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PDF LM2677T-ADJ Data sheet ( Hoja de datos )

Número de pieza LM2677T-ADJ
Descripción SIMPLE SWITCHER High Efficiency 5A Step-Down Voltage Regulator with Sync
Fabricantes National Semiconductor 
Logotipo National Semiconductor Logotipo



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April 2000
LM2677
SIMPLE SWITCHER® High Efficiency 5A Step-Down
Voltage Regulator with Sync
General Description
The LM2677 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 characteris-
tics. 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 adjust-
able output version.
The SIMPLE SWITCHER concept provides for a complete
design using a minimum number of external components.
The switching clock frequency can be provided by an inter-
nal fixed frequency oscillator (260KHz) or from an externally
provided clock in the range of 280KHz to 400Khz which al-
lows the use of physically smaller sized components. A fam-
ily of standard inductors for use with the LM2677 are avail-
able from several manufacturers to greatly simplify the
design process. The external Sync clock provides direct and
precise control of the output ripple frequency for consistent
filtering or frequency spectrum positioning.
The LM2677 series also has built in thermal shutdown, cur-
rent 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.
Features
n Efficiency up to 92%
n Simple and easy to design with (using off-the-shelf
external components)
n 100 mDMOS 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 External Sync clock capability (280KHz to 400KHz)
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
n Communications and radio equipment regulator with
synchronized clock frequency
Typical Application
DS101301-3
SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation.
© 2000 National Semiconductor Corporation DS101301
www.national.com

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LM2677T-ADJ pdf
(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) meth-
ods. 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 12 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 12 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.
5 www.national.com

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LM2677T-ADJ arduino
Application Hints (Continued)
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 low-
est 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, be-
ing 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 ca-
pacitor, 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 ca-
pacitor 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 ca-
pacitance and the ESR value create a zero. These fre-
quency response effects together with the internal frequency
compensation circuitry of the LM2677 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
frequency. With the fixed 260KHz switching frequency of the
LM2677, the output capacitor is selected to provide a unity
gain bandwidth of 40KHz maximum. Each recommended ca-
pacitor value has been chosen to achieve this result.
In some cases multiple capacitors are required either to re-
duce the ESR of the output capacitor, to minimize output
ripple (a ripple voltage of 1% of Vout or less is the assumed
performance condition), or to increase the output capaci-
tance to reduce the closed loop unity gain bandwidth (to less
than 40KHz). When parallel combinations of capacitors are
required it has been assumed that each capacitor is the ex-
act same part type.
The RMS current and working voltage (WV) ratings of the
output capacitor are also important considerations. In a typi-
cal step-down switching regulator, the inductor ripple current
(set to be no more than 30% of the maximum load current by
the inductor selection) is the current that flows through the
output capacitor. The capacitor RMS current rating must be
greater than this ripple current. The voltage rating of the out-
put capacitor should be greater than 1.3 times the maximum
output voltage of the power supply. If operation of the system
at elevated temperatures is required, the capacitor voltage
rating may be de-rated to less than the nominal room tem-
perature rating. Careful inspection of the manufacturer’s
specification for de-rating of working voltage with tempera-
ture is important.
INPUT CAPACITOR
Fast changing currents in high current switching regulators
place a significant dynamic load on the unregulated power
source. An input capacitor helps to provide additional current
to the power supply as well as smooth out input voltage
variations.
Like the output capacitor, the key specifications for the input
capacitor are RMS current rating and working voltage. The
RMS current flowing through the input capacitor is equal to
one-half of the maximum dc load current so the capacitor
should be rated to handle this. Paralleling multiple capacitors
proportionally increases the current rating of the total capaci-
tance. The voltage rating should also be selected to be 1.3
times the maximum input voltage. Depending on the unregu-
lated input power source, under light load conditions the
maximum input voltage could be significantly higher than
normal operation and should be considered when selecting
an input capacitor.
The input capacitor should be placed very close to the input
pin of the LM2677. Due to relative high current operation
with fast transient changes, the series inductance of input
connecting wires or PCB traces can create ringing signals at
the input terminal which could possibly propagate to the out-
put or other parts of the circuitry. It may be necessary in
some designs to add a small valued (0.1µF to 0.47µF) ce-
ramic type capacitor in parallel with the input capacitor to
prevent or minimize any ringing.
CATCH DIODE
When the power switch in the LM2677 turns OFF, the current
through the inductor continues to flow. The path for this cur-
rent is through the diode connected between the switch out-
put and ground. This forward biased diode clamps the switch
output to a voltage less than ground. This negative voltage
must be greater than −1V so a low voltage drop (particularly
at high current levels) Schottky diode is recommended. Total
efficiency of the entire power supply is significantly impacted
by the power lost in the output catch diode. The average cur-
rent through the catch diode is dependent on the switch duty
cycle (D) and is equal to the load current times (1-D). Use of
a diode rated for much higher current than is required by the
actual application helps to minimize the voltage drop and
power loss in the diode.
During the switch ON time the diode will be reversed biased
by the input voltage. The reverse voltage rating of the diode
should be at least 1.3 times greater than the maximum input
voltage.
BOOST CAPACITOR
The boost capacitor creates a voltage used to overdrive the
gate of the internal power MOSFET. This improves efficiency
by minimizing the on resistance of the switch and associated
power loss. For all applications it is recommended to use a
0.01µF/50V ceramic capacitor.
SYNC COMPONENTS
When synchronizing the LM2677 with an external clock it is
recommended to connect the clock to pin 5 through a series
100pf capacitor and connect a 1Kresistor to ground from
pin 5. This RC network creates a short 100nS pulse on each
positive edge of the clock to reset the internal ramp oscilla-
tor. The reset time of the oscillator is approximately 300nS.
11 www.national.com

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