LM2678
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SNVS029I –MARCH 2000–REVISED APRIL 2013
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
number 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
Texas Instrument's Internet site located at http://www.ti.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 ensured 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 minimizes conduction losses in the power switch to maintain
high efficiency. The recommended value for C Boost is 0.01μF.
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 recommended that a broad ground plane be used
to minimize 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
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 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 required pin 7 should be left
open circuited.
DAP (VSON 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 www.ti.com/lsds/ti/analog/powermanagement/power_portal.page.
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