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SNVS225H –FEBRUARY 2003–REVISED JUNE 2015
8.2.2.3 Capacitor Characteristics
8.2.2.3.1 Ceramic
For values of capacitance in the 10-µF to 100-µF range, ceramics are usually larger and more costly than
tantalum capacitors but give superior AC performance for bypassing high frequency noise because of very low
ESR (typically less than 10 mΩ). However, some dielectric types do not have good capacitance characteristics
as a function of voltage and temperature.
Z5U and Y5V dielectric ceramics have capacitance that drops severely with applied voltage. A typical Z5U or
Y5V capacitor can lose 60% of its rated capacitance with half of the rated voltage applied to it. The Z5U and Y5V
also exhibit a severe temperature effect, losing more than 50% of nominal capacitance at high and low limits of
the temperature range.
X7R and X5R dielectric ceramic capacitors are strongly recommended if ceramics are used, as they typically
maintain a capacitance range within ±20% of nominal over full operating ratings of temperature and voltage. Of
course, they are typically larger and more costly than Z5U/Y5U types for a given voltage and capacitance.
8.2.2.3.2 Tantalum
Solid tantalum capacitors are recommended for use on the output because their typical ESR is very close to the
ideal value required for loop compensation. They also work well as input capacitors if selected to meet the ESR
requirements previously listed.
Tantalums also have good temperature stability: a good quality tantalum will typically show a capacitance value
that varies less than 10-15% across the full temperature range of 125°C to −40°C. ESR will vary only about 2X
going from the high to low temperature limits.
The increasing ESR at lower temperatures can cause oscillations when marginal quality capacitors are used (if
the ESR of the capacitor is near the upper limit of the stability range at room temperature).
8.2.2.3.3 Aluminum
This capacitor type offers the most capacitance for the money. The disadvantages are that they are larger in
physical size, not widely available in surface mount, and have poor AC performance (especially at higher
frequencies) due to higher ESR and ESL.
Compared by size, the ESR of an aluminum electrolytic is higher than either tantalum or ceramic, and it also
varies greatly with temperature. A typical aluminum electrolytic can exhibit an ESR increase of as much as 50×
when going from 25°C down to −40°C.
It must also be noted that many aluminum electrolytics only specify impedance at a frequency of 120 Hz, which
indicates they have poor high frequency performance. Only aluminum electrolytics that have an impedance
specified at a higher frequency (between 20 kHz and 100 kHz) must be used for the LP387X. Derating must be
applied to the manufacturer's ESR specification, since it is typically only valid at room temperature.
Any applications using aluminum electrolytics must be thoroughly tested at the lowest ambient operating
temperature where ESR is maximum.
8.2.2.4 RFI/EMI Susceptibility
Radio frequency interference (RFI) and electromagnetic interference (EMI) can degrade the performance of any
integrated circuit because of the small dimensions of the geometries inside the device. In applications where
circuit sources are present which generate signals with significant high frequency energy content (> 1 MHz), care
must be taken to ensure that this does not affect the device regulator.
If RFI/EMI noise is present on the input side of the regulator (such as applications where the input source comes
from the output of a switching regulator), good ceramic bypass capacitors must be used at the input pin of the
device.
If a load is connected to the device output which switches at high speed (such as a clock), the high-frequency
current pulses required by the load must be supplied by the capacitors on the device output. Since the bandwidth
of the regulator loop is less than 100 kHz, the control circuitry cannot respond to load changes above that
frequency. This means the effective output impedance of the device at frequencies above 100 kHz is determined
only by the output capacitor(s).
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