AAT3238 datasheet(10/14 Pages) ANALOGICTECH

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AAT3238

300mA MicroPower™

High Performance LDO Linear Regulator

3238.2006.11.1.6

has very low ESR, is lower cost, has a smaller PCB

footprint, and is non-polarized. Line and load tran-

sient 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

resistance, 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.

Enable Function

The AAT3238 features an LDO regulator enable/

disable function. This pin (EN) is active high and is

compatible with CMOS logic. To assure the LDO

regulator will switch on, the EN turn-on control level

must be greater than 1.5V. The LDO regulator will

go into the disable shutdown mode when the volt-

age on the EN pin falls below 0.6V. If the enable

function is not needed in a specific application, it

may be tied to VIN to keep the LDO regulator in a

continuously on state.

When the LDO regulator is in shutdown mode, an

internal 1.5k

Ω resistor is connected between VOUT

and GND. This is intended to discharge COUT when

the LDO regulator is disabled. The internal 1.5k

has no adverse effect on device turn-on time.

Short-Circuit Protection

The AAT3238 contains an internal short-circuit pro-

tection circuit that will trigger when the output load

current exceeds the internal threshold limit. Under

short-circuit conditions, the output of the LDO reg-

ulator will be current limited until the short-circuit

condition is removed from the output or the LDO

regulator package power dissipation exceeds the

device thermal limit.

Thermal Protection

The AAT3238 has an internal thermal protection cir-

cuit which will turn on when the device die temper-

ature exceeds 145°C. The internal thermal protec-

tion circuit will actively turn off the LDO regulator

output pass device to prevent the possibility of over-

temperature damage. The LDO regulator output

will remain in a shutdown state until the internal die

temperature falls back below the 145°C trip point.

The combination and interaction between the short-

circuit and thermal protection systems allow the

LDO regulator to withstand indefinite short-circuit

conditions without sustaining permanent damage.

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