LP2987, LP2988
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SNVS004J –MARCH 1999–REVISED APRIL 2013
CAPACITOR CHARACTERISTICS
TANTALUM: A solid tantalum capacitor is the best choice for the output capacitor on the LM2987/8. 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).
CERAMIC: The ESR of ceramic capacitor can be low enough to cause an LDO regulator to oscillate: a 2.2 µF
ceramic was measured and found to have an ESR of 15 mΩ.
If a ceramic capacitor is to be used on the LP2987/8 output, a 1Ω resistor should be placed in series with the
capacitor to provide a minimum ESR for the regulator.
A 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 50% as the temperature goes from 25°C to 80°C.
This means you have to buy a capacitor with twice the minimum COUT to assure stable operation up to 80°C.
ALUMINUM: The large physical size of aluminum electrolytics makes them unsuitable for most applications.
Their ESR characteristics are also not well suited to the requirements of LDO regulators. The ESR of a typical
aluminum electrolytic is higher than a tantalum, and it also varies greatly with temperature.
A typical aluminum electrolytic can exhibit an ESR increase of 50X when going from 20°C to −40°C. Also, some
aluminum electrolytics can not be used below −25°C because the electrolyte will freeze.
POWER-ON RESET DELAY
A power-on reset function can be easily implemented using the LP2987/8 by adding a single external capacitor to
the Delay pin. The Error output provides the power-on reset signal when input power is applied to the regulator.
The reset signal stays low for a pre-set time period after power is applied to the regulator, and then goes high
(see Timing Diagram below).
Figure 46. Timing Diagram for Power-Up
The external capacitor cDLY sets the delay time (TDELAY). The value of capacitor required for a given time delay
may be calculated using the formula:
CDLY = TDELAY/(5.59 X 105)
To simplify design, a plot is provided below which shows values of CDLY versus delay time.
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