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PDF LM2678 Data sheet ( Hoja de datos )
| Número de pieza | LM2678 | |
| Descripción | SIMPLE SWITCHER High Efficiency 5A Step-Down Voltage Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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April 2005
LM2678
SIMPLE SWITCHER® High Efficiency 5A Step-Down
Voltage Regulator
General Description
The LM2678 series of regulators are monolithic integrated
circuits which provide all of the active functions for a step-
down (buck) switching regulator capable of driving up to 5A
loads with excellent line and load regulation characteristics.
High efficiency (>90%) is obtained through the use of a low
ON-resistance DMOS power switch. The series consists of
fixed output voltages of 3.3V, 5V and 12V and an adjustable
output version.
The SIMPLE SWITCHER concept provides for a complete
design using a minimum number of external components. A
high fixed frequency oscillator (260KHz) allows the use of
physically smaller sized components. A family of standard
inductors for use with the LM2678 are available from several
manufacturers to greatly simplify the design process.
The LM2678 series also has built in thermal shutdown,
current limiting and an ON/OFF control input that can power
down the regulator to a low 50µA quiescent current standby
condition. The output voltage is guaranteed to a ±2% toler-
ance. The clock frequency is controlled to within a ±11%
tolerance.
Features
n Efficiency up to 92%
n Simple and easy to design with (using off-the-shelf
external components)
n 120 mΩ DMOS output switch
n 3.3V, 5V and 12V fixed output and adjustable (1.2V to
37V ) versions
n 50µA standby current when switched OFF
n ±2%maximum output tolerance over full line and load
conditions
n Wide input voltage range: 8V to 40V
n 260 KHz fixed frequency internal oscillator
n −40 to +125˚C operating junction temperature range
Applications
n Simple to design, high efficiency (>90%) step-down
switching regulators
n Efficient system pre-regulator for linear voltage
regulators
n Battery chargers
Typical Application
10088603
SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation.
© 2005 National Semiconductor Corporation DS100886
www.national.com
1 page  
All Output Voltage Versions
Electrical Characteristics (Continued)
Note 4: All limits are guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
tested during production with TA = TJ = 25˚C. All limits at temperature extremes are guaranteed via correlation using standard standard Quality Control (SQC)
methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Note 5: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with 1⁄2 inch leads in a socket, or on a PC
board with minimum copper area.
Note 6: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with 1⁄2 inch leads soldered to a PC board
containing approximately 4 square inches of (1 oz.) copper area surrounding the leads.
Note 7: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.136 square inches (the same size as the
TO-263 package) of 1 oz. (0.0014 in. thick) copper.
Note 8: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.4896 square inches (3.6 times the area
of the TO-263 package) of 1 oz. (0.0014 in. thick) copper.
Note 9: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board copper area of 1.0064 square inches (7.4 times the
area of the TO-263 package) of 1 oz. (0.0014 in. thick) copper. Additional copper area will reduce thermal resistance further. See the thermal model in Switchers
Made Simple® software.
Note 10: Junction to ambient thermal resistance for the 14-lead LLP mounted on a PC board copper area equal to the die attach paddle.
Note 11: Junction to ambient thermal resistance for the 14-lead LLP mounted on a PC board copper area using 12 vias to a second layer of copper equal to die
attach paddle. Additional copper area will reduce thermal resistance further. For layout recommendations, refer to Application Note AN-1187.
5 www.national.com
5 Page 
Application Hints (Continued)
FIGURE 2. Basic circuit for adjustable output voltage applications
10088608
Power supply design using the LM2678 is greatly simplified
by using recommended external components. A wide range
of inductors, capacitors and Schottky diodes from several
manufacturers have been evaluated for use in designs that
cover the full range of capabilities (input voltage, output
voltage and load current) of the LM2678. A simple design
procedure using nomographs and component tables pro-
vided in this data sheet leads to a working design with very
little effort. Alternatively, the design software, LM267X Made
Simple (version 6.0), can also be used to provide instant
component selection, circuit performance calculations for
evaluation, a bill of materials component list and a circuit
schematic.
The individual components from the various manufacturers
called out for use are still just a small sample of the vast
array of components available in the industry. While these
components are recommended, they are not exclusively the
only components for use in a design. After a close compari-
son of component specifications, equivalent devices from
other manufacturers could be substituted for use in an ap-
plication.
Important considerations for each external component and
an explanation of how the nomographs and selection tables
were developed follows.
INDUCTOR
The inductor is the key component in a switching regulator.
For efficiency the inductor stores energy during the switch
ON time and then transfers energy to the load while the
switch is OFF.
Nomographs are used to select the inductance value re-
quired for a given set of operating conditions. The nomo-
graphs assume that the circuit is operating in continuous
mode (the current flowing through the inductor never falls to
zero). The magnitude of inductance is selected to maintain a
maximum ripple current of 30% of the maximum load cur-
rent. If the ripple current exceeds this 30% limit the next
larger value is selected.
The inductors offered have been specifically manufactured
to provide proper operation under all operating conditions of
input and output voltage and load current. Several part types
are offered for a given amount of inductance. Both surface
mount and through-hole devices are available. The inductors
from each of the three manufacturers have unique charac-
teristics.
Renco: ferrite stick core inductors; benefits are typically
lowest cost and can withstand ripple and transient peak
currents above the rated value. These inductors have an
external magnetic field, which may generate EMI.
Pulse Engineering: powdered iron toroid core inductors;
these also can withstand higher than rated currents and,
being toroid inductors, will have low EMI.
Coilcraft: ferrite drum core inductors; these are the smallest
physical size inductors and are available only as surface
mount components. These inductors also generate EMI but
less than stick inductors.
OUTPUT CAPACITOR
The output capacitor acts to smooth the dc output voltage
and also provides energy storage. Selection of an output
capacitor, with an associated equivalent series resistance
(ESR), impacts both the amount of output ripple voltage and
stability of the control loop.
The output ripple voltage of the power supply is the product
of the capacitor ESR and the inductor ripple current. The
capacitor types recommended in the tables were selected
for having low ESR ratings.
In addition, both surface mount tantalum capacitors and
through-hole aluminum electrolytic capacitors are offered as
solutions.
Impacting frequency stability of the overall control loop, the
output capacitance, in conjunction with the inductor, creates
a double pole inside the feedback loop. In addition the
capacitance and the ESR value create a zero. These fre-
quency response effects together with the internal frequency
compensation circuitry of the LM2678 modify the gain and
phase shift of the closed loop system.
As a general rule for stable switching regulator circuits it is
desired to have the unity gain bandwidth of the circuit to be
limited to no more than one-sixth of the controller switching
11 www.national.com
11 Page | ||
| Páginas | Total 27 Páginas | |
| PDF Descargar | [ Datasheet LM2678.PDF ] | |
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