LM2852YMXAX-3.3_技术文档

January 2005

LM2852

™

2A 500/1500kHz SIMPLE SYNCHRONOUS Buck

Regulator

General Description

Features

n Input voltage range of 2.85 to 5.5V

n Factory EEPROM set output voltages from 0.8V to 3.3V

in 100mV increments

™

The LM2852 SIMPLE SYNCHRONOUS buck regulator is

a high frequency step-down switching voltage regulator ca-

pable of driving up to a 2A load with excellent line and load

regulation. The LM2852 can accept an input voltage be-

tween 2.85V and 5.5V and deliver a customizable output

voltage that is factory programmable from 0.8V to 3.3V in

100mV increments. The LM2852 is available with a choice of

two switching frequencies - 500kHz (LM2852Y) or 1.5MHz

(LM2852X). It also features internal compensation to deliver

n Maximum Load Current of 2A

n Voltage Mode Control

n Internal type three compensation

n Switching frequency of 500kHz or 1.5MHz

n Low standby current of 10µA

n Internal 60 mΩ MOSFET switches

n Standard voltage options 1.0/1.2/1.5/1.8/2.5/3.3 volts

a

low component count solution. The exposed-pad

TSSOP-14 package enhances the thermal performance of

the LM2852.

Applications

n Low voltage point of load regulators

n Local solution for FPGA/DSP/ASIC core power

n Broadband networking and communications

infrastructure

n Portable computing

Typical Application Circuit

20127001

20127002

DS201270

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Connection Diagram

TOP VIEW

20127003

MXA14A

NC (Pins 5, 12 and 13): No-connect. These pins must be

tied to ground or left floating in the application.

Pin Descriptions

AVIN (Pin 1): Chip bias input pin. This provides power to the

logic of the chip. Connect to the input voltage or a separate

rail.

PVIN (Pins 6, 7): Input supply pin. PVIN is connected to the

input voltage. This rail connects to the source of the internal

power PFET.

EN (Pin 2): Enable. Connect this pin to ground to disable the

chip, connect to AVIN or leave floating to enable the chip;

enable is internally pulled up.

SW (Pins 8, 9): Switch pin. Connect to the output inductor.

PGND (Pins 10, 11): Power ground. Connect this to an

internal ground plane or other large ground plane.

SGND (Pin 3): Low-noise ground.

SNS (Pin 14): Output voltage sense pin. Connect this pin to

the output voltage as close to the load as possible.

SS (Pin 4): Soft-start pin. Connect this pin to a small capaci-

tor to control startup and soften inrush current. The soft-start

capacitance range is restricted to values 1 nF to 50 nF.

Exposed Pad: Connect to ground.

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Ordering Information

Order Number

LM2852YMXA-1.0

LM2852YMXAX-1.0

Frequency

Voltage Option

Package Type

Package Drawing

Supplied As

94 Units, Rail

2500 Units, Tape and

Reel

1.0

LM2852YMXA-1.2

LM2852YMXAX-1.2

1.2

1.5

1.8

2.5

3.3

94 Units, Rail

2500 Units, Tape and

Reel

LM2852YMXA-1.5

LM2852YMXAX-1.5

94 Units, Rail

2500 Units, Tape and

Reel

500kHz

LM2852YMXA-1.8

LM2852YMXAX-1.8

94 Units, Rail

2500 Units, Tape and

Reel

LM2852YMXA-2.5

LM2852YMXAX-2.5

94 Units, Rail

2500 Units, Tape and

Reel

TSSOP-14 exposed pad

MXA14A

LM2852YMXA-3.3

LM2852YMXAX-3.3

94 Units, Rail

2500 Units, Tape and

Reel

LM2852XMXA-1.0

LM2852XMXAX-1.0

LM2852XMXA-1.2

LM2852XMXAX-1.2

LM2852XMXA-1.5

LM2852XMXAX-1.5

LM2852XMXA-1.8

LM2852XMXAX-1.8

LM2852XMXA-2.5

LM2852XMXAX-2.5

LM2852XMXA-3.3

LM2852XMXAX-3.3

1.0

1.2

1.5

1.8

2.5

3.3

1500kHz

Coming Soon

Note: Contact factory for other voltage options.

3

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Absolute Maximum Ratings (Note 1)

Operating Ratings

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

PVIN to GND

AVIN to GND

Junction Temperature

θ_JA

1.5V to 5.5V

2.85V to 5.5V

−40˚C to +125˚C

37.6˚C/W

PVIN, AVIN, EN, SNS

ESD Susceptibility (Note 2)

Power Dissipation

−0.3V to 6.5V

2kV

Internally Limited

−65˚C to +150˚C

150˚C

Storage Temperature Range

Maximum Junction Temp.

14-Pin Exposed Pad TSSOP

Package

220˚C

215˚C

260˚C

Infrared (15 sec)

Vapor Phase (60 sec)

Soldering (10 sec)

Electrical Characteristics AVIN = PVIN = 5V unless otherwise indicated under the Conditions column.

Typicals and limits appearing in plain type apply for T_A= T_J= +25˚C. Limits appearing in boldface type apply over full Oper-

ating Junction Temperature Range (−40˚C to +125˚C). Datasheet min/max specification limits are guaranteed by design, test,

or statistical analysis.

Symbol

Parameter

Conditions

Min

Typ

Max

Units

SYSTEM PARAMETERS

V_OUT

Voltage Tolerance³V_OUT= 1.0V option

V_OUT= 1.2V option

0.9775

1.1730

1.4663

1.7595

2.4437

3.2257

1.0225

1.2270

1.5337

1.8405

2.5563

3.3743

0.6

V

V_OUT= 1.5V option

V_OUT= 1.8V option

V_OUT= 2.5V option

V_OUT= 3.3V option

Line Regulation³V_OUT= 0.8V, 1.0V, 1.2V, 1.5V, 1.8V or

∆V_OUT/∆AVIN

0.2

%

2.5V

2.85V ≤ AVIN ≤ 5.5V

V_OUT= 3.3V

0.6

3.5V ≤ AVIN ≤ 5.5V

∆V_OUT/∆I_O

Load Regulation Normal operation

UVLO Threshold Rising

8

mV/A

V

V_ON

2.47

150

75

2.85

210

140

(AVIN)

Falling Hysteresis

Isw = 2A

85

mV

mΩ

r_DSON-P

r_DSON-N

R_SS

PFET On

Resistance

NFET On

Isw = 2A

55

400

3

120

mΩ

kΩ

A

Resistance

Soft-start

resistance

I_CL

Peak Current Limit

Threshold

2.25

3.65

I_Q

Operating Current Non-switching

0.85

10

2

mA

µA

I_SD

Shut Down

Quiescent Current

Sense pin

EN = 0V

25

R_SNS

400

kΩ

resistance

PWM

f_osc

LM2852X

LM2852Y

1500kHz option.

500kHz option.

TBD

325

0

1500

500

TBD

625

100

kHz

%

D_range

Duty Cycle Range

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Electrical Characteristics AVIN = PVIN = 5V unless otherwise indicated under the Conditions column.

Typicals and limits appearing in plain type apply for T_A= T_J= +25˚C. Limits appearing in boldface type apply over full

Operating Junction Temperature Range (−40˚C to +125˚C). Datasheet min/max specification limits are guaranteed by design,

test, or statistical analysis. (Continued)

Symbol

ENABLE CONTROL⁴

Parameter

Conditions

Min

75

Typ

Max

25

Units

V_IH

V_IL

I_EN

EN Pin Minimum

High Input

% of

AVIN

% of

AVIN

µA

EN Pin Maximum

Low Input

EN Pin Pullup

Current

EN = 0V

1.2

THERMAL CONTROLS

T_SD

T_Jfor Thermal

165

10

˚C

Shutdown

Hysteresis for

T_SD-hys

Thermal Shutdown

Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating Range indicates conditions for which the device is

intended to be functional, but does not guarantee specfic performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: Human body model: 1.5kΩ in series with 100pF. SW and PVIN pins are derated to 1.5kV

Note 3: V

measured in a non-switching, closed-loop configuration at the SNS pin.

OUT

Note 4: The enable pin is internally pulled up, so the LM2852 is automatically enabled unless an external enable voltage is applied.

5

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Typical Performance Characteristics

Efficiency vs I_Load

V_OUT= 1.5V

Efficiency vs I_Load

V_OUT= 2.5V

20127024

20127004

Efficiency vs I_Load

V_OUT= 3.3V

Quiescent Current (Non-Switching) vs V_INand Temp.

20127006

20127007

Shut-Down Current vs V_INand Temp.

Frequency vs Temperature and V_IN

20127009

20127008

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Typical Performance Characteristics (Continued)

NMOS Switch R_DSONvs Temperature and PVIN

PMOS Switch R_DSONvs Temperature and PVIN

20127010

20127011

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Block Diagram

20127012

Applications Information

The LM2852 is a DC-DC buck converter belonging to Na-

tional Semiconductor’s SIMPLE SYNCHRONOUS^®family.

Integration of the PWM controller, power switches and com-

pensation network greatly reduces the component count

required to implement a switching power supply. A typical

application requires only four components: an input capaci-

tor, a soft-start capacitor, an output filter capacitor and an

output filter inductor.

permanently attached to the reference voltage node which is

also connected to the soft-start pin, SS. Adding a soft-start

capacitor externally increases the time it takes for the output

voltage to reach its final level.

The charging time required for the reference voltage can be

estimated using the RC time constant of the DAC resistor

and the capacitance connected to the SS pin. Three RC time

constant periods are needed for the reference voltage to

reach 95% of its final value. The actual start-up time will vary

with differences in the DAC resistance and higher-order

effects.

INPUT CAPACITOR (C_IN

)

Fast switching of large currents in the buck converter places

a heavy demand on the voltage source supplying PVIN. The

input capacitor, C_IN, supplies extra charge when the switcher

needs to draw a burst of current from the supply. The RMS

current rating and the voltage rating of the C_INcapacitor are

therefore important in the selection of C_IN. The RMS current

specification can be approximated to be the load current

times the square root of the duty cycle:

If little or no soft-start capacitance is connected, then the

start-up time may be determined by the time required for the

current limit current to charge the output filter capacitance.

The capacitor charging equation I = C ∆V/∆t can be used to

estimate the start-up time in this case. For example, a part

with a 3V output, a 100 µF output capacitance and a 3A

current limit threshold would require a time of 100 µs:

where D is the duty cycle, V_OUT/V_IN. C_INalso provides

filtering of the supply. Trace resistance and inductance de-

grade the benefits of the input capacitor, so C_INshould be

placed very close to PVIN in the layout. A 22 µF or 47 µF

ceramic capacitor is typically sufficient for C_IN. In parallel

with the large input capacitance a smaller capacitor may be

added such as a 1µF ceramic for higher frequency filtering.

Since it is undesirable for the power supply to start up in

current limit, a soft-start capacitor must be chosen to force

the LM2852 to start up in a more controlled fashion based on

the charging of the soft-start capacitance. In this example,

suppose a 3 ms start time is desired. Three time constants

are required for charging the soft-start capacitor to 95% of

the final reference voltage. So in this case RC=1ms. The

DAC resistor, R, is 400 kΩ so C can be calculated to be

2.5nF. A 2.7nF ceramic capacitor can be chosen to yield

approximately a 3ms start-up time.

SOFT-START CAPACITOR (C_SS

)

The DAC that sets the reference voltage of the error amp

sources a current through a resistor to set the reference

voltage. The reference voltage is one half of the output

voltage of the switcher due to the 200kΩ divider connected

to the SNS pin. Upon start-up, the output voltage of the

switcher tracks the reference voltage with a two to one ratio

as the DAC current charges the capacitance connected to

the reference voltage node. Internal capacitance of 20pF is

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type three, is included on-chip. The benefit to integrated

compensation is straight-forward, simple power supply de-

sign. Since the output filter capacitor and inductor values

impact the compensation of the control loop, the range of L,

C and C_ESRvalues is restricted in order to ensure stability.

Applications Information (Continued)

SOFT-START CAPACITOR (C_SS) AND FAULT

CONDITIONS

Various fault conditions such as short circuit and UVLO of

the LM2852 activate internal circuitry designed to control the

voltage on the soft-start capacitor. For example, during a

short circuit current limit event, the output voltage typically

falls to a low voltage. During this time, the soft-start voltage

is forced to track the output so that once the short is re-

moved, the LM2852 can restart gracefully from whatever

voltage the output reached during the short circuit event. The

range of soft-start capacitors is therefore restricted to values

1nF to 50nF.

OUTPUT FILTER VALUES

Table 1 details the recommended inductor and capacitor

ranges for the LM2852 that are suggested for various typical

output voltages. Values slightly different than those recom-

mended may be used, however the phase margin of the

power supply may be degraded.

COMPENSATION

The LM2852 provides a highly integrated solution to power

supply design. The compensation of the LM2852, which is

TABLE 1. Output Filter Values

L (µH)

C (µF)

C_ESR(mΩ)

Min

Frequency Option

V_OUT(V)

0.8

1

PVIN (V)

Min

10

15

10

22

10

22

6.8

15

Max

15

22

15

22

15

33

10

22

Min

100

68

Max

220

120

180

120

100

Max

200

275

3.3

5

70

3.3

5

70

1

70

1.2

1.5

1.8

2.5

3.3

5

70

LM2852Y

(500kHz)

3.3

5

70

3.3

5

100

95

3.3

5

68

95

5

68

100

CHOOSING AN INDUCTANCE VALUE

The current ripple present in the output filter inductor is

determined by the input voltage, output voltage, switching

frequency and inductance according to the following equa-

tion:

The maximum inductor current for a 2A load would therefore

be 2A plus 60.8 mA, 2.0608A. As shown in the ripple equa-

tion, the current ripple is inversely proportional to induc-

tance.

where ∆I_Lis the peak to peak current ripple, D is the duty

cycle V_OUT/V_IN, V_INis the input voltage applied to the output

stage, V_OUTis the output voltage of the switcher, f is the

switching frequency and L is the inductance of the output

filter inductor. Knowing the current ripple is important for

inductor selection since the peak current through the induc-

tor is the load current plus one half the ripple current. Care

must be taken to ensure the peak inductor current does not

reach a level high enough to trip the current limit circuitry of

the LM2852.

OUTPUT FILTER INDUCTORS

Once the inductance value is chosen, the key parameter for

selecting the output filter inductor is its saturation current

(I_sat) specification. Typically I_satis given by the manufacturer

as the current at which the inductance of the coil falls to a

certain percentage of the nominal inductance. The I_satof an

inductor used in an application should be greater than the

maximum expected inductor current to avoid saturation. Be-

low is a table of inductors that may be suitable in LM2852

applications.

As an example, consider a 5V to 1.2V conversion and a

500kHz switching frequency. According to Table 1, a 15µH

inductor may be used. Calculating the expected peak-to-

peak ripple,

9

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Applications Information (Continued)

TABLE 2. (LM2852Y Output Filter Inductors (500kHz)

Inductance (µH)

Part Number

DO3316P-682

Vendor

Coilcraft

6.8

7

MSS1038-702NBC

DO3316P-103

10

12

15

18

22

27

33

MSS1038-103NBC

MSS1038-123NBC

D03316P-153

MSS1038-153NBC

MSS1038-183NBC

DO3316P-223

MSS1038-223NBC

DO3340P-223

MSS1038-273NBC

MSS1038-333NBC

DO3340P-333

OUTPUT FILTER CAPACITORS

The capacitors that may be used in the output filter with the

LM2852 are limited in value and ESR range according to

Table 1. Below are some examples of capacitors that can

typically be used in an LM2852 application.

TABLE 3. LM2852Y Output Filter Capacitors (500kHz)

Capacitance (µF)

Part Number

Chemistry

Tantalum

Vendor

68

595D686X_010C2T

595D686X_016D2T

595D017X_6R3C2T

595D107X_016D2T

NOSC107M004R0150

NOSD107M006R0100

595D127X_004C2T

595D127X_010D2T

595D157X_004C2T

595D157X_016D2T

NOSC157M004R0150

NOSD157M006R0100

595D227X_004D2T

NOSD227M004R0100

NOSE227M006R0100

Vishay - Sprague

AVX

68

Tantalum

100

120

150

220

Tantalum

Niobium Oxide

Tantalum

AVX

Vishay - Sprague

AVX

Tantalum

Niobium Oxide

Tantalum

AVX

Vishay - Sprague

AVX

Niobium Oxide

AVX

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PVIN is the supply for the power FETs. The output filter

components need to be chosen based on the value of PVIN.

For PVIN levels lower than 3.3V, use output filter component

values recommended for 3.3V. PVIN must always be equal

to or less than AVIN.

Applications Information (Continued)

SPLIT-RAIL OPERATION

The LM2852 can be powered using two separate voltages

for AVIN and PVIN. AVIN is the supply for the control logic;

20127014

Layout Hints

node) to minimize interference.

These are several guidelines to follow while designing the

PCB layout for an LM2852 application.

4. The switch node connections should be low resistance

to reduce power losses. Low resistance means the trace

between the switch pin and the inductor should be wide.

However, the area of the switch node should not be too

large since EMI increases with greater area. So connect

the inductor to the switch pin with a short, but wide trace.

Other high current connections in the application such

as PVIN and V_OUTassume the same trade off between

low resistance and EMI.

1. The input bulk capacitor, C_IN, should be placed very

close to the PVIN pin to keep the resistance as low as

possible between the capacitor and the pin. High current

levels will be present in this connection.

2. All ground connections must be tied together. Use a

broad ground plane, for example a completely filled back

plane, to establish the lowest resistance possible be-

tween all ground connections.

5. Allow area under the chip to solder the entire exposed

die attach pad to ground. Lab measurements show im-

proved regulation performance when the exposed pad is

well grounded.

3. The sense pin connection should be made as close to

the load as possible so that the voltage at the load is the

expected regulated value. The sense line should not run

too close to nodes with high EMI (such as the switch

LM2852Y Example Circuit Schematic (500kHz)

20127020

FIGURE 1.

Bill of Materials for 3.3V_INto 1.8 V_OUTConversion

ID

U₁

Part Number

LM2852YMXA-1.8

Type

Size

Parameters

Qty

1

Vendor

2A Buck

Inductor

Capacitor

Resistor

Capacitor

ETSSOP-14

NSC

Coilcraft

L_O

DO3316P-153

15 µH

1

C_O*

C_IN

C_INX

C_SS

R_f

595D107X_6R3C2T

GRM32ER60J476ME20B

GRM21BR71C105KA01B

VJ0805Y272KXXA

Case Code “C”

1210

100 µF 20%

47µF/X5R/6.3V

1µF/X7R/16V

2.7nF 10%

10Ω 10%

1

Vishay-Sprague

Murata

1

0805

1

Murata

0805

1

Vishay-Vitramon

Vishay-Dale

Murata

CRCW060310R0F

0603

1

C_f

GRM21BR71C105KA01B

0805

1µF/X7R/16V

1

* If a “non-tantalum” solution is desired use an NOSC107M004R0150, 100 µF capacitor from AVX for C

.

O

11

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12

Physical Dimensions inches (millimeters)

unless otherwise noted

14-Lead ETSSOP Package

NS Package Number MXA14A

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves

the right at any time without notice to change said circuitry and specifications.

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

LIFE SUPPORT POLICY

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

WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR

CORPORATION. As used herein:

1. Life support devices or systems are devices or systems

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.

2. A critical component is any component of 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.

BANNED SUBSTANCE COMPLIANCE

National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship

Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned

Substances’’ as defined in CSP-9-111S2.

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Support Center

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型号：	LM2852YMXAX-3.3
厂家：	National Semiconductor
描述：	2A 500/1500kHz SIMPLE SYNCHRONOUS Buck Regulator 2A 500 / 1500kHz简单的同步降压稳压器稳压器
文件：	总13页 (文件大小：617K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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