LM22676TJ-ADJ_技术文档

November 21, 2008

LM22676

3A SIMPLE SWITCHER®, Step-Down Voltage Regulator

with Precision Enable

General Description

Features

The LM22676 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 LM22676 series also

has built in thermal shutdown, current limiting and an enable

control input that can power down the regulator to a low 25

µA quiescent current standby condition.

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

-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

30076501

300765

www.national.com

Connection Diagrams

30076540

8-Lead Plastic PSOP-8 Package

NS Package Number MRA08B

30076502

7-Lead Plastic TO-263 THIN Package

NS Package Number TJ7A

Ordering Information

Output Voltage

Order Number

LM22676MR-ADJ

LM22676MRE-ADJ

LM22676MRX-ADJ

LM22676TJE-ADJ

LM22676TJ-ADJ

LM22676MR-5.0

LM22676MRE-5.0

LM22676MRX-5.0

LM22676TJE-5.0

LM22676TJ-5.0

Package Type

NSC Package Drawing

Supplied As

ADJ

5.0

PSOP-8 Exposed Pad

MRA08B

95 Units in Rails

250 Units in Tape and Reel

2500 Units in Tape and Reel

250 Units in Tape and Reel

1000 Units in Tape and Reel

95 Units in Rails

TO-263 THIN Exposed Pad

PSOP-8 Exposed Pad

TJ7A

MRA08B

5.0

250 Units in Tape and Reel

2500 Units in Tape and Reel

250 Units in Tape and Reel

1000 Units in Tape and Reel

5.0

TO-263 THIN Exposed Pad

TJ7A

5.0

www.national.com

2

Pin Descriptions

Pin Numbers

PSOP-8

Package

Pin Numbers

TO-263 THIN

Package

Name Description

Application Information

1

3

5

BOOT Bootstrap input

Provides the gate voltage for the high side NFET.

2, 3

NC

Not Connected

Pins are not electrically connected inside the chip. Pins do

function as thermal conductor.

4

5

6

7

4

FB

EN

Feedback pin

Inverting input to the internal voltage error amplifier.

When pulled low regulator turns off.

Precision enable pin

System ground

GND

Provide good capacitive decoupling between VIN and this

7

8

2

1

VIN

SW

Source input voltage

Switch pin

Input to the regulator. Operates from 4.5V to 42V.

Attaches to the switch node

3

www.national.com

Junction Temperature

Soldering Information

Infrared (5 sec.)

ESD Rating (Note 3)

Human Body Model

Storage Temperature Range

150°C

260°C

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

±2 kV

-65°C to +150°C

VIN to GND

43V

-0.5V to 6V

-5V to V_IN

EN Pin Voltage

SW to GND (Note 2)

BOOT Pin Voltage

FB Pin Voltage

Operating Ratings (Note 1)

Supply Voltage (V_IN)

V_SW+ 7V

4.5V to 42V

-40°C to +125°C

-0.5V to 7V

Junction Temperature Range

Power Dissipation

Internally Limited

Electrical Characteristics Limits in standard type are for T_J= 25°C only; limits in boldface type apply over the

junction temperature (T_J) range of -40°C to +125°C. Minimum and Maximum limits are guaranteed through test, design, or statistical

correlation. Typical values represent the most likely parametric norm at T_A= T_J= 25°C, and are provided for reference purposes

only. Unless otherwise specified: V_IN= 12V.

Symbol

Parameter

Conditions

Min

Typ

Max

Units

(Note 5)

(Note 4)

(Note 5)

LM22676-5.0

V_FB

Feedback Voltage

V_IN= 8V to 42V

V_IN= 4.7V to 42V

V_FB= 5V

4.925/4.9

5.0

5.075/5.1

V

LM22676-ADJ

V_FB

1.266/1.259

1.285

1.304/1.311

All Output Voltage Versions

I_Q

I_STDBY

I_CL

Quiescent Current

3.4

25

6

40

mA

µA

A

Standby Quiescent Current EN Pin = 0V

Current Limit

3.4/3.35

4.2

0.2

0.1

0.12

0.10

500

300

100

230

1.6

6

5.3/5.5

2

I_L

Output Leakage Current

V_IN= 42V, EN Pin = 0V, V_SW= 0V

µA

Ω

V_SW= -1V

3

R_DS(ON)

Switch On-Resistance

TO-263 THIN Package

PSOP-8 Package

0.16/0.22

0.16/0.20

600

f_O

T_OFFMIN

T_ONMIN

I_BIAS

Oscillator Frequency

Minimum Off-time

400

1.3

kHz

ns

nA

V

Minimum On-time

Feedback Bias Current

Enable Threshold Voltage

Enable Input Current

V_FB= 1.3V (ADJ Version Only)

EN Input = 0V

V_EN

1.9

I_EN

µA

°C

T_SD

Thermal Shutdown

Threshold

150

Thermal Resistance

TJ Junction to ambient temperature

resistance (Note 6)

22

60

°C/W

θ_JA

Thermal Resistance

MR Package, Junction to ambient

temperature resistance (Note 7)

www.national.com

4

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).

Note 6: The value of θ_JAfor the TO-263 THIN (TJ) package of 22°C/W is valid if package is mounted to 1 square inch of copper. The θ_JAvalue 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 θ_JAfor the PSOP-8 exposed pad (MR) package of 60°C/W is valid if package is mounted to 1 square inch of copper. The θ_JAvalue 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, T_J= 25°C.

Efficiency vs I_OUTand V_IN

V_OUT= 3.3V

Normalized Switching Frequency vs Temperature

30076504

30076527

Current Limit vs Temperature

Normalized R_DS(ON)vs Temperature

30076508

30076503

5

www.national.com

Feedback Bias Current vs Temperature

Normalized Enable Threshold Voltage vs Temperature

30076505

30076510

Standby Quiescent Current vs Input Voltage

Normalized Feedback Voltage vs Temperature

30076507

30076506

Normalized Feedback Voltage vs Input Voltage

30076509

www.national.com

6

Typical Application Circuit and Block Diagram

30076514

FIGURE 1. 3.3V V_OUT, 3A

7

www.national.com

Detailed Operating Description

Minimum Duty-Cycle

The LM22676 switching regulator features all of the functions

necessary to implement an efficient high voltage buck regu-

lator using a minimum of external components. This easy to

use regulator integrates a 42V N-Channel switch with an out-

put current capability of 3A. The regulator control method is

based on voltage mode control with input voltage feed for-

ward. The loop compensation is integrated into the LM22676

so that no external compensation components need to be se-

lected or utilized. Voltage mode control offers short minimum

on-times allowing short duty-cycles necessary in high input

voltage applications. The operating frequency is fixed at

500kHz to allow for small external components while avoiding

excessive switching losses. The output voltage can be set as

low as 1.285V with the -ADJ device. Fault protection features

include current limiting, thermal shutdown and remote shut-

down capability. The device is available in the TO-263 THIN

and PSOP-8 packages featuring an exposed pad to aid ther-

mal dissipation.

Besides a minimum off-time, there is also a minimum on-time

which will take effect when the output voltage is adjusted very

low and the input voltage is very high. Should the operation

require a shorter minimum on-time than the typical 100 ns,

individual switching pulses will be skipped.

where D is the duty-cycle.

Current Limit

When the power switch turns on, the slight capacitance load-

ing of the Schottky diode, D1, causes a leading-edge current

spike with an extended ringing period. This spike can cause

the current limit comparator to trip prematurely. A leading

edge blanking time (T_BLK) of 110 ns (typical) is used to avoid

sampling the spike.

The functional block diagram with typical application of the

LM22676 are shown in Figure 1.

When the switch current reaches the current limit threshold,

the switch is immediately turned off and the internal switching

frequency is reduced. This extends the off time of the switch

to prevent a steady state high current condition. As the switch

current falls below the current limit threshold, the switch cur-

rent will attempt to turn on. If a load fault continues, the switch

will again exceed the threshold and turn off. This will result in

a low duty-cycle pulsing of the power switch to minimize the

overall fault condition power dissipation.

The internal compensation of the -ADJ option of the LM22676

is optimized for output voltages up to 5V. If an output voltage

of 5V or higher is needed, the -5.0 fixed output voltage option

with an additional external resistive feedback voltage divider

may also be used.

Precision Enable

The precision enable pin (EN) can be used to shut down the

power supply. Connecting this pin to ground or to a voltage

less than typical 1.6V will completely turn off the regulator.

The current drain from the input supply when off is typically

25 µA with 12V input voltage. The power consumed during

this off state is mostly defined by an internal 2 MΩ resistor to

VIN. The enable pin has an internal pull-up current source of

approximately 6 µA. When driving the enable pin, the high

voltage level for the on condition should not exceed the 6V

absolute maximum limit. When enable control is not required,

the EN pin should be left floating. The precision feature en-

ables simple sequencing of multiple power supplies with a

resistor divider from another power supply.

The switching frequency will reduce (fold back) if the overload

condition causes the output voltage to be 72.4% (typical) of

the adjusted output voltage.

The current limit will only protect the inductor from a runaway

condition if the LM22676 is operating in its safe operating

area. A runaway condition of the inductor is potentially catas-

trophic to the application. For every design, the safe operating

area needs to be calculated. Factors in determining the safe

operating area are the switching frequency, input voltage,

output voltage, minimum on-time and feedback voltage dur-

ing an over current condition.

As a first pass check, if the following equation holds true, a

given design is considered in a safe operating area and the

current limit will protect the circuit:

Maximum Duty-Cycle / Dropout

Voltage

The typical maximum duty-cycle is 85% at 500 kHz switching

frequency. This corresponds to a typical minimum off-time of

300 ns. When operating at switching frequencies higher than

500 kHz, the 300 ns minimum off-time results in a lower max-

imum duty-cycle limit than 85%. This forced off-time is impor-

tant to provide enough time for the Cboot capacitor to charge

during each cycle.

V_INx T_BLKx F < V_OUTx 0.724

If the equation above does not hold true, the following sec-

ondary equation will need to hold true to be in safe operating

area:

The lowest input voltage required to maintain operation is:

If both equations do not hold true, a particular design will not

have an effective current limit function which might damage

the circuit during startup, over current conditions, or steady

state over current and short circuit condition. Oftentimes a

reduction of the maximum input voltage will bring a design into

the safe operating area.

Where V_Dis the forward voltage drop across the re-circulating

Schottky diode and V_Qis the voltage drop across the internal

power N-FET of the LM22676. The R_DS(ON)of the FET is

specified in the electrical characteristics section of this

datasheet to calculate V_Qaccording to the FET current. F is

the switching frequency.

Soft-Start

The soft-start feature allows the regulator to gradually reach

the initial steady state operating point, thus reducing start-up

stresses and surges. The soft-start is fixed to 500 µs (typical)

start-up time and cannot be modified.

www.national.com

8

The peak ramp level of the oscillator signal feeding into the

PWM comparator is V_IN/10 which equals a gain of 20dB of

this modulator stage of the IC. The -5.0 fixed output voltage

option has twice the gain of the compensation transfer func-

tion compared to the -ADJ option which is 43.5dB instead of

37.5dB.

Boot Pin

The LM22676 integrates an N-Channel FET switch and as-

sociated floating high voltage level shift / gate driver. This gate

driver circuit works in conjunction with an internal diode and

an external bootstrap capacitor. A 0.01 µF ceramic capacitor

connected with short traces between the BOOT pin and the

SW pin is recommended to effectively drive the internal FET

switch. During the off-time of the switch, the SW voltage is

approximately -0.5V and the external bootstrap capacitor is

charged from the internal supply through the internal boot-

strap diode. When operating with a high PWM duty-cycle, the

buck switch will be forced off each cycle to ensure that the

bootstrap capacitor is recharged. See the maximum duty-cy-

cle section for more details.

Generally, calculation as well as simulation can only aid in

selecting good power stage components. A good design prac-

tice is to test for stability with load transient tests or loop

measurement tests. Application note AN-1889 shows how to

easily perform a loop transfer function measurement with only

an oscilloscope and a function generator.

Application Information

EXTERNAL COMPONENTS

Thermal Protection

The following design procedures can be used to design a non-

synchronous buck converter with the LM22676.

Internal Thermal Shutdown circuitry protects the LM22676 in

the event the maximum junction temperature is exceeded.

When activated, typically at 150°C, the regulator is forced into

a low power reset state. There is a typical hysteresis of 15

degrees.

Inductor

The inductor value is determined based on the load current,

ripple current, and the minimum and maximum input voltage.

To keep the application in continuous current conduction

mode (CCM), the maximum ripple current, I_RIPPLE, should be

less than twice the minimum load current.

Internal Compensation

The LM22676 has internal compensation designed for a sta-

ble loop with a wide range of external power stage compo-

nents.

The general rule of keeping the inductor current peak-to-peak

ripple around 30% of the nominal output current is a good

compromise between excessive output voltage ripple and ex-

cessive component size and cost. When selecting the induc-

tor ripple current ensure that the peak current is below the

minimum current limit as given in the Electrical Characteris-

tics section. Using this value of ripple current, the value of

inductor, L, is calculated using the following formula:

Insuring stability of a design with a specific power stage (in-

ductor and output capacitor) can be tricky. The LM22676

stability can be verified over varying loads and input and out-

put voltages using WEBENCH® Designer online circuit sim-

ulation tool at www.national.com. A quick start spreadsheet

can also be downloaded from the online product folder.

The internal compensation of the -ADJ option of the LM22676

is optimized for output voltages below 5V. If an output voltage

of 5V or higher is needed, the -5.0 option with an additional

external resistor divider may also be used.

The typical location of the internal compensation poles and

zeros as well as the DC gain is given in Table 1. The LM22676

has internal type III compensation allowing for the use of most

output capacitors including ceramics.

where F is the switching frequency which is 500 kHz (typical).

This procedure provides a guide to select the value of the

inductor L. The nearest standard value will then be used in

the circuit.

This information can be used to calculate the transfer function

from the FB pin to the internal compensation node (input to

the PWM comparator in the block diagram).

Increasing the inductance will generally slow down the tran-

sient response but reduce the output voltage ripple amplitude.

Reducing the inductance will generally improve the transient

response but increase the output voltage ripple.

TABLE 1.

The inductor must be rated for the peak current, I_PK+, to pre-

vent saturation. During normal loading conditions, the peak

current occurs at maximum load current plus maximum ripple.

Under an overload condition as well as during load transients,

the peak current is limited to 4.2A typical (5.5A maximum).

This requires that the inductor be selected such that it can run

at the maximum current limit and not only the steady state

current.

Corners

Pole 1

Pole 2

Pole 3

Zero 1

Zero 2

DC gain

Frequency

150 kHz

250 kHz

100 Hz

1.5 kHz

15 kHz

Depending on inductor manufacturer, the saturation rating is

defined as the current necessary for the inductance to reduce

by 30% at 20°C. In typical designs the inductor will run at

higher temperatures. If the inductor is not rated for enough

current, it might saturate and due to the propagation delay of

the current limit circuitry, the power supply may get damaged.

37.5 dB

For the power stage transfer function the standard voltage

mode formulas for the double pole and the ESR zero apply:

Input Capacitor

Good quality input capacitors are necessary to limit the ripple

voltage at the VIN pin while supplying most of the switch cur-

rent during on-time. When the switch turns on, the current into

the VIN pin steps to the peak value, then drops to zero at turn-

9

www.national.com

off. The average current into VIN during switch on-time is the

load current. The input capacitance should be selected for

RMS current, I_RMS, and minimum ripple voltage. A good ap-

proximation for the required ripple current rating necessary is

I_RMS> I_OUT/ 2.

-5.0 option:

Quality ceramic capacitors with a low ESR should be selected

for the input filter. To allow for capacitor tolerances and volt-

age effects, multiple capacitors may be used in parallel. If step

input voltage transients are expected near the maximum rat-

ing of the LM22676, a careful evaluation of ringing and pos-

sible voltage spikes at the VIN pin should be completed. An

additional damping network or input voltage clamp may be

required in these cases.

Where V_FB= 1.285V typical for the -ADJ option and 5V for the

-5.0 option

Usually putting a higher ESR electrolytic input capacitor in

parallel to the low ESR bypass capacitor will help to reduce

excessive voltages during a line transient and will also move

the resonance frequency of the input filter away from the reg-

ulator bandwidth.

Output Capacitor

The output capacitor can limit the output ripple voltage and

provide a source of charge for transient loading conditions.

Multiple capacitors can be placed in parallel. Very low ESR

capacitors such as ceramic capacitors reduce the output rip-

ple voltage and noise spikes, while larger higher ESR capac-

itors in parallel provide large bulk capacitance for transient

loading conditions. An approximation for the output voltage

ripple is:

30076523

FIGURE 2. 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.

where ΔI_Lis the inductor ripple current.

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.

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.

Catch Diode

A Schottky type re-circulating diode is required for all

LM22676 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 LM22676. 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 V_IN, 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.

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 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 LM22676 this current can

be as high as 4.2A (typical). Assuming a worst case 1V drop

The resistor values can be determined by the following equa-

tions:

-ADJ option:

www.national.com

10

across the diode, the maximum diode power dissipation can

be as high as 4.2W.

Thermal Considerations

The two highest power dissipating components are the re-

circulating diode and the LM22676 regulator IC. The easiest

method to determine the power dissipation within the

LM22676 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:

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.

P = (1 - D) x I_OUTx V_D

An approximation for the output inductor power is:

P = I_OUT²x 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 LM22676 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 LM22676 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

LM22676 TO-263 THIN and PSOP-8 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.

The most important layout rule is to keep the AC current loops

as small as possible. Figure 3 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 LM22676, 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 LM22676.

Not soldering the DAP to the board may result in erroneous

operation due to excessive noise on the board.

30076524

FIGURE 3. Current Flow in a Buck Application

11

www.national.com

PCB Layout Example for TO-263 THIN Package

30076525

www.national.com

12

PCB Layout Example for PSOP-8 Package

30076541

Schematic for Buck/Boost

(Inverting) Application

See AN-1888 for more information on the inverting (buck-

boost) application generating a negative output voltage from

a positive input voltage.

30076526

13

www.national.com

Physical Dimensions inches (millimeters) unless otherwise noted

7-Lead Plastic TO-263 THIN Package

NS Package Number TJ7A

8-Lead PSOP Package

NS Package Number MRA08B

www.national.com

14

Notes

15

www.national.com

Notes

For more National Semiconductor product information and proven design tools, visit the following Web sites at:

Products

www.national.com/amplifiers

Design Support

Amplifiers

WEBENCH® Tools

App Notes

www.national.com/webench

www.national.com/appnotes

www.national.com/refdesigns

www.national.com/samples

www.national.com/evalboards

www.national.com/packaging

www.national.com/quality/green

www.national.com/contacts

Audio

www.national.com/audio

www.national.com/timing

www.national.com/adc

www.national.com/interface

www.national.com/lvds

www.national.com/power

www.national.com/switchers

www.national.com/ldo

Clock and Timing

Data Converters

Interface

Reference Designs

Samples

Eval Boards

LVDS

Packaging

Power Management

Switching Regulators

LDOs

Green Compliance

Distributors

Quality and Reliability www.national.com/quality

LED Lighting

Voltage Reference

PowerWise® Solutions

www.national.com/led

Feedback/Support

Design Made Easy

Solutions

www.national.com/feedback

www.national.com/easy

www.national.com/vref

www.national.com/powerwise

www.national.com/solutions

www.national.com/milaero

www.national.com/solarmagic

www.national.com/AU

Serial Digital Interface (SDI) www.national.com/sdi

Mil/Aero

Temperature Sensors

Wireless (PLL/VCO)

www.national.com/tempsensors Solar Magic®

www.national.com/wireless

Analog University®

THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION

(“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY

OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO

SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS,

IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS

DOCUMENT.

TESTING AND OTHER QUALITY CONTROLS ARE USED TO THE EXTENT NATIONAL DEEMS NECESSARY TO SUPPORT

NATIONAL’S PRODUCT WARRANTY. EXCEPT WHERE MANDATED BY GOVERNMENT REQUIREMENTS, TESTING OF ALL

PARAMETERS OF EACH PRODUCT IS NOT NECESSARILY PERFORMED. NATIONAL ASSUMES NO LIABILITY FOR

APPLICATIONS ASSISTANCE OR BUYER PRODUCT DESIGN. BUYERS ARE RESPONSIBLE FOR THEIR PRODUCTS AND

APPLICATIONS USING NATIONAL COMPONENTS. PRIOR TO USING OR DISTRIBUTING ANY PRODUCTS THAT INCLUDE

NATIONAL COMPONENTS, BUYERS SHOULD PROVIDE ADEQUATE DESIGN, TESTING AND OPERATING SAFEGUARDS.

EXCEPT AS PROVIDED IN NATIONAL’S TERMS AND CONDITIONS OF SALE FOR SUCH PRODUCTS, NATIONAL ASSUMES NO

LIABILITY WHATSOEVER, AND NATIONAL DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY RELATING TO THE SALE

AND/OR USE OF NATIONAL PRODUCTS INCLUDING LIABILITY OR WARRANTIES RELATING TO FITNESS FOR A PARTICULAR

PURPOSE, MERCHANTABILITY, OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY

RIGHT.

LIFE SUPPORT POLICY

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR

SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE CHIEF EXECUTIVE OFFICER AND GENERAL

COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b) support or sustain life and

whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected

to result in a significant injury to the user. A critical component is any component in a life support device or system whose failure to perform

can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness.

National Semiconductor and the National Semiconductor logo are registered trademarks of National Semiconductor Corporation. All other

brand or product names may be trademarks or registered trademarks of their respective holders.

For the most current product information visit us at www.national.com

National Semiconductor

Americas Technical

Support Center

Email: support@nsc.com

Tel: 1-800-272-9959

National Semiconductor Europe

Technical Support Center

Email: europe.support@nsc.com

German Tel: +49 (0) 180 5010 771

English Tel: +44 (0) 870 850 4288

National Semiconductor Asia

Pacific Technical Support Center

Email: ap.support@nsc.com

National Semiconductor Japan

Technical Support Center

Email: jpn.feedback@nsc.com

www.national.com


型号：	LM22676TJ-ADJ
厂家：	National Semiconductor
描述：	3A SIMPLE SWITCHER Step-Down Voltage Regulator with Precision Enable 3A SIMPLE SWITCHER降压稳压器具有精密启用稳压器开关输出元件 PC
文件：	总16页 (文件大小：609K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM22676TJ-ADJ [NSC]

相关型号：

LM22676TJ-ADJ/NOPB

LM22676TJE-5.0

LM22676TJE-5.0/NOPB

LM22676TJE-ADJ

LM22676TJE-ADJ/NOPB

LM22676_13

LM22676_15

LM22677

LM22677

LM22677-Q1

LM22677Q

LM22677QTJ-5.0