LM3940IS-3.3/NOPB - National Semiconductor

July 2007

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 ap-

plications 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

■Output voltage specified over temperature

■Excellent load regulation

■Guaranteed 1A output current

■Requires only one external component

■Built-in protection against excess temperature

■Short circuit protected

Applications

■Laptop/Desktop Computers

■Logic Systems

Typical Application

1208001

*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

1208002

3-Lead TO-220 Package

(Front View)

Order Part Number LM3940IT-3.3

NSC Drawing Number TO3B

1208003

3-Lead TO-263 Package

(Front View)

Order Part Number LM3940IS-3.3

NSC Drawing Number TS3B

1208010

3-Lead SOT-223

(Front View)

Order Part Number LM3940IMP-3.3

Package Marked L52B

NSC Drawing Number MP04A

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

1208027

16-Lead Ceramic Dual-in-Line Package

(Top View)

Order Part Number LM3940J-3.3-QML

5962-9688401QEA

NSC Drawing Number J16A

1208028

16-Lead Ceramic Surface-Mount Package

(Top View)

Order Part Number LM3940WG-3.3-QML

5962-9688401QXA

NSC Drawing Number WG16A

8-Lead LLP

1208030

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

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LM3940

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

Lead Temperature (Soldering, 5 seconds) 260°C

Power Dissipation (Note 2) Internally Limited

Input Supply Voltage 7.5V

ESD Rating (Note 3) 2 kV

Operating Ratings (Note 1)

Junction Temperature Range, TJ−40°C to +125°C

Input Supply Voltage, VIN(MIN) VO + VDO

Electrical Characteristics

Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range. Unless

otherwise specified: VIN = 5V, IL = 1A, COUT = 33 μF.

Symbol Parameter Conditions Typical LM3940 (Note 4) Units

min max

VOOutput Voltage 5 mA ≤ IL ≤ 1A 3.3 3.20

3.13

3.40

3.47 V

Line Regulation IL = 5 mA

4.5V ≤ VIN ≤ 5.5V 20 40

Load Regulation 50 mA ≤ IL ≤ 1A 35 50

ZOOutput Impedance

IL (DC) = 100 mA

IL (AC) = 20 mA (rms)

f = 120 Hz

35 mΩ

IQQuiescent Current

4.5V ≤ VIN ≤ 5.5V

IL = 5 mA 10 15

20 mA

VIN = 5V

IL = 1A 110 200

250

enOutput Noise Voltage BW = 10 Hz–100 kHz

IL = 5 mA 150 μV (rms)

VDO

Dropout Voltage

(Note 5)

IL = 1A 0.5 0.8

1.0 V

IL = 100 mA 110 150

200 mV

IL(SC) Short Circuit Current RL = 0 1.7 1.2 A

Thermal Performance

Thermal Resistance

Junction-to-Case, θJC

3-Lead TO-220 4 °C/W

3-Lead TO-263 4 °C/W

8-Lead LLP 6 °C/W

Thermal Resistance

Junction-to-Ambient, θJA

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. The θJA rating for the LLP is with a JESD51-7 test board having 6 thermal vias

under the exposed pad.

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.

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LM3940

Typical Performance Characteristics

Dropout Voltage

1208013

Dropout Voltage vs. Temperature

1208014

Output Voltage vs. Temperature

1208015

Quiescent Current vs. Temperature

1208016

Quiescent Current vs. VIN

1208017

Quiescent Current vs. Load

1208018

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LM3940

Line Transient Response

1208019

Load Transient Response

1208020

Ripple Rejection

1208021

Low Voltage Behavior

1208022

Output Impedance

1208023

Peak Output Current

1208024

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LM3940

Application Hints

EXTERNAL CAPACITORS

The output capacitor is critical to maintaining regulator stabil-

ity, 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 stabil-

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

values of output capacitance will give improved transient re-

sponse.

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.

1208005

FIGURE 1. ESR Limits

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

fied 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 temperature

operation.

Solid tantalum capacitors have a more stable ESR over tem-

perature, but are more expensive than aluminum electrolyt-

ics. 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 par-

allel 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 pow-

er dissipation and maximum ambient temperature of the ap-

plication. Under all possible operating conditions, the junction

temperature must be within the range specified under Abso-

lute Maximum Ratings.

To determine if a heatsink is required, the power dissipated

by the regulator, PD, 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:

1208006

IIN = IL + IG

PD = (VIN − VOUT) IL + (VIN) IG

FIGURE 2. Power Dissipation Diagram

The next parameter which must be calculated is the maximum

allowable temperature rise, TR (max). This is calculated by

using the formula:

TR (max) = TJ (max) − TA (max)

Where: TJ (max) is the maximum allowable junction tem-

perature, which is 125°C for commercial

grade parts.

TA (max) is the maximum ambient temperature

which will be encountered in the applica-

tion.

Using the calculated values for TR(max) and PD, the maxi-

mum allowable value for the junction-to-ambient thermal re-

sistance, θ(JA), can now be found:

θ(JA) = TR (max)/PD

IMPORTANT: If the maximum allowable value for θ(JA) is

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 package,

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 secured

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.

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LM3940

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

heatsink-to-ambient thermal resistance, θ(H−A), must first be

calculated:

θ(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 cal-

culation.

θ(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 θ(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.

θ(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.

1208007

FIGURE 3. θ(JA) vs. Copper (1 ounce) Area for the TO-263

Package

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).

1208008

FIGURE 4. Maximum Power Dissipation vs. TAMB for the

TO-263 Package

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 junc-

tion temperature of 125°C.

1208011

FIGURE 5. θ(JA) vs. Copper (2 ounce) Area for the SOT-223

Package

1208012

FIGURE 6. Maximum Power Dissipation vs. TAMB for the

SOT-223 Package

Please see AN1028 for power enhancement techniques to be

used with the SOT-223 package.

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LM3940

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

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LM3940

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

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LM3940

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

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LM3940

Notes

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LM3940

Notes

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

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