AAT3258ITS-3.0-R-T1 datasheet(12/16 Pages) AAT

12 / 16 page

AAT3258

300mA LDO Linear Regulator with µP Reset

3258.2006.03.1.6

Bypass Capacitor and Low Noise

Applications

A bypass capacitor pin is provided to enhance the

low noise characteristics of the AAT3258 LDO reg-

ulator. The bypass capacitor is not necessary for

operation. However, for best device performance, a

small ceramic capacitor should be placed between

the bypass pin (BYP) and the device ground pin

(GND). The value of C

BYP may range from 470pF

to 10nF. For lowest noise and best possible power

supply ripple rejection performance, a 10nF capac-

itor should be used. To practically realize the high-

est power supply ripple rejection and lowest output

noise performance, it is critical that the capacitor

connection between the BYP pin and GND pin be

direct and PCB traces should be as short as possi-

ble. Refer to the Evaluation Board Layout section

of this document for examples.

There is a relationship between the bypass capac-

itor value and the LDO regulator turn-on and turn-

off time. In applications where fast device turn-on

and turn-off time are desired, the value of C

BYP

should be reduced.

In applications where low noise performance and/

or ripple rejection are less of a concern, the bypass

capacitor may be omitted. The fastest device turn-

on time will be realized when no bypass capacitor

is used.

DC leakage on this pin can affect the LDO regula-

tor output noise and voltage regulation perform-

ance. For this reason, the use of a low leakage,

high quality, ceramic (NPO or C0G type) or film

capacitor is highly recommended.

Capacitor Characteristics

Ceramic composition capacitors are highly recom-

mended over all other types of capacitors.

Ceramic capacitors offer many advantages over

their tantalum and aluminum electrolytic counter-

parts. A ceramic capacitor typically has very low

ESR, is lower cost, has a smaller PCB footprint,

and is non-polarized.

Line and load transient

response of the LDO regulator is improved by

using low ESR ceramic capacitors. Since ceramic

capacitors are non-polarized, they are not prone to

incorrect connection damage.

Equivalent Series Resistance: ESR is a very

important characteristic to consider when selecting a

capacitor.

ESR is the internal series resistance

associated with a capacitor that includes lead resist-

ance, internal connections, size and area, material

composition, and ambient temperature.

Typically,

capacitor ESR is measured in milliohms for ceramic

capacitors and can range to more than several ohms

for tantalum or aluminum electrolytic capacitors.

Ceramic Capacitor Materials:

Ceramic capaci-

tors less than 0.1µF are typically made from NPO

or C0G materials. NPO and C0G materials gener-

ally have tight tolerance and are very stable over

temperature. Larger capacitor values are usually

composed of X7R, X5R, Z5U, or Y5V dielectric

materials. These two material types are not recom-

mended for use with LDO regulators since the

capacitor tolerance can vary more than ±50% over

the operating temperature range of the device. A

2.2µF Y5V capacitor could be reduced to 1µF over

temperature; this could cause problems for circuit

operation. X7R and X5R dielectrics are much more

desirable.

The temperature tolerance of X7R

dielectric is better than ±15%.

Capacitor area is another contributor to ESR.

Capacitors that are physically large in size will have

a lower ESR when compared to a smaller sized

capacitor of an equivalent material and capaci-

tance value. These larger devices can improve cir-

cuit transient response when compared to an equal

value capacitor in a smaller package size.

Consult capacitor vendor datasheets carefully

when selecting capacitors for LDO regulators.

Shutdown Function

The shutdown pin is designed to turn off the LDO

regulator when the device is not in use. This pin is

active high and is compatible with CMOS logic. To

assure the LDO regulator will switch on, the SHDN

turn-on control level must be greater than 1.5V.

The LDO regulator will go into the disable shut-

down mode when the voltage falls below 0.6V. If

the shutdown function is not needed in a specific

application, it may be tied to V

IN to keep the LDO

regulator in a continuously on state.

AAT3258ITS-3.0-R-T1 Datasheet(PDF) 12 Page - Advanced Analog Technology, Inc.

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