MOSFET above Vin to fully turn it ON. This minimizes con-
duction losses in the power switch to maintain high efficiency.
The recommended value for C Boost is 0.01μF.
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.
GROUND
This is the ground reference connection for all components in
the power supply. In fast-switching, high-current applications
such as those implemented with the LM2678, it is recom-
mended that a broad ground plane be used to minimize signal
coupling throughout the circuit
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 de-
signed quickly. The software is provided free of charge and
can be downloaded from National Semiconductor's Internet
site located at http://www.national.com.
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 volt-
age. 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.
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 con-
trol 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 volt-
age on pin 1 switches between Vin (switch ON) and below
ground by the voltage drop of the external Schottky diode
(switch OFF).
ON/OFF
This input provides an electrical ON/OFF control of the power
supply. Connecting this pin to ground or to any voltage 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 approximately 20μA and
a protection clamp zener diode of 7V to ground. When elec-
trically 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 required pin 7 should be
left open circuited.
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 ca-
pacitor located close to pin 2.
DAP (LLP PACKAGE)
The Die Attach Pad (DAP) can and should be connected to
PCB Ground plane/island. For CAD and assembly guidelines
refer
to
Application
Note
AN-1187
at
http://
C BOOST
power.national.com.
A capacitor must be connected from pin 3 to the switch output,
pin 1. This capacitor boosts the gate drive to the internal
DESIGN CONSIDERATIONS
10088607
FIGURE 1. Basic circuit for fixed output voltage applications.
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