lm2678sx-adj National Semiconductor Corporation, lm2678sx-adj Datasheet - Page 10

• Manufacturer Part Number: lm2678sx-adj
• Description: Simple Switcher High Efficiency 5a Step-down Voltage Regulator
• Manufacturer: National Semiconductor Corporation

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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
efficient operation.
The LM2678 is part of the SIMPLE SWITCHER family of
power converters. A complete design uses a minimum num-
ber of 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
supply can be designed quickly. The software is provided
free of charge and can be downloaded from National Semi-
conductor’s Internet site located at http://www.national.com.
SWITCH OUTPUT
This is the output of a power MOSFET switch connected
directly 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
oscillator. 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
voltage. The voltage on pin 1 switches between Vin (switch
ON) and below ground by the voltage drop of the external
Schottky diode (switch OFF).
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
supply the input pin should always be bypassed with an input
capacitor located close to pin 2.
C BOOST
A capacitor must be connected from pin 3 to the switch
output, pin 1. This capacitor boosts the gate drive to the
internal MOSFET above Vin to fully turn it ON. This mini-
mizes conduction losses in the power switch to maintain high
efficiency. The recommended value for C Boost is 0.01µF.
DESIGN CONSIDERATIONS
FIGURE 1. Basic circuit for fixed output voltage applications.
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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
recommended that a broad ground plane be used to mini-
mize signal coupling throughout the circuit
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
outputs), a direct wire connection to the output is all that is
required 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
coupling of any inductor flux to the feedback input.
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.
DAP (LLP PACKAGE)
The Die Attach Pad (DAP) can and should be connected to
PCB Ground plane/island. For CAD and assembly guide-
lines
power.national.com.
refer
to Application
Note AN-1187
10088607
at
http://