NSC LM2679T-ADJ, LM2679T-3.3, LM2679T-12, LM2679SX-ADJ, LM2679SX-5.0 Datasheet

LM2679
SIMPLE SWITCHER

®

5A Step-Down Voltage Regulator

with Adjustable Current Limit

General Description

The LM2679 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 characteris­tics. 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 adjust­able 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.Afamilyofstandardin­ductors for use with the LM2679 are available from several
manufacturers to greatly simplify the design process.

Other features include the ability to reduce the input surge
current at power-ON by adding a softstart timing capacitor to
gradually turn on the regulator. The LM2679 series also has
built in thermal shutdown and resistor programmable current
limit of the power MOSFET switch to protect the device and
load circuitry under fault conditions. The output voltage is
guaranteed to a

±

2% tolerance. The clock frequency is con-

trolled to within a

±

11% tolerance.

Features

n Efficiency up to 92%
n Simple and easy to design with (using off-the-shelf

external components)

n Resistor programmable peak current limit over a range

of 3A to 7A.

n 120 mΩ DMOS output switch
n 3.3V, 5V and 12V fixed output and adjustable (1.2V to

37V ) versions

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 Softstart capability
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.

DS100847-3

March 2000

LM2679 SIMPLE SWITCHER 5A Step-Down Voltage Regulator with Adjustable Current Limit

© 2000 National Semiconductor Corporation DS100847 www.national.com

Connection Diagram and Ordering Information

TO-263 Package

Top View

DS100847-1

Order Number

LM2679S-3.3, LM2679S-5.0,

LM2679S-12 or LM2679S-ADJ

See NSC Package Number TS7B

TO-220 Package

Top View

DS100847-2

Order Number

LM2679T-3.3, LM2679T-5.0,

LM2679T-12 or LM2679T-ADJ

See NSC Package Number TA07B

LM2679

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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
Softstart Pin Voltage −0.1V to 6V
Switch Voltage to Ground −1V to V

IN

Boost Pin Voltage VSW+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=TJ

= 25˚C. R

ADJ

= 5.6KΩ

LM2679-3.3

Symbol Parameter Conditions Typical Min Max Units

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

V

OUT

Output Voltage VIN= 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 %

LM2679-5.0

Symbol Parameter Conditions Typical Min Max Units

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

V

OUT

Output Voltage VIN= 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 %

LM2679-12

Symbol Parameter Conditions Typical Min Max Units

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

V

OUT

Output Voltage VIN= 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 %

LM2679-ADJ

Symbol Parameter Conditions Typ Min Max Units

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

V

FB

Feedback
Voltage

VIN= 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 %

LM2679

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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 TA=TJ= 25˚C. Unless otherwise specified VIN=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

V

ADJ

Current Limit
Adjust Voltage

1.21 1.181/1.169 1.229/1.246 V

I

CL

Current Limit R

ADJ

= 5.6KΩ, (Note 5) 6.3 5.5/5.3 7.6/8.1 A

I

L

Output
Leakage
Current

V

IN

= 40V, Softstart Pin = 0V

V

SWITCH

=0V

V

SWITCH

= −1V

1.0
6

1.5
15

mA
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

SFST

Softstart
Threshold
Voltage

0.63 0.53 0.74 V

I

SFST

Softstart Pin
Current

Softstart Pin = 0V

3.7 6.9 µA

θ

JA

Thermal
Resistance

T Package, Junction to Ambient 65
(Note 6)

θ

JA

T Package, Junction to Ambient 45
(Note 7)

θ

JC

T Package, Junction to Case 2

θ

JA

S Package, Junction to Ambient 56 ˚C/W
(Note 8)

θ

JA

S Package, Junction to Ambient 35
(Note 9)

θ

JA

S Package, Junction to Ambient 26
(Note 10)

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 Char­acteristics 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=TJ

= 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=TJ

= 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: The peak switch current limit is determined by the following relationship: I

CL

=37,125/ R

ADJ

.

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

1

⁄2inch leads in a socket, or on a PC

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

1

⁄2inch leads soldered to a PC board

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

LM2679

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(Continued)

Note 8: 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 9: 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 10: 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.

LM2679

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Typical Performance Characteristics

Normalized
Output Voltage

DS100847-4

Line Regulation

DS100847-5

Efficiency vs Input Voltage

DS100847-6

Efficiency vs I

LOAD

DS100847-7

Switch Current Limit

DS100847-8

Operating Quiescent Current

DS100847-9

Switching Frequency

DS100847-12

Feedback Pin Bias Current

DS100847-13

LM2679

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Block Diagram

DS100847-14

* Active Inductor Patent Number 5,514,947

†

Active Capacitor Patent Number 5,382,918

LM2679

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Typical Performance Characteristics

Continuous Mode Switching Waveforms

V

IN

= 20V, V

OUT

=5V,I

LOAD

=5A

L = 10 µH, C

OUT

= 400 µF, C

OUT

ESR=13mΩ

DS100847-15

A: VSWPin Voltage, 10 V/div.
B: Inductor Current, 2 A/div
C: Output Ripple Voltage, 20 mV/div AC-Coupled

Horizontal Time Base: 1 µs/div

Discontinuous Mode Switching Waveforms

V

IN

= 20V, V

OUT

=5V,I

LOAD

= 500 mA

L = 10 µH, C

OUT

= 400 µF, C

OUT

ESR=13mΩ

DS100847-16

A: VSWPin Voltage, 10 V/div.
B: Inductor Current, 1 A/div
C: Output Ripple Voltage, 20 mV/div AC-Coupled

Horizontal Time Base: 1 µs//iv

Load Transient Response for Continuous Mode

V

IN

= 20V, V

OUT

=5V

L = 10 µH, C

OUT

= 400 µF, C

OUT

ESR=13mΩ

DS100847-17

A: Output Voltage, 100 mV//div, AC-Coupled.
B: Load Current: 500 mA to 5A Load Pulse

Horizontal Time Base: 100 µs/div

Load Transient Response for Discontinuous Mode

V

IN

= 20V, V

OUT

=5V,

L = 10 µH, C

OUT

= 400 µF, C

OUT

ESR=13mΩ

DS100847-18

A: Output Voltage, 100 mV/div, AC-Coupled.
B: Load Current: 200 mA to 3A Load Pulse

Horizontal Time Base: 200 µs/div

LM2679

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