5962-9688401QXA - National Semiconductor

LM3940

1A Low Dropout Regulator for 5V to 3.3V Conversion

General Description

The LM3940 is a 1A low dropout regulator designed to

provide 3.3V from a 5V supply.

The LM3940 is ideally suited for systems which contain both

5V and 3.3V logic, with prime power provided from a 5V bus.

Because the LM3940 is a true low dropout regulator, it can

hold its 3.3V output in regulation with input voltages as low

as 4.5V.

The T0-220 package of the LM3940 means that in most

applications the full 1A of load current can be delivered

without using an additional heatsink.

The surface mount TO-263 package uses minimum board

space, and gives excellent power dissipation capability when

soldered to a copper plane on the PC board.

Features

nOutput voltage specified over temperature

nExcellent load regulation

nGuaranteed 1A output current

nRequires only one external component

nBuilt-in protection against excess temperature

nShort circuit protected

Applications

nLaptop/Desktop Computers

nLogic Systems

Typical Application

01208001

*Required if regulator is located more than 1" from the power supply filter capacitor or if battery power is used.

**See Application Hints.

Connection Diagram/Ordering Information

01208002

3-Lead TO-220 Package

(Front View)

Order Part Number LM3940IT-3.3

NSC Drawing Number TO3B

01208003

3-Lead TO-263 Package

(Front View)

Order Part Number LM3940IS-3.3

NSC Drawing Number TS3B

01208010

3-Lead SOT-223

(Front View)

Order Part Number LM3940IMP-3.3

Package Marked L52B

NSC Drawing Number MP04A

January 2003

LM3940 1A Low Dropout Regulator for 5V to 3.3V Conversion

Connection Diagram/Ordering

Information (Continued)

01208027

16-Lead Ceramic Dual-in-Line Package

(Top View)

Order Part Number LM3940J-3.3-QML

5962-9688401QEA

NSC Drawing Number J16A

01208028

16-Lead Ceramic Surface-Mount Package

(Top View)

Order Part Number LM3940WG-3.3-QML

5962-9688401QXA

NSC Drawing Number WG16A

8-Lead LLP

01208030

Pin 2 and pin 7 are fused to center DAP

Pin 5 and 6 need to be tied together on PCB board

(Top View)

Order Part Number LM3940LD-3.3

NSC Drawing Number LDC08A

LM3940

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Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Storage Temperature Range −65˚C to +150˚C

Operating Junction Temperature Range −40˚C to +125˚C

Lead Temperature (Soldering, 5 seconds) 260˚C

Power Dissipation (Note 2) Internally Limited

Input Supply Voltage 7.5V

ESD Rating (Note 3) 2 kV

Electrical Characteristics

Limits in standard typeface are for T

= 25˚C, and limits in boldface type apply over the full operating temperature range. Un-

less otherwise specified: V

= 5V, I

= 1A, C

OUT

= 33 µF.

Symbol Parameter Conditions Typical LM3940 (Note 4) Units

min max

Output Voltage 5 mA ≤I

≤1A 3.3 3.20 3.40 V

3.13 3.47

Line Regulation I

=5mA 20 40 mV

4.5V ≤V

≤5.5V

Load Regulation 50 mA ≤I

≤1A 35 50

Output Impedance I

(DC) = 100 mA

(AC) = 20 mA (rms) 35 mΩ

f = 120 Hz

Quiescent Current 4.5V ≤V

≤5.5V 10 15 mA

=5mA 20

= 5V 110 200

=1A 250

Output Noise Voltage BW = 10 Hz–100 kHz 150 µV (rms)

=5mA

−V

Dropout Voltage I

= 1A 0.5 0.8 V

(Note 5) 1.0

= 100 mA 110 150 mV

200

(SC) Short Circuit Current R

= 0 1.7 1.2 A

Thermal Performance

Thermal Resistance

Junction-to-Case

3-Lead TO-220 4 ˚C/W

3-Lead TO-263 4 ˚C/W

Thermal Resistance

Junction-to-Ambient

3-Lead TO-220 60 ˚C/W

3-Lead TO-263 80 ˚C/W

8-Lead LLP (Note 2) 35 ˚C/W

Note 1: Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when operating the

device outside of its rated operating conditions.

Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ, the junction-to-ambient thermal resistance, θJA, and the

ambient temperature, TA. Exceeding the maximum allowable power dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown.

The value of θJA (for devices in still air with no heatsink) is 60˚C/W for the TO-220 package, 80˚C/W for the TO-263 package, and 174˚C/W for the SOT-223 package.

The effective value of θJA can be reduced by using a heatsink (see Application Hints for specific information on heatsinking). The value of θJA for the LLP package

is specifically dependant on PCB trace area, trace material, and the number of layers and thermal vias. For improved thermal resistance and power dissipation for

the LLP package, refer to Application Note AN-1187. It is recommended that 6 vias be placed under the center pad to improve thermal performance.

Note 3: ESD rating is based on the human body model: 100 pF discharged through 1.5 kΩ.

Note 4: All limits guaranteed for TJ= 25˚C are 100% tested and are used to calculate Outgoing Quality Levels. All limits at temperature extremes are guaranteed

via correlation using standard Statistical Quality Control (SQC) methods.

Note 5: Dropout voltage is defined as the input-output differential voltage where the regulator output drops to a value that is 100 mV below the value that is

measured at VIN =5V.

LM3940

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Typical Performance Characteristics

Dropout Voltage Dropout Voltage vs. Temperature

01208013 01208014

Output Voltage vs. Temperature Quiescent Current vs. Temperature

01208015 01208016

Quiescent Current vs. V

Quiescent Current vs. Load

01208017 01208018

LM3940

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Typical Performance Characteristics (Continued)

Line Transient Response Load Transient Response

01208019 01208020

Ripple Rejection Low Voltage Behavior

01208021 01208022

Output Impedance Peak Output Current

01208023 01208024

LM3940

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Application Hints

EXTERNAL CAPACITORS

The output capacitor is critical to maintaining regulator sta-

bility, and must meet the required conditions for both ESR

(Equivalent Series Resistance) and minimum amount of ca-

pacitance.

MINIMUM CAPACITANCE:

The minimum output capacitance required to maintain sta-

bility is 33 µF (this value may be increased without limit).

Larger values of output capacitance will give improved tran-

sient response.

ESR LIMITS:

The ESR of the output capacitor will cause loop instability if

it is too high or too low. The acceptable range of ESR plotted

versus load current is shown in the graph below. It is essen-

tial that the output capacitor meet these requirements,

or oscillations can result.

It is important to note that for most capacitors, ESR is

specified only at room temperature. However, the designer

must ensure that the ESR will stay inside the limits shown

over the entire operating temperature range for the design.

For aluminum electrolytic capacitors, ESR will increase by

about 30X as the temperature is reduced from 25˚C to

−40˚C. This type of capacitor is not well-suited for low tem-

perature operation.

Solid tantalum capacitors have a more stable ESR over

temperature, but are more expensive than aluminum elec-

trolytics. A cost-effective approach sometimes used is to

parallel an aluminum electrolytic with a solid Tantalum, with

the total capacitance split about 75/25% with the Aluminum

being the larger value.

If two capacitors are paralleled, the effective ESR is the

parallel of the two individual values. The “flatter” ESR of the

Tantalum will keep the effective ESR from rising as quickly at

low temperatures.

HEATSINKING

A heatsink may be required depending on the maximum

power dissipation and maximum ambient temperature of the

application. Under all possible operating conditions, the junc-

tion temperature must be within the range specified under

Absolute Maximum Ratings.

To determine if a heatsink is required, the power dissipated

by the regulator, P

, must be calculated.

The figure below shows the voltages and currents which are

present in the circuit, as well as the formula for calculating

the power dissipated in the regulator:

The next parameter which must be calculated is the maxi-

mum allowable temperature rise, T

(max). This is calcu-

lated by using the formula:

(max) = T

(max) − T

(max)

Where: T

(max) is the maximum allowable junction tem-

perature, which is 125˚C for commercial

grade parts.

(max) is the maximum ambient temperature

which will be encountered in the

application.

Using the calculated values for T

(max) and P

, the maxi-

mum allowable value for the junction-to-ambient thermal

resistance, θ

(JA)

, can now be found:

(JA)

(max)/P

IMPORTANT: If the maximum allowable value for θ

(JA)

found to be ≥60˚C/W for the TO-220 package, ≥80˚C/W for

the TO-263 package, or ≥174˚C/W for the SOT-223 pack-

age, no heatsink is needed since the package alone will

dissipate enough heat to satisfy these requirements.

If the calculated value for θ

(JA)

falls below these limits, a

heatsink is required.

HEATSINKING TO-220 PACKAGE PARTS

The TO-220 can be attached to a typical heatsink, or se-

cured to a copper plane on a PC board. If a copper plane is

to be used, the values of θ

(JA)

will be the same as shown in

the next section for the TO-263.

01208005

FIGURE 1. ESR Limits

01208006

IIN =I

L+I

PD=(V

IN −V

OUT)I

L+(V

IN)I

FIGURE 2. Power Dissipation Diagram

LM3940

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Application Hints (Continued)

If a manufactured heatsink is to be selected, the value of

heatsink-to-ambient thermal resistance, θ

(H−A)

, must first be

calculated:

(H−A)

=θ

(JA)

−θ

(C−H)

−θ

(J−C)

Where: θ

(J−C)

is defined as the thermal resistance from the

junction to the surface of the case. A value of

4˚C/W can be assumed for θ

(J−C)

for this

calculation.

(C−H)

is defined as the thermal resistance between

the case and the surface of the heatsink. The

value of θ

(C−H)

will vary from about 1.5˚C/W to

about 2.5˚C/W (depending on method of at-

tachment, insulator, etc.). If the exact value is

unknown, 2˚C/W should be assumed for

(C−H)

When a value for θ

(H−A)

is found using the equation shown,

a heatsink must be selected that has a value that is less than

or equal to this number.

(H−A)

is specified numerically by the heatsink manufacturer

in the catalog, or shown in a curve that plots temperature rise

vs. power dissipation for the heatsink.

HEATSINKING TO-263 AND SOT-223 PACKAGE PARTS

Both the TO-263 (“S”) and SOT-223 (“MP”) packages use a

copper plane on the PCB and the PCB itself as a heatsink.

To optimize the heat sinking ability of the plane and PCB,

solder the tab of the package to the plane.

Figure 3 shows for the TO-263 the measured values of θ

(JA)

for different copper area sizes using a typical PCB with 1

ounce copper and no solder mask over the copper area used

for heatsinking.

As shown in the figure, increasing the copper area beyond 1

square inch produces very little improvement. It should also

be observed that the minimum value of θ

(JA)

for the TO-263

package mounted to a PCB is 32˚C/W.

As a design aid, Figure 4 shows the maximum allowable

power dissipation compared to ambient temperature for the

TO-263 device (assuming θ

(JA)

is 35˚C/W and the maximum

junction temperature is 125˚C).

Figure 5 and Figure 6 show the information for the SOT-223

package. Figure 6 assumes a θ

(JA)

of 74˚C/W for 1 ounce

copper and 51˚C/W for 2 ounce copper and a maximum

junction temperature of 125˚C.

Please see AN1028 for power enhancement techniques to

be used with the SOT-223 package.

01208007

FIGURE 3. θ

(JA)

vs. Copper (1 ounce) Area for the

TO-263 Package

01208008

FIGURE 4. Maximum Power Dissipation vs. T

AMB

for

the TO-263 Package

01208011

FIGURE 5. θ

(JA)

vs. Copper (2 ounce) Area for the

SOT-223 Package

01208012

FIGURE 6. Maximum Power Dissipation vs. T

AMB

for

the SOT-223 Package

LM3940

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Physical Dimensions inches (millimeters) unless otherwise noted

3-Lead SOT-223 Package

Order Part Number LM3940IMP-3.3

NSC Package Number MP04A

3-Lead TO-220 Package

Order Part Number LM3940IT-3.3

NSC Package Number TO3B

LM3940

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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

3-Lead TO-263 Package

Order Part Number LM3940IS-3.3

NSC Package Number TS3B

16-Lead Ceramic Dual-in-Line Package

Order Part Number LM3940J-3.3-QML

5962-9688401QEA

NSC Drawing Number J16A

LM3940

www.national.com9

Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

16-Lead Ceramic Surface-Mount Package

Order Part Number LM3940WG-3.3-QML

5962-9688401QXA

NSC Package Number WG16A

8-Lead LLP

Order Part Number LM3940LD-3.3

NSC Package Number LDC08A

LM3940

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Notes

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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.

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www.national.com

LM3940 1A Low Dropout Regulator for 5V to 3.3V Conversion

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.