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PDF LP2957AIS Data sheet ( Hoja de datos )
| Número de pieza | LP2957AIS | |
| Descripción | 5V Low-Dropout Regulator for P Applications | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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June 1998
LP2957/LP2957A
5V Low-Dropout Regulator for µP Applications
General Description
The LP2957 is a 5V micropower voltage regulator with elec-
tronic shutdown, error flag, very low quiescent current
(150 µA typical at 1 mA load), and very low dropout voltage
(470 mV typical at 250 mA load current).
Output can be wired for snap-on/snap-off operation to elimi-
nate transition voltage states where µP operation may be un-
predictable.
Output crowbar (50 mA typical pull-down current) will bring
down the output quickly when the regulator snaps off or
when the shutdown function is activated.
The part has tight line and load regulation (0.04% typical)
and low output temperature coefficient (20 ppm/˚C typical).
The accuracy of the 5V output is guaranteed at room tem-
perature and over the full operating temperature range.
The LP2957 is available in the five-lead TO-220 and TO-263
packages.
Features
n 5V output within 1.4% over temperature (A grade)
n Easily programmed for snap-on/snap-off output
n Guaranteed 250 mA output current
n Extremely low quiescent current
n Low Input-Output voltage required for regulation
n Reverse battery protection
n Extremely tight line and load regulation
n Very low temperature coefficient
n Current and thermal limiting
n Error flag signals when output is out of regulation
Applications
n High-efficiency linear regulator
n Battery-powered regulator
Package Outline
Bent, Staggered Leads
5-Lead TO-220 (T)
DS011340-16
Top View
Order Number LP2957AIT or LP2957IT
See NS Package Number T05D
Plastic Surface Mount Package
5-Lead TO-263 (S)
Top View
DS011340-17
DS011340-18
Side View
Order Number LP2957AIS or LP2957IS
See NS Package Number TS5B
© 1999 National Semiconductor Corporation DS011340
www.national.com
1 page  
Typical Performance Characteristics Unless otherwise specified: VIN = 6V, IL = 1 mA, CL = 2.2 µF,
V SD = 3V, TA = 25˚C (Continued)
Load Transient
Response
Load Transient
Response
Dropout
Characteristics
Enable Transient
DS011340-31
Enable Transient
DS011340-32
DS011340-33
Short-Circuit Output
Current and Maximum
Output Current
DS011340-34
Thermal Regulation
Error Output
Sink Current
DS011340-35
Dropout Detection
Threshold Voltages
DS011340-36
DS011340-37
DS011340-38
DS011340-39
5 www.national.com
5 Page 
Application Hints (Continued)
Solving for R2:
DESIGN EXAMPLE #2:
A 5V regulated output is to be powered from a battery made
up of six NiCad cells. The cell data is:
cell voltage (full charged): 1.4V
cell voltage (90% discharged): 1.0V
The internal impedance of a typical battery is low enough
that source loading during regulator turn-on is not usually a
problem.
In a battery-powered application, the turn-off voltage VOFF
should be selected so that the regulator is shut down when
the batteries are about 90% discharged (over discharge can
damage rechargeable batteries).
In this case, the battery voltage will be 6.0V at the 90% dis-
charge point (since there are six cells at 1.0V each). That
means for this application, VOFF will be set to 6.0V.
Selecting the optimum voltage for VON requires understand-
ing battery behavior. If a Ni-Cad battery is nearly discharged
(cell voltage 1.0V) and the load is removed , the cell volt-
age will drift back up. The voltage where the regulator turns
on must be set high enough to keep the regulator from
re-starting during this time, or an on-off pulsing mode can oc-
cur.
If the regulator restarts when the discharged cell voltage
drifts up, the load on the battery will cause the cell voltage to
fall below the turn-off level, which causes the regulator to
shut down. The cell voltage will again float up and the on-off
cycling will continue.
For NiCad batteries, a good cell voltage to use to calculate
VON is about 1.2V per cell. In this application, this will yield a
value for VON of 7.2V.
We can now find R1, R2 and R3 assuming:
VOFF = 6.0V V ON = 7.2V R3 = 49.9k
Solving for R1:
Solving for R2:
11 www.national.com
11 Page | ||
| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet LP2957AIS.PDF ] | |
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