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PDF LM22670 Data sheet ( Hoja de datos )
| Número de pieza | LM22670 | |
| Descripción | Step-Down Voltage Regulator | |
| Fabricantes | National Semiconductor Corporation | |
| Logotipo |  |
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October 17, 2008
LM22670
3A SIMPLE SWITCHER®, Step-Down Voltage Regulator
www.dawtasihteeht4u.Scomynchronization or Adjustable Switching Frequency
General Description
The LM22670 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 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 LM22670 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
■ 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 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
■ 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 300760
30076001
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
Normalized Switching Frequency vs Temperature
30076027
Current Limit vs Temperature
30076004
Normalized RDS(ON) vs Temperature
30076003
5
30076008
www.national.com
5 Page 
where ΔIL is the inductor ripple current.
If the switching frequency is set higher than 500 kHz, the ca-
pacitance value may not be reduced accordingly due to sta-
bility requirements. The internal compensation is optimized
for circuits with a 500 kHz switching frequency. See the in-
ternal compensation section for more details.
www.datasheet4u.com
Cboot Capacitor
The bootstrap capacitor between the BOOT pin and the SW
pin supplies the gate current to turn on the N-channel MOS-
FET. The recommended value of this capacitor is 10 nF and
should be a good quality, low ESR ceramic capacitor.
It is possible to put a small resistor in series with the Cboot
capacitor to slow down the turn-on transition time of the in-
ternal N-channel MOSFET. Resistors in the range of 10Ω to
50Ω can slow down the transition time. This can reduce EMI
of a switched mode power supply circuit. Using such a series
resistor is not recommended for every design since it will in-
crease the switching losses of the application and makes
thermal considerations more challenging.
Resistor Divider
For the -5.0 option no resistor divider is required for 5V output
voltage. The output voltage should be directly connected to
the FB pin. Output voltages above 5V can use the -5.0 option
with a resistor divider as an alternative to the -ADJ option.
This may offer improved loop bandwidth in some applications.
See the Internal Compensation section for more details.
For the -ADJ option no resistor divider is required for 1.285V
output voltage. The output voltage should be directly con-
nected to the FB pin. Other output voltages can use the -ADJ
option with a resistor divider.
The resistor values can be determined by the following equa-
tions:
-ADJ option:
-5.0 option:
Where VFB = 1.285V typical for the -ADJ option and 5V for the
-5.0 option
30076023
FIGURE 4. Resistive Feedback Divider
A maximum value of 10 kΩ is recommended for the sum of
R1 and R2 to keep high output voltage accuracy for the –ADJ
option. A maximum of 2 kΩ is recommended for the -5.0 out-
put voltage option. For the 5V fixed output voltage option, the
total internal divider resistance is typically 9.93 kΩ.
At loads less than 5 mA, the boot capacitor will not hold
enough charge to power the internal high side driver. The
output voltage may droop until the boot capacitor is
recharged. Selecting a total feedback resistance to be below
3 kΩ will provide some minimal load and can keep the output
voltage from collapsing in such low load conditions.
Catch Diode
A Schottky type re-circulating diode is required for all
LM22670 applications. Ultra-fast diodes which are not Schot-
tky diodes are not recommended and may result in damage
to the IC due to reverse recovery current transients. The near
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 LM22670. 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
are significantly reduced 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 LM22670 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.
11 www.national.com
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| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet LM22670.PDF ] | |
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