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PDF LM22677 Data sheet ( Hoja de datos )

Número de pieza LM22677
Descripción Step-Down Voltage Regulator
Fabricantes National Semiconductor Corporation 
Logotipo National Semiconductor Corporation Logotipo



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No Preview Available ! LM22677 Hoja de datos, Descripción, Manual

October 17, 2008
LM22677
5A SIMPLE SWITCHER®, Step-Down Voltage Regulator
www.dawtasihteeht4u.Scomynchronization or Adjustable Switching Frequency
General Description
The LM22677 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 N-channel MOSFET. The series consists of a
fixed 5V output and an adjustable version.
The SIMPLE SWITCHER® concept provides for an easy to
use complete design using a minimum number of external
components and National’s WEBENCH® design tool.
National’s WEBENCH® tool includes features such as exter-
nal component calculation, electrical simulation, thermal sim-
ulation, and Build-It boards for easy design-in. The switching
clock frequency is provided by an internal fixed frequency os-
cillator which operates at 500 kHz. The switching frequency
can also be adjusted with an external resistor or synchronized
to an external clock up to 1MHz. It is also possible to self-
synchronize multiple regulators to share the same switching
frequency. The LM22677 series also has built in thermal shut-
down, current limiting and an enable control input that can
power down the regulator to a low 25 µA quiescent current
standby condition.
Features
Wide input voltage range: 4.5V to 42V
Internally compensated voltage mode control
Stable with low ESR ceramic capacitors
100 mN-channel MOSFET
Output voltage options:
-ADJ (outputs as low as 1.285V)
-5.0 (output fixed to 5V)
±1.5% feedback reference accuracy
Switching frequency of 500 kHz, adjustable between 200
kHz and 1 MHz
-40°C to 125°C operating junction temperature range
Precision enable pin
Integrated boot diode
Integrated soft-start
Fully WEBENCH® enabled
Step-down and inverting buck-boost applications
Package
TO-263 THIN (Exposed Pad)
Applications
Industrial Control
Telecom and Datacom Systems
Embedded Systems
Automotive Telematics and Body Electronics
Conversions from Standard 24V, 12V and 5V Input Rails
Simplified Application Schematic
© 2008 National Semiconductor Corporation 300741
30074101
www.national.com

1 page




LM22677 pdf
Feedback Bias Current vs Temperature
Normalized Enable Threshold Voltage vs Temperature
www.datasheet4u.com
30074105
Standby Quiescent Current vs Input Voltage
30074110
Normalized Feedback Voltage vs Temperature
30074106
Normalized Feedback Voltage vs Input Voltage
30074107
Switching Frequency vs RT/SYNC Resistor
30074109
5
30074113
www.national.com

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LM22677 arduino
Circuit Board Layout
Board layout is critical for switching power supplies. First, the
ground plane area must be sufficient for thermal dissipation
purposes. Second, appropriate guidelines must be followed
to reduce the effects of switching noise. Switch mode con-
verters are very fast switching devices. In such devices, the
rapid increase of input current combined with the parasitic
trace inductance generates unwanted L di/dt noise spikes.
wwTcuwhre.rdemanttaasginhnceitreuetda4esue.ocsof.mthTihsisnopisaeratseitnicdssptoikiencnreoaissee
as the output
may turn into
electromagnetic interference (EMI) and can also cause prob-
lems in device performance. Therefore, care must be taken
in layout to minimize the effect of this switching noise.
The most important layout rule is to keep the AC current loops
as small as possible. Figure 5 shows the current flow of a buck
converter. The top schematic shows a dotted line which rep-
resents the current flow during the FET switch on-state. The
middle schematic shows the current flow during the FET
switch off-state.
The bottom schematic shows the currents referred to as AC
currents. These AC currents are the most critical since current
is changing in very short time periods. The dotted lines of the
bottom schematic are the traces to keep as short as possible.
This will also yield a small loop area reducing the loop induc-
tance. To avoid functional problems due to layout, review the
PCB layout example. Providing 5A of output current in a very
low thermal resistance package such as the TO-263 THIN is
challenging considering the trace inductances involved. Best
results are achieved if the placement of the LM22677, the by-
pass capacitor, the Schottky diode and the inductor are
placed as shown in the example. It is also recommended to
use 2oz copper boards or thicker to help thermal dissipation
and to reduce the parasitic inductances of board traces.
It is very important to ensure that the exposed DAP on the
TO-263 THIN package is soldered to the ground area of the
PCB to reduce the AC trace length between the bypass ca-
pacitor ground and the ground connection to the LM22677.
Not soldering the DAP to the board may result in erroneous
operation due to excessive noise on the board.
Thermal Considerations
The two highest power dissipating components are the re-
circulating diode and the LM22677 regulator IC. The easiest
method to determine the power dissipation within the
LM22677 is to measure the total conversion losses (Pin –
Pout) then subtract the power losses in the Schottky diode
and output inductor. An approximation for the Schottky diode
loss is:
P = (1 - D) x IOUT x VD
An approximation for the output inductor power is:
P = IOUT2 x R x 1.1,
where R is the DC resistance of the inductor and the 1.1 factor
is an approximation for the AC losses. The regulator has an
exposed thermal pad to aid power dissipation. Adding several
vias under the device to the ground plane will greatly reduce
the regulator junction temperature. Selecting a diode with an
exposed pad will aid the power dissipation of the diode. The
most significant variables that affect the power dissipated by
the LM22677 are the output current, input voltage and oper-
ating frequency. The power dissipated while operating near
the maximum output current and maximum input voltage can
be appreciable. The junction-to-ambient thermal resistance of
the LM22677 will vary with the application. The most signifi-
cant variables are the area of copper in the PC board, the
number of vias under the IC exposed pad and the amount of
forced air cooling provided. The integrity of the solder con-
nection from the IC exposed pad to the PC board is critical.
Excessive voids will greatly diminish the thermal dissipation
capacity. The junction-to-ambient thermal resistance of the
LM22677 TO-263 THIN package is specified in the electrical
characteristics table under the applicable conditions. For
more information regarding the TO-263 THIN package, refer
to Application Note AN-1797 at www.national.com.
30074124
FIGURE 5. Current Flow in a Buck Application
11
www.national.com

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