LM2678S-3.3 datasheet(9/23 Pages) NSC

9 / 23 page

Application Hints

The LM2678 provides all of the active functions required for

a step-down (buck) switching regulator. The internal power

switch is a DMOS power MOSFET to provide power supply

designs with high current capability, up to 5A, and highly ef-

ficient operation.

The LM2678 is part of the

SIMPLE SWITCHER family of

power converters. A complete design uses a minimum num-

ber

external

components,

which

have

been

pre-determined from a variety of manufacturers. Using either

this data sheet or a design software program called

LM267X

Made Simple (version 2.0) a complete switching power sup-

ply can be designed quickly. The software is provided free of

charge and can be downloaded from National Semiconduc-

tor’s Internet site located at http://www.national.com.

PIN 1 - Switch Output

This is the output of a power MOSFET switch connected di-

rectly to the input voltage. The switch provides energy to an

inductor, an output capacitor and the load circuitry under

control of an internal pulse-width-modulator (PWM). The

PWM controller is internally clocked by a fixed 260KHz oscil-

lator. In a standard step-down application the duty cycle

(Time ON/Time OFF) of the power switch is proportional to

the ratio of the power supply output voltage to the input volt-

age. The voltage on pin 1 switches between Vin (switch ON)

and below ground by the voltage drop of the external Schot-

tky diode (switch OFF).

PIN 2 - Input

The input voltage for the power supply is connected to pin 2.

In addition to providing energy to the load the input voltage

also provides bias for the internal circuitry of the LM2678.

For guaranteed performance the input voltage must be in the

range of 8V to 40V. For best performance of the power sup-

ply the input pin should always be bypassed with an input ca-

pacitor located close to pin 2.

PIN3-C Boost

A capacitor must be connected from pin 3 to the switch out-

put, pin 1. This capacitor boosts the gate drive to the internal

MOSFET above Vin to fully turn it ON. This minimizes con-

duction losses in the power switch to maintain high effi-

ciency. The recommended value for C Boost is 0.01µF.

PIN 4 - Ground

This is the ground reference connection for all components

in the power supply. In fast-switching, high-current applica-

tions such as those implemented with the LM2678, it is rec-

ommended that a broad ground plane be used to minimize

signal coupling throughout the circuit

PIN5-No Connection

PIN 6 - Feedback

This is the input to a two-stage high gain amplifier, which

drives the PWM controller. It is necessary to connect pin 6 to

the actual output of the power supply to set the dc output

voltage. For the fixed output devices (3.3V, 5V and 12V out-

puts), a direct wire connection to the output is all that is re-

quired as internal gain setting resistors are provided inside

the LM2678. For the adjustable output version two external

resistors are required to set the dc output voltage. For stable

operation of the power supply it is important to prevent cou-

pling of any inductor flux to the feedback input.

PIN 7 - ON/OFF

This input provides an electrical ON/OFF control of the

power supply. Connecting this pin to ground or to any volt-

age less than 0.8V will completely turn OFF the regulator.

The current drain from the input supply when OFF is only

50µA. Pin 7 has an internal pull-up current source of approxi-

mately 20µA and a protection clamp zener diode of 7V to

ground. When electrically driving the ON/OFF pin the high

voltage level for the ON condition should not exceed the 6V

absolute maximum limit. When ON/OFF control is not re-

quired pin 7 should be left open circuited.

DESIGN CONSIDERATIONS

DS100886-7

FIGURE 1. Basic circuit for fixed output voltage applications.

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