ESR is extremely low. However, it is very important in Tan-

talum and aluminum electrolytic capacitors. Both show in-

creasing ESR at colder temperatures, but the increase in

aluminum electrolytic capacitors is so severe they may not

be feasible for some applications (see Capacitor Character-

istics Section).

CAPACITOR CHARACTERISTICS

CERAMIC: For values of capacitance in the 10 to 100 µF

range, ceramics are usually larger and more costly than

tantalums 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 nomi-

nal capacitance at high and low limits of the temperature

range.

X7R and X5R dielectric ceramic capacitors are strongly rec-

ommended if ceramics are used, as they typically maintain a

ing ratings of temperature and voltage. Of course, they are

typically larger and more costly than Z5U/Y5U types for a

given voltage and capacitance.

TANTALUM: Solid Tantalum capacitors are typically recom-

mended for use on the output because their ESR is very

close to the ideal value required for loop compensation.

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 oscil-

lations when marginal quality capacitors are used (if the ESR

of the capacitor is near the upper limit of the stability range at

room temperature).

ALUMINUM: This capacitor type offers the most capaci-

tance 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 fre-

quencies) 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 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 (between 20 kHz and 100 kHz) should be used for

the LP385X. Derating must be applied to the manufacturer’s

ESR specification, since it is typically only valid at room

temperature.

Any applications using aluminum electrolytics should be

thoroughly tested at the lowest ambient operating tempera-

ture where ESR is maximum.

PCB LAYOUT

Good PC layout practices must be used or instability can be

induced because of ground loops and voltage drops. The

input and output capacitors must be directly connected to the

input, output, and ground pins of the regulator using traces

which do not have other currents flowing in them (Kelvin

connect).

The best way to do this is to lay out C

device with short traces to the V

The regulator ground pin should be connected to the exter-

nal circuit ground so that the regulator and its capacitors

have a "single point ground".

It should be noted that stability problems have been seen in

applications where "vias" to an internal ground plane were

used at the ground points of the IC and the input and output

capacitors. This was caused by varying ground potentials at

these nodes resulting from current flowing through the

ground plane. Using a single point ground technique for the

regulator and it’s capacitors fixed the problem.

Since high current flows through the traces going into V

IN

and coming from V

these pins so there is no voltage drop in series with the input

and output capacitors.

RFI/EMI SUSCEPTIBILITY

RFI (radio frequency interference) and EMI (electromagnetic

interference) can degrade any integrated circuit’s perfor-

mance because of the small dimensions of the geometries

inside the device. In applications where circuit sources are

present which generate signals with significant high fre-

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 ca-

pacitors 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. 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 IC at frequencies above 100 kHz is

determined only by the output capacitor(s).

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 capaci-

tors be placed directly across the load.

PCB layout is also critical in high noise environments, since

RFI/EMI is easily radiated directly into PC traces. Noisy

circuitry should be isolated from "clean" circuits where pos-

sible, and grounded through a separate path. At MHz fre-

quencies, ground planes begin to look inductive and RFI/

EMI can cause ground bounce across the ground plane.

In multi-layer 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.

OUTPUT NOISE

Noise is specified in two ways-

Spot Noise or Output noise density is the RMS sum of all

noise sources, measured at the regulator output, at a spe-

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