NSC LM2678SX-5.0, LM2678SX-3.3, LM2678SX-12, LM2678S-5.0, LM2678S-3.3 Datasheet

LM2678

SIMPLE SWITCHER

®

High Efficiency 5A Step-Down

Voltage Regulator

General Description

The LM2678 series of regulators are monolithic integrated

circuits which provide all of the active functions for a

step-down (buck) switching regulator capable of driving up to

5A loads with excellent line and load regulation characteris-

tics. High efficiency (

>

90%) is obtained through the use of a

low ON-resistance DMOS power switch. The series consists

of fixed output voltages of 3.3V, 5V and 12V and an adjust-

able output version.

The SIMPLE SWITCHER concept provides for a complete

design using a minimum number of external components. A

high fixed frequency oscillator (260KHz) allows the use of

physically smaller sized components.Afamilyofstandardin-

ductors for use with the LM2678 are available from several

manufacturers to greatly simplify the design process.

The LM2678 series also has built in thermal shutdown, cur-

rent limiting and an ON/OFF control input that can power

down the regulator to a low 50µA quiescent current standby

condition. The output voltage is guaranteed to a

±

2% toler-

ance. The clock frequency is controlled to within a

±

11%tol-

erance.

Features

n Efficiency up to 92%

n Simple and easy to design with (using off-the-shelf

external components)

n 120 mΩ DMOS output switch

n 3.3V, 5V and 12V fixed output and adjustable (1.2V to

37V ) versions

n 50µA standby current when switched OFF

n

±

2%maximum output tolerance over full line and load

conditions

n Wide input voltage range: 8V to 40V

n 260 KHz fixed frequency internal oscillator

n −40 to +125˚C operating junction temperature range

Applications

n Simple to design, high efficiency (

>

90%) step-down

switching regulators

n Efficient system pre-regulator for linear voltage

regulators

n Battery chargers

Typical Application

SIMPLE SWITCHER

®

is a registered trademark of National Semiconductor Corporation.

DS100886-3

March 2000

LM2678 SIMPLE SWITCHER High Efficiency 5A Step-Down Voltage Regulator

© 2000 National Semiconductor Corporation DS100886 www.national.com

Connection Diagram and Ordering Information

TO-263 Package

Top View

DS100886-1

Order Number

LM2678S-3.3, LM2678S-5.0,

LM2678S-12 or LM2678S-ADJ

See NSC Package Number TS7B

TO-220 Package

Top View

DS100886-2

Order Number

LM2678T-3.3, LM2678T-5.0,

LM2678T-12 or LM2678T-ADJ

See NSC Package Number TA07B

LM2678

www.national.com 2

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Input Supply Voltage 45V

ON/OFF Pin Voltage −0.1V to 6V

Switch Voltage to Ground −1V to V

IN

Boost Pin Voltage V

SW

+8V

Feedback Pin Voltage −0.3V to 14V

Power Dissipation Internally Limited

ESD (Note 2) 2 kV

Storage Temperature Range −65˚C to 150˚C

Soldering Temperature

Wave 4 sec, 260˚C

Infrared 10 sec, 240˚C

Vapor Phase 75 sec, 219˚C

Operating Ratings

Supply Voltage 8V to 40V

Junction Temperature Range (T

J

) −40˚C to 125˚C

Electrical Characteristics Limits appearing in bold type face apply over the entire junction temperature

range of operation, −40˚C to 125˚C. Specifications appearing in normal type apply for T

A

=T

J

= 25˚C.

LM2678-3.3

Symbol Parameter Conditions Typical Min Max Units

(Note 3) (Note 4) (Note 4)

V

OUT

Output Voltage V

IN

= 8V to 40V, 100mA ≤ I

OUT

≤ 5A 3.3 3.234/3.201 3.366/3.399 V

η Efficiency V

IN

= 12V, I

LOAD

=5A 82 %

LM2678-5.0

Symbol Parameter Conditions Typical Min Max Units

(Note 3) (Note 4) (Note 4)

V

OUT

Output Voltage V

IN

= 8V to 40V, 100mA ≤ I

OUT

≤ 5A 5.0 4.900/4.850 5.100/5.150 V

η Efficiency V

IN

= 12V, I

LOAD

=5A 84 %

LM2678-12

Symbol Parameter Conditions Typical Min Max Units

(Note 3) (Note 4) (Note 4)

V

OUT

Output Voltage V

IN

= 15V to 40V, 100mA ≤ I

OUT

≤ 5A 12 11.76/11.64 12.24/12.36 V

η Efficiency V

IN

= 24V, I

LOAD

=5A 92 %

LM2678-ADJ

Symbol Parameter Conditions Typ Min Max Units

(Note 3) (Note 4) (Note 4)

V

FB

Feedback

Voltage

V

IN

= 8V to 40V, 100mA ≤ I

OUT

≤ 5A

V

OUT

Programmed for 5V

1.21 1.186/1.174 1.234/1.246 V

η Efficiency V

IN

= 12V, I

LOAD

=5A 84 %

LM2678

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All Output Voltage Versions

Electrical Characteristics

Limits appearing in bold type face apply over the entire junction temperature range of operation, −40˚C to 125˚C.

Specifications appearing in normal type apply for T

A

=T

J

= 25˚C. Unless otherwise specified V

IN

=12V for the 3.3V, 5V and

Adjustable versions and V

IN

=24V for the 12V version.

Symbol Parameter Conditions Typ Min Max Units

DEVICE PARAMETERS

I

Q

Quiescent

Current V

FEEDBACK

= 8V 4.2 6 mA

For 3.3V, 5.0V, and ADJ Versions

V

FEEDBACK

= 15V

For 12V Versions

I

STBY

Standby

Quiescent

Current

ON/OFF Pin = 0V

50 100/150 µA

I

CL

Current Limit 7 6.1/5.75 8.3/8.75 A

I

L

Output

Leakage

Current

V

IN

= 40V, ON/OFF Pin = 0V

V

SWITCH

=0V

V

SWITCH

= −1V

1

6

200

15

µA

mA

R

DS(ON)

Switch

On-Resistance

I

SWITCH

= 5A 0.12 0.14/0.225 Ω

f

O

Oscillator

Frequency

Measured at Switch Pin 260 225 280 kHz

D Duty Cycle Maximum Duty Cycle 91 %

Minimum Duty Cycle 0 %

I

BIAS

Feedback Bias

Current

V

FEEDBACK

= 1.3V

ADJ Version Only

85 nA

V

ON/OFF

ON/OFF

Threshold

Voltage

1.4 0.8 2.0 V

I

ON/OFF

ON/OFF Input

Current

ON/OFF Input = 0V

20 45 µA

θ

JA

Thermal

Resistance

T Package, Junction to Ambient 65

(Note 5)

θ

JA

T Package, Junction to Ambient 45

(Note 6)

θ

JC

T Package, Junction to Case 2

θ

JA

S Package, Junction to Ambient 56 ˚C/W

(Note 7)

θ

JA

S Package, Junction to Ambient 35

(Note 8)

θ

JA

S Package, Junction to Ambient 26

(Note 9)

θ

JC

S Package, Junction to Case 2 ++

Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings indicate conditions under which of the device is

guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test condition, see the electrical Char-

acteristics tables.

Note 2: ESD was applied using the human-body model, a 100pF capacitor discharged through a 1.5 kΩ resistor into each pin.

Note 3: Typical values are determined with T

A

=T

J

= 25˚C and represent the most likely norm.

Note 4: All limits are guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%

tested during production with T

A

=T

J

= 25˚C. All limits at temperature extremes are guaranteed via correlation using standard standard Quality Control (SQC) meth-

ods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

Note 5: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with

1

⁄

2

inch leads in a socket, or on a PC

board with minimum copper area.

Note 6: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with

1

⁄

2

inch leads soldered to a PC board

containing approximately 4 square inches of (1 oz.) copper area surrounding the leads.

LM2678

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(Continued)

Note 7: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.136 square inches (the same size as the

TO-263 package) of 1 oz. (0.0014 in. thick) copper.

Note 8: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.4896 square inches (3.6 times the area of

the TO-263 package) of 1 oz. (0.0014 in. thick) copper.

Note 9: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board copper area of 1.0064 square inches (7.4 times the

area of the TO-263 package) of 1 oz. (0.0014 in. thick) copper.Additionalcopper area will reduce thermal resistance further.See the thermal model in Switchers Made

Simple

®

software.

LM2678

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Typical Performance Characteristics

Normalized

Output Voltage

DS100886-9

Line Regulation

DS100886-10

Efficiency vs Input Voltage

DS100886-11

Efficiency vs I

LOAD

DS100886-12

Switch Current Limit

DS100886-4

Operating Quiescent Current

DS100886-5

Standby Quiescent Current

DS100886-40

ON/OFF Threshold Voltage

DS100886-13

ON/OFF Pin Current (Sourcing)

DS100886-14

Switching Frequency

DS100886-15

Feedback Pin Bias Current

DS100886-16

LM2678

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Block Diagram

DS100886-6

* Active Inductor Patent Number 5,514,947

†

Active Capacitor Patent Number 5,382,918

LM2678

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