AAT3218 Datasheet, PDF(11/18 Page) List of Unclassifed Manufacturers

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

AAT3218 Datasheet, PDF (11/18 Pages) List of Unclassifed Manufacturers – 150mA MicroPower™ High Performance LDO

◁

AAT3218

150mA MicroPowerâ¢ High Performance LDO

Applications Information

In applications where there is a possibility of VOUT

exceeding VIN for brief amounts of time during nor-

mal operation, the use of a larger value CIN capaci-

tor is highly recommended. A larger value of CIN

with respect to COUT will effect a slower CIN decay

rate during shutdown, thus preventing VOUT from

exceeding VIN. In applications where there is a

greater danger of VOUT exceeding VIN for extended

periods of time, it is recommended to place a schot-

tky diode across VIN to VOUT (connecting the cath-

ode to VIN and anode to VOUT). The Schottky diode

forward voltage should be less than 0.45 volts.

Thermal Considerations and High

Output Current Applications

The AAT3218 is designed to deliver a continuous

output load current of 150mA under normal operat-

ing conditions.

The limiting characteristic for the maximum output

load current safe operating area is essentially

package power dissipation and the internal preset

thermal limit of the device. In order to obtain high

operating currents, careful device layout and circuit

operating conditions need to be taken into account.

The following discussions will assume the LDO reg-

ulator is mounted on a printed circuit board utilizing

the minimum recommended footprint as stated in

the layout considerations section of the document.

At any given ambient temperature (TA) the maxi-

mum package power dissipation can be deter-

mined by the following equation:

PD(MAX) = [TJ(MAX) - TA] / ÎJA

Constants for the AAT3218 are TJ(MAX), the maxi-

mum junction temperature for the device which is

125Â°C and ÎJA = 190Â°C/W, the package thermal

resistance. Typically, maximum conditions are cal-

culated at the maximum operating temperature

where TA = 85Â°C, under normal ambient conditions

TA = 25Â°C. Given TA = 85Â°, the maximum package

power dissipation is 211mW. At TA = 25Â°CÂ°, the

maximum package power dissipation is 526mW.

The maximum continuous output current for the

AAT3218 is a function of the package power dissi-

pation and the input to output voltage drop across

the LDO regulator. Refer to the following simple

equation:

IOUT(MAX) < PD(MAX) / (VIN - VOUT)

For example, if VIN = 5V, VOUT = 3V and TA = 25Â°,

IOUT(MAX) < 264mA. If the output load current were to

exceed 264mA or if the ambient temperature were to

increase, the internal die temperature will increase.

If the condition remained constant, the LDO regula-

tor thermal protection circuit will activate.

To figure what the maximum input voltage would be

for a given load current refer to the following equa-

tion. This calculation accounts for the total power

dissipation of the LDO Regulator, including that

caused by ground current.

PD(MAX) = (VIN - VOUT)IOUT + (VIN x IGND)

This formula can be solved for VIN to determine the

maximum input voltage.

VIN(MAX) = (PD(MAX) + (VOUT x IOUT)) / (IOUT + IGND)

The following is an example for an AAT3218 set for

a 2.5 volt output:

From the discussion above, PD(MAX) was deter-

mined to equal 526mW at TA = 25Â°C.

VOUT = 2.5 volts

IOUT = 150mA

IGND = 150ÂµA

VIN(MAX)=(526mW+(2.5Vx150mA))/(150mA +150ÂµA)

VIN(MAX) = 6.00V

Thus, the AAT3218 can sustain a constant 2.5V

output at a 150mA load current as long as VIN is â¤

6.00V at an ambient temperature of 25Â°C. 6.0V is

the absolute maximum voltage where an AAT3218

would never be operated, thus at 25Â°C, the device

would not have any thermal concerns or opera-

tional VIN(MAX) limits.

This situation can be different at 85Â°C. The follow-

ing is an example for an AAT3218 set for a 2.5 volt

output at 85Â°C:

From the discussion above, PD(MAX) was deter-

mined to equal 211mW at TA = 85Â°C.

3218.2004.02.1.0

▷