LP3871, LP3874

www.ti.com

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 m

Ω). 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).

Copyright © 2003–2015, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LP3871 LP3874