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PDF LP3871EMP-1.8 Data sheet ( Hoja de datos )
| Número de pieza | LP3871EMP-1.8 | |
| Descripción | 0.8A Fast Ultra Low Dropout Linear Regulators | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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April 2004
LP3871/LP3874
0.8A Fast Ultra Low Dropout Linear Regulators
General Description
The LP3871/LP3874 series of fast ultra low-dropout linear
regulators operate from a +2.5V to +7.0V input supply. Wide
range of preset output voltage options are available. These
ultra low dropout linear regulators respond very quickly to
step changes in load, which makes them suitable for low
voltage microprocessor applications. The LP3871/LP3874
are developed on a CMOS process which allows low quies-
cent current operation independent of output load current.
This CMOS process also allows the LP3871/LP3874 to op-
erate under extremely low dropout conditions.
Dropout Voltage: Ultra low dropout voltage; typically 24mV
at 80mA load current and 240mV at 0.8A load current.
Ground Pin Current: Typically 6mA at 0.8A load current.
Shutdown Mode: Typically 10nA quiescent current when
the shutdown pin is pulled low.
Error Flag: Error flag goes low when the output voltage
drops 10% below nominal value.
SENSE: Sense pin improves regulation at remote loads.
Precision Output Voltage: Multiple output voltage options
are available ranging from 1.8V to 5.0V with a guaranteed
accuracy of ±1.5% at room temperature, and ±3.0% over all
conditions (varying line, load, and temperature).
Features
n Ultra low dropout voltage
n Low ground pin current
n Load regulation of 0.04%
n 10nA quiescent current in shutdown mode
n Guaranteed output current of 0.8A DC
n Available in TO-263, TO-220 and SOT-223 packages
n Output voltage accuracy ± 1.5%
n Error flag indicates output status
n Sense option improves load regulation
n Minimum output capacitor requirements
n Overtemperature/overcurrent protection
n −40˚C to +125˚C junction temperature range
Applications
n Microprocessor power supplies
n GTL, GTL+, BTL, and SSTL bus terminators
n Power supplies for DSPs
n SCSI terminator
n Post regulators
n High efficiency linear regulators
n Battery chargers
n Other battery powered applications
Typical Application Circuits
20063101
*SD and ERROR pins must be pulled high through a 10kΩ pull-up resistor. Connect the ERROR pin to ground if this function is not used. See application hints
for more information.
© 2004 National Semiconductor Corporation DS200631
www.national.com
1 page  
Ordering Information (Continued)
Output
Voltage
5.0
5.0
3.3
3.3
2.5
2.5
1.8
1.8
TABLE 1. Package Marking and Ordering Information (Continued)
Order Number
Description
(Current, Option)
Package
Type
Package
Marking
Supplied As:
LP3874EMP-5.0
0.8A, SENSE
SOT223-5 LHJB
1000 Units on Tape and Reel
LP3874EMPX-5.0 0.8A, SENSE
SOT223-5 LHJB
2000 Units on Tape and Reel
LP3874EMP-3.3
0.8A, SENSE
SOT223-5 LHHB
1000 Units on Tape and Reel
LP3874EMPX-3.3 0.8A, SENSE
SOT223-5 LHHB
2000 Units on Tape and Reel
LP3874EMP-2.5
0.8A, SENSE
SOT223-5 LHFB
1000 Units on Tape and Reel
LP3874EMPX-2.5 0.8A, SENSE
SOT223-5 LHFB
2000 Units on Tape and Reel
LP3874EMP-1.8
0.8A, SENSE
SOT223-5 LHEB
1000 Units on Tape and Reel
LP3874EMPX-1.8 0.8A, SENSE
SOT223-5 LHEB
2000 Units on Tape and Reel
5 www.national.com
5 Page 
Application Hints
VIN RESTRICTIONS FOR PROPER START-UP
To prevent misoperation, ensure that VIN is below 50mV
before start-up is initiated. This scenario can occur in sys-
tems with a backup battery using reverse-biased "blocking"
diodes which may allow enough leakage current to flow into
the VIN node to raise it’s voltage slightly above ground when
the main power is removed. Using low leakage diodes or a
resistive pull down can prevent the voltage at VIN from rising
above 50mV. Large bulk capacitors connected to VIN may
also cause a start-up problem if they do not discharge fully
before re-start is initiated (but only if VIN is allowed to fall
below 1V). A resistor connected across the capacitor will
allow it to discharge more quickly. It should be noted that the
probability of a "false start" caused by incorrect logic states
is extremely low.
EXTERNAL CAPACITORS
Like any low-dropout regulator, external capacitors are re-
quired to assure stability. These capacitors must be correctly
selected for proper performance.
INPUT CAPACITOR: An input capacitor of at least 10µF is
required. Ceramic, Tantalum, or Electrolytic capacitors may
be used, and capacitance may be increased without limit.
OUTPUT CAPACITOR: An output capacitor is required for
loop stability. It must be located less than 1 cm from the
device and connected directly to the output and ground pins
using traces which have no other currents flowing through
them (see PCB Layout section).
The minimum value of output capacitance that can be used
for stable full-load operation is 10µF, but it may be increased
without limit. The output capacitor must have an ESR value
as shown in the stable region of the curve (below).Tantalum
capacitors are recommended for the output capacitor.
ESR Curve
20063170
SELECTING A CAPACITOR
It is important to note that capacitance tolerance and varia-
tion with temperature must be taken into consideration when
selecting a capacitor so that the minimum required amount
of capacitance is provided over the full operating tempera-
ture range. In general, a good Tantalum capacitor will show
very little capacitance variation with temperature, but a ce-
ramic may not be as good (depending on dielectric type).
Aluminum electrolytics also typically have large temperature
variation of capacitance value.
Equally important to consider is a capacitor’s ESR change
with temperature: this is not an issue with ceramics, as their
ESR is extremely low. However, it is very important in Tan-
talum and aluminum electrolytic capacitors. Both show in-
creasing ESR at colder temperatures, but the increase in
aluminum electrolytic capacitors is so severe they may not
be feasible for some applications (see Capacitor Character-
istics Section).
CAPACITOR CHARACTERISTICS
CERAMIC: For values of capacitance in the 10 to 100 µF
range, ceramics are usually larger and more costly than
tantalums but give superior AC performance for bypassing
high frequency noise because of very low ESR (typically less
than 10 mΩ). However, some dielectric types do not have
good capacitance characteristics as a function of voltage
and temperature.
Z5U and Y5V dielectric ceramics have capacitance that
drops severely with applied voltage. A typical Z5U or Y5V
capacitor can lose 60% of its rated capacitance with half of
the rated voltage applied to it. The Z5U and Y5V also exhibit
a severe temperature effect, losing more than 50% of nomi-
nal capacitance at high and low limits of the temperature
range.
X7R and X5R dielectric ceramic capacitors are strongly rec-
ommended if ceramics are used, as they typically maintain a
capacitance range within ±20% of nominal over full operat-
ing ratings of temperature and voltage. Of course, they are
typically larger and more costly than Z5U/Y5U types for a
given voltage and capacitance.
TANTALUM: Solid Tantalum capacitors are recommended
for use on the output because their typical ESR is very close
to the ideal value required for loop compensation. They also
work well as input capacitors if selected to meet the ESR
requirements previously listed.
Tantalums also have good temperature stability: a good
quality Tantalum will typically show a capacitance value that
varies less than 10-15% across the full temperature range of
125˚C to −40˚C. ESR will vary only about 2X going from the
high to low temperature limits.
The increasing ESR at lower temperatures can cause oscil-
lations when marginal quality capacitors are used (if the ESR
of the capacitor is near the upper limit of the stability range at
room temperature).
ALUMINUM: This capacitor type offers the most capaci-
tance for the money. The disadvantages are that they are
larger in physical size, not widely available in surface mount,
and have poor AC performance (especially at higher fre-
quencies) due to higher ESR and ESL.
Compared by size, the ESR of an aluminum electrolytic is
higher than either Tantalum or ceramic, and it also varies
greatly with temperature. A typical aluminum electrolytic can
exhibit an ESR increase of as much as 50X when going from
25˚C down to −40˚C.
It should also be noted that many aluminum electrolytics only
specify impedance at a frequency of 120 Hz, which indicates
they have poor high frequency performance. Only aluminum
electrolytics that have an impedance specified at a higher
frequency (between 20 kHz and 100 kHz) should be used for
the LP387X. Derating must be applied to the manufacturer’s
ESR specification, since it is typically only valid at room
temperature.
11 www.national.com
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| PDF Descargar | [ Datasheet LP3871EMP-1.8.PDF ] | |
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