MAXIMUM INPUT VOLTAGE (V)

5.0

60 40

30 25

20 18

17 16

, MAXIMUM INPUT VOLTAGE (V) V

680

2200

Figure 19. LM2574–3.3

470

330

220

150

IL, MAXIMUM LOAD CURRENT (A)

100

0.50.40.30.20.150.1 0.50.40.30.20.150.1

1000

30 20

15 12 10

9.0

8.0

, MAXIMUM INPUT VOLTAGE (V)V

7.0

Figure 20. LM2574–5

680

470

IL, MAXIMUM LOAD CURRENT (A)

Figure 21. LM2574–12 Figure 22. LM2574–15

2200

1500

1000

680

470

330

220

0.50.40.30.20.150.1 0.50.40.30.20.150.1

IL, MAXIMUM LOAD CURRENT (A)

40 30 25

22 20

, MAXIMUM INPUT VOLTAGE (V)

1500

1000

680

IL, MAXIMUM LOAD CURRENT (A)

330

220

150

470

330

220

250 200 150

100

80 60

50 40

ET, VOLTAGE TIME (V s)

20 15

MOTOROLA ANALOG IC DEVICE DATA

Figure 23. LM2574–ADJ

2200

1500

1000

680

470

330

220

150

100

0.50.40.30.20.150.1

IL, MAXIMUM LOAD CURRENT (A)

Page 14

LM2574

T able 1. Diode Selection Guide gives an overview about through–hole diodes

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

1.0 Amp Diodes

Schottky Fast Recovery

20 V

30 V

40 V

50 V MBR150 60 V MBR160

1N5817

MBR120P

1N5818

MBR130P

1N5819

MBR140P

MUR110

(rated to 100 V)

T able 2. Inductor Selection Guide

Inductor

Value Pulse Engineering Tech 39 Renco NPI

68 µH * 55 258 SN RL–1284–68 NP5915 100 µH * 55 308 SN RL–1284–100 NP5916 150 µH 52625 55 356 SN RL–1284–150 NP5917 220 µH 52626 55 406 SN RL–1284–220 NP5918/5919 330 µH 52627 55 454 SN RL–1284–330 NP5920/5921 470 µH 52628 * RL–1284–470 NP5922 680 µH 52629 55 504 SN RL–1284–680 NP5923

1000 µH 52631 55 554 SN RL–1284–1000 * 1500 µH * * RL–1284–1500 * 2200 µH * * RL–1284–2200 *

* : Contact Manufacturer

T able 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

+ 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

MOTOROLA ANALOG IC DEVICE DATA

Page 15

LM2574

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:

rms

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

out

and

Output Capacitor (C

For low output ripple voltage and good stability, low ESR output capacitors are recommended. An output capacitor has two main functions: it filters the output and provides regulator loop stability. The ESR of the output capacitor and the peak–to–peak value of the inductor ripple current are the main factors contributing to the output ripple voltage value. Standard aluminium electrolytics could be adequate for some applications but for quality design, low ESR types are recommended.

An aluminium electrolytic capacitor’s ESR value is related to many factors, such as the capacitance value, the voltage rating, the physical size and the type of construction. In most cases, the higher voltage electrolytic capacitors have lower ESR value. Often capacitors with much higher voltage ratings may be needed to provide low ESR values, that are required for low output ripple voltage.

1.2 x d x I

out |)V

)

out

Load

out

for a buck–boost regulator.

The Output Capacitor Requires an ESR Value that has an Upper and Lower Limit

As mentioned above, a low ESR value is needed for low output ripple voltage, typically 1% to 2% of the output voltage. But if the selected capacitor’s ESR is extremely low (below

0.03 Ω), there is a possibility of an unstable feedback loop, resulting in oscillation at the output. This situation can occur when a tantalum capacitor, that can have a very low ESR, is used as the only output capacitor.

At Low Temperatures, Put in Parallel Aluminium Electrolytic Capacitors with Tantalum Capacitors

Electrolytic capacitors are not recommended for temperatures below –25°C. The ESR rises dramatically at cold temperatures and typically rises 3 times at –25°C and as much as 10 times at –40°C. Solid tantalum capacitors have much better ESR spec at cold temperatures and are recommended for temperatures below –25°C. They can be also used in parallel with aluminium electrolytics. The value of the tantalum capacitor should be about 10% or 20% of the total capacitance. The output capacitor should have at least 50% higher RMS ripple current rating at 52 kHz than the peak–to–peak inductor ripple current.

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.

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.

MOTOROLA ANALOG IC DEVICE DATA

Page 16

LM2574

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.

0.5 A

Inductor

Current

Waveform

0.5 A

Power

Switch

Current

Waveform

Figure 24. Continuous Mode Switching

Current Waveforms

0 A

HORIZONTAL TIME BASE: 5.0 µs/DIV

0.1 A

Inductor

Current

Waveform

Current

VERTRICAL RESOLUTION 200 mADV

Waveform

Power

Switch

0 A

0.1 A 0 A

Figure 25. Continuous Mode Switching

Current Waveforms

VERTICAL RESOLUTION 100 mADV

HORIZONTAL TIME BASE: 5.0 µs/DIV

MOTOROLA ANALOG IC DEVICE DATA

Page 17

LM2574

GENERAL RECOMMENDATIONS

Output V oltage 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.

Figure 26. Output Ripple Voltage Waveforms

Voltage spikes caused by switching action of the output switch and the parasitic inductance of the output capacitor

Unfiltered

Output

Voltage

Filtered

Output

Voltage

HORIZONTAL TIME BASE: 5.0 µs/DIV

VERTRICAL RESOLUTION 20 mV/DIV

Minimizing the Output Ripple

In order to minimize the output ripple voltage it is possible to enlarge the inductance value of the inductor L1 and/or to use a larger value output capacitor. There is also another way to smooth the output by means of an additional LC filter (20 µH, 100 µ F), 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 a 8–pin DIP package. When used in the typical application the copper lead frame conducts

the majority of the heat from the die, through the leads, to the printed circuit copper. The copper and the board are the heatsink for this package and the other heat producing components, such as the catch diode and inductor.

For the best thermal performance, wide copper traces should be used and all ground and unused pins should be soldered to generous amounts of printed circuit board copper, such as a ground plane. Large areas of copper provide the best transfer of heat to the surrounding air. One exception to this is the output (switch) pin, which should not have large areas of copper in order to minimize coupling to sensitive circuitry.

Additional improvement in heat dissipation can be achieved even by using of double sided or multilayer boards which can provide even better heat path to the ambient. Using a socket for the 8–pin DIP package is not recommended because socket represents an additional thermal resistance, and as a result the junction temperature will be higher.

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 1 10°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

θJC θJA

– package thermal resistance junction–case. – package thermal resistance junction–ambient.

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

θJC

and R

θJA

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

IQ (quiescent current) and V

can be found in the

sat

LM2574 data sheet,

Vin is minimum input voltage applied,

VO is the regulator output voltage,

is the load current.

Load

MOTOROLA ANALOG IC DEVICE DATA

Page 18

LM2574

Figure 27. Inverting Buck–Boost Develops –12 V

8.0 to 25 V

Unregulated

DC Input

in 5

LM2574–12

2 Sig

Gnd

Feedback

1 Output

34ON

/OFFPwr

D1 MBR150

–12 V @ 100 mA

C 680

Regulated

Output

out

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:

where (R

TJ = (R

)(PD) represents the junction temperature rise

θJA

)(PD) + T

caused by the dissipated power and TA is the maximum ambient temperature.

Some Aspects That can Influence Thermal Design

It should be noted that the package thermal resistance and the junction temperature rise numbers are all approximate, and there are many factors that will affect these numbers, such as PC board size, shape, thickness, physical position, location, board temperature, as well as whether the surrounding air is moving or still. At higher power levels the thermal resistance decreases due to the increased air current activity.

Other factors are trace width, total printed circuit copper area, copper thickness, single– or double–sided, multilayer board, the amount of solder on the board or even color of the traces.

The size, quantity and spacing of other components on the board can also influence its effectiveness to dissipate the heat. Some of them, like the catch diode or the inductor will generate some additional heat.

Since the switch currents in this buck–boost configuration are higher than in the standard buck converter topology, the available output current is lower.

This type of buck–boost inverting regulator can also require a larger amount of startup input current, even for light loads. This may overload an input power source with a current limit less than 0.6 A.

Because of the relatively high startup currents required by this inverting regulator topology, the use of a delayed startup or an undervoltage lockout circuit is recommended.

While using a delayed startup arrangement, the input capacitor can charge up to a higher voltage before the switch–mode regulator begins to operate.

The high input current needed for startup is now partially supplied by the input capacitor Cin.

Design Recommendations:

The inverting regulator operates in a different manner than the buck converter and so a different design procedure has to be used to select the inductor L1 or the output capacitor 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 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.

Figure 28. Inverting Buck–Boost Regulator with

Delayed Startup

12 to 25 V

Unregulated

DC Input

0.1

/50 V

LM2574–12

47 k

/OFF Pwr

43ON

R2 47 k

Gnd

Feedback

1 Output

2 Sig

Gnd

D1 MBR150

out

680 /16 V

ADDITIONAL APPLICATIONS

Inverting Regulator

An inverting buck–boost regulator using the LM2574–12 is shown in Figure 27. This circuit converts a positive input voltage to a negative output voltage with a common ground by bootstrapping the regulators ground to the negative output voltage. By grounding the feedback pin, the regulator senses the inverted output voltage and regulates it.

In this example the LM2574–12 is used to generate a –12 V output. The maximum input voltage in this case cannot exceed 28 V because the maximum voltage appearing across the regulator is the absolute sum of the input and output voltages and this must be limited to a maximum of 40 V.

This circuit configuration is able to deliver approximately

0.1 A to the output when the input voltage is 8.0 V or higher. At lighter loads the minimum input voltage required drops to approximately 4.7 V, because the buck–boost regulator topology can produce an output voltage that, in its absolute value, is either greater or less than the input voltage.

–12 V @ 100 mA

Regulated

Output

The following formula is used to obtain the peak inductor

current:

where

peak

ton+

[

|VO|

Vin)

Load

|VO|

Vin)

1.0

osc

|VO|

, and f

)

Vinxt

= 52 kHz.

osc

Under normal continuous inductor current operating

conditions, the worst case occurs when Vin is minimal.

It has been already mentioned above, that in some situations, the delayed startup or the undervoltage lockout features could be very useful. A delayed startup circuit

MOTOROLA ANALOG IC DEVICE DATA

Page 19

LM2574

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 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 (1.3 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.

Figure 29. Inverting Buck–Boost Regulator Shutdown

Circuit Using an Optocoupler

Shutdown

5.0 V 0

Input

Off

NOTE: This picture does not show the complete circuit.

470

47 k

MOC8101

LM2574–XX

3 Gnds

Figure 30. Inverting Buck–Boost Regulator Shutdown

Circuit Using a PNP Transistor

5.6 k

Shutdown Input

Q1 2N3906

LM2574–XX

/OFF

3 Gnds

and

R1 12 k

NOTE: This picture does not show the complete circuit.

Off

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 .

R2 47 k

2 4

/OFF

Pins

–V

/OFF pin

and

Pins

–V

out

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.

Figure 31. Negative Boost Regulator

–5.0 to –12 V

LM2574–12

Sig

Gnd

330

1 Feedback

Output 7

/OFFPwr

1N5817

Load Current 60 mA for Vin = –5.2 V 120 mA for Vin = –7.0 V

C 1000

out

= –12 V

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 a

out

standard buck converter. Low input voltages or high output currents require a large value output capacitor (in the range of thousands of µF). The recommended range of inductor values for the negative boost regulator is the same as for inverting converter design.

Another important point is that these negative boost converters cannot provide any current limiting load protection in the event of a short in the output so some other means, such as a fuse, may be necessary to provide the load protection.

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 .

MOTOROLA ANALOG IC DEVICE DATA

Page 20

LM2574

Figure 32. Delayed Startup Circuitry

0.1

LM2574–XX

3 Gnds

47 k

R2 47 k

/OFF

and

Pins

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 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:

Vth[

VZ1)

1.0

)

R2 R1

(Q1)

Figure 34. Undervoltage Lockout Circuit for

Buck–Boost Converter

15 k

1N5242

15 k

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

68 k

Q1 2N3904

LM2574–XX

3 Gnds

/OFF

and

Pins

–V

out

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.

Figure 33. Undervoltage Lockout Circuit for

Buck Converter

10 k

1N5242B

10 k

NOTE: This picture does not show the complete circuit.

47 k

Q1 2N3904

LM2574–XX

3 Gnds

/OFF

and

Pins

MOTOROLA ANALOG IC DEVICE DATA

Page 21

LM2574

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

40 V Max Unregulated DC Input

LM2574–ADJ

Gnd

2 Sig

Gnd

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 1

Output

/OFFPwr

34ON

150

D1 1N5819

out

1000

R2 50 k

1.1 k

100

Optional Output

Ripple Filter

Output

Voltage

1.2 to 35 V @ 0.5 A

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

Unregulated

DC Input

+Vin = 7.0 to 40 V

Gnd Gnd

LM2574–5

Gnd

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

Figure 37. PC Board Layout Component Side

LM2574–5.0

C1 C2

2 Sig

Gnd

Feedback 1

Output 7

34ON

/OFFPwr

330

D1 1N5819

Figure 38. PC Board Layout Copper Side

Gnd

C2 220

Regulated Output +V

= 5.0 V @ 0.5 A

out

Gnd

NOTE: Not to scale.

MOTOROLA ANALOG IC DEVICE DATA

out

NOTE: Not to scale.

Page 22

LM2574

The LM2574–ADJ Step–Down Voltage Regulator with 5.0 V @ 0.5 A Output Power Capability Typical

Application With Through–Hole PC Board Layout

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)

Feedback

Unregulated DC Input

+Vin = 7.0 to 40 V

Gnd Gnd

LM2574–ADJ

2 Sig

Gnd

Output

/OFFPwr

34ON

330

D1 1N5819

C2 220

Ω

6.12 k

2.0 k

Ω

100

Regulated Output Filtered

= 5.0 V @ 0.5 A

out

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

Figure 40. PC Board Layout Component Side

LM2574

NOTE: Not to scale.

R1 R2

Gnd

C3+Gnd

Figure 41. PC Board Layout Copper Side

out

Ripple Filter

NOTE: Not to scale.

References

• National Semiconductor LM2574 Data Sheet and Application Note

• National Semiconductor LM2594 Data Sheet and Application Note

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

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

Output

MOTOROLA ANALOG IC DEVICE DATA

Page 23

NOTE 2

–T–

SEATING PLANE

OUTLINE DIMENSIONS

–B–

–A–

0.13 (0.005) B

LM2574

N SUFFIX

PLASTIC PACKAGE

CASE 626–05

(DIP–8)

ISSUE K

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 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

STYLE 1:

PIN 1. AC IN

2. DC + IN

3. DC – IN

4. AC IN

5. GROUND

6. OUTPUT

7. AUXILIARY

8. V

INCHESMILLIMETERS

MOTOROLA ANALOG IC DEVICE DATA

Page 24

LM2574

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MOTOROLA ANALOG IC DEVICE DATA

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LM2574/D