LP2986 Datasheet, PDF (17/33 Pages) National Semiconductor (TI) – Micropower, 200 mA Ultra Low-Dropout Fixed or Adjustable Voltage Regulator

www.ti.com
LP2986
SNVS137I – MARCH 1999 – REVISED SEPTEMBER 2015
Figure 31. ESR Curves For 5-V Output
Figure 32. ESR Curves for 2.5-V Output
NOTE
The output capacitor must maintain its ESR in the stable region over the full operating
temperature range of the application to assure stability.
The minimum required amount of output capacitance is 4.7 µF. Output capacitor size can be increased without
limit.
It is important to remember that capacitor tolerance and variation with temperature must be taken into
consideration when selecting an output capacitor so that the minimum required amount of output capacitance is
provided over the full operating temperature range. A good tantalum capacitor will show very little variation with
temperature, but a ceramic may not be as good (see Capacitor Characteristics).
8.2.2.3 Capacitor Characteristics
8.2.2.3.1 Tantalum
The best choice for size, cost, and performance are solid tantalum capacitors. Available from many sources, their
typical ESR is very close to the ideal value required on the output of many LDO regulators.
Tantalums also have good temperature stability: a 4.7 µF was tested and showed only a 10% decline in
capacitance as the temperature was decreased from +125°C to −40°C. The ESR increased only about 2:1 over
the same range of temperature.
However, it should be noted that the increasing ESR at lower temperatures present in all tantalums can cause
oscillations when marginal quality capacitors are used (where the ESR of the capacitor is near the upper limit of
the stability range at room temperature).
8.2.2.3.2 Ceramic
For a given amount of a capacitance, ceramics are usually larger and more costly than tantalums.
Be warned that the ESR of a ceramic capacitor can be low enough to cause instability: a 2.2-µF ceramic
capacitor was measured and found to have an ESR of about 15 mΩ.
If a ceramic capacitor is to be used on the LP2986 output, a 1-Ω resistor should be placed in series with the
capacitor to provide a minimum ESR for the regulator.
Another disadvantage of ceramic capacitors is that their capacitance varies a lot with temperature:
Large ceramic capacitors are typically manufactured with the Z5U temperature characteristic, which results in the
capacitance dropping by a 50% as the temperature goes from +25°C to 80°C.
This means you have to buy a capacitor with twice the minimum C_OUT to assure stable operation up to 80°C.
8.2.2.3.3 Aluminum
The large physical size of aluminum electrolytics makes them unattractive for use with the LP2986. Their ESR
characteristics are also not well suited to the requirements of LDO regulators.
The ESR of an aluminum electrolytic is higher than a tantalum, and it also varies greatly with temperature.
Copyright © 1999–2015, Texas Instruments Incorporated
Product Folder Links: LP2986
Submit Documentation Feedback
17