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PDF LM22673 Data sheet ( Hoja de datos )
| Número de pieza | LM22673 | |
| Descripción | Step-Down Voltage Regulator | |
| Fabricantes | National Semiconductor Corporation | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LM22673 (archivo pdf) en la parte inferior de esta página. Total 16 Páginas | ||
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November 5, 2008
LM22673
3A SIMPLE SWITCHER®, Step-Down Voltage Regulator
www.dawtasihteeht4u.Acomdjustable Soft-Start and Current Limit
General Description
The LM22673 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 3A
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 external component
calculation, electrical simulation, thermal simulation, and
Build-It boards for easy design-in. The switching clock fre-
quency is provided by an internal fixed frequency oscillator
which operates at 500 kHz. The LM22673 series also has built
in thermal shutdown and current limiting. The current limit
threshold can be adjusted using an external resistor. An ad-
justable soft-start feature is provided by selecting an appro-
priate external soft-start capacitor.
Features
■ Wide input voltage range: 4.5V to 42V
■ Internally compensated voltage mode control
■ Stable with low ESR ceramic capacitors
■ 120 mΩ N-channel MOSFET TO-263 THIN package
■ 100 mΩ N-channel MOSFET PSOP-8 package
■ Output voltage options:
-ADJ (outputs as low as 1.285V)
-5.0 (output fixed to 5V)
■ ±1.5% feedback reference accuracy
■ Switching frequency of 500kHz
■ -40°C to 125°C operating junction temperature range
■ Adjustable soft-start
■ Adjustable current limit
■ Integrated boot diode
■ Fully Webench® enabled
■ Step-down and inverting buck-boost applications
Package
■ PSOP-8 (Exposed Pad)
■ 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 300762
30076201
www.national.com
1 page  
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability
and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in
the recommended Operating Ratings is not implied. The recommended Operating Ratings indicate conditions at which the device is functional and should not be
operated beyond such conditions.
Note 2: The absolute maximum specification of the ‘SW to GND’ applies to DC voltage. An extended negative voltage limit of -10V applies to a pulse of up to 50
ns.
Note 3: ESD was applied using the human body model, a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.
Note 4: Typical values represent most likely parametric norms at the conditions specified and are not guaranteed.
Note 5: Min and Max limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical
Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL).
wwNwot.ed6a:taTshheevealtu4euo.cf oθJmA for the TO-263 THIN (TJ) package of 22°C/W is valid if package is mounted to 1 square inch of copper. The θJA value can range from
20 to 30°C/W depending on the amount of PCB copper dedicated to heat transfer. See application note AN-1797 for more information.
Note 7: The value of θJA for the PSOP-8 exposed pad (MR) package of 60°C/W is valid if package is mounted to 1 square inch of copper. The θJA value can
range from 42 to 115°C/W depending on the amount of PCB copper dedicated to heat transfer.
Typical Performance Characteristics Unless otherwise specified the following conditions apply: Vin =
12V, TJ = 25°C.
Efficiency vs IOUT and VIN
VOUT = 3.3V
Current Limit vs Temperature
30076227
Normalized Switching Frequency vs Temperature
30076203
Feedback Bias Current vs Temperature
30076204
5
30076205
www.national.com
5 Page 
ideal reverse recovery characteristics and low forward volt-
age drop of Schottky diodes are particularly important diode
characteristics for high input voltage and low output voltage
applications common to the LM22673. The reverse recovery
characteristic determines how long the current surge lasts
each cycle when the N-channel MOSFET is turned on. The
reverse recovery characteristics of Schottky diodes mini-
mizes the peak instantaneous power in the switch occurring
during turn-on for each cycle. The resulting switching losses
wwawre.dsaigtansifhiceaent4tluy.croemduced when using a Schottky diode. The
reverse breakdown rating should be selected for the maxi-
mum VIN, plus some safety margin. A rule of thumb is to select
a diode with the reverse voltage rating of 1.3 times the max-
imum input voltage.
The forward voltage drop has a significant impact on the con-
version efficiency, especially for applications with a low output
voltage. ‘Rated’ current for diodes varies widely from various
manufacturers. The worst case is to assume a short circuit
load condition. In this case the diode will carry the output cur-
rent almost continuously. For the LM22673 this current can
be as high as 4.2A (typical). Assuming a worst case 1V drop
across the diode, the maximum diode power dissipation can
be as high as 4.2W.
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.
The magnitude of this noise tends to increase as the output
current increases. This parasitic spike noise 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 4 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 3A 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 LM22673, 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 LM22673.
Not soldering the DAP to the board may result in erroneous
operation due to excessive noise on the board.
30076224
FIGURE 4. Current Flow in a Buck Application
Thermal Considerations
The two highest power dissipating components are the re-
circulating diode and the LM22673 regulator IC. The easiest
method to determine the power dissipation within the
LM22673 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 LM22673 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 LM22673 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
LM22673 TO-263 THIN and PSOP packages are specified in
the electrical characteristics table under the applicable con-
ditions. For more information regarding the TO-263 THIN
package, refer to Application Note AN-1797 at
www.national.com.
11 www.national.com
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LM22673.PDF ] | |
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