Datasheet

LP3988
www.ti.com SNVS161D –OCTOBER 2001–REVISED MAY 2013
APPLICATION INFORMATION
External Capacitors
Like any low-dropout regulator, the LP3988 requires external capacitors for regulator stability. The LP3988 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 ≊ 1 µF is required between the LP3988 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 ensured 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 ≊ 1 µF over the entire operating
temperature range.
Output Capacitors
The LP3988 is designed specifically to work with very small ceramic output capacitors. A ceramic capacitor
(dielectric types Z5U, Y5V or X7R) in 1 to 22 µF range with 5mΩ to 500mΩ ESR range is suitable in the LP3988
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 LP3988 will remain stable and in regulation with no external load. This is specially important in CMOS RAM
keep-alive applications.
Capacitor Characteristics
The LP3988 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 1 µF to 4.7 µF 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 1 µF ceramic capacitor is in the range of 20 mΩ to 40 mΩ, which easily meets the ESR
requirement for stability by the LP3988.
The ceramic capacitor's capacitance can vary with temperature. Most large value ceramic capacitors (≊ 2.2 µF)
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
±15%.
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 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.
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