AAT3221 Datasheet, PDF(11/16 Page) Advanced Analogic Technologies

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AAT3221 Datasheet, PDF (11/16 Pages) Advanced Analogic Technologies – 150mA NanoPower™ LDO Linear Regulator

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AAT3221/2

150mA NanoPowerâ¢ LDO Linear Regulator

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

power dissipation is 267mW. At TA = 25Â°CÂ°, the

maximum package power dissipation is 667mW.

The maximum continuous output current for the

AAT3221/2 is a function of the package power dis-

sipation 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 = 2.5V and TA = 25Â°,

IOUT(MAX) < 267mA. The output short circuit protec-

tion threshold is set between 150mA and 300mA. If

the output load current were to exceed 267mA or if

the ambient temperature were to increase, the inter-

nal die temperature will increase. If the condition

remained constant and the short circuit protection

did not activate, there would be a potential damage

hazard to LDO regulator since the thermal protection

circuit will only activate after a short circuit event

occurs on the LDO regulator output.

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 AAT3221/2 set

for a 2.5 volt output:

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

mined to equal 667mW at TA = 25Â°C.

VOUT = 2.5 volts

IOUT = 150mA

IGND = 1.1ÂµA

VIN(MAX)=(667mW+(2.5Vx150mA))/(150mA +1.1ÂµA)

VIN(MAX) = 6.95V

Thus, the AAT3221/2 can sustain a constant 2.5V

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

6.95V at an ambient temperature of 25Â°C. 5.5V is

the maximum input operating voltage for the

AAT3221/2, thus at 25Â°C, the device would not have

any thermal concerns or operational VIN(MAX) limits.

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

ing is an example for an AAT3221/2 set for a 2.5

volt output at 85Â°C:

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

mined to equal 267mW at TA = 85Â°C.

VOUT = 2.5 volts

IOUT = 150mA

IGND = 1.1ÂµA

VIN(MAX)=(267mW+(2.5Vx150mA))/(150mA +1.1ÂµA)

VIN(MAX) = 4.28V

Higher input to output voltage differentials can be

obtained with the AAT3221/2, while maintaining

device functions in the thermal safe operating area.

To accomplish this, the device thermal resistance

must be reduced by increasing the heat sink area

or by operating the LDO regulator in a duty cycled

mode.

For example, an application requires VIN = 5.0V

while VOUT = 2.5V at a 150mA load and TA = 85Â°C.

VIN is greater than 4.28V, which is the maximum

safe continuous input level for VOUT = 2.5V at

150mA for TA = 85Â°C. To maintain this high input

voltage and output current level, the LDO regulator

must be operated in a duty cycled mode. Refer to

the following calculation for duty cycle operation:

PD(MAX) is assumed to be 267mW

IGND = 1.1ÂµA

IOUT = 150mA

VIN = 5.0 volts

VOUT = 2.5 volts

%DC = 100(PD(MAX) / ((VIN - VOUT)IOUT + (VIN x IGND))

%DC=100(267mW/((5.0V-2.5V)150mA+(5.0Vx1.1ÂµA))

%DC = 71.2%

For a 150mA output current and a 2.5 volt drop

across the AAT3221/2 at an ambient temperature

of 85Â°C, the maximum on time duty cycle for the

device would be 71.2%.

The following family of curves shows the safe oper-

ating area for duty cycled operation from ambient

room temperature to the maximum operating level.

3221.2002.03.0.94

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