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LP2985IBPX-1.8 Schematic ( PDF Datasheet ) - National Semiconductor

Teilenummer LP2985IBPX-1.8
Beschreibung Micropower 150 mA Low-Noise Low-Dropout Regulator in SOT-23 and micro SMD packages for Applications
Hersteller National Semiconductor
Logo National Semiconductor Logo 




Gesamt 17 Seiten
LP2985IBPX-1.8 Datasheet, Funktion
May 2004
LP2985LV
Micropower 150 mA Low-Noise Low-Dropout Regulator
in SOT-23 and micro SMD packages for Applications
with Output Voltages 2.3V
Designed for Use with Very Low ESR Output Capacitors
General Description
The LP2985LV is a 150 mA, fixed-output voltage regulator
designed to providehigh performance and low noise in ap-
plications requiring output voltages < 2.3V.
Using an optimized VIP(Vertically Integrated PNP) pro-
cess, the LP2985LV delivers unequalled performance in all
specifications critical to battery-powered designs:
Ground Pin Current: Typically 825 µA @ 150 mA load, and
75 µA @ 1 mA load.
Enhanced Stability: The LP2985LV is stable with output
capacitor ESR as low as 5 m, which allows the use of
ceramic capacitors on the output.
Sleep Mode: Less than 1 µA quiescent current when ON/
OFF pin is pulled low.
Smallest Possible Size: micro SMD package uses absolute
minimum board space.
Precision Output: 1% tolerance output voltages available
(A grade).
Low Noise: By adding a 10 nF bypass capacitor, output
noise can be reduced to 30 µV (typical).
Features
n Guaranteed 150 mA output current
n Smallest possible size (micro SMD)
n Requires minimum external components
n Stable with low-ESR output capacitor
n <1 µA quiescent current when shut down
n Low ground pin current at all loads
n Output voltage accuracy 1% (A Grade)
n High peak current capability
n Wide supply voltage range (16V max)
n Low ZOUT: 0.3typical (10 Hz to 1 MHz)
n Overtemperature/overcurrent protection
n −40˚C to +125˚C junction temperature range
n Custom voltages available
Applications
n Cellular Phone
n Palmtop/Laptop Computer
n Personal Digital Assistant (PDA)
n Camcorder, Personal Stereo, Camera
Block Diagram
VIPis a trademark of National Semiconductor Corporation.
© 2004 National Semiconductor Corporation DS101295
10129501
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LP2985IBPX-1.8 Datasheet, Funktion
Typical Performance Characteristics Unless otherwise specified: CIN = 1µF, COUT = 4.7µF,
VIN = VOUT(NOM) +1, VOUT = 1.8V, TA = 25˚C, ON/OFF pin is tied to VIN
VOUT vs Temperature
Short-Circuit Current
Short-Circuit Current
10129507
10129516
Short-Circuit Current vs Output Voltage
10129517
Ripple Rejection
COUT = 4.7µF, Bypass = 10nF
10129518
Ripple Rejection
COUT = 4.7µF, No Bypass
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LP2985IBPX-1.8 pdf, datenblatt
Application Hints (Continued)
One disadvantage of ceramic capacitors is that their capaci-
tance can vary with temperature. Most large value ceramic
capacitors (2.2 µF) are manufactured with the Z5U or Y5V
temperature characteristic, which results in the capacitance
dropping by more than 50% as the temperature goes from
25˚C to 85˚C.
This could cause problems if a 2.2 µF capacitor were used
on the output since it will drop down to approximately 1 µF at
high ambient temperatures (which could cause the
LP2985LV to oscillate). If Z5U or Y5V capacitors are used on
the output, a minimum capacitance value of 4.7 µF must be
observed.
A better choice for temperature coefficient in ceramic capaci-
tors is X7R, which holds the capacitance within ±15%. Un-
fortunately, the larger values of capacitance are not offered
by all manufacturers in the X7R dielectric.
Tantalum
Tantalum capacitors are less desirable than ceramics for use
as output capacitors because they are more expensive when
comparing equivalent capacitance and voltage ratings in the
1 µF to 4.7 µF range.
Another important consideration is that Tantalum capacitors
have higher ESR values than equivalent size ceramics. This
means that while it may be possible to find a Tantalum
capacitor with an ESR value within the stable range, it would
have to be larger in capacitance (which means bigger and
more costly) than a ceramic capacitor with the same ESR
value.
It should also be noted that the ESR of a typical Tantalum will
increase about 2:1 as the temperature goes from 25˚C down
to −40˚C, so some guard band must be allowed.
On/off Input Operation
The LP2985LV is shut off by driving the ON/OFF input low,
and turned on by pulling it high. If this feature is not to be
used, the ON/OFF input should be tied to VIN to keep the
regulator output on at all times.
To assure proper operation, the signal source used to drive
the ON/OFF input must be able to swing above and below
the specified turn-on/turn-off voltage thresholds listed in the
Electrical Characteristics section under VON/OFF. To prevent
mis-operation, the turn-on (and turn-off) voltage signals ap-
plied to the ON/OFF input must have a slew rate which is
40 mV/µs.
Caution: the regulator output voltage can not be guaranteed
if a slow-moving AC (or DC) signal is applied that is in the
range between the specified turn-on and turn-off voltages
listed under the electrical specification VON/OFF (see Electri-
cal Characteristics).
REVERSE INPUT-OUTPUT VOLTAGE
The PNP power transistor used as the pass element in the
LP2985LV has an inherent diode connected between the
regulator output and input. During normal operation (where
the input voltage is higher than the output) this diode is
reverse-biased.
However, if the output is pulled above the input, this diode
will turn ON and current will flow into the regulator output. In
such cases, a parasitic SCR can latch which will allow a high
current to flow into VIN (and out the ground pin), which can
damage the part.
In any application where the output may be pulled above the
input, an external Schottky diode must be connected from
VIN to VOUT (cathode on VIN, anode on VOUT), to limit the
reverse voltage across the LP2985LV to 0.3V (see Absolute
Maximum Ratings).
MICRO SMD MOUNTING
The micro SMD package requires specific mounting tech-
niques which are detailed in National Semiconductor Appli-
cation Note # 1112. Referring to the section Surface Mount
Technology (SMT) Assembly Considerations, it should be
noted that the pad style which must be used with the 5-pin
package is the NSMD (non-solder mask defined) type.
For best results during assembly, alignment ordinals on the
PC board may be used to facilitate placement of the micro
SMD device.
MICRO SMD LIGHT SENSITIVITY
Exposing the micro SMD device to direct sunlight will cause
misoperation of the device. Light sources such as Halogen
lamps can also affect electrical performance if brought near
to the device.
The wavelenghts which have the most detrimental effect are
reds and infra-reds, which means that the fluorescent light-
ing used inside most buildings has very little effect on per-
formance. A micro SMD test board was brought to within 1
cm of a fluorescent desk lamp and the effect on the regu-
lated output voltage was negligible, showing a deviation of
less than 0.1% from nominal.
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