LP3872, LP3875

SNVS227H – FEBRUARY 2003 – REVISED JANUARY 2015

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8.2.2.4.3

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 equivalent series inductance (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 50X

when going from 25°C down to

−40°C.

It should 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 (from 20 kHz to 100 kHz) should be used for the LP387x. Derating must be

applied to the manufacturer's ESR specification, because it is typically only valid at room temperature.

Any applications using aluminum electrolytics should be thoroughly tested at the lowest ambient operating

temperature where ESR is maximum.

8.2.2.5 Turnon Characteristics for Output Voltages Programmed to 2 V or Less

voltage (typically about 2.2 V). For output voltages programmed to 2 V or less, the regulator output may

momentarily exceed its programmed output voltage during start-up. Outputs programmed to voltages above 2 V

are not affected by this behavior.

8.2.2.6 RFI/EMI Susceptibility

Radio frequency interference (RFI) and electromagnetic interference (EMI) can degrade the performance of any

IC 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 IC 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 IC.

If a load is connected to the IC 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 IC output. Because 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 IC at frequencies above 100 kHz is determined only by the

output capacitors.

In applications where the load is switching at high speed, the output of the IC may need RF isolation from the

load. It is recommended that some inductance be placed between the output capacitor and the load, and good

RF bypass capacitors be placed directly across the load.

PCB layout is also critical in high noise environments, because RFI/EMI is easily radiated directly into PC traces.

Noisy circuitry should be isolated from "clean" circuits where possible, and grounded through a separate path. At

MHz frequencies, ground planes begin to look inductive and RFI/EMI can cause ground bounce across the

ground plane.

In multilayer PCB applications, care should be taken in layout so that noisy power and ground planes do not

radiate directly into adjacent layers which carry analog power and ground.

8.2.2.7 Output Noise

Noise is specified in two ways:

•

Spot Noise (or Output Noise Density): the RMS sum of all noise sources, measured at the regulator output, at

a specific frequency (measured with a 1-Hz bandwidth). This type of noise is usually plotted on a curve as a

function of frequency.

•

Total Output Noise (or Broad-Band Noise): the RMS sum of spot noise over a specified bandwidth, usually

several decades of frequencies.

Attention should be paid to the units of measurement. Spot noise is measured in units µV/

√Hz or nV/√Hz and

total output noise is measured in µV(rms).

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