Datasheet
LP3981
SNVS159G –OCTOBER 2001–REVISED MAY 2013
www.ti.com
APPLICATION HINTS
POWER DISSIPATION AND DEVICE OPERATION
The permissible power dissipation for any package is a measure of the capability of the device to pass heat from
the power source, the junctions of the IC, to the ultimate heat sink, the ambient environment. Thus, the power
dissipation is dependant on the ambient temperature and the thermal resistance across the various interfaces
between the die and ambient air.
As stated in the notes for Absolute Maximum Ratings and Operating Ratings, the allowable power dissipation for
the device in a given package can be calculated using the equation:
(1)
With a θ
JA
= 50°C/W, the device in the WSON package returns a value of 2.0W with a maximum junction
temperature of 125°C and an ambient temperature of 25°C. The device in a VSSOP package returns a figure of
0.476W, ( θ
JA
= 210°C/W).
The actual power dissipation across the device can be represented by the following equation:
P
D
= (V
IN
− V
OUT
) x I
OUT
(2)
This establishes the relationship between the power dissipation 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 failures 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 specified by the manufacturer to have a surge current rating sufficient for the application.
There are no requirements for the ESR on the input capacitor, but tolerance and temperature coefficient must be
considered when selecting the capacitor to ensure the capacitance 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 CAPACITOR CHARACTERISTICS).
The output capacitor must meet the requirement for minimum amount of capacitance and also have an ESR
(Equivalent 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.
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