LM3940WG3.3-SCD [NSC]
3.3V FIXED POSITIVE LDO REGULATOR, 1V DROPOUT, CDSO16, CERAMIC, SOIC-16;
| 型号： | LM3940WG3.3-SCD |
| 厂家： | 
National Semiconductor |
| 描述： | 3.3V FIXED POSITIVE LDO REGULATOR, 1V DROPOUT, CDSO16, CERAMIC, SOIC-16 CD |
| 文件： | 总12页 (文件大小：399K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
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January 2003
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.
Features
n Output voltage specified over temperature
n Excellent load regulation
n Guaranteed 1A output current
n Requires only one external component
n Built-in protection against excess temperature
n Short circuit protected
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.
Applications
n Laptop/Desktop Computers
n Logic Systems
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.
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
01208010
01208002
3-Lead TO-220 Package
(Front View)
3-Lead SOT-223
(Front View)
Order Part Number LM3940IMP-3.3
Package Marked L52B
Order Part Number LM3940IT-3.3
NSC Drawing Number TO3B
NSC Drawing Number MP04A
01208003
3-Lead TO-263 Package
(Front View)
Order Part Number LM3940IS-3.3
NSC Drawing Number TS3B
© 2003 National Semiconductor Corporation
DS012080
www.national.com
Connection Diagram/Ordering
Information (Continued)
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
01208027
16-Lead Ceramic Dual-in-Line Package
(Top View)
Order Part Number LM3940J-3.3-QML
5962-9688401QEA
(Top View)
Order Part Number LM3940LD-3.3
NSC Drawing Number LDC08A
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
www.national.com
2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Lead Temperature (Soldering, 5 seconds)
Power Dissipation (Note 2)
Input Supply Voltage
260˚C
Internally Limited
7.5V
ESD Rating (Note 3)
2 kV
Storage Temperature Range
−65˚C to +150˚C
Operating Junction Temperature Range −40˚C to +125˚C
Electrical Characteristics
Limits in standard typeface are for TJ = 25˚C, and limits in boldface type apply over the full operating temperature range. Un-
less otherwise specified: VIN = 5V, IL = 1A, COUT = 33 µF.
Symbol
Parameter
Output Voltage
Conditions
5 mA ≤ IL ≤ 1A
Typical
LM3940 (Note 4)
Units
V
min
3.20
3.13
max
3.40
3.47
40
VO
3.3
Line Regulation
Load Regulation
Output Impedance
IL = 5 mA
mV
20
35
4.5V ≤ VO ≤ 5.5V
50 mA ≤ IL ≤ 1A
50
80
ZO
IL (DC) = 100 mA
IL (AC) = 20 mA (rms)
f = 120 Hz
35
10
mΩ
IQ
Quiescent Current
4.5V ≤ VIN ≤ 5.5V
IL = 5 mA
15
20
mA
VIN = 5V
110
150
0.5
110
1.7
200
250
IL = 1A
en
Output Noise Voltage
BW = 10 Hz–100 kHz
IL = 5 mA
µV (rms)
VO − VIN
Dropout Voltage
(Note 5)
IL = 1A
0.8
1.0
V
mV
A
IL = 100 mA
RL = 0
150
200
IL(SC)
Short Circuit Current
1.2
Thermal Performance
Thermal Resistance
3-Lead TO-220
4
˚C/W
Junction-to-Case
3-Lead TO-263
4
˚C/W
˚C/W
˚C/W
˚C/W
Thermal Resistance
Junction-to-Ambient
3-Lead TO-220
60
80
35
3-Lead TO-263
8-Lead LLP (Note 2)
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, T , the junction-to-ambient thermal resistance, θ , and the
J
JA
ambient temperature, T . Exceeding the maximum allowable power dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown.
A
The value of θ (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.
JA
The effective value of θ can be reduced by using a heatsink (see Application Hints for specific information on heatsinking). The value of θ for the LLP package
JA
JA
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 T = 25˚C are 100% tested and are used to calculate Outgoing Quality Levels. All limits at temperature extremes are guaranteed
J
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 V = 5V.
IN
3
www.national.com
Typical Performance Characteristics
Dropout Voltage
Dropout Voltage vs. Temperature
01208013
01208014
Output Voltage vs. Temperature
Quiescent Current vs. Temperature
01208015
01208016
Quiescent Current vs. VIN
Quiescent Current vs. Load
01208017
01208018
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4
Typical Performance Characteristics (Continued)
Line Transient Response
Load Transient Response
01208020
01208019
Ripple Rejection
Low Voltage Behavior
01208022
01208021
Output Impedance
Peak Output Current
01208024
01208023
5
www.national.com
HEATSINKING
Application Hints
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.
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.
To determine if a heatsink is required, the power dissipated
by the regulator, PD, must be calculated.
MINIMUM CAPACITANCE:
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 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.
01208006
I
= I + I
L G
IN
P
= (V − V
) I + (V ) I
OUT L IN G
D
IN
FIGURE 2. Power Dissipation Diagram
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, TR (max). This is calcu-
lated 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.
01208005
TA (max) is the maximum ambient temperature
which will be encountered in the
application.
FIGURE 1. ESR Limits
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.
Using the calculated values for TR(max) and PD, the maxi-
mum allowable value for the junction-to-ambient thermal
resistance, θ(JA), can now be found:
θ(JA) = TR (max)/PD
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.
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 pack-
age, no heatsink is needed since the package alone will
dissipate enough heat to satisfy these requirements.
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 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.
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.
www.national.com
6
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:
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
calculation.
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
01208008
FIGURE 4. Maximum Power Dissipation vs. TAMB for
the TO-263 Package
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.
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.
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.
01208011
FIGURE 5. θ(JA) vs. Copper (2 ounce) Area for the
SOT-223 Package
01208007
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.
01208012
FIGURE 6. Maximum Power Dissipation vs. TAMB for
the SOT-223 Package
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).
Please see AN1028 for power enhancement techniques to
be used with the SOT-223 package.
7
www.national.com
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
www.national.com
8
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
9
www.national.com
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
www.national.com
10
Notes
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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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