National Semiconductor LM3678 Technical data

April 30, 2008

LM3678 High-Performance, Miniature 1.5A Step-Down DC­DC Converter for Handheld Applications

LM3678 High-Performance, Miniature 1.5A Step-Down DC-DC Converter for Handheld

Applications

General Description

The LM3678 step-down DC-DC converter is optimized for
powering low voltage circuits from a single Li-Ion cell battery
and input voltage rails from 2.5V to 5.5V. It provides up to 1.5A
load current, over the entire input voltage range. LM3678 of­fers a 0.8V/1.2V option. One of the pair of voltages is set
through the VSELECT pin.

LM3678 operates in PWM mode with a fixed frequency of

3.3MHz. Internal synchronous rectification provides high ef­ficiency during PWM mode operation. In shutdown mode, the
device turns off and reduces battery consumption to 0.01µA
(typ).

The LM3678 is available in a 3mm x 3mm LLP-10 package.
A high switching frequency of 3.3MHz (typ.) allows use of tiny
surface-mount components. Only three external surface­mount components, an inductor and two ceramic capacitors,
are required (solution size less than 33 mm2). For voltages
other than the voltage shown, please refer to ordering infor­mation section or contact National Semiconductor.

Typical Application Circuit

Features

V

■

■

■

■

■

■

■

■

■

= 0.8V/1.2V

OUT

VIN = 2.5V to 5.5V

1.5A maximum load capability

3.3MHz PWM fixed switching frequency (typ.) allows the
use of 1µH inductor

+/- 3% DC output voltage precision

0.01µA typical shutdown current
Internal synchronous rectification for high efficiency
Internal soft start
Current overload and thermal shutdown protection

Applications

PDAs and Smart Phones

■

Personal Media Players

■

W-LAN

■

USB Modem Applications

■

Digital still cameras

■

Portable Hard disk drives

■

Efficiency vs. Output Current

( V

OUT

= 1.2V)

Note: V

© 2008 National Semiconductor Corporation 202045 www.national.com

H = 1.2V, V

SEL

L = 0.8V

SEL

FIGURE 1.

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Functional Block Diagram

LM3678

FIGURE 2. LM3678 Block Diagram

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Connection Diagram and Package Mark Information

LM3678

Top View

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Bottom View

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Pin Descriptions

Pin # Name Description

1 GND Power Ground pin.

2 GND Analog Ground Pin

3 SW Switching node connection to the internal PFET switch and NFET synchronous rectifier

4 VDD Analog supply input. Connect to the input filter capacitor (Figure 1).

5 VDD Power supply Input. Connect to the input filter capacitor (Figure 1).

6 VSELECT Output voltage select ( For example)

VSELECT = LOW, V
VSELECT = HIGH , V

7 PGOOD Power Good Flag. This commom drain logic output is pulled to ground when the output

voltage is not within +/-7.5% of regulation.

8 PWM Connect PWM pin to VIN.

9 EN Enable pin. The device is in shutdown mode when voltage to this pin is <0.4V and enabled

when >1.0V. Do not leave this pin floating.

10 FB Feedback analog input. Connect directly to the output filter capacitor for fixed voltage

versions.

DAP DAP Die Attach Pad, connect the DAP to GND on PCB layout to enhance thermal

performance. It should not be used as a primary ground connection.

OUT

OUT

= 0.8V

= 1.2V

Ordering Information

Voltage Option Order Number Package Marking Supplied As

LM3678SDE-1.2 S021B 250 units, Tape-and-Reel

0.8V/1.2V

LM3678SD-1.2 S021B 1000 units, Tape-and-Reel

LM3678SDX-1.2 S021B 4500 units, Tape-and-Reel

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Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

LM3678

please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.

VIN Pin: Voltage to GND −0.2V to 6.0V

EN Pin: −0.2V to 6.0V
FB, SW Pin: (GND−0.2V) to

Continuous Power Dissipation
(Note 3)

Junction Temperature (T

J-MAX

) +150°C

Internally Limited

(VIN + 0.2V)

Storage Temperature Range −65°C to +150°C
Maximum Lead Temperature

260°C

(Soldering, 10 sec.)

Operating Ratings (Notes 1, 2)

Input Voltage Range 2.5V to 5.5V
Recommended Load Current 0mA to 1.5A
Junction Temperature (TJ) Range −30°C to +125°C

Ambient Temperature (TA) Range (Note4)−30°C to +85°C

Thermal Properties

Junction-to-Ambient Thermal
Resistance (θJA) (LLP-10) for
4 layer board (Note 5)

49.8°C/W

Electrical Characteristics (Notes 2, 6, 8) Limits in standard typeface are for T

= 25°C. Limits in boldface type

J

apply over the full operating ambient temperature range (−30°C ≤ TA ≤ +85°C). Unless otherwise noted, specifications apply to
the LM3678 Typical Application Circuit (pg. 1) with VIN = EN = 3.6V

Symbol Parameter Condition Min Typ Max Units

V

FB

V

REF

R

DSON (P)

R

DSON (N)

I

LIM

I

SHDN

EN

IH

EN

IL

I

EN

F

OSC

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of
the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see
the Electrical Characteristics tables.

Note 2: All voltages are with respect to the potential at the GND pin.

Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ= 150°C (typ.) and disengages at

TJ= 130°C (typ.).

Note 4: In Applications where high power dissipation and/or poor package resistance is present, the maximum ambient temperature may have to be derated.
Maximum ambient temperature (T
the application (P

− (θJAx P

Note 5: Junction to ambient thermal resistance is highly application and board layout dependent. In applications where high power dissipation exists, special care
must be given to thermal dissipation issues in board design.

Note 6: Refer to datasheet curves for closed loop data and its variation with regards to supply voltage and temperature. Electrical Characteristic table reflects
open loop data (FB=0V and current drawn from SW pin ramped up until cycle by cycle current limit is activated). Closed loop current limit is the peak inductor
current measured in the application circuit by increasing output current until output voltage drops by 10%.

Note 7: Min and Max limits are guaranteed by design, test or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm.

Note 8: The parameters in the electrical characteristic table are tested at VIN= 3.6V unless otherwise specified. For performance over the input voltage range

refer to datasheet curves.

Feedback Voltage VSELECT = Low & High -3 +3 %

Internal Reference Voltage (Note 7) 0.5 V

Pin-Pin Resistance for PFET VIN= VGS= 3.6V 150 200

Pin-Pin Resistance for NFET VIN= VGS= 3.6V 110 150

Switch Peak Current Limit Open loop 1.9 2.15 2.4 A

Shutdown Supply Current EN = 0V 1 µA

Logic High Input VIN = 3.6V 1.2 V

Logic Low Input VIN = 3.6V 0.4 V

Enable (EN) Input Current 0.01 1 µA

Internal Oscillator Frequency PWM Mode 2.7 3.3 3.6 MHz

D-MAX

) is dependent on the maximum operating junction temperature (T

) and the junction to ambient thermal resistance of the package (θJA) in the application, as given by the following equation: T

D-MAX

).

A-MAX

), the maximum power dissipation of the device in

J-MAX

A-MAX

= T

mΩ

mΩ

J-MAX

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Typical Performance Characteristics

LM3678SD, Circuit of Figure 1, VIN= 3.6V, V

= 1.2V, CIN = 10µF, C

OUT

= 22µF, and TA= 25°C, unless otherwise noted.

OUT

LM3678

Quiescent Supply Current vs. Temperature

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NFET_ R

vs. Temperature

DSON

Switching Frequency vs. Temperature

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PFET_R

vs. Temperature

DSON

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ILIMIT vs. Temperature (Open Loop)

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Efficiency PWM Mode vs. ILOAD (0.8V)

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