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PDF LP3856ESX-2.5 Data sheet ( Hoja de datos )
| Número de pieza | LP3856ESX-2.5 | |
| Descripción | 3A Fast Response Ultra Low Dropout Linear Regulators | |
| Fabricantes | National Semiconductor | |
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March 2004
LP3853/LP3856
3A Fast Response Ultra Low Dropout Linear Regulators
General Description
The LP3853/LP3856 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 LP3853/LP3856
are developed on a CMOS process which allows low quies-
cent current operation independent of output load current.
This CMOS process also allows the LP3853/LP3856 to op-
erate under extremely low dropout conditions.
Dropout Voltage: Ultra low dropout voltage; typically 39mV
at 300mA load current and 390mV at 3A load current.
Ground Pin Current: Typically 4mA at 3A 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 Stable with selected ceramic capacitors
n Low ground pin current
n Load regulation of 0.08%
n 10nA quiescent current in shutdown mode
n Guaranteed output current of 3A DC
n Available in TO-263 and TO-220 packages
n Output voltage accuracy ± 1.5%
n Error flag indicates output status
n Sense option improves load regulation
n Overtemperature/overcurrent protection
n −40˚C to +125˚C junction temperature range
Applications
n Microprocessor power supplies
n Stable with ceramic output capacitors
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
20030901
**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 DS200309
www.national.com
1 page  
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
Lead Temperature
(Soldering, 5 sec.)
ESD Rating (Note 3)
Power Dissipation (Note 2)
Input Supply Voltage (Survival)
Shutdown Input Voltage
(Survival)
Output Voltage (Survival), (Note
6), (Note 7)
−65˚C to +150˚C
260˚C
2 kV
Internally Limited
−0.3V to +7.5V
−0.3V to 7.5V
−0.3V to +6.0V
IOUT (Survival)
Short Circuit Protected
Maximum Voltage for ERROR
Pin VIN
Maximum Voltage for SENSE Pin
VOUT
Operating Ratings
Input Supply Voltage (Note 11)
Shutdown Input Voltage
Maximum Operating Current (DC)
Junction Temperature
2.5V to 7.0V
−0.3V to 7.0V
3A
−40˚C to +125˚C
Electrical Characteristics
LP3853/LP3856
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 = VO(NOM) + 1V, IL = 10 mA, COUT = 10µF, VSD = 2V.
Symbol
Parameter
Conditions
Typ LP3853/6 (Note 5) Units
(Note
4)
Min
Max
Output Voltage
VO Tolerance
(Note 8)
VOUT +1V ≤ VIN ≤ 7.0V
10 mA ≤ IL ≤ 3A
-1.5 +1.5
0
-3.0 +3.0
%
∆V OL
Output Voltage Line
Regulation (Note 8)
VOUT +1V ≤ VIN ≤ 7.0V
0.02
0.06
%
∆VO/ ∆IOUT
Output Voltage Load
Regulation
10 mA ≤ IL ≤ 3A
0.08
0.14
%
(Note 8)
VIN - VOUT
IL = 300 mA
39
50
Dropout Voltage
(Note 10)
IL = 3A
65
mV
390 450
600
IGND
Ground Pin Current In
Normal Operation Mode
IL = 300 mA
IL = 3A
4
4
9
10
mA
9
10
IGND
Ground Pin Current In
Shutdown Mode
IO(PK)
Peak Output Current
Short Circuit Protection
VSD ≤ 0.3V
-40˚C ≤ TJ ≤ 85˚C
VO ≥ VO(NOM) - 4%
0.01
4.5
10 µA
50
A
ISC Short Circuit Current
6A
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 or Tantalum may be used, and capaci-
tance 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 amount of output capacitance that can be
used for stable operation is 10µF. For general usage across
all load currents and operating conditions, the part was
characterized using a 10µF Tantalum input capacitor. The
minimum and maximum stable ESR range for the output
capacitor was then measured which kept the device stable,
assuming any output capacitor whose value is greater than
10µF (see Figure 1 below).
20030970
FIGURE 1. ESR Curve for COUT (with 10µF Tantalum
Input Capacitor)
It should be noted that it is possible to operate the part with
an output capacitor whose ESR is below these limits, as-
suming that sufficient ceramic input capacitance is provided.
This will allow stable operation using ceramic output capaci-
tors (see next section).
OPERATION WITH CERAMIC OUTPUT CAPACITORS
LP385X voltage regulators can operate with ceramic output
capacitors if the values of input and output capacitors are
selected appropriately. The total ceramic output capacitance
must be equal to or less than a specified maximum value in
order for the regulator to remain stable over all operating
conditions. This maximum amount of ceramic output capaci-
tance is dependent upon the amount of ceramic input ca-
pacitance used as well as the load current of the application.
This relationship is shown in Figure 2, which graphs the
maximum stable value of ceramic output capacitance as a
function of ceramic input capacitance for load currents of 1A,
2A, and 3A. For example, if the maximum load current is 1A,
a 10µF ceramic input capacitor will allow stable operation for
values of ceramic output capacitance from 10µF up to about
500µF.
20030985
FIGURE 2. Maximum Ceramic Output Capacitance vs
Ceramic Input Capacitance
If the maximum load current is 2A and a 10µF ceramic input
capacitor is used, the regulator will be stable with ceramic
output capacitor values from 10µF up to about 50µF. At 3A of
load current, the ratio of input to output capacitance required
approaches 1:1, meaning that whatever amount of ceramic
output capacitance is used must also be provided at the
input for stable operation. For load currents between 1A, 2A,
and 3A, interpolation may be used to approximate values on
the graph. When calculating the total ceramic output capaci-
tance present in an application, it is necessary to include any
ceramic bypass capacitors connected to the regulator out-
put.
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
11 www.national.com
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| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet LP3856ESX-2.5.PDF ] | |
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