LP3981IMM-3.3/NOPB National Semiconductor, LP3981IMM-3.3/NOPB Datasheet - Page 9

• Manufacturer Part Number: LP3981IMM-3.3/NOPB
• Description: IC REG LDO 300MA 3.3V 8MSOP
• Manufacturer: National Semiconductor

Specifications of LP3981IMM-3.3/NOPB

• Regulator Topology: Positive Fixed
• Voltage - Output: 3.3V
• Voltage - Input: Up to 6V
• Voltage - Dropout (typical): 0.132V @ 300mA
• Number Of Regulators: 1
• Current - Output: 300mA
• Current - Limit (min): 300mA
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LP3981IMM-3.3; LP3981IMM-3.3TR

Application Hints
125˚C and an ambient temperature of 25˚C. The device in a
MSOP package returns a figure of 0.476W, ( θ
W).
The actual power dissipation across the device can be rep-
resented by the following equation:
This establishes the relationship between the power dissipa-
tion allowed due to thermal considerations, the voltage drop
across the device, and the continuous current capability of
the device. The device can deliver 300mA but care must be
taken when choosing the continuous current output for the
device under the operating load conditions.
EXTERNAL CAPACITORS
Like any low-dropout regulator, the LP3981 requires external
capacitors for regulator stability. The LP3981 is specifically
designed for portable applications requiring minimum board
space and smallest components. These capacitors must be
correctly selected for good performance.
INPUT CAPACITOR
An input capacitance of ) 2.2µF is required between the
LP3981 input pin and ground (the amount of the capacitance
may be increased without limit).
This capacitor must be located a distance of not more than
1cm from the input pin and returned to a clean analog
ground. Any good quality ceramic, tantalum, or film capacitor
may be used at the input.
Important: Tantalum capacitors can suffer catastrophic fail-
ures due to surge current when connected to a low-
impedance source of power (like a battery or a very large
capacitor). If a tantalum capacitor is used at the input, it must
be guaranteed by the manufacturer to have a surge current
rating sufficient for the application.
There are no requirements for the ESR on the input capaci-
tor, but tolerance and temperature coefficient must be con-
sidered when selecting the capacitor to ensure the capaci-
tance will be ) 2.2µF over the entire operating temperature
range.
OUTPUT CAPACITOR
The LP3981 is designed specifically to work with very small
ceramic output capacitors. A ceramic capacitor (dielectric
types Z5U, Y5V or X7R) in 2.2 to 22 µF range with 5mΩ to
500mΩ ESR range is suitable in the LP3981 application
circuit.
It may also be possible to use tantalum or film capacitors at
the output, but these are not as attractive for reasons of size
and cost (see next section Capacitor Characteristics).
The output capacitor must meet the requirement for mini-
mum amount of capacitance and also have an ESR (Equiva-
lent Series Resistance) value which is within a stable range
(5 mΩ to 500 mΩ).
NO-LOAD STABILITY
The LP3981 will remain stable and in regulation with no
external load. This is specially important in CMOS RAM
keep-alive applications.
P
D
= (V
IN
− V
OUT
(Continued)
) x I
OUT
JA
= 210˚C/
9
NOISE BYPASS CAPACITOR
Connecting a 0.033µF capacitor between the C
ground significantly reduces noise on the regulator output.
This cap is connected directly to a high impedance node in
the bad gap reference circuit. Any significant loading on this
node will cause a change on the regulated output voltage.
For this reason, DC leakage current through this pin must be
kept as low as possible for best output voltage accuracy.
The types of capacitors best suited for the noise bypass
capacitor are ceramic and film. Hight-quality ceramic capaci-
tors with either NPO or COG dielectric typically have very
low leakage. Polypropolene and polycarbonate film capaci-
tors are available in small surface-mount packages and
typically have extremely low leakage current.
Unlike many other LDO’s, addition of a noise reduction
capacitor does not effect the transient response of the de-
vice.
CAPACITOR CHARACTERISTICS
The LP3981 is designed to work with ceramic capacitors on
the output to take advantage of the benefits they offer: for
capacitance values in the range of 1µF to 4.7µF range,
ceramic capacitors are the smallest, least expensive and
have the lowest ESR values (which makes them best for
eliminating high frequency noise). The ESR of a typical 1µF
ceramic capacitor is in the range of 20 mΩ to 40 mΩ, which
easily meets the ESR requirement for stability by the
LP3981.
The ceramic capacitor’s capacitance can vary with tempera-
ture. Most large value ceramic capacitors () 2.2µF) are
manufactured with Z5U or Y5V temperature characteristics,
which results in the capacitance dropping by more than 50%
as the temperature goes from 25˚C to 85˚C.
A better choice for temperature coefficient in a ceramic
capacitor is X7R, which holds the capacitance within
Tantalum capacitors are less desirable than ceramic for use
as output capacitors because they are more expensive when
comparing equivalent capacitance and voltage ratings in the
1µF to 4.7µF 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 LP3981 is turned off by pulling the V
turned on by pulling it high. If this feature is not used, the V
pin should be tied to V
time. To assure proper operation, the signal source used to
drive the V
the specified turn-on/off voltage thresholds listed in the Elec-
trical Characteristics section under V
FAST ON-TIME
The LP3981 utilizes a speed up circuitry to ramp up the
internal V
output turn on time.
REF
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input must be able to swing above and below
voltage to its final value to achieve a fast
IN
to keep the regulator output on at all
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and V
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