LP3985
SNVS087AE –OCTOBER 2000–REVISED MAY 2015
www.ti.com
This capacitor must be located a distance of not more than 1 cm from the input pin and returned to a clean
analog ground. A ceramic capacitor is recommended although a good quality tantalum or film capacitor may be
used at the input.
NOTE
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 verified 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 remain within the operational range over
the full range of temperature and operating conditions.
8.2.2.3 Output Capacitor
Correct selection of the output capacitor is important 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. (See the next section Capacitor
Characteristics).
The LP3985 is designed specifically to work with very small ceramic output capacitors. A 1-µF ceramic capacitor
(dialectric type X7R) with ESR between 5 mΩto 500 mΩis suitable in the LP3985 application circuit. X5R
capacitors may be used but have a narrower temperature range. With these and other capacitor types (Y5V,
Z6U) that may be used, selection is dependant on the range of operating conditions and temperature range for
that application. (see Capacitor Characteristics ).
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).
It is also recommended that the output capacitor be placed within 1 cm from the output pin and returned to a
clean ground line.
8.2.2.4 Capacitor Characteristics
The LP3985 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, 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 mΩto 40 mΩ, which easily meets the ESR requirement for
stability by the LP3985.
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 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. As an example Figure 38 shows a typical graph showing a comparison of
capacitor case sizes in a Capacitance vs. DC Bias plot. As shown in the graph, as a result of the DC Bias
condition the capacitance value may drop below the minimum capacitance value given in the recommended
capacitor table (0.7 µF in this case). Note that the graph shows the capacitance out of spec for the 0402 case
size capacitor at higher bias voltages. It is therefore recommended that the capacitor manufacturers'
specifications for the nominal value capacitor are consulted for all conditions as some capacitor sizes (for
example, 0402) may not be suitable in the actual application.
16 Submit Documentation Feedback Copyright © 2000–2015, Texas Instruments Incorporated
Product Folder Links: LP3985