LM2831XMF - National Semiconductor

LM2831

January 17, 2012

High Frequency 1.5A Load - Step-Down DC-DC Regulator

General Description

The LM2831 regulator is a monolithic, high frequency, PWM

step-down DC/DC converter in a 5 pin SOT23 and a 6 Pin

LLP package. It provides all the active functions to provide

local DC/DC conversion with fast transient response and ac-

curate regulation in the smallest possible PCB area. With a

minimum of external components, the LM2831 is easy to use.

The ability to drive 1.5A loads with an internal 130 mΩ PMOS

switch using state-of-the-art 0.5 µm BiCMOS technology re-

sults in the best power density available. The world-class

control circuitry allows on-times as low as 30ns, thus sup-

porting exceptionally high frequency conversion over the en-

tire 3V to 5.5V input operating range down to the minimum

output voltage of 0.6V. Switching frequency is internally set

to 550 kHz, 1.6 MHz, or 3.0 MHz, allowing the use of ex-

tremely small surface mount inductors and chip capacitors.

Even though the operating frequency is high, efficiencies up

to 93% are easy to achieve. External shutdown is included,

featuring an ultra-low stand-by current of 30 nA. The LM2831

utilizes current-mode control and internal compensation to

provide high-performance regulation over a wide range of op-

erating conditions. Additional features include internal soft-

start circuitry to reduce inrush current, pulse-by-pulse current

limit, thermal shutdown, and output over-voltage protection.

Features

■Space Saving SOT23-5 Package

■Input voltage range of 3.0V to 5.5V

■Output voltage range of 0.6V to 4.5V

■1.5A output current

■High Switching Frequencies

1.6MHz (LM2831X)

0.55MHz (LM2831Y)

3.0MHz (LM2831Z)

■130mΩ PMOS switch

■0.6V, 2% Internal Voltage Reference

■Internal soft-start

■Current mode, PWM operation

■Thermal Shutdown

■Over voltage protection

Applications

■Local 5V to Vcore Step-Down Converters

■Core Power in HDDs

■Set-Top Boxes

■USB Powered Devices

■DSL Modems

Typical Application Circuit

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LM2831 High Frequency 1.5A Load - Step-Down DC-DC Regulator

Connection Diagrams

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6-Pin LLP

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5-Pin SOT-23

Ordering Information

Order Number Frequency

Option Package Type NSC Package

Drawing Top Mark Supplied As Features

LM2831XMF

1.6MHz

SOT23-5 MF05A SKYB 1000 units Tape and Reel

LM2831XMFX 3000 units Tape and Reel

LM2831XSD LLP-6 SDE06A L193B 1000 units Tape and Reel

LM2831XSDX 4500 units Tape and Reel

LM2831YMF

0.55MHz

SOT23-5 MF05A SKZB 1000 units Tape and Reel

LM2831YMFX 3000 units Tape and Reel

LM2831YSD LLP-6 SDE06A L194B 1000 units Tape and Reel

LM2831YSDX 4500 units Tape and Reel

LM2831ZMF

3MHz

SOT23-5 MF05A SLAB 1000 units Tape and Reel

LM2831ZMFX 3000 units Tape and Reel

LM2831ZSD LLP-6 SDE06A L195B 1000 units Tape and Reel

LM2831ZSDX 4500 units Tape and Reel

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LM2831

Pin Descriptions 5-Pin SOT23

Pin Name Function

1 SW Output switch. Connect to the inductor and catch diode.

2 GND Signal and power ground pin. Place the bottom resistor of the feedback network as close as

possible to this pin.

3 FB Feedback pin. Connect to external resistor divider to set output voltage.

4 EN Enable control input. Logic high enables operation. Do not allow this pin to float or be greater

than VIN + 0.3V.

5 VIN Input supply voltage.

Pin Descriptions 6-Pin LLP

Pin Name Function

1 FB Feedback pin. Connect to external resistor divider to set output voltage.

2 GND Signal and power ground pin. Place the bottom resistor of the feedback network as close

as possible to this pin.

3 SW Output switch. Connect to the inductor and catch diode.

4 VIND Power Input supply.

5 VINA Control circuitry supply voltage. Connect VINA to VIND on PC board.

6 EN Enable control input. Logic high enables operation. Do not allow this pin to float or be greater

than VINA + 0.3V.

DAP Die Attach Pad Connect to system ground for low thermal impedance, but it cannot be used as a primary

GND connection.

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LM2831

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the Texas Instruments Sales Office/

Distributors for availability and specifications.

VIN -0.5V to 7V

FB Voltage -0.5V to 3V

EN Voltage -0.5V to 7V

SW Voltage -0.5V to 7V

ESD Susceptibility 2kV

Junction Temperature (Note 2) 150°C

Storage Temperature −65°C to +150°C

Soldering Information

Infrared or Convection Reflow

(15 sec) 220°C

Operating Ratings

VIN 3V to 5.5V

Junction Temperature −40°C to +125°C

Electrical Characteristics VIN = 5V unless otherwise indicated under the Conditions column. Limits in standard

type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) 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 TJ = 25°C, and are provided for reference purposes only.

Symbol Parameter Conditions Min Typ Max Units

VFB Feedback Voltage LLP-6 and SOT23-5

Package

0.588 0.600 0.612 V

ΔVFB/VIN Feedback Voltage Line Regulation VIN = 3V to 5V 0.02 %/V

IBFeedback Input Bias Current 0.1 100 nA

UVLO Undervoltage Lockout VIN Rising 2.73 2.90 V

VIN Falling 1.85 2.3

UVLO Hysteresis 0.43 V

FSW Switching Frequency

LM2831-X 1.2 1.6 1.95

MHzLM2831-Y 0.4 0.55 0.7

LM2831-Z 2.25 3.0 3.75

DMAX Maximum Duty Cycle

LM2831-X 86 94

%LM2831-Y 90 96

LM2831-Z 82 90

DMIN Minimum Duty Cycle

LM2831-X 5

%LM2831-Y 2

LM2831-Z 7

RDS(ON) Switch On Resistance LLP-6 Package 150 mΩ

SOT23-5 Package 130 195

ICL Switch Current Limit VIN = 3.3V 1.8 2.5 A

VEN_TH

Shutdown Threshold Voltage 0.4 V

Enable Threshold Voltage 1.8

ISW Switch Leakage 100 nA

IEN Enable Pin Current Sink/Source 100 nA

Quiescent Current (switching)

LM2831X VFB = 0.55 3.3 5mA

LM2831Y VFB = 0.55 2.8 4.5

LM2831Z VFB = 0.55 4.3 6.5

Quiescent Current (shutdown) All Options VEN = 0V 30 nA

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LM2831

Symbol Parameter Conditions Min Typ Max Units

θJA

Junction to Ambient

0 LFPM Air Flow (Note 3)

LLP-6 Package 80 °C/W

SOT23-5 Package 118

θJC Junction to Case (Note 3)LLP-6 Package 18 °C/W

SOT23-5 Package 80

TSD Thermal Shutdown Temperature 165 °C

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: Thermal shutdown will occur if the junction temperature exceeds the maximum junction temperature of the device.

Note 3: Applies for packages soldered directly onto a 3” x 3” PC board with 2oz. copper on 4 layers in still air.

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LM2831

Typical Performance Characteristics

All curves taken at VIN = 5.0V with configuration in typical application circuit shown in Application Information section of this

datasheet. TJ = 25°C, unless otherwise specified.

η vs Load "X" Vin = 5V, Vo = 1.8V & 3.3V

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η vs Load - "Y" Vin = 5V, Vo = 3.3V & 1.8V

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η vs Load "Z" Vin = 5V, Vo = 3.3V & 1.8V

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η vs Load "X, Y and Z" Vin = 3.3V, Vo = 1.8V

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LM2831

Load Regulation

Vin = 3.3V, Vo = 1.8V (All Options)

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Load Regulation

Vin = 5V, Vo = 1.8V (All Options)

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Load Regulation

Vin = 5V, Vo = 3.3V (All Options)

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Oscillator Frequency vs Temperature - "X"

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Oscillator Frequency vs Temperature - "Y"

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Oscillator Frequency vs Temperature - "Z"

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LM2831

Current Limit vs Temperature

Vin = 3.3V

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RDSON vs Temperature (LLP-6 Package)

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RDSON vs Temperature (SOT23-5 Package)

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LM2831X IQ (Quiescent Current)

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LM2831Y IQ (Quiescent Current)

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LM2831Z IQ (Quiescent Current)

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LM2831

Line Regulation

Vo = 1.8V, Io = 500mA

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VFB vs Temperature

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Gain vs Frequency

(Vin = 5V, Vo = 1.2V @ 1A)

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Phase Plot vs Frequency

(Vin = 5V, Vo = 1.2V @ 1A)

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LM2831

Simplified Block Diagram

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FIGURE 1.

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LM2831

Applications Information

THEORY OF OPERATION

The LM2831 is a constant frequency PWM buck regulator IC

that delivers a 1.5A load current. The regulator has a preset

switching frequency of 550kHz, 1.6MHz, or 3.0MHz. This high

frequency allows the LM2831 to operate with small surface

mount capacitors and inductors, resulting in a DC/DC con-

verter that requires a minimum amount of board space. The

LM2831 is internally compensated, so it is simple to use and

requires few external components. The LM2831 uses current-

mode control to regulate the output voltage. The following

operating description of the LM2831 will refer to the Simplified

Block Diagram (Figure 1) and to the waveforms in Figure 2.

The LM2831 supplies a regulated output voltage by switching

the internal PMOS control switch at constant frequency and

variable duty cycle. A switching cycle begins at the falling

edge of the reset pulse generated by the internal oscillator.

When this pulse goes low, the output control logic turns on

the internal PMOS control switch. During this on-time, the SW

pin voltage (VSW) swings up to approximately VIN, and the in-

ductor current (IL) increases with a linear slope. IL is measured

by the current sense amplifier, which generates an output

proportional to the switch current. The sense signal is

summed with the regulator’s corrective ramp and compared

to the error amplifier’s output, which is proportional to the dif-

ference between the feedback voltage and VREF. When the

PWM comparator output goes high, the output switch turns

off until the next switching cycle begins. During the switch off-

time, inductor current discharges through the Schottky catch

diode, which forces the SW pin to swing below ground by the

forward voltage (VD) of the Schottky catch diode. The regu-

lator loop adjusts the duty cycle (D) to maintain a constant

output voltage.

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FIGURE 2. Typical Waveforms

SOFT-START

This function forces VOUT to increase at a controlled rate dur-

ing start up. During soft-start, the error amplifier’s reference

voltage ramps from 0V to its nominal value of 0.6V in approx-

imately 600 µs. This forces the regulator output to ramp up in

a controlled fashion, which helps reduce inrush current.

OUTPUT OVERVOLTAGE PROTECTION

The over-voltage comparator compares the FB pin voltage to

a voltage that is 15% higher than the internal reference

VREF. Once the FB pin voltage goes 15% above the internal

reference, the internal PMOS control switch is turned off,

which allows the output voltage to decrease toward regula-

tion.

UNDERVOLTAGE LOCKOUT

Under-voltage lockout (UVLO) prevents the LM2831 from op-

erating until the input voltage exceeds 2.73V (typ). The UVLO

threshold has approximately 430 mV of hysteresis, so the part

will operate until VIN drops below 2.3V (typ). Hysteresis pre-

vents the part from turning off during power up if VIN is non-

monotonic.

CURRENT LIMIT

The LM2831 uses cycle-by-cycle current limiting to protect

the output switch. During each switching cycle, a current limit

comparator detects if the output switch current exceeds 2.5A

(typ), and turns off the switch until the next switching cycle

begins.

THERMAL SHUTDOWN

Thermal shutdown limits total power dissipation by turning off

the output switch when the IC junction temperature exceeds

165°C. After thermal shutdown occurs, the output switch

doesn’t turn on until the junction temperature drops to ap-

proximately 150°C.

Design Guide

INDUCTOR SELECTION

The Duty Cycle (D) can be approximated quickly using the

ratio of output voltage (VO) to input voltage (VIN):

The catch diode (D1) forward voltage drop and the voltage

drop across the internal PMOS must be included to calculate

a more accurate duty cycle. Calculate D by using the following

formula:

VSW can be approximated by:

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LM2831

VSW = IOUT x RDSON

The diode forward drop (VD) can range from 0.3V to 0.7V de-

pending on the quality of the diode. The lower the VD, the

higher the operating efficiency of the converter. The inductor

value determines the output ripple current. Lower inductor

values decrease the size of the inductor, but increase the

output ripple current. An increase in the inductor value will

decrease the output ripple current.

One must ensure that the minimum current limit (1.8A) is not

exceeded, so the peak current in the inductor must be calcu-

lated. The peak current (ILPK) in the inductor is calculated by:

ILPK = IOUT + ΔiL

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FIGURE 3. Inductor Current

In general,

ΔiL = 0.1 x (IOUT) → 0.2 x (IOUT)

If ΔiL = 20% of 1.50A, the peak current in the inductor will be

1.8A. The minimum guaranteed current limit over all operating

conditions is 1.8A. One can either reduce ΔiL, or make the

engineering judgment that zero margin will be safe enough.

The typical current limit is 2.5A.

The LM2831 operates at frequencies allowing the use of ce-

ramic output capacitors without compromising transient re-

sponse. Ceramic capacitors allow higher inductor ripple

without significantly increasing output ripple. See the output

capacitor section for more details on calculating output volt-

age ripple. Now that the ripple current is determined, the

inductance is calculated by:

Where

When selecting an inductor, make sure that it is capable of

supporting the peak output current without saturating. Induc-

tor saturation will result in a sudden reduction in inductance

and prevent the regulator from operating correctly. Because

of the speed of the internal current limit, the peak current of

the inductor need only be specified for the required maximum

output current. For example, if the designed maximum output

current is 1.0A and the peak current is 1.25A, then the induc-

tor should be specified with a saturation current limit of >

1.25A. There is no need to specify the saturation or peak cur-

rent of the inductor at the 2.5A typical switch current limit. The

difference in inductor size is a factor of 5. Because of the op-

erating frequency of the LM2831, ferrite based inductors are

preferred to minimize core losses. This presents little restric-

tion since the variety of ferrite-based inductors is huge. Lastly,

inductors with lower series resistance (RDCR) will provide bet-

ter operating efficiency. For recommended inductors see Ex-

ample Circuits.

INPUT CAPACITOR

An input capacitor is necessary to ensure that VIN does not

drop excessively during switching transients. The primary

specifications of the input capacitor are capacitance, voltage,

RMS current rating, and ESL (Equivalent Series Inductance).

The recommended input capacitance is 22 µF.The input volt-

age rating is specifically stated by the capacitor manufacturer.

Make sure to check any recommended deratings and also

verify if there is any significant change in capacitance at the

operating input voltage and the operating temperature. The

input capacitor maximum RMS input current rating (IRMS-IN)

must be greater than:

Neglecting inductor ripple simplifies the above equation to:

It can be shown from the above equation that maximum RMS

capacitor current occurs when D = 0.5. Always calculate the

RMS at the point where the duty cycle D is closest to 0.5. The

ESL of an input capacitor is usually determined by the effec-

tive cross sectional area of the current path. A large leaded

capacitor will have high ESL and a 0805 ceramic chip capac-

itor will have very low ESL. At the operating frequencies of the

LM2831, leaded capacitors may have an ESL so large that

the resulting impedance (2πfL) will be higher than that re-

quired to provide stable operation. As a result, surface mount

capacitors are strongly recommended.

Sanyo POSCAP, Tantalum or Niobium, Panasonic SP, and

multilayer ceramic capacitors (MLCC) are all good choices for

both input and output capacitors and have very low ESL. For

MLCCs it is recommended to use X7R or X5R type capacitors

due to their tolerance and temperature characteristics. Con-

sult capacitor manufacturer datasheets to see how rated

capacitance varies over operating conditions.

OUTPUT CAPACITOR

The output capacitor is selected based upon the desired out-

put ripple and transient response. The initial current of a load

transient is provided mainly by the output capacitor. The out-

put ripple of the converter is:

When using MLCCs, the ESR is typically so low that the ca-

pacitive ripple may dominate. When this occurs, the output

ripple will be approximately sinusoidal and 90° phase shifted

from the switching action. Given the availability and quality of

MLCCs and the expected output voltage of designs using the

LM2831, there is really no need to review any other capacitor

technologies. Another benefit of ceramic capacitors is their

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LM2831

ability to bypass high frequency noise. A certain amount of

switching edge noise will couple through parasitic capaci-

tances in the inductor to the output. A ceramic capacitor will

bypass this noise while a tantalum will not. Since the output

capacitor is one of the two external components that control

the stability of the regulator control loop, most applications will

require a minimum of 22 µF of output capacitance. Capaci-

tance often, but not always, can be increased significantly

with little detriment to the regulator stability. Like the input ca-

pacitor, recommended multilayer ceramic capacitors are X7R

or X5R types.

CATCH DIODE

The catch diode (D1) conducts during the switch off-time. A

Schottky diode is recommended for its fast switching times

and low forward voltage drop. The catch diode should be

chosen so that its current rating is greater than:

ID1 = IOUT x (1-D)

The reverse breakdown rating of the diode must be at least

the maximum input voltage plus appropriate margin. To im-

prove efficiency, choose a Schottky diode with a low forward

voltage drop.

OUTPUT VOLTAGE

The output voltage is set using the following equation where

R2 is connected between the FB pin and GND, and R1 is

connected between VO and the FB pin. A good value for R2

is 10k. When designing a unity gain converter (Vo = 0.6V), R1

should be between 0Ω and 100Ω, and R2 should be equal or

greater than 10kΩ.

VREF = 0.60V

PCB LAYOUT CONSIDERATIONS

When planning layout there are a few things to consider when

trying to achieve a clean, regulated output. The most impor-

tant consideration is the close coupling of the GND connec-

tions of the input capacitor and the catch diode D1. These

ground ends should be close to one another and be connect-

ed to the GND plane with at least two through-holes. Place

these components as close to the IC as possible. Next in im-

portance is the location of the GND connection of the output

capacitor, which should be near the GND connections of CIN

and D1. There should be a continuous ground plane on the

bottom layer of a two-layer board except under the switching

node island. The FB pin is a high impedance node and care

should be taken to make the FB trace short to avoid noise

pickup and inaccurate regulation. The feedback resistors

should be placed as close as possible to the IC, with the GND

of R1 placed as close as possible to the GND of the IC. The

VOUT trace to R2 should be routed away from the inductor and

any other traces that are switching. High AC currents flow

through the VIN, SW and VOUT traces, so they should be as

short and wide as possible. However, making the traces wide

increases radiated noise, so the designer must make this

trade-off. Radiated noise can be decreased by choosing a

shielded inductor. The remaining components should also be

placed as close as possible to the IC. Please see Application

Note AN-1229 for further considerations and the LM2831 de-

mo board as an example of a four-layer layout.

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LM2831

Calculating Efficiency, and Junction

Temperature

The complete LM2831 DC/DC converter efficiency can be

calculated in the following manner.

Calculations for determining the most significant power loss-

es are shown below. Other losses totaling less than 2% are

not discussed.

Power loss (PLOSS) is the sum of two basic types of losses in

the converter: switching and conduction. Conduction losses

usually dominate at higher output loads, whereas switching

losses remain relatively fixed and dominate at lower output

loads. The first step in determining the losses is to calculate

the duty cycle (D):

VSW is the voltage drop across the internal PFET when it is

on, and is equal to:

VSW = IOUT x RDSON

VD is the forward voltage drop across the Schottky catch

diode. It can be obtained from the diode manufactures Elec-

trical Characteristics section. If the voltage drop across the

inductor (VDCR) is accounted for, the equation becomes:

The conduction losses in the free-wheeling Schottky diode

are calculated as follows:

PDIODE = VD x IOUT x (1-D)

Often this is the single most significant power loss in the cir-

cuit. Care should be taken to choose a Schottky diode that

has a low forward voltage drop.

Another significant external power loss is the conduction loss

in the output inductor. The equation can be simplified to:

PIND = IOUT2 x RDCR

The LM2831 conduction loss is mainly associated with the

internal PFET:

If the inductor ripple current is fairly small, the conduction

losses can be simplified to:

PCOND = IOUT2 x RDSON x D

Switching losses are also associated with the internal PFET.

They occur during the switch on and off transition periods,

where voltages and currents overlap resulting in power loss.

The simplest means to determine this loss is to empirically

measuring the rise and fall times (10% to 90%) of the switch

at the switch node.

Switching Power Loss is calculated as follows:

PSWR = 1/2(VIN x IOUT x FSW x TRISE)

PSWF = 1/2(VIN x IOUT x FSW x TFALL)

PSW = PSWR + PSWF

Another loss is the power required for operation of the internal

circuitry:

PQ = IQ x VIN

IQ is the quiescent operating current, and is typically around

2.5mA for the 0.55MHz frequency option.

Typical Application power losses are:

Power Loss Tabulation

VIN 5.0V

VOUT 3.3V POUT 4.125W

IOUT 1.25A

VD0.45V PDIODE 188mW

FSW 550kHz

IQ2.5mA PQ12.5mW

TRISE 4nS PSWR 7mW

TFALL 4nS PSWF 7mW

RDS(ON) 150mΩPCOND 156mW

INDDCR 70mΩPIND 110mW

D 0.667 PLOSS 481mW

η88% PINTERNAL 183mW

ΣPCOND + PSW + PDIODE + PIND + PQ = PLOSS

ΣPCOND + PSWF + PSWR + PQ = PINTERNAL

PINTERNAL = 183mW

Thermal Definitions

TJ = Chip junction temperature

TA = Ambient temperature

RθJC = Thermal resistance from chip junction to device case

RθJA = Thermal resistance from chip junction to ambient air

Heat in the LM2831 due to internal power dissipation is re-

moved through conduction and/or convection.

Conduction: Heat transfer occurs through cross sectional ar-

eas of material. Depending on the material, the transfer of

heat can be considered to have poor to good thermal con-

ductivity properties (insulator vs. conductor).

Heat Transfer goes as:

Silicon → package → lead frame → PCB

Convection: Heat transfer is by means of airflow. This could

be from a fan or natural convection. Natural convection occurs

when air currents rise from the hot device to cooler air.

Thermal impedance is defined as:

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LM2831

Thermal impedance from the silicon junction to the ambient

air is defined as:

The PCB size, weight of copper used to route traces and

ground plane, and number of layers within the PCB can great-

ly effect RθJA. The type and number of thermal vias can also

make a large difference in the thermal impedance. Thermal

vias are necessary in most applications. They conduct heat

from the surface of the PCB to the ground plane. Four to six

thermal vias should be placed under the exposed pad to the

ground plane if the LLP package is used.

Thermal impedance also depends on the thermal properties

of the application operating conditions (Vin, Vo, Io etc), and

the surrounding circuitry.

Silicon Junction Temperature Determination Method 1:

To accurately measure the silicon temperature for a given

application, two methods can be used. The first method re-

quires the user to know the thermal impedance of the silicon

junction to top case temperature.

Some clarification needs to be made before we go any further.

RθJC is the thermal impedance from all six sides of an IC

package to silicon junction.

RΦJC is the thermal impedance from top case to the silicon

junction.

In this data sheet we will use RΦJC so that it allows the user

to measure top case temperature with a small thermocouple

attached to the top case.

RΦJC is approximately 30°C/Watt for the 6-pin LLP package

with the exposed pad. Knowing the internal dissipation from

the efficiency calculation given previously, and the case tem-

perature, which can be empirically measured on the bench

we have:

Therefore:

Tj = (RΦJC x PLOSS) + TC

From the previous example:

Tj = (RΦJC x PINTERNAL) + TC

Tj = 30°C/W x 0.189W + TC

The second method can give a very accurate silicon junction

temperature.

The first step is to determine RθJA of the application. The

LM2831 has over-temperature protection circuitry. When the

silicon temperature reaches 165°C, the device stops switch-

ing. The protection circuitry has a hysteresis of about 15°C.

Once the silicon temperature has decreased to approximately

150°C, the device will start to switch again. Knowing this, the

RθJA for any application can be characterized during the early

stages of the design one may calculate the RθJA by placing

the PCB circuit into a thermal chamber. Raise the ambient

temperature in the given working application until the circuit

enters thermal shutdown. If the SW-pin is monitored, it will be

obvious when the internal PFET stops switching, indicating a

junction temperature of 165°C. Knowing the internal power

dissipation from the above methods, the junction tempera-

ture, and the ambient temperature RθJA can be determined.

Once this is determined, the maximum ambient temperature

allowed for a desired junction temperature can be found.

An example of calculating RθJA for an application using the

National Semiconductor LM2831 LLP demonstration board is

shown below.

The four layer PCB is constructed using FR4 with ½ oz copper

traces. The copper ground plane is on the bottom layer. The

ground plane is accessed by two vias. The board measures

3.0cm x 3.0cm. It was placed in an oven with no forced airflow.

The ambient temperature was raised to 144°C, and at that

temperature, the device went into thermal shutdown.

From the previous example:

PINTERNAL = 189mW

If the junction temperature was to be kept below 125°C, then

the ambient temperature could not go above 109°C

Tj - (RθJA x PLOSS) = TA

125°C - (111°C/W x 189mW) = 104°C

LLP Package

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FIGURE 4. Internal LLP Connection

For certain high power applications, the PCB land may be

modified to a "dog bone" shape (see Figure 6). By increasing

the size of ground plane, and adding thermal vias, the RθJA

for the application can be reduced.

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LM2831

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FIGURE 5. 6-Lead LLP PCB Dog Bone Layout

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LM2831

LM2831X Design Example 1

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FIGURE 6. LM2831X (1.6MHz): Vin = 5V, Vo = 1.2V @ 1.5A

Bill of Materials

Part ID Part Value Manufacturer Part Number

U1 1.5A Buck Regulator NSC LM2831X

C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M

C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M

D1, Catch Diode 0.3Vf Schottky 1.5A, 30VRTOSHIBA CRS08

L1 3.3µH, 2.2A TDK VLCF5020T-3R3N2R0-1

R2 15.0kΩ, 1% Vishay CRCW08051502F

R1 15.0kΩ, 1% Vishay CRCW08051502F

R3 100kΩ, 1% Vishay CRCW08051003F

17 www.ti.com

LM2831

LM2831X Design Example 2

20174860

FIGURE 7. LM2831X (1.6MHz): Vin = 5V, Vo = 0.6V @ 1.5A

Bill of Materials

Part ID Part Value Manufacturer Part Number

U1 1.5A Buck Regulator NSC LM2831X

C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M

C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M

D1, Catch Diode 0.3Vf Schottky 1.5A, 30VRTOSHIBA CRS08

L1 3.3µH, 2.2A TDK VLCF5020T- 3R3N2R0-1

R2 10.0kΩ, 1% Vishay CRCW08051000F

R1 0Ω

R3 100kΩ, 1% Vishay CRCW08051003F

www.ti.com 18

LM2831

LM2831X Design Example 3

20174808

FIGURE 8. LM2831X (1.6MHz): Vin = 5V, Vo = 3.3V @ 1.5A

Bill of Materials

Part ID Part Value Manufacturer Part Number

U1 1.5A Buck Regulator NSC LM2831X

C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M

C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M

D1, Catch Diode 0.3Vf Schottky 1.5A, 30VRTOSHIBA CRS08

L1 2.7µH 2.3A TDK VLCF5020T-2R7N2R2-1

R2 10.0kΩ, 1% Vishay CRCW08051002F

R1 45.3kΩ, 1% Vishay CRCW08054532F

R3 100kΩ, 1% Vishay CRCW08051003F

19 www.ti.com

LM2831

LM2831Y Design Example 4

20174808

FIGURE 9. LM2831Y (550kHz): Vin = 5V, Vout = 3.3V @ 1.5A

Bill of Materials

Part ID Part Value Manufacturer Part Number

U1 1.5A Buck Regulator NSC LM2831Y

C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M

C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M

D1, Catch Diode 0.3Vf Schottky 1.5A, 30VRTOSHIBA CRS08

L1 4.7µH 2.1A TDK SLF7045T-4R7M2R0-PF

R1 45.3kΩ, 1% Vishay CRCW080545K3FKEA

R2 10.0kΩ, 1% Vishay CRCW08051002F

www.ti.com 20

LM2831

LM2831Y Design Example 5

20174807

FIGURE 10. LM2831Y (550kHz): Vin = 5V, Vout = 1.2V @ 1.5A

Bill of Materials

Part ID Part Value Manufacturer Part Number

U1 1.5A Buck Regulator NSC LM2831Y

C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M

C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M

D1, Catch Diode 0.3Vf Schottky 1.5A, 30VRTOSHIBA CRS08

L1 6.8µH 1.8A TDK SLF7045T-6R8M1R7

R1 10.0kΩ, 1% Vishay CRCW08051002F

R2 10.0kΩ, 1% Vishay CRCW08051002F

21 www.ti.com

LM2831

LM2831Z Design Example 6

20174808

FIGURE 11. LM2831Z (3MHz): Vin = 5V, Vo = 3.3V @ 1.5A

Bill of Materials

Part ID Part Value Manufacturer Part Number

U1 1.5A Buck Regulator NSC LM2831Z

C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M

C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M

D1, Catch Diode 0.3Vf Schottky 1.5A, 30VRTOSHIBA CRS08

L1 1.6µH 2.0A TDK VLCF4018T-1R6N1R7-2

R2 10.0kΩ, 1% Vishay CRCW08051002F

R1 45.3kΩ, 1% Vishay CRCW08054532F

R3 100kΩ, 1% Vishay CRCW08051003F

www.ti.com 22

LM2831

LM2831Z Design Example 7

20174807

FIGURE 12. LM2831Z (3MHz): Vin = 5V, Vo = 1.2V @ 1.5A

Bill of Materials

Part ID Part Value Manufacturer Part Number

U1 1.5A Buck Regulator NSC LM2831Z

C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M

C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M

D1, Catch Diode 0.3Vf Schottky 1.5A, 30VRTOSHIBA CRS08

L1 1.6µH, 2.0A TDK VLCF4018T- 1R6N1R7-2

R2 10.0kΩ, 1% Vishay CRCW08051002F

R1 10.0kΩ, 1% Vishay CRCW08051002F

R3 100kΩ, 1% Vishay CRCW08051003F

23 www.ti.com

LM2831

LM2831X Dual Converters with Delayed Enabled Design Example 8

20174862

FIGURE 13. LM2831X (1.6MHz): Vin = 5V, Vo = 1.2V @ 1.5A & 3.3V @1.5A

Bill of Materials

Part ID Part Value Manufacturer Part Number

U1, U2 1.5A Buck Regulator NSC LM2831X

U3 Power on Reset NSC LP3470M5X-3.08

C1, C3 Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M

C2, C4 Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M

C7 Trr delay capacitor TDK

D1, D2 Catch Diode 0.3Vf Schottky 1.5A, 30VRTOSHIBA CRS08

L1, L2 3.3µH, 2.2A TDK VLCF5020T-3R3N2R0-1

R2, R4, R5 10.0kΩ, 1% Vishay CRCW08051002F

R1, R6 45.3kΩ, 1% Vishay CRCW08054532F

R3 100kΩ, 1% Vishay CRCW08051003F

www.ti.com 24

LM2831

LM2831X Buck Converter & Voltage Double Circuit with LDO Follower Design

Example 9

20174863

FIGURE 14. LM2831X (1.6MHz): Vin = 5V, Vo = 3.3V @ 1.5A & LP2986-5.0 @ 150mA

Bill of Materials

Part ID Part Value Manufacturer Part Number

U1 1.5A Buck Regulator NSC LM2831X

U2 200mA LDO NSC LP2986-5.0

C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M

C2, Output Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M

C3 – C6 2.2µF, 6.3V, X5R TDK C1608X5R0J225M

D1, Catch Diode 0.3Vf Schottky 1.5A, 30VRTOSHIBA CRS08

D2 0.4Vf Schottky 20VR, 500mA ON Semi MBR0520

L2 10µH, 800mA CoilCraft ME3220-103

L1 3.3µH, 2.2A TDK VLCF5020T-3R3N2R0-1

R2 45.3kΩ, 1% Vishay CRCW08054532F

R1 10.0kΩ, 1% Vishay CRCW08051002F

25 www.ti.com

LM2831

Physical Dimensions inches (millimeters) unless otherwise noted

5-Lead SOT-23 Package

NS Package Number MF05A

6-Lead LLP Package

NS Package Number SDE06A

www.ti.com 26

LM2831

Notes

27 www.ti.com

LM2831

Notes

LM2831 High Frequency 1.5A Load - Step-Down DC-DC Regulator

www.ti.com

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