Datasheet

LP3991
www.ti.com SNVS296H –DECEMBER 2006–REVISED MAY 2013
APPLICATION INFORMATION
EXTERNAL CAPACITORS
In common with most regulators, the LP3991 requires external capacitors for regulator stability. The LP3991 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 capacitor is required for stability. It is recommended that a 1.0µF capacitor be connected between the
LP3991 input pin and ground (this capacitance value 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
analogue 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 ensured by the manufacturer to have a surge current rating sufficient for the application.
There are no requirements for the ESR (Equivalent Series Resistance) on the input capacitor, but tolerance,
temperature, and voltage coefficients must be considered when selecting the capacitor to ensure the capacitance
will remain ≊ 1.0µF over the entire operating temperature range.
OUTPUT CAPACITOR
Correct selection of the output capacitor is critical to ensure stable operation in the intended application.
The output capacitor must meet all the requirements specified in the recommended capacitor table over all
conditions in the application. these conditions include DC bias, frequency and temperature. Unstable operation
will result if the capacitance drops below the minimum specified value.
The LP3991 is designed specifically to work with very small ceramic output capacitors. For voltage options of
1.5V and higher, A 4.7µF ceramic capacitor (dielectric type X7R or X5R) with an ESR between 5mΩ to 500mΩ,
is suitable in the LP3991 application circuit. However, on lower V
OUT
options a 2.2µF may be employed with only
a small increase in load transient.
Other ceramic types such as Y5V and Z5U are less suitable owing to their inferior temperature characteristics.
(See section on Capacitor Characteristics).
It is also recommended that the output capacitor is placed within 1cm of the output pin and returned to a clean,
low impedance, ground connection.
It is possible to use tantalum or film capacitors at the device output, V
OUT
, but these are not as attractive for
reasons of size and cost (see the section Capacitor Characteristics).
NO-LOAD STABILITY
The LP3991 will remain stable and in regulation with no external load. This is an important consideration in some
circuits, for example CMOS RAM keep-alive applications.
CAPACITOR CHARACTERISTICS
The LP3991 is designed to work with ceramic capacitors on the input and output to take advantage of the
benefits they offer. For capacitance values around 4.7µF, ceramic capacitors give the circuit designer the best
design options in terms of low cost and minimal area.
For both input and output capacitors, careful interpretation of the capacitor specification is required to ensure
correct device operation. The capacitor value can change greatly dependant on the conditions of operation and
capacitor type.
In particular, to ensure stability, the output capacitor selection should take account of all the capacitor
parameters, to ensure that the specification is met within the application. Capacitance value can vary with DC
bias conditions as well as temperature and frequency of operation. Capacitor values will also show some
decrease over time due to aging. The capacitor parameters are also dependant on the particular case size with
smaller sizes giving poorer performance figures in general.
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